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Old 26th Oct 2003, 10:14 pm   #1
Chris_Peacock
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Default Telequipment D31 oscillosope - no X deflection

Hi,

I've just bought a Telequipment D31 oscilloscope. The only problem with it appears to be that there is no movement of the beam(s) on the X axis whatsoever. I did intend to repair it myself, but I'm a little stumped being a beginner with these things and not being able to find any service information for this model.

Can anyone provide any information on this model? Also, what are the common causes of a problem such as this?

I've poked around to some extent with a multimeter. The external sweep output seems to have a fluctuating signal. The terminals on the sides of the CRT have positive voltages (am I right in thinking these are just for the Y deflection and the focus?), but I didn't want to test the terminals at the end of the CRT as my meter only goes to 1000V and I don't know which pins are which!

Any help would be greatly appreciated.

Thanks.
 
Old 26th Oct 2003, 11:20 pm   #2
jim_beacon
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Default Re: Telequipment D31 oscillosope - no X deflection

Hi Chris,

firstly welcome!

Do you hava spot on the face of the tube? If so, do the focus and intenesity controls seem to work correctly? If these are OK, it is fairly safe to assume that the tube voltages are OK.

Next, do the X and Y shift controls work? Controls work, then the amplifiers are working.

From what you say, I suspect that the X amp has failed, and would suspect that the valves are the most likely culprits.

You can get a manual for the S31 (single trace version) from Duke Nukems site, at

http://www.thevalvepage.com/servdata...p/telequip.htm

Good Luck

Jim.
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Old 26th Oct 2003, 11:34 pm   #3
Duke_Nukem
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Default Re: Telequipment D31 oscillosope - no X deflection

If you are used to modern scopes then you need to be aware of the triggering behaviour of these old telequipment scopes. I bought one cheap cos the guy said " tube lights but no trace " ... this was just because the triggering wasn't set right on the front panel ! I though this fussyness was a fault, but having now aquired a few I can see that they are all like this.

Upshot is a static spot just off left of screen.If there is no spot on screen, turn up brightness and adjust X shift to see if you get anything appear on the left (as soon as something does appear reduce brightness since dont want to burn the phosphor). Try different timebase settings as these switch in different timning caps, and dodgy timing caps can stop the timebase from running.

How big is the signal on the external sweep input ? If pretty small (volts) then the timebase certainly isn't running. If big then as Jim says its probably the X-amp, though I'd suspect dodgy caps long before a dodgy valve ; a dodgy valve often limits or shifts the X movement but not stops it altogether. If the X-shift control has an effect then blame the trigger circuits first closely followed by the timebase.

TTFN,
Jon
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Old 26th Oct 2003, 11:45 pm   #4
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Default Re: Telequipment D31 oscillosope - no X deflection

Thanks very much for your really quick and helpful replies!!!!

From what you both say, I suspect the X amp - there are two stationary (with regard to X) spots, the focus and intensity controls seem to work OK. The Y shift controls work fine, the X shift has no effect whatsoever. I've tried all the timebase settings, none of them work. The signal on the external sweep is about 40 volts.

Thanks for the link and the service data too - I'd already downloaded what I thought would be the closest, for the D43, but that one has transistors in!

Does the S31 use the same CRT and amp circuits as the D31?

Many thanks.
 
Old 27th Oct 2003, 1:42 pm   #5
Duke_Nukem
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Default Re: Telequipment D31 oscillosope - no X deflection

The S31 and D31 will use different CRT's, though I'm not familiar with the D31.

Are the stationary spots in the center of the screen ? The X-amp is usually driven by two pentodes arranged differentially and it would be a strange coincidence indeed if failure would give spots in the middle of the screen, particularly if the X-shift isn't doing anything either.

It almost feels as if the connections to the plates are missing. I don't know the D31's CRT, but some CRT's have deflection plate connections on the side of the tube instead of the base, so if tube has been removed at some time these could be disconnected ? Deflection could be killed even by a single plate disconnection I think.

If you can access the plate connections, measure volts from each plate to HT, then each plate to 0V ; If you get similar readings then connection is o/c or there is no HT.

So, another possibility is loss of HT to the amp stage.

Another is the X-Gain control (if there is one); this usually sits between the cathodes of the two output pentodes. Try both extremes of this control.

X-gain ... hmmm ... a short circuit between the two cathodes of the output pentodes would give no deflection. Also check that cathode-to-ground resistor is not open circuit. Can't do ascii art but you may well find that the basic o/p circuit is two pentodes whose cathodes are connected via a resistance (well, lots of resistors and caps actually), the center of which goes to ground (or a.n.other voltage) and it is this latter resistance to check for o/c.

TTFN,
Jon
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Old 27th Oct 2003, 8:54 pm   #6
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Default Re: Telequipment D31 oscillosope - no X deflection

Jon / Chris

deflection is not always killed by a single plate disconnection, but it does do some very odd things, as the isolated plate tends to build up charge, and push the beam one way or the other, in a seemingly random way!

The voltage level om the X output seems about right - though without knowing the scope, it is difficult to be sure. Modern scopes tend to put out about 15V, where as my old Cosor 339 will give over 100V on the X out terminal! (you need it to drive one of those Cossor alignment generators / wobbulators)

I would agre with Jon that the fault lies in the X defelction amp. This is a DC coupled stage, and is basically a " long tail pair " , with the grid of one pentode held at constant potential (via a pot to give acoarse X position), the X signal being fed to the grid of the second pentode, which also has a variable Dc potential to give the front panel X shift control. The stage gain is altered by offsetting the cathode currents between the valves.

The X plates are connected to the anodes of the valves. The DC potential on the X plates helps with the linearity of the trace. In some configurations of electrostatic deflection, you will find that the plates float at the tube final anode voltage, and the signal from the amplifier is capacitivly coupled to the plate (some of the early homebuilt scopes and televisors based on the VCR 97 use this principle.

There is very little to go wrong with the circuit, so you should be able to find the fault with some resistance and continuity checks around the amplifier stage.

Its at this point you wish you had another scope........

Jim.
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Old 28th Oct 2003, 11:32 pm   #7
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Default Re: Telequipment D31 oscillosope - no X deflection

Thanks for all your helpful suggestions!

There seems to be some differences in the X amp from the S31 schematic (has anyone got the D31's schematic?).

The valves seem to be OK (well, swapping them with some of the Y amp's ones has no effect).

The spots on the screen are slightly left of the center, but both are vertically in line with each other.

And yes, Chris_C it was the one at the Leeds radio fair!!! I know there'd be someone on here who saw it! I wouldn't have ended up with it if my wife hadn't phoned when she did to say that I hadn't won the D52 I was bidding on on Ebay! I did make a mad rush for it after I got the call....

Thanks everyone, I'll let you know how I get on.
 
Old 29th Oct 2003, 12:28 am   #8
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Default Re: Telequipment D31 oscillosope - no X deflection

Chris

I have schematics for a Telequipment D31 if you still need them (about 12 a4 sheets). They were supplied by a helpful lady in the service dept at Telequipment in the mid 70's, long after the product was deleted.

I have repaired several of these some years ago, and the major problem was those awful Wima toffee coloured paper capacitors (used in the timebase switch and for various coupling and decoupling purposes). They go leaky and can prevent the timebase running. They all should be changed.

There is also a 4.7M resistor and an 820k resistor coupling the timebase to the X amp which traditionally go o/c or high causing non-linear sweep as I recall.

Setting of the trigger sensitivity is quite critical and the timebase will not run if there is no trigger at V7b cathode. Operation of the trigger circuit is made more tricky if the bias resistors in the trigger sensitivity circuit have drifted out of tolerance.

Ron

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Old 30th Oct 2003, 10:03 pm   #9
Chris_Peacock
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Default Re: Telequipment D31 oscillosope - no X deflection

Bingo! It was really straightforward after all - a 10K resistor in the HT supply to the anodes of the X amp output valves had gone open circuit. I'd checked the HT before - but only by checking that each valve had power, I didn't check against the pin outs/schematic -daw!.

Thanks for all the help, I'll save the contents of this topic in case anything goes wrong with it in future!

Thanks for the offer of the schematics, Ron. I won't trouble you for them at the moment (while it's working!) with them being paper copies, thanks very much anyway.
 
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