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Retirement Home for Vintage Oscilloscopes.

NAVIGATION: You can access the posts from the page "Equipment List & Posts" above, the items I have written about are in blue and clicking on them will take you to the relevant post.
Note; CLICKING on the pictures and diagrams will enlarge them!

Oscilloscopes fascinated me as a kid, the ability to see what is going on in a circuit is magical and like many other kids interested in electronics in those days, I built my own simple oscilloscope, based on the Heathkit design that was published in the fourth edition (1968) of the Radio Society of Great Britain handbook. My first professional oscilloscope was a Cossor 1035 MKIII which started my odd obsession! I now have one in the collection, you may locate it by going to the "Equipment List & Posts Guide".

I am expressing this aspect of my interest in vacuum tube (thermionic valve) technology by creating something of a retirement home for oscilloscopes and some other test gear. I say that there was a golden age from the time that Hewlett Packard was formed lasting until perhaps the early 80s. I am particularly interested in Tektronix scopes; Tektronix was created by Howard Vollum and Jack Murdock in 1947. Howard and Jack were interested in creating not only truly excellent products but also that their employees and their families had great lives, a holistic approach. And that was before anybody started talking about holistics; I think that for people of the calibre of Vollum, Murdock, Hewlett and Packard, it was a matter of common sense and common decency. Quoting from Marshall Lee "Why did Murdock and Vollum institute a broad range of profit sharing and employee ownership in the company? Because neither man had as a goal the accumulation of personal material wealth", and "it just seemed the right thing to do." Compare this attitude to the bunch of rapacious thugs that constitute the body of corporate CEOs and CFOs today. I recently met somebody who knew Howard as a customer and said that he was his nicest customer. Of Jack Murdock, one of the original four partners, Milt Bave, said of him "He was probably the nicest person that I've ever met". And Tektronix was hugely successful so what was that about nice guys coming last? The corporate environment of today, savaged by the nonsense of Thatcher-Reagan “market forces” – and as a result inhuman and predatory - is a far cry from that golden age.

Speaking about Tektronix scopes, Jim Williams (author of Analogue Circuit Design) says of them that they simply radiate intellectual honesty. I respectfully agree, in fact, my experience is that truly well engineered devices (of all kinds) are imbued with the spirit of their creators. Other than Tektronix, I also have scopes by Cossor, Telequipment, Solartron, Heathkit, Siemens, Triumph and a couple of AN/USM units along with two solid state HP AN/USM 281 units, a solid state 180A, a 122A and an early HP 150A. Currently, my collection stands at some 36 oscilloscopes along with signal generators, meters e.t.c.

I will be documenting my collection here together with what I have learned that may be of use to you.

Historical and technical sources include:
Oscilloscopes, Selecting and Restoring a Classic (Tektronix) by Stan Griffiths, W7NI,
Winning With People: The First 40 Years Of Tektronix by Marshall M. Lee,
My Years At Tektronix by Miles Tippery,
Tube Testers and Classic Electronic Test Gear by Alan Douglas and, website associated with the Oregon Tektronix museum.

GE Type TP-13 Regulated PSU

Having decided to dispose of my collection, this is the first item I put on ebay and it sold in the first day, clearly I priced it too low at $135 however, it is the first thing I have ever sold on ebay, live and learn!
My ebay name is triodeguy.
I have many items that have not yet made it to the blog and as I sell them I will post the description and pictures so as at complete this documentation project, albeit without the technical and restoration narrative.
Here is the description I used on ebay:
GE regulated power supply used to power a transmitter modulator. Who knows, you may have listened to your favorite station when powered by this unit!
I fitted a terminal block to the cut wire ends. The wire nut connects the line to the B+ transformer to allow the B+ to be remotely switched. A B+ time delay could be installed here.
The raw B+ supply is configured as choke input, given that the massive 10H choke is rated for 340mA and that there are 3 6AS7/6080 series tubes, I would expect this power supply to be conservatively rated for 300mA.
The rotary switch allows the meter to indicate total current, current through each section of each series tube or output voltage.
I have powered it up and it delivers 300V DC that can be varied from 255V to 415V.
I load tested from no load to 300mA (at 300V) and the voltage did not shift more than a volt however, it IS old and I will not guarantee your satisfacton.
This is pro gear and has just one electrolytic capacitor, the rest are paper in oil.
Use vintage the pictures to form your own impression of the condition of this unit, NOTE, while testing it, I did find one dead 6AS7 section so I replaced that tube with a good 6080.
Again, being vintage, I DO NOT guarantee proper operation so this unit is sold as is.
It weighs 58 lb and measures 13″ x 10.5″ x 18.5″
And here are the pictures I used:









