Skip to content

Telequipment S31

June 24, 2011

What is there to say about this one? First, it was designed and marketed as a service scope, not a lab scope. The design is basic and functional with no power supply regulation. It does what it was designed to do well, far better than the plethora of recurring sweep scopes that were the mainstay of the low-end market at the time. Every time I turn it on, the display comes up just as I left it, clear and stable, no nonsense, it just works. The circuit description talks about how it is designed to be minimally affected by line voltage changes, even without power supply regulation. I tried this, varying the line from 115V to 125V. I noted no change in sweep rate and about 10% in sensitivity so, it works as intended, somewhat. For servicing purposes, it is accurate enough to permit diagnosis of problems. Unlike the recurring sweep scopes, it can make measurements of both voltage and time with perhaps +/- 10% accuracy in voltage and +/- 5% in time. Sensitivity is 0.1v/cm to 50v/cm with a passband of dc to 6MHz; the sweep rate is 500mS/cm to 1μS/cm. Useful. Actually, the passband is wider than that of the contemporary and far more expensive (and complex) Tektronix 310.

The tube line up is, 3WP1 CRT, EZ81 rectifier and 9 Mullard gold pin ECF80s (as always, hands off audiophiles!)

The amplitude calibrator is derived from B+ winding and uses NT-2 type neons to clip the sine wave to something resembling a squarewave and an adjustable attenuator to set the output to 1v p-p. This explains the soft rise time of the calibrator waveform shown. The only problem I had with it was that the time base would quit from time-to-time. I traced this to some carbon composition resistors that had aged high (which is “normal” for vintage equipment). Padding these back into tolerance restored proper operation. These were R58, R87 & R88.

Here are pictures of the “works”:

In the left picture, the metal brace mostly conceals the CRT shield. In comparison with laboratory scopes, there are few tubes. The right picture shows the quite neat tagboard construction. It really is quite nicely built. The power transformer is a double C core unit that seems out of place in a budget design!

The only other thing I have to say is that this is a triggered design and I think that people who are not familiar with the triggered concept sometimes think it does not work! (Of course, sometimes it actually does not work!) If it is properly set up then a base line should always be present when the trigger level is set to auto. This (in common with many Tektronic designs) causes the trigger circuit to operate in astable mode at some 50Hz causing the time base to run; when a signal is applied, it responds as a bistable causing the time base to lock onto the signal. Most of the time this is adequate, but for tricky signals, the triggering stability and level can be adjusted to obtain a stable display. Look at the top right of the front and you can clearly see the trigger level and stability controls.

Advertisements

From → All Posts

Leave a Comment

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: