Attempting the repair of Telequipment D75 - UK Vintage Radio Repair and Restoration Discussion Forum

pixeltwisted · 7th Jul 2020, 5:21 pm

Hello folks this is my first post on the forums.

Stupidly I recently purchased (cheaply) a mostly dead Telequipment D75 scope as a project.
The reset neon lamp V61 was lit in the picture offered by the seller.

Considering the age and unknown previous repair attempts decided not to power on.
Upon opening on PC 149 I was greeted with leaky filter caps C404, C405 and C406 (image attached)
and following further testing of components cap C408 47uF read less than 10nF. Also it appears diodes
D401-D413 have previously been replaced.

Cleaned PC 149 (image attached) and replaced C404, C405, C406 and C408 and applied power
with the board outside the chassis (chassis ground connected). R406 then started smoking
luckily I switched off before R406 was destroyed, further diagnosis found TR401 BF305 was
shorted between emitter and collector! Am pretty sure TR401 tested good and the fact that the seller
provided a picture with V61 lit and no smoke has me worried.

Removed TR401 while I await replacement to be delivered, no smoke and outputs read:

Unregulated 30v = 33.7v
+24 = 26.44v
+105 = 105.1v
-24 = 23.93v

So a small success, before I destroy the replacement any ideas as to why TR401 might have shorted?
It makes me laugh that TR401, TR404 and R406 form the short cicuit protection.

Another issue is with the unobtainable EHT oscillator transformer, it appears that the Primary
and S1 secondary are shorted. S2 and S3 check out OK. I removed it and tested by injecting a 1khz
sine wave at 0.5v. S2 and S3 pass it between each other in both directions but P and S1 are dead.

Have attached schematic excerpt (thanks to Pinörkel's amazing work) and picture of the Quintupler
melted wire and crispy C320 capacitor, before spending more money repairing PC 148 does anybody
know where I might source a replacement transformer?

Apologies for the length of the post. Any help from you folks is greatly appreciated.

Kindest Regards,

Pixeltwisted.

MotorBikeLes · 7th Jul 2020, 8:44 pm

You say that primary winding of EHT transformer and Sec 1 are shorted, but you can inject a sine wave into one secondary and see it in another.
But surely if you have any wires shorted, the result will be a heavily damped transformer that would not see an injected wave in one winding appearing in another?
Most EHT faults are in the high resistance chain, which on early production were carbon resistors. Only VR37 types should be used as replacements, with fewer resistors of higher values being used. If you search, you should find where I have detailed all these changes previously.
Les.

pixeltwisted · 7th Jul 2020, 9:01 pm

Many thanks for your reply MotorBikeLes, my understanding of transformers is clearly lacking. My mistaken thought was that if I inject a sine wave into the primary I would get something on all three secondaries. Also If I did the same on a secondary it would appear on the other windings as occured between S2 and S3.

As suggested I will search for and read all of your previous posts before attempting repair of PC 148 and returning everything into the case.

Again thankyou for your time.

Regards,

P.

Chris55000 · 7th Jul 2020, 9:59 pm

Hi!

The step–up ratio between P/S1 and S2/S3 is very large, and also 1kHz is much too low a frequency to test this transformer with, hence you're not getting anything measurable on P/S1!

Turn up your generator's frequency to 25 kHz and the sine–wave amplitude to maximum (about 5V RMS with most of the cheaper generators), then connect the generator's main 50 ohm output to winding P with TR304 disconnected, this time you should get some output from S2 and S3 which you can measure at the quintupler's output.

If you get little or nothing with your generator's maximum output across winding P, the quintupler module may have a duffy diode or capacitor in it, in which case you'll have to try and dig into it to attempt a repair (a heat–gun will soften the epoxy filling) or get one from another Member!

These e.h.t. oscillator transformers do sometimes develop short–circuited turns in the windings but not as commonly as generally imagined – it's more generally the paxolin insulation panel for the tags of the transformer that breaks down, and the effects of this are usually self–evident as black burn/arc–tracks along the material itself!

Chris Williams

pixeltwisted · 7th Jul 2020, 10:29 pm

Hello Chris55000 your reply gives me some hope that it is my ignorance that makes me think the transformer is at fault.

My test gear is minimal and the 1khZ sine wave was being generated by an op-amp bubba oscillator I put together for troubleshooting an audio mixer earlier this year.

I have not powered the scope up since I have owned it so I think once I replace the faulty electrolytic caps on PC 148 I will install it and the transformer and test in circuit. The paxolin board definitely has some burn marks, not tracks though, it has clearly been extremely hot at some point. I will take a photograph tomorrow when I can light it properly.

MotorBikeLes mentioned about resistors in EHT section, one of his previous posts lists resistors for the D83. Here it seems I have lucked out I seem to have a later revision of the
board as all are VR37 and measure spot on, apart from R323 which is a 3M9 not 5M6. Am still trying to read and understand all previous posts regarding this scope.

Must take a rest from this machine and come back to it tomorrow.

Many thanks to you both for your help. This forum is amazing.

pixeltwisted · 8th Jul 2020, 4:38 pm

Have taken some images of the state of the EHT transformer and two of PC 148, second image of PC 148 shows R315 which I think is a cermet pot? Also R318 which is a VR37 resistor like the others in the high resistance chain as recommended by MotorBikeLes in another post.

Does anybody have any ideas as to why TR401 BF305 on PC 149 shorted? Would rather not destroy the new one coming.

The 20nF capacitors C307 and C310 have burnt/crispy ends would you suggest I replace them before testing the board in circuit, they measure OK for capacitance but I do not have
the ability to test for leakage. All transistors resistors and other capacitors seem to be OK. Have now replaced electrolytic capacitors C202 and C304.

Will be a few days before I receive replacement for TR401 so will update when everything is re-installed and tested.

Kindest Regards, P.

pixeltwisted · 8th Jul 2020, 5:58 pm

Following your advice Chris55000 I knocked up a breadboard amplifier to test the EHT transformer. (image attached)

Primary P passes a 20kHz sine wave to S1, S2 and S3 secondaries. Am very relieved that
I do not have to get a replacement just to test the rest of the unit.

Much cheaper to learn something new than blindly buy replacement parts.

Once again many thanks for your advice.

MotorBikeLes · 8th Jul 2020, 9:55 pm

Those caps on the EHT board all look OK to me.
Yes, a later board with VR37 resistors. Not likely to have problems with those. The values were quite different to the early carbon types, as well as type.
Upright cermet pot also correct for later models. I always replaced them on D75 and D82 if carbon.
Les.

pixeltwisted · 8th Jul 2020, 10:16 pm

Thanks Les for looking at the images, the help already given is very much appreciated.
Am very glad this forum exists and people like yourself are willing to help learners like myself.

As mentioned earlier I am awaiting replacement TR401 transistor, have actually two in the
post just in case. Am going to go over PC 149 with a fine tooth comb looking for potential
problems before reinstating.

Pinörkel · 8th Jul 2020, 11:32 pm

Hello,
nice to see my scanned manual was of some use.
The diodes D401-D413 in your first post look quite original to me.

Your reading on the unregulated 30 V of 33.7 V, and consequently the +24V line, seems to be higher than what I would expect. Have you checked if all the rectifier diodes are working, especially D405-D408? Maybe one is not and you only get half rectified 50 Hz output (best to see with another scope). I had a similar issue on my D755. However, the defective diode was undetectable with a DMM in diode test mode.

Also check the large through hole vias on PC149. Those tend to have cold soldering joints. I have some reference values of the PSU ripple in this post.

Seeing that you replaced the leaky filter caps C404, C405 and C406. Before they are otherwise thrown in the trash, I would be interested in the defective set.

Denis

pixeltwisted · 8th Jul 2020, 11:51 pm

Denis your manual was one of the main reasons I even began this repair, without it I would
have been lost.