Time To Let Go….

3/19/2015: I sold 7 scopes, 2 meters a power supply and a signal generator this weekend, 4 of the scopes are full size (Tek 500 series and HP 150A). I am experiencing a sense of relief that this wonderful equipment is wanted and going to good homes. I am also relieved to know that the burden of stuff in my home is steadily reducing. I have moved into a new phase of my life and want to be able to move freely without having to deal with (literally) tons of stuff.

11/5/2014: I have decided that it is time to start letting go of stuff. If you are interested in acquiring any of the items I have written about, please comment on this post and I will get back to you.

80M (3.7MHz) Loop Antenna

8/9/14, UPDATE: I took and passed the general class today. Now the learning (leaning) curve begins……

One area of vacuum tube technology that I have not (yet) explored is (amateur) radio. I have a technician license and have been putting off doing anything more because I found the antenna (and ham jargon) issue daunting. One thing is that I want the feedpoint at the house, not in the middle of the yard! This makes a classic dipole less attractive given my house and yard. A friend (ham mentors are called Elmers) suggested a loop and sent me an excellent article describing the setup and performance of a loop antenna. Because it is a loop, the feedpoint can be anywhere so I have mine where I want it! The loop length is made equal to the wavelength of the lowest frequency of interest, in this case 3.7MHz giving 272ft. The larger the area enclosed by the loop the better, so the ideal shape is a circle. Mine is a pentagon because that gives me the best compromise given the wire length and the trees I have. The feed is RG8X 50Ω coax. I now need to learn how to use it!

I have a Heathkit SB-102 transceiver that I have repaired and checked out into a dummy load. It has a solid state Variable Frequency Oscillator (VFO) that is supposed to be significantly more stable than a tubed VFO. Using a MFJ Deluxe Versa Tuner II with the Heathkit, I obtained Standing Wave Ratio (SWR) figures of 80M, 1 to 1.1; 40M 1.2 to 1.3, 20M, 1 to 1.1 and 10M 2.5 at best. I need to upgrade my license so that I can use voice (SSB) on bands other than 10M! Some people use a 2:1 balun with a loop and I do wonder if this would yield better performance at 10M? I am a beginner….

By the way, the J in MFJ may stand for junk. The vanes of the variable condensers in my unit were rubbing due to massive slop in the spindle bushings that allowed the vane assemblies to flop around. I bought the unit on ebay however, it shows no signs of use much less abuse, so I think this issue is due to rubbish quality parts. I was able to slip plastic strips into the spindle bushings and remedy the problem. Even so, I would not use it with solid state transmitter finals.

The wire is 14ga stranded and insulated house wire. I placed loops of rope around the trees / branches that looked to be good locations for suspension. The loops are fitted with pulleys. Each suspension point on the wire is fed through an insulator tied to another rope (1/4in Daycron) that passes through the pulley down to ground level. In this way, the suspension points can be dropped down to ground level when the wire snaps (it will for sure). The length combined with the elasticity of the suspension ropes provides some accommodation for tree sway however, the mid-point suspension has a dead weight load consisting of two house bricks. (The dead weight tension method is used for the suspension of railway power catenaries.) At the lowest point, the chimney it is 24′ high and at the highest, around 40′. I hope this arrangement will accommodate most wind activity! The feed point is tied to the chimney of my house (which seems a better scheme than tying it to my neighbours house). It consists of a small plastic Radio Shack project box fitted with an eye bolt for the anchor. The ends of the antenna come in, then out and back in again through small holes providing sufficient grip to prevent them pulling out. The feed cable comes in through the bottom of the box and a tie wrap inside prevents it pulling out. All the entry points are sealed with silicon goop.

Oh, I had great fun playing with a potato gun to launch 3 of the suspension points……….

Here are pics of the suspension points:
Feed Point,
Feed Point
West corner suspension,
West Corner Suspension
West side suspension,
West Side Suspension
North apex suspension,
North Apex Tension Weights
Tension weights,
Tension Weights
East side suspension,
East Side Suspension

GR 1217-B Unit Pulse Generator.


I have decided to take on writing up at least some of the GR kit in this collection. This unit was for sale at an attractive price given that it included a very neat GR 1201-B unit regulated power supply. It did not work properly, ceasing to pulse whenever I turned the pulse width control much either side of centre. I did not do well in diagnosing the problem, diving in too deep before simply studying the thing. The result was a cycle of picking it up and putting it aside. A very perplexing issue was that the pulse width was off by a factor of ten and I got into all sorts of trouble with that one, sure that it was acting as a frequency divider even though that made no sense. Finally I saw it, a previous muddler had (re)connected the pulse width pot incorrectly, damn! And so it now works quite well. The Pulse Repetition Frequency was low and again, manual in hand went through all sorts of component value conniptions (based on the manual) only to realise upon studying the circuit calmly, that the tube condition would affect the PRF. The manual makes no mention of this yet looking at the circuit it must be, and is. I simply went through a number of tubes until the PRF came in correct at the calibrated position of the PRF pot. Such dependence on the tube is not necessary and is surprising to me considering the reputation GR has.

I don’t have a lot more to say about it. It works well after many the self-made detours. The manual claims that it can deliver rise times of less than 18nS and fall times less than 10nS. I was able to see 18nS and 10nS on my Tek 475 (250MHz bandwidth) but not less. It does find quite a bit of use on my bench.

GR 1217-B 1


Repetition Rate, 2.5Hz to 500kHz with calibrated points in 1-3 sequence from 10Hz to 300kHz plus 500kHz. Continuous coverage with uncalibrated control.

Duration, 100nS to 1S in 7 decade ranges.

Pulse Output Levels: + and – 40mA pulses, each 40Vpk into internal 1k load. DC coupled with DC component negative wrt ground.

The 1201-B power supply provides a regulated floating 300VDC up to 70mA and 6.3VAC at 4A. In the pulse generator, the floating voltage is referenced to ground at -150V and +150V.


The pulse repetition generator is a Schmitt trigger with a RC circuit charge/discharge appended. The C of the time constant is charged from the plate circuit of the A section via the R of the time constant that is between the plate and the grid, thus the charge and discharge of the grid drives the A section.  If the A section is off (plate high), the capacitor will charge through the R until the A section turns on*, turning the “B” section off. This action happens suddenly due to the regenerative switching action of a Schmitt trigger. The B section has an inductor in its plate circuit and the sudden release of energy due to the B section turning off produces a sharp pulse that is used to trigger the output pulse timing circuit. When the A section turns on, its plate voltage will fall and the capacitor will discharge until the A section turns off again. In this way, the oscillation swings takes place within the hysteresis of the trigger. The reason for PRF dependency on the tube as well as the RC values is that the hysteresis of the circuit will vary from tube to tube. The PRF ranges are controlled by switching in different values of C and continuous control is provided by making R variable from the front panel.

* The manual incorrectly states this as off. Here is the repetition generator circuit:

GR 1217-B Repetition Gen Circuit

V101 serves as a current source when the circuit is in generate mode (switch position O), allowing the Schmitt trigger to freely swing around its hysteresis point. In position A, it acts as a driver for the Schmitt trigger to allow the circuit to be driven from an external source.

The positive trigger pulse from the repetition generator is applied to the pulse timing circuit and then the whole bloody thing goes belly up, or at least it did until I spotted the wiring blunder.

