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Old 26th Nov 2017, 3:22 pm   #1
Hans Steenman
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Join Date: Nov 2017
Location: Doorn, Netherlands
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Default Telequipment D43 scope type C amplifier

My D43 scope works fine after restauration.
The only remaining problem is in the type C amplifier:
when I set the sensitivity to it's maximum level (100 uV/cm) the trace becomes very jumpy. I jumps up and down over several divisions.
Only back to a sensitivity of 1mV the trace is almost back to normal.

The unit has a regulated voltage supply for the preamplifier stage. Measurements are hard, because I do not have an extension cable to operate the plugin outside the scope.
I have replaced the regulated supply by a properly installed OA2.
It doesn't make a difference.
What else can I do?
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Old 26th Nov 2017, 5:13 pm   #2
ronbryan
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Default Re: Telequipment D43 scope type C amplifier

Hans

Is your observation of a jumpy trace at 100uV/cm on the 'C' plug-in made with the input grounded or open?

On my D43 'C' with the input grounded at 100uV/cm there is noise on the trace of around 0.1 divisions, a short term variation in the horizontal trace position of about 0.2 to 0.3 divisions plus a longer term random jumping of the trace of about 1 division, which might be due to mains voltage fluctuations. There is also drift of the zero position with time.

The D43 is a relatively cheap scope and you should not expect too much from it at that sensitivity, you cannot expect the same performance as you would have got from a similar vintage high sensitivity valve scope such as the Tektronix 502A. Improvements can be made by replacing carbon resistors in the front end amplifier with metal film types and any Plessey electrolytics (HT, and cathode decouplers and in the V1 heater DC supply) may well be past their best. The 5k pot RV8 may need cleaning as it is a gain adjustment. I presume you have subbed V1 (ECC82?) and cleaned the valve pins and socket.

It is possible to extend the plug-in power connections to enable bench testing with wires tacked on to the back of the module connectors. In addition one plug-in can be removed and the covers taken off the remaining unit in the lower deck, to permit in-chassis testing of the plug-ins one at a time (with care).

Ron

Last edited by ronbryan; 26th Nov 2017 at 5:25 pm.
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Old 27th Nov 2017, 2:21 pm   #3
Hans Steenman
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Default Re: Telequipment D43 scope type C amplifier

Hallo Ron,
thanks for your reply.
I know the TEK 502 well, and realize they're not the same. The Tek has similar sensitivity as our C plugin, but then DC coupled!
The fluctuations which bother me are totally due to mains variations. I hooked up a Variac and at the slightest movement of the voltage control I see the trace jump up and down.
I actually have 2 type C units, and they behave the same.
So my conclusion now is that something is wrong in the HT+2 245V supply, which they have in common. Not much choice there: must be C431 or resistor R433. Or in the preceding 380V section.
I am going to locate these elco's (the manual isn't very clear on physical lay-out) and bridge or replace them with a good elco. Etc....
Initially I left the elco's alone because the voltages and ripple are just about correct.
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Old 27th Nov 2017, 11:22 pm   #4
MotorBikeLes
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Default Re: Telequipment D43 scope type C amplifier

Hans, there are two 100uFd ca[ps in the HT+ voltage doubler. I have known these go low capacitance (but a bit rare).
I can't remember what is "under the bonnet" in the Type "C", but in the type "A" there is a 10uFd bipolar capacitor. Usually a bit dry and low capacitance. I have used "normal" non-electrolytic types to replace in the past. It is across the valve heaters. C20 I think looking at a blurry type "A" circuit.
Les.
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Old 29th Nov 2017, 1:29 pm   #5
Hans Steenman
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Default Re: Telequipment D43 scope type C amplifier

I have tried just about everything, but the trace stays jumpy.
Easily a division, occasionally more.
At 1mV/cm everything is normal, only at even higher sensitivity it gets jumpy.
According to the spec the C pluging will jump 1/3 division at 100uV/cm.
Maybe they had a more stable mains supply!

I'm happy with this scope and close the book on further improvements.
Thanks for your contributions.
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Old 29th Nov 2017, 4:11 pm   #6
ronbryan
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Default Re: Telequipment D43 scope type C amplifier

Hans

I think that Telequipment must have been dissatisfied with the performance of the x100 amplifier in the 'C' unit, as they appear to have designed an updated version, the C-2. This plug-in amplifier x100 range has a zener stabilised supply and a FET input followed by a bipolar transistor, the remaining valve circuitry for x10 and x1 being similar to the standard 'C'.

The C-2 manual even has a specification for x 100 range hum and noise, which is more than I could find for the standard 'C' unit.

Ron
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