Your point regarding the diodes makes sense now that I think about it, I have only checked
using a DMM so tomorrow I will break out my Rigol scope and take a look.
Thanks for the suggestion.

With regards the leaky caps I have kept them, I have used cheap no name replacements
in the scope until I am sure it is worth using Nichicon/Rubycon eventually. Was even
contemplating re-stuffing them but to be honest I won't. You are more than welcome to them
as thanks for your manual scanning work.

Gavin

Pinörkel · 15th Jul 2020, 9:34 pm

Quote:

Originally Posted by pixeltwisted

Denis your manual was one of the main reasons I even began this repair, without it I would have been lost.

Then my scan has served its purpose.

Quote:

Originally Posted by pixeltwisted

Your point regarding the diodes makes sense now that I think about it, I have only checked using a DMM so tomorrow I will break out my Rigol scope and take a look. Thanks for the suggestion.

This one took nearly a day to detect it on my D755. Luckily I had a D75 for debugging at hand.

Quote:

Originally Posted by pixeltwisted

With regards the leaky caps I have kept them, I have used cheap no name replacements in the scope until I am sure it is worth using Nichicon/Rubycon eventually. Was even contemplating re-stuffing them but to be honest I won't. You are more than welcome to them as thanks for your manual scanning work.

Gavin

Thank you for the kind offer. I'll try to restuff them and put them in my D755 which has been quite tinkered with by its previous owners.

If you are searching for 2200 µF 3-pin replacement caps, try to search with google for "A723283". The only issue those cheap sicsafco caps have is that the positive and the remaining not connected pin have swapped positions in contrast to the original ones. However, this should not be an issue. Since the third pin is unconnected, it can simply be shorted to the positive pin.

pixeltwisted · 16th Jul 2020, 4:04 pm

Apologies for the delay in updating this thread Life is getting in the way of my hobby.

Denis PM me and I can arrange to have the caps sent to you. May take some time but will arrange.

Replacement transistor finally arrived and installed tested OK no magic smoke, deep joy.

Decided not to put my Rigol scope in the firing line just yet as am unemployed and have no way
financially of replacing it, one reason for getting the D75 is it can be sacrificial.

So replaced D405-D408 with 1N4007's, now have 33.25V rather than the 33.7V previously, obviously
above the 30v required. Tested the 26v winding feeding the full wave bridge and it measures 25.96V?

Could it just be the board is not in circuit and not under any load?

Any ideas greatly appreciated.

Pinörkel · 16th Jul 2020, 8:08 pm

Quote:

Originally Posted by pixeltwisted

Decided not to put my Rigol scope in the firing line just yet as am unemployed and have no way
financially of replacing it, one reason for getting the D75 is it can be sacrificial.

So replaced D405-D408 with 1N4007's, now have 33.25V rather than the 33.7V previously, obviously
above the 30v required. Tested the 26v winding feeding the full wave bridge and it measures 25.96V?

Could it just be the board is not in circuit and not under any load?

Any ideas greatly appreciated.

I just took PC149 out of my D755 for you and it measures 33.5 V on that line without any load. With load it is very close to 30 V. If you have a DMM with frequency measurement mode, you could just measure over the filter cap contacts and should get 100 Hz, if the diodes are at least working to some degree. If the diodes are only sort-of-working you could well debug that later with your D75 probing itself. If there was a change between the original diodes and your beefy 1N4007 replacements, I would suspect an improvement there. So for now, I think your measurements look OK to me. However, do not forget to monitor the stability of the values under load.

pixeltwisted · 16th Jul 2020, 9:02 pm

Cannot thank you enough for doing that, removing any of the boards in this scope is akin to contortionism so fair play.

You now give me the confidence to reinstate PC 148 & 149 with replaced electrolytics and see what happens.

Reading one of your earlier posts you mention that the green power light came back to life inexplicably, mine did too......

Again thankyou for your help.

