Telequipment D83

[MrJohn_IOM]

Got a problem I'm not too sure about with this old scope, there is a similar post on here back from 2012

The trace is really weak, no line, just a dot moving across the screen and only then with the intensity at max.

All the main PSU rails are at the correct values, with the exception of the +55v rail at pin 14 on PC126 - the value is jumping around, up to 100V ish. i have checked all the resistors on the board, a few of which are low. I'm going to blanket replace all of them, but I'd like to know if anyone else has any ideas, where I may look?

[MotorBikeLes]

Obviously your starting point is to get all power rails correct FIRST. That should be straightforward fault finding. The manual has a good circuit description of all functions, and should be read carefully first.

Then move to your intensity problem. That will almost certainly be down to the high value carbon resistors in the EHT module. Difficult box to get into, but it can be done. Note SOME later production replaced the carbon resistors with a FEWER number of higher value VR37 resistors. They are the type to use as replacements.

Somewhere here I provided a list of resistor replacements for use in D63, DM63, D75 and D83. A search should find them.

Les.

[MrJohn_IOM]

Thanks, I'll search for that, also completely off topic, I just noticed you're from the island, I moved from Farmhill to Sheffield in July.

[Pinörkel]

Hello, as far as I remember, in the D83, there is a series of resistors on PC129, namely R317, R318, R319, R321 and R322, that tend to go bad. They are located near the grid cut-off pot R315 and can cause issues like this when bad. However, be careful on PC129 with the high voltages.

[MrJohn_IOM]

Originally Posted by MotorBikeLes

Obviously your starting point is to get all power rails correct FIRST. That should be straightforward fault finding. The manual has a good circuit description of all functions, and should be read carefully first.
Then move to your intensity problem. That will almost certainly be down to the high value carbon resistors in the EHT module. Difficult box to get into, but it can be done. Note SOME later production replaced the carbon resistors with a FEWER number of higher value VR37 resistors. They are the type to use as replacements.
Somewhere here I provided a list of resistor replacements for use in D63, DM63, D75 and D83. A search should find them.
Les.

Found them. thank you

[DouglasDenny]

I don't want to appear to be 'teaching granny to suck eggs' but assume you know that in any old equipment, especially valve (tube) equipment it is the _capacitors_ that are the first thing to check and preferably replace all ( except the silver mica caps which are usually very stable for decades) - but especially the old paper capacitors and especially in the power supply. I did this in an old oscilloscope just to restore it to good working order - a 'Sontranic' probably circa 1950s. Checking valve voltages usually gives a good indication if the resistors are in need of attention: especially the screen resistor.

Douglas Denny.

[fetteler]

The D83 is of the transistor generation.

Failure of components in the resistor ladder in the HV module is a classic fault. The board is tricky (but not impossible if you are careful) to remove - use nice modern high voltage resistors to replace the ones in the ladder and you should be fine. It's also worth cleaning the board to reduce the chances of tracking.

Nice 'scopes!

Cheers,
Steve.

[MrJohn_IOM]

Originally Posted by Pinörkel

Hello, as far as I remember, in the D83, there is a series of resistors on PC129, namely R317, R318, R319, R321 and R322, that tend to go bad. They are located near the grid cut-off pot R315 and can cause issues like this when bad. However, be careful on PC129 with the high voltages.

love it when people have this knowledge. after some minor annoyance taking the board out, you were spot on. R318 was OL, and the rest were low. ill do the resistor swap MotorBikeLes described in another post.

[WME_bill]

Telequipment D83 and D75, D63, DM63.
Failure of the resistors in the high voltage chain down the focus controls is the usual fault, as MotorBikeLes, Steve Fetteler and others have said. (R304 to R313 and R323 to R326.)
When replacing them, bear in mind that the exact values are of little importance. The key thing is that the ratio of the total resistance each side of the focus control is about the same. The current is negligible (perhaps 20 microamp for the tube cathode, no current to the focus, so a few calculations with what resistors you have around will do. As long as the total resistance is well up around 50M.
You can always use conventional carbon or metal film resistors as Telequipment did originally but be prepared to replace them every year. They are rated at 200v / 250v, and go high with voltage stress. One watt carbon composition are better and rated at 500v but bulky.
The metal glaze light blue VR25 / 37 are rated at 2kv. I keep a small stock and always fit them, just at MotorbikeLes does!