Q101 is normally on and with it, V103A and diode V103B, holding the voltage at the junction of R118 & R122 at a level determined by the setting of R125, the Pulse Duration control. Since Q101 is on, V105 is also on producing a current in the positive pulse output load resistor, R130.

A positive pulse trigger pulse from V102 (the pulse repetition generator Schmitt circuit) turns Q101 off and with it, V105 and V103A. Q102 turns on, and with it V106, producing a current in the negative pulse output load, R133. C2 begins to charge (ramp up) via R118 and the grid voltage of V104 rises until this tube conducts and the Schmitt circuit V104A and B changes state, sending a positive triggering pulse to Q102, turning Q102 off and Q101 on, re-establishing the initial circuit conditions. The circuit is now ready for the next cycle initiation pulse from the Pulse Repetition Generator. The pulse duration is therefore controlled by the time constant R118, C2 and the initial potential on C2 established by the Pulse Duration control.

GR 1217-B Pulse Timing and Output Circuit


The pulse output circuit consists of pentode V105 and V106 that are switched as described above by the transistor bi-stable, Q101 and Q 102. In the initial state, Q101 is off and with it, V105 causing the output pulse to rise to ground, going low when the circuit changes state. V106 operates in the opposite direction. The screen voltages set the zero bias currents at 40 to 45mA and since the output loads are 1k, the pulse amplitudes are 40 to 45 volts in maximum amplitude.

GR 1217-B 2





GR 1201-B Unit Regulated PSU

I have one of these neat power supplies each for my 1217-B Pulse Generator and 1210-C RC Oscillator so I thought I would do a quick post on it. It supplies a floating precision 300V at 70mA and 6.3V at 4A.

A search on the net did not bring up the schematic for this unit so I have traced it out:

(P.S. After doing this I did find a link to the manual for this unit! Go here)

GR 1201-B PSU Schematic

Apart from the primary control loop from the output to the series pass tube via the error comparator (12AT7), amplifier (6AN8 P) and cathode follower (6AN8 T), there are two other signal paths: C531 and R533 feed input fluctuations forward to the grid of the series pass tube in cancelation phase via the amplifier (6AN8 P) and R540 serves to make the open loop gain infinite thereby allowing very low output impedance. The technique of regenerative feedback to allow infinite open loop gain coupled with degenerative feedback to realise extremely solid stability and freedom from drift is also used in the TS-375 A/U VTVM DC amplifier which is the subject of my previous post.

I haven’t characterised the impedance vs frequency however, the use of a grounded grid pentode amplifier with a cathode follower to drive the grid of the series pass tube clearly indicates that it was designed to have excellent performance at high frequencies. My own regulator designs usually include feed-forward (DC coupled) but I have never attempted to make the open loop gain infinite.

Here is the inside with the PCB flipped up:

GR 1201_B PSU


INTRODUCTION: This unit potentially has practical value to me since it is both an AC and a DC Valve-Voltmeter, also as an AC meter at 100MHz plus, it has a much wider bandwidth than my AC only HP 400H (4MHz). (By the way, Valve Voltmeter because I am of English origin and I think that valves are SO much more interesting than toobs, you see.) I have had this VVM for quite a while and it was one of those projects that I flirted with and then set aside, again and again. The reasons for this were 1, it did not work properly and 2, I could not understand the circuit; I finally re-drew the DC amplifier and meter circuit for clarity and I have included this diagram in the theory section below.

CONDITION: It is in excellent condition with tubes that are effectively free from microphonics and balance perfectly, and the balance once set, stays put. However, I could never get it to read accurately and consistently. I had removed the meter cover and gently blown on the needle, it seemed completely free however, I finally realised that it was in fact sticky. The disturbing force due to blowing is quite a bit stronger than that due to the available magnetomotive force! Since the meter is quite well sealed, I felt that the problem was not likely to be particles in the gap so I gingerly backed off the top pivot. Sure enough, the meter cleared and now does move under electrical stimulus freely and can be reversed at any point without sticking. I cannot offer any explanation as to why the pivots were dragging other than perhaps corrosion. Having said that, I do wonder at the quality of the (Simpson) movement; The mass balance of the needle is very poor despite the presence of balance weights, and it can only be calibrated and used either flat or vertical. I set it up flat because I will use it on a bench, not sitting on a shelf. The picture below shows the large mechanical zero error due to the mass unbalance:


DESCRIPTION: It is an extremely neat unit, and typically for military equipment is enclosed in a grey aluminium case. (By the way, I do have the knobs from the terminals, I simply forgot to replace them before taking the picture.) Both the AC and DC probes are present however, other than the power cord and spares fuses and lamps, none of the other clips and cords are present. Here it is showing the probe compartments open:


The AC input at the terminals is routed via the rectifier probe which is stored on a mounting clip that connects to the probe tip and probe grounding ring. To make accurate measurements above 100MHz, it is necessary to connect the source signal ground directly to the probe ground ring and test tip directly. Even at 40MHz, the leads should not be longer than 3 or 4 inches, according to the manual. The probe tip and ground ring with their respective cradle contacts are both shown in the picture below. The ground wire should not be more than at the most 4″ long. This means that the source ground and test points must be close together. One problem was that the tip of the AC probe was shorting onto the case, you can see where the paint has been scratched. I managed to adjust the cradle a bit but it is still close:


Here is the spares compartment:


And here is what is contains:


And finally, before we get in deeper, here is the inside, note the two spare knobs and spare rectifier on the left by the pots:



AC and DC voltage ranges, 1.2, 3, 12, 30, 120, DC only, 300.

Input loading, DC 30MΩ, that is useful for me!

AC, 5MΩ shunted by 70pF at the panel terminals or 5pF at the AC probe tip.  The manual has a graph showing that the resistive component of the loading falls (the load increases) with frequency, two points from the graph being 5MΩ at 10KHz and 90,000KΩ at 100MHz. A good scope with a top quality X10 probe can do better, typically 10M shunted by 2pF. However, when this unit was produced, scopes that could match the bandwidth of this meter existed only in engineer’s dreams! (Engineers are weird like that, I KNOW!) Also, this unit is much more handy than a lab grade scope of the era in any case.

Claimed Accuracy:

DC, all ranges 3% FSD.

AC, 10 to 50Hz, 5% with correction curve

50Hz to 100MHz, 4% without correction

50MHz to 150MHz, 3% with correction curve

150MHz to 300MHz, 8% with correction curve

Observed accuracy:

The meter meets the % FSD specification on the AC and DC ranges. I made at least 3 spot checks on each range after some tweaking. It is worth noting that as is normal for meters, the specification is given as % FSD; The % of actual values is often quite large but within 5% except for the actual value at 1V on the 3VDC range which is -6.25%, this translates to -2.08% FSD. Most DC errors were on the low side which suggests that I might be able to set the calibration better. Having said that, the AC errors were a mix of high and low so to actually accomplish better calibration would be very tricky and most likely not stable, so I will not attempt to “improve” it.

It is essential to carefully zero the meter with the input terminals shorted on each AC range, you cannot simply switch from range to range. The zero variation on the DC ranges is negligible. This is very likely due to diode contact potential about which I say more in the theory section below.

I checked the frequency response. It is claimed to be flat over the range 100Hz to 100MHz which if true is excellent, for the period of this meter at least. I checked this aspect out using a HP 8601A generator terminated into a 50Ω through BNC with the AC probe and ground ring connected right at the termination, on the 3VAC range I observed a 1dB increase up to 97MHz increasing to +1.5dB at 110 MHz which is the generator limit. Clearly there is a resonance somewhere in the signal path yet I am impressed. This meter is better than it seemed to me at first, second and third blush. This is the connection arrangement to the signal generator:

TS-375A/U HF Test Connection to Generator

Overall, this seems to be a capable instrument, at least in the context of its vintage. However, it is essential confirm to that it is reading accurately in the range you desire to use by connecting a known accurate meter in parallel with it and testing it using a DC or true sine AC supply. This is not as silly as it sounds because the point of this meter is the extremely light load it applies to the circuit under test and its wide bandwidth. Your DVM probably cannot match it in these aspects.



