My Telequipment S31's

[britishrocket]

Finally got into one of the S31s. Here are some photos.

[britishrocket]

Greetings,

Well, I blew the unit out with compressed air yesterday. Lots of dust.

Can anyone suggest a method to test the eht selenium rectifiers? I'm told they can't be tested with a diode checker. Also, the "brown German caps that go s/c"; are these the WIMA ones?

[ronbryan]

The Wima caps shown on your post #21 are of the later Wima TFF or TFM type, which I have found to be quite reliable. The change on sight flat axial leaded dark brown Wimas were fitted to older versions of the S31 that used point-to-point wiring, not the ones that used printed circuit boards.

Ron

[Diabolical Artificer]

Are they K8-50's ? If I remember right, you can test them using a PSU, say 15v and put a resistor in series (1k) with the the DUT. A good one should give 12v forward biased, a bad one 15v.

To be honest though I wouldn't bother. I tested quite a few, got mostly dud's, a "good" one died after several day's. I replaced mine with Vishay BYX10GP's which I got off ebay . You need to put a resistor in series with them.

There's quite a few thread's on here about them. They were used in quite a few Telequipment scope's.

Andy.

[WME_bill]

S31. Testing EHT rectifiers.
They are made up of a chain of elements in series. You can test them quite satisfactorily on the high ohms range of an Avo or similar. What is needed is the 15V test voltage. You would not try to test a dozen silicon diodes in series on a normal ohms setting, which uses 1.5V, as they would never switch on.
If your meter only tests on 1.5v range, then use a 10v/15v supply, in series with a sensitive meter, and a current limiting resistor. Don't burn out your meter.
I have just tested my stock of spare K8-15, K8-25, K8-40 STC rectifiers on my Avo8 ohms x100 setting, and they all test fine. Conduction one way, no conduction the other. What more do you want of a rectifier.
I would not worry about changing capacitors unless they are faulty. You run the risk of causing more problems than you cure.
The manual gives a detailed list of working voltages. If they are about right (+-10%), then OK.
But that will identify what could be a leaking or failed capacitor. On the TB range capacitors, a range hopelessly out, or non-linear is your best indicator.
On my S32, I had to replace one electrolytic (voltage rails out) and one TB timing capacitor. Not too bad for equipment made in early 1960s.
wme_bill

[britishrocket]

Thank you for all the informative replies. I think on balance that replacing the eht sticks with high voltage silicon diodes and a series resistor sounds like the way forward

I did a general visual inspection of the unit earlier today and discovered a damaged capacitor on the time/division switch. I have appended a couple of photos of this.

WME bill, I'd agree that a couple of faults like those you describe on an instrument made in 1962 or so is a highly creditable showing.

Carl.

[Diabolical Artificer]

Perhap's temporaraly repair it with a bolb of solder and see if the CRT is good after you fixed the EHT supply.It's a pig to get in there. Once you have a trace you can tweek the rest.

Good to see your making progress Carl, Andy.

[britishrocket]

The cap reformer is almost finished and I have ordered the 1.6kv diodes for the eht multiplier. I'm thinking about 100 ohm in series with each one. Anyone concur?

[MotorBikeLes]

I vaguely remember using much higher, 4K7, but it may be poor memory.
Check EHT- BEFORE changing, then AFTER changing. You are aiming for the same value as shown in the manual.
If you use 4K7 and you get less than manual shows, reduce to say 1K0 and try again. Repeat until correct.
Note you MUST check mains voltage and voltage setting on scope are the same before doing the above. Once you get the PSU and EHT voltages correct, I recall that this scope will compensate pretty well against mains voltage variation in use.
Les.

[britishrocket]

Thank you Les,

I will go with 4K7 initially. If the eht is low then I can alter the value. Better to start off too low, if anything.

[Argus25]

Originally Posted by britishrocket

The cap reformer is almost finished and I have ordered the 1.6kv diodes for the eht multiplier. I'm thinking about 100 ohm in series with each one. Anyone concur?

I would tend to want to use a much higher voltage rated silicon rectifier to avoid future failure. One cheap and easy to get type is the common garden microwave oven diode (a plethora of them cheap as chips on ebay) rated at around 10kv usually.The forward drops of these are still lower than the vintage stick, despite the much higher reverse breakdown voltages.

I agree, a suitable resistor would be in the 500 ohm to 5k region, as noted by MotorBikeLes, found by experiment. But be aware of the EHT voltage measurement, depending on the meter & probe ,that you don't get an artificially low reading there due to loading by the meter, hopefully the Ohms per volt of the meter that the manufacturers used to measure the EHT value was documented.

The HV stick rectifiers in my D52, were taken out by the leaky high voltage filter caps, these were replaced with high voltage poly caps.

(As another rectifier option; there are some beautiful high voltage rectifiers made by VMI, Voltage multipliers inc, their products are outstanding;
http://www.voltagemultipliers.com/html/diodes.html )

[MotorBikeLes]

Argus25, no worry about a 1.6kv rectifier in this case. The tube runs with about 1400v, so I would guess 350v+ HT, and -1050v for the EHT-.
Your D52 has a (fairly low voltage) PDA tube, so 4 or 5 KV accel voltage, far more than the S31.
Les

[britishrocket]

Thank you both Gentlemen for your experience and knowledge.

I just have to fit the 12 way rotary switch to my cap reformer now. Barring any teething problems it'll then be usable.

My plan is to replace the eht sticks, reform all the big electrolytic and then apply mains via a lamp limiter.