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Old 1st Nov 2007, 2:59 pm   #1
sg4instruments
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Default Telequipment D75 Oscilloscope

I have an old telequipment D75 oscilloscope that I am currretly working on. Do you have any experience on these? It has a number of faults. but the main one is no intensity control. So far I have found a faulty 130v zener, open circuit resistor, broken high voltage ceramic cap and it seems the HV rectifier on the flyback tranformer (bottom half) is open circuit. Do you have any idea where to get the parts for these? The diode is rated at 6K 10mA. I believe that's double working voltage which I think is 3K

I would appreciate any advice you can give please?

Last edited by Station X; 1st Nov 2007 at 5:19 pm. Reason: Moved to new thread, as it hijacked an existing one.
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Old 1st Nov 2007, 11:30 pm   #2
WME_bill
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Default Re: Telequipment D75 Oscilloscope

I worked a lot a some years ago on the D83, which is identical apart from a larger tube to your D75. I found the Oscillator transistor tended to go, a Motorola 2N5191. The catch is it has a different pin-out (ECB)) to the more usual TIP33 (BCE), so it was tricky to fit an alternative. A replacement HV rectifier is not particular (D303, D304). Any high voltage TV type does, eg BY8406 etc . Older numbers are Mullard BY140, BY176, BY182, BY187, BY476, BY713. All 8kv upwards, about 2 to 3 m/a, which should be ample for that use. Listed by RadioSpares, or even rummage inside a scap TV. Incidentally, I have a brand new unused EHT transformer for that scope, if anyone wants that. The ceramic capacitors I used to get from Cricklewood Electronics. Are they still trading?
The other frequent trouble was the brilliance fading as it warmed up. This was usually the resistors in the EHT chain changing in value. They are standard Welwyn etc types, not high voltage versions. Maplin used to stock a few of the Mullard VR25, which are rated at 1kv. You may have to juggle values to get the right ratio either side of the focus control. An odd meg ohm either way not important, as modern tubes have virtually no focus anode current- a few microamps only. Your burnt out 130V zener probably occurred if the grid voltage feed failed, as it protects the grid/cathode of the CRT. Better look at that resistor bleed chain (50M). The resistors were changed in later models of these scopes, leaving stray holes on the printed circuit board, which suggests that Tektronix/Telequipment were learning about failures in use. That might be why you have no brilliance. The grid is tied to the cathode, by the burnt out zener, and the whole floating grid supply is not. Check the bright up circuit transistors (TR351,2,3,4,5). I have had trouble here also. I assume you have the circuit. Bill
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Old 2nd Nov 2007, 9:59 am   #3
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Default Re: Telequipment D75 Oscilloscope

Quote:
and it seems the HV rectifier on the flyback tranformer (bottom half) is open circuit
If its teh SenterCel type (long stick thing) then try using a 9V battery + series resistor to test current in both directions. I've had instances when the low volts used by most meters to measure resistance wasn't quite enough to get conduction, but EHT generated just fine.

TTFN,
Jon
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Old 5th Nov 2007, 12:32 am   #4
sg4instruments
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Default Re: Telequipment D75 Oscilloscope

Hi Bill,
Many thanks for your reply. Criclewood electronics is apparently still around and they seem to be able to cater for all of the high voltage components I need for the task. Interestingly, many of the electronic suppliers I browsed could not, including Maplin. I do have the circuit diagram fortunatly. I would be interested in that EHT transformer just incase anything has gone wrong with S2 I think its called. But only if its not good for dazzlevision as he asked first.
Bye for now,
Carl
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Old 5th Nov 2007, 4:33 pm   #5
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Default Re: Telequipment D75 Oscilloscope

The EHT transformer seems pretty reliable, so don't look out for one until you know you want it. Thats what I did, and found the trouble was in the resistor chains and the bright up circuit. I have had several of these to repair, and the transformer always came out OK in the end. I endorse what Duke?? says about Stentecel or Westinghouse HV rectifiers. But in your D83/D75, they are silicon. Appropos Cricklewood, I have looked for them, and they are still around, and offer HV ceramic and diodes. Not it seems the high voltage resistors. Mullard/Philips metal glaze VR37 listed 1978/9 & 1989, rated at 0.5W,2500v, up to 33M, and also VR25, listed 1989 at 0.25w, 1150v to 22M.
Normally available metal film MR series only rated to about 200/250v. The old fashioned Welvyn 1w cracked carbon would go to about 500v. Bill
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Old 9th Nov 2007, 12:16 am   #6
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Default Re: Telequipment D75 Oscilloscope

Hi Bill,

I have checked PC128 which I think refers to the bright-up circuit you mentioned. I checked the test voltages shown on the circuit and they all checked out fine, so I am asuming this board works ok. However, on the circuit diagram it appears to illustrate a 6v square wave coming from TR355 and TR354 -which I actually measured at 60v; does the 6v really mean 60 or do you think I have a problem here? Re. Criclewood components, they dont appear to have the 10n cap (C311 ) is this value critical or will a 2n2 do? They have a ceramic at this value rated at 4k which the correct voltage rating according to the manual. Yes, I think the transformer is probably ok- Ill get to that a little later. Thanks for your advice.