Essentially, it is a DC amplifier and meter. A good DC amplifier is not a trivial proposition because of drift. Most VVMs rely on large amounts of degenerative feedback to stabilize the circuit, however this can result in problems with HF oscillation. Perhaps that is why the HP 400H mentioned above is limited to 4MHz. This design relies on both regenerative and degenerative feedback that I will attempt to explain with the aid of the manual. (The HP 425 Micro-Volt/Ammeter uses a light chopper that produces a rough squarewave that is proportional to the DC current and in this way, completely removes drift. This is followed by an amplifier having a passband at the chop frequency that suppresses other frequencies (including 60Hz) heavily. In fact the chop frequency is chosen not to be harmonically related to line frequency to further ensure no hum on the extremely fragile signals. The amplified chop is then demodulated, rectified and applied to the meter. I think this exemplifies that DC amplifiers are truly non-trivial!)

Because it is a DC device, AC currents must be rectified and this takes place in the AC probe. The result is a peak voltage and this is corrected such that the resulting DC voltage presented to the DC amplifier is equal to the RMS voltage, if the AC current is truly sinusoidal*. This correction is applied by R-111 in TS-375/U and fixed R-133 plus variable R-134 in TS-375A/U. If the waveform is other than sinusoidal, a form or crest factor must be applied. The AC rectifier is right at the probe tip so that the AC signal does not have to travel along a wire and this is how the wide bandwidth is realised.

* The GR 1201-C oscillator that I was using to test and calibrate the AC ranges with has a slight clip on the positive peak (I don’t like GR equipment other than their bridges, heresy I know) and this was sufficient distortion to confound my efforts. The HP DVM I used as a reference would indicate the true RMS of this signal but the VVM would not. I ended up using a HP 205AG that produces a clean waveform with enough available amplitude to test the 120V range.

First here is the simplified circuit:

VTVM TS-375 A:U DC Amplifier Simplified Schematic

The screen cross-coupling provides the regenerative paths while the DC plate to grid offsets, (coupling batteries in the diagram) provide the degenerative paths and the output terminals. The plate resistors and the tubes comprise the 4 arms of a bridge and so you can think of the meter as being across the bridge from one plate/plate resistor node to the other plate/plate resistor node, via the coupling batteries. This visualisation may help you to see the bridge.

Degenerative feedback brings stability, any error is returned to the input as a cancellation voltage. However there is a limit to the degree of degenerative feedback possible with a conventional amplifier due to the limits of the single stage amplification. In this design, the additional gain needed for stability is obtained by regeneration, permitting a higher level of degenerative feedback and thus higher voltage stability in this single stage circuit than would otherwise be possible. Quoting from the manual “By this method a gain ratio greater than the actual gain of the amplifier tubes proper can be realized.”VTVM TS-375 A:U DC Amplifier Schematic

Now to the actual circuit: The “batteries” have been replaced by gas voltage stabiliser tubes, the internal resistance of the tubes being low enough to act effectively as batteries in this circuit. The strike currents for the stabiliser tubes is provided by the keep-alive resistors, R-106 and R-107, each being 100k. These terminate at the negative supply lead. The keep alive currents are too great for each plate circuit so the positive end of the strike tubes are fed via V-103 cathode followers from the + supply lead, the cathode followers being driven by each screen grid. In this way, the practical circuit emulates the simplified circuit. The resistor R-108 is included in the cathode circuit to allow for the voltage dropped across R-106 and R-107.

I found the circuit as drawn difficult to follow so I re-drew it thus:

VTVM TS-375 A:U DC Amplifier Schematic Redrawn

The regenerative (positive) feedback paths are obvious, R-104 limits the regeneration to a prescribed level. What was not obvious to me was the degenerative (negative) paths and the meter circuit so my re-draw show how the meter is connected to each plate/plate resistor node of the bridge via the screen CFs and the gas tubes. The gas tube keep alive resistors are effectively in parallel (Thevenin) with the plate load resistors. The degenerative feedback paths are shown in blue and red. Taking the blue first, the path is plate to ground then via the input voltage, to the input grid. Since the whole circuit is floating, the ground connection does not affect the operation. The red feedback path includes the meter which has a negligible resistance of 1K. Any grid current is orders of magnitude too small to cause sensible deflections of the meter which is driven by the current due to the unbalancing of the plate voltages.

(At this point a question arose for me: Why isn’t the meter simply connected directly across the plates? The answer is that the resistance value of the meter path would reduce the regenerative feedback too much. As it is, the CFs isolate the R-104 regeneration path from the meter path. It is a clever circuit.)

At the point of optimum adjustment, the screen-grid circuit is critically regenerated and on the point of self-sustaining oscillation if the degenerative paths to the control grids were disconnected. The manual suggests thinking of the circuit as an amplifier having infinite gain due to the critical regeneration which is completely degenerated by external feedback paths (the blue and red paths); thus the stability improvement possible due to degenerative feedback around any amplifier is in this case, carried to the limit.

At this point of critical regeneration, the screen grids do the actual work of unbalancing the bridge to produce an output (proportional to the input) and the control grids simply serve to initiate the unbalancing action. For any steady value of input, the potential of each control grid with respect to its cathode is the same and the potential difference across the grids is zero. The value of zero grid voltage excursion is that it entirely removes the effect of the curved grid voltage, plate current relationship and the amplifier is strictly linear in its input to output relationship. As I said, it is a clever circuit!

I mentioned diode contact potential earlier: A hot cathode diode will develop a negative potential on the plate with no ac input, this potential is termed the “contact potential” of the plate wrt the cathode. The diode in the probe is compensated by adding a similar diode to the red side grid circuit. I suspect that the very slight change of DC resistance across the compensating diode as the range is changed may account for the need to re-zero the balance every time the ac range is changed.





GR 1568-A Wave Analyser


This is the first item of GR equipment that I have written about. I have been resisting (and continue to resist) GR gear that in it’s way, is superb and accurate, but fussy. The mechanical construction tends to be tricky and every item I have is hard to work on. I think that the electronics must have been designed by mathematicians, not engineers. This is not helped by skimpy, confusing manuals that I need a magnifying glass to read. Given GR’s reputation, there is no excuse for such poor manuals that contain difficult to read schematics with obscure descriptions and instructions that contain many errors. It is almost as though the construction and the manuals conspire to protect the secrets within!

GR 1568-A Wave Analyser

The 1568-A usually comes with an XLR input connector at the top left (the third pin was used to provide a microphone energisation potential), this one has an input level control. (I have marked a full clockwise arrow to indicate calibrate next to this control.) I replaced the God-forsaken 1/4″ jack input with a RCA connector. My experience of the reliability of jacks is dismal.


The GR 1568-A is a wave analyser that operates over the range 10Hz to 20KHz. According to the manual it was “inexpensive”, I take that with a grain of salt. The filter is of the constant percentage bandwidth type where the width of the response peak measured between -3dB points is always 1% of the centre frequency resulting in more than 75dB attenuation an octave away from the center frequency. Again, according to the manual, it is particularly suited to analyse closely spaced components common to the vibration spectra produced by various types of machines (presumably including record cutting and transcription machines).

The analyser consists of three main sections, an input amplifier, the variable peak filter and an output amplifier / meter driver. There is also a precision step attenuator that consists of a maximum input control and a meter range control.

The filter consists of a synchronous cascade of two resonant (second order) filters, the first filter having high Q and the second low Q. This filter system is continuously tuneable from 10Hz to 20KHz, the synchronisation being accomplished by two precision wirewound frequency potentiometers that are geared together.