Pinorkel's comments about the other resistor chain R317 to R322 are useful, but changes there will have a smaller effect.

If you are having trouble with brightness, check the bright-up circuit Tr351- 355. Failure of the transistors here is the second most common fault. Replace with higher voltage rated ones.
The HV capacitors on these are generally reliable.
Thoroughly clean around the terminals on the EHT transformer. The paxolin board near D304 and D303 sometimes fails. Comments upon this here some years ago - do a search.

Your varying voltage on PC126, X Amplifier output to CRT plates sounds a different problem. Perhaps you are merely measuring the effect of a slow sweep.
Otherwise as the feed to the X output stages is from the 105V rail, and you imply that the other X plate output is steady, then the output transistors may be failing.
I assume that you made that measurement with the time Base off with no sweep, by setting to External X and No trigger. Otherwise all measurements around the TB and X Amplifier are quite invalid.
Incidentally, does switching to External X or Trace Locate affect the brightness. Both should give an increase.
wme_bill .

[fetteler]

Originally Posted by WME_bill

....Thoroughly clean around the terminals on the EHT transformer. The paxolin board near D304 and D303 sometimes fails. Comments upon this here some years ago - do a search.
.

Yes indeed! I have had this happen to my own D83 - the board sort of de-laminated and there was tracking within the layers of the board. It took me ages to find out just what was happening. For those of you who are interested, this is the (dark brown of course) paxolin board fixed to the top of the EHT transformer and (if memory serves) the diodes D303 and D304 have a connection directly to pins on this board.

I have a great deal of affection for the D83 and all this reminds me that I have some work to do on my own one which has been waiting far too long !

Cheers,
Steve.

[MrJohn_IOM]

ok, the saga continues. replaced all the components. cleaned the board, checked all my work and connections (maybe 4 times)

turned it on, and Magic-Smoke(TM) . from TH301 - a 130 ohm thermistor.... it blew the fuse on the 30v rail too. hopefully, that, and the resister in parallel will be the only thing to replace. ill recheck everything again... unless im missing something?

it connects directly to the EHT transformer, with any luck it hasnt damaged that too.

[MrJohn_IOM]

Update 2,

Thermistor failed short, fuse blew, but not fast enough as the EHT transformer has shorted, between pins 3 and 5. so its pretty much the end of this repair until i can find a replacement or repair/rewind the damage.

anyone ever repaired damage to one of these?

[micheal]

Are you sure the transformer has gone short? have you checked that the mica insulator is in place behind the power oscillator transistor?

As this did sort of work before you took it apart it may have been something you have done.

Mike.

[MrJohn_IOM]

Originally Posted by micheal

Are you sure the transformer has gone short? have you checked that the mica insulator is in place behind the power oscillator transistor?

As this did sort of work before you took it apart it may have been something you have done.

Mike.

you know when you read a comment, and you immediately close your eyes and realise.....

- i put it on the wrong side. urgh...... im glad this forum exists, wouldn't have given it a second look.

thanks for the suggestion Mike.

[micheal]

My pleasure, easily done.

Mike

[fetteler]

Wow! What a relief!!!
Fingers crossed all will come good soon - keep us posted.

Cheers,
Steve.

[MrJohn_IOM]

Originally Posted by fetteler

Wow! What a relief!!!
Fingers crossed all will come good soon - keep us posted.

Cheers,
Steve.

placing the washer on the correct side, a new thermisor, and, i finally have a stable-ish trace. this thing has other issues though, need to do my research and have another look through the manual...