Regards
Carl
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Old 9th Nov 2007, 1:19 pm   #7
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Default Re: Telequipment D75 Oscilloscope

Glad you are making progress. Bright up circuit. The output from T354/355, waveform point 5 rests at about 6V no trace, and goes to +32 for a bright trave, and about +22V for a dim trace, varying with the brilliance control R351 which sets the bright up top voltage level. If at 60v, then is the brilliance(intensity to use the US term) control working? I am puzzled about your C311.Do you mean C311, connecting between the geometry anode p9, at +48V and the mesh pin11 at 0V. My manual listing quotes 10n at 250v, which is easy, and is what I'd expect for that part of the circuit. If. as I suspect you do, you mean the HV capacitors C306,7,9,10, then they are all high voltage.(Also C303 across the feedback chain to regulator EHT). Value not critical. If you replace with a lower value, at worst you will get ripple on the supply, which will give shading on the trace. If that happens, put another one across the existing. I've looked back at some old notes. EHT voltage for cathode supply,at C307 is 2.5kv, so the grid will be about the same. A capacitor rating 3kv up should be fine to get it working. Ripple on the grid supply (129/15) is <1v at 20us, and on cathode (129/16) is <10v. But these are both at -2.5kv, so don't use a normal scope probe, it is only rated to 500V at best, and will burn out, and probably the input attenuator of the test scope. Stick to the old faithful Avo8 on 2.5kv/3kv range, and don't measure at the grid, it is high impedance, and the 50ua of the Avo will probaby destroy the tube by swinging the grid vastly positive of the k. Measure on the cathode is usually alright if done quickly. The fuzzing and little sparks as you touch the probe is quite enough to make one nervous and very quick. I usually wear a rubber kitchen glove as well. Whether it is false security I don't know, but it gives me confidence!
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Old 11th Nov 2007, 5:23 pm   #8
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Default Re: Telequipment D75 Oscilloscope

Hi Bill,

Many thanks for your reply: C311 on the D75 appears to be strapped between points 4 and 2 on the crt, between the focus point and the cathode. Its physically a large ceramic (broken on mine) rated at 10n-4K. The manual I have appears to corroborate this so I am asuming its correct. The value of C311 at this voltage seems hard to get hold of but cricklewood do have a 2n at 4K, So, thinking its not critical, I was going to run with that. Bright up. --- the brilliance pot checked out ok as I recall, but I was wondering if my digial meter was incorrectly reading the square wave, it could be perhaps that I was using the wrong earth, sitting lower than zero, and the difference seemed higher than normal. But, unless I made a mistake 105 volts supply halved through pot and test votages checked out to that point. I am now getting the replacmement bits I need which so far consist of 130 zener HV rectifier 10n (2n) ceramic cap. Kitchen glove is a good idea and one that I use . Do you have a dedicated D75 manual? if not I have one in pdf format- your welcome to have it if you dont mind a lengthy upload.
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Old 10th Dec 2007, 11:58 pm   #9
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Default Re: Telequipment D75 Oscilloscope

Hi Agiain,

I still have not been able to track down the problem relating to the intensity control and consequently the brightness being either full up or too high. I have checked the bleeder chain and the values of each resistor checks out ok when cold. I have changed grid 130v zener and cap, also a blown rectifying diode at the bottom half of the flyback transformer has been changed. Experimentally, I removed the connection from the bright-up circuit and this made no difference at all. The Brilliance control checks out ok, the only thing I can see is the voltage levels are too high from the output of the bright-up circuit, a constant 60 volts (as I say, when removed no difference) Any further advice please I am a relative novice to oscilloscope theory of operation (that might be the other problem I have tested the various voltage levels in the bright-up circuit and noted most of it is flooded with 60 volts.

Regards

Carl



Quote:
Originally Posted by WME_bill View Post
Glad you are making progress.
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Old 18th Apr 2008, 3:19 pm   #10
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Default Re: Telequipment D75 Oscilloscope

I have a telequipment D75 osciloscope with resistors burnt out on a board, so cannot identify the values.
Can anybody advise where to get a cct. diagram please, or could anyone e-mail me a scanned copy please?
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Old 18th Apr 2008, 7:29 pm   #11
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Default Re: Telequipment D75 Oscilloscope

A copy of the D83 scope manual, which is a.f.a.i.k. a re-packaged D75, using the same plug-ins is available on the BAMA mirror site.

http://bama.edebris.com/manuals/telequipment/


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Old 18th Apr 2008, 8:40 pm   #12
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Default Re: Telequipment D75 Oscilloscope

The D75 and D83 are very similar. The D83 is "upright" with the screen above the plugins. The screen is larger than average too, larger than on the D75 which is "horizontal" format with the plugins each side of the screen.

I've used both in the early 1980s and never much liked them. My employer at the time had a fair selection of scopes and when I had the choice I used an Advance OS3000. Only 40MHz against the 50MHz D83 but I thought is was nicer to use. I ended up buying a secondhand OS3000 for home use, not sure what happened to it. I wouldn't have chosen a D75/D83.
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