Well, in a word, tricky, this is my first experience of such a machine and I have found it extremely challenging. In principle, the first thing to do is to set it in calibrate mode at the centre of the frequency range of interest. (The manual does not specify whether centre refers to the log centre of the linear centre. For example for a range of of 1KHz to 10KHz, the log centre is 3.16KHz and the linear centre is 5.5KHz. This seems like pedantry I suppose and maybe if I understood the mathematics properly I would know.) The calibrator is interesting: When in calibrate mode, the output of the filter amplifier is fed back to the input. The forward gain is set using the input and meter range step attenuator by placing white dots on each control over each other at 12 o’ clock. The loop gain is controlled by the cal control and is to be adjusted such that the gain of the analyser is equal to the loss in the feedback network such that the system oscillates, the resulting signal being displayed on the meter. The cal control is operated so that the meter needle lies in the calibrate region on the meter face.

The system then may be adjusted to indicate the fundamental frequency of the signal to be analysed at some indicated reference level, either voltage or 0dB. Then sweep the frequency upwards, noting the frequency and amplitude of the harmonics. It is nice to set it in automatic so that the frequency range indexes automatically. The meter is scaled 1-10 and 1-3 also +2dB to -15dB. If the amplitude of the signal falls too low to be resolved accurately on the meter, the meter range may be stepped downwards in 10dB increments or corresponding voltage increments, thereby increasing the sensitivity by a known amount. The overall range is 100µV to 300V full scale. The analysing range of the system is given as 80dB so for example, if the fundamental is 1Vrms, then the smallest measurable harmonic would be 100µVrms in amplitude. I would think that battery operation is necessary to realise the highest sensitivity of the unit.

The intention of the automatic mode is interesting: That is the frequency control may be rotated by a drive from a GR1521 chart recorder so that as the pen moves across the paper, the frequency sweeps upwards, the range automatically indexing with each rotation. Type GR 1521 is fitted with a drive sprocket. The knob on the 1568 is replaced with a corresponding sprocket and placed on top of the 1521. The drive is hooked up with a chain and set to a slight backlash with an idler sprocket. The days before computer based analysis! It would be fun to see this set-up working, preferably demonstrated by a competent operator, not me!


Completely dead. I expected that the two 9.6V Ni-cad batteries would be defunct however, it turned out that the meter coil was open-circuit. The wire was so fine that I could barely see it using a magnifying glass, and I am quite good at finding broken ends of fine windings and repairing them. It was a Jewell taut-band 100µA device. I scoured the panel meter offerings on Ebay and eventually took a chance on a nice looking 100µA taut band meter, it that looked as though I might be able to transplant the movement from this meter  into the GR meter case. I got lucky, it worked out beautifully though the needle is a little short. I find it does not hinder reading the meter significantly and the linearity of the scaling matches the GR scale well. The picture of the analyser above shows it with the rebuilt meter. Here is the original movement:

Original Meter Movement

Here is the transplant:

Replacement Movement API 7035

Another issue was a failed zener in the high Q section power supply, easily replaced. Later, one section of the 4 section precision wirewound frequency pot in the high Q section went open. I got lucky here in two respects, 1, it was the top section than is relatively easy to open and 2, all it took to repair it was to re-flow the resistance wire connection at one end. Phew! Here is the open pot:

Frequency Pot, first section

Here is a view showing showing the high Q and low Q pots that are geared together. Without doubt I got lucky with the location and nature of the pot fault! NOTE, I took GREAT CARE to place index marks on the potentiometer shaft, rotor and case before disassembly!

Frequency Pots

Here is an overall view. Partially visible on the bottom right is the potentiometer rotor. The high Q board is on the left, the low Q to the right of it. Underneath the filter boards is the frequency multiplier switch that is rotated via bevel gears (visible in the picture behind the rotary switch) either directly from the front panel or by a motor when the unit is operating in automatic mode. This compartment is completely shielded, closure being completed by a plate over the top. To the right of the filter compartment is the PSU board and above that is the meter drive board. The capacitors are for the 10Hz range with the attenuator between. The two frequency potentiometer shafts are visible at the bottom, the printed switch cuts the filter path when the dial is crossing between ranges and the micro switch on the right indexes the frequency range motor when the unit is in automatic mode or when operating manually and crossing over up to the next range. This mode is not available when the unit is powered from the batteries.

GR 1568 -1