Pinörkel · 16th Jul 2020, 10:08 pm

Quote:

Originally Posted by pixeltwisted

Cannot thank you enough for doing that, removing any of the boards in this scope is akin to contortionism so fair play.

You now give me the confidence to reinstate PC 148 & 149 with replaced electrolytics and see what happens.

Reading one of your earlier posts you mention that the green power light came back to life inexplicably, mine did too......

Again thankyou for your help.

The green power light is fed by a ~7 V transformer winding that is connected trough the solder pads 7,8,9,10 near the fuse on PC149. They are just for connecting the transformer to the lamp and for some internal 50 Hz referencing. So, If the transformer comes on, the lamp is automatically powered. Temporary failures of the lamp are often caused by bad soldering joints of these 4 contacts on PC149 and by bad lamp socket contacts. To solve the latter, just clean the lamp socket contacts. You can reach them behind the front panel, by just removing the bottom cover of the scope. There is a small plastic assembly on a metal tab that you can just pull back to access the lamp without disassembling the front panel (see image).

BTW: Good thing you made me pull out PC149 on my D755 again. After I put it back in, the strange issue, I observed earlier occurred again and the trace started twitching and drifting to the left. But this time, the error seems to be permanent, so I could find out that there is at least something wrong with TR408. Seeing the many issues I had to resolve with the 24 V line on my D755, I am starting to suspect that it was soldered together by the devil himself.

pixeltwisted · 16th Jul 2020, 10:22 pm

Thanks to your PDF I was very aware of the 7V winding and the circuit involved, am pretty sure it was just a dry solder joint.
Just made me smile that you mentioned it in one of your posts from another thread and it happened to me also.

Am quite sure the designers of these scopes had no regard for the assemblers or repairers of their equipment... so possibly demonic.

Regards P.

pixeltwisted · 20th Jul 2020, 8:01 pm

Hello all hope you are safe and well?

Denis I hope you are having more luck with your scope than I am with mine.
Still waiting for you to PM me with an address to post the three capacitors to.

Spent quite a frustrating time resoldering EHT board PC148 back into the case
after replacing C302 & C304 electrolytics. Then plugged PC149 into PC147 and
with V4 and S2A plugins installed (possibly a big mistake?) powered on the unit
and the magic smoke appeared. Rapidly turned off but the damage is done.

On PC 149 Fuse FS402 which is on the unregulated 30v is intact?
Resistor R406 2R7 roasted completely, TR401 BF305 that I replaced and
tested OK is now open between base and emitter. As previously posted PC 149
worked fine out of the case. Am also pretty sure I saw some smoke coming
from behind PC150 but can find no burn marks or obvious failed components
all transistors on the board all test fine with a DMM.

I Should have tested without the plugins installed to reduce the number of
possible failure points. Will have to go through everything with a fine tooth
comb trying to find shorts as the components that failed form part of the
short circuit protection.

As always any ideas greatly appreciated.

Regards. P.

P.s I have replacement resistors and transistors on order.

Pinörkel · 20th Jul 2020, 10:09 pm

Quote:

Originally Posted by pixeltwisted

Denis I hope you are having more luck with your scope than I am with mine.
Still waiting for you to PM me with an address to post the three capacitors to.

Will do that soon.

Quote:

Originally Posted by pixeltwisted

Spent quite a frustrating time resoldering EHT board PC148 back into the case
after replacing C302 & C304 electrolytics. Then plugged PC149 into PC147 and
with V4 and S2A plugins installed (possibly a big mistake?) powered on the unit and the magic smoke appeared. Rapidly turned off but the damage is done.

Outch! That is sad to hear. It is usually a good strategy to power things on step by step. Also before plugging things in, it is also a good idea to check for short circuits by measuring the resistance of the power lines on the unconnected consumers, e.g. the PC147 connector pins, to ground. The same goes for the power lines going into the unconnected plugins. If anything has a low resistance there, you should investigate that before powering anything on.

Quote:

Originally Posted by pixeltwisted

On PC 149 Fuse FS402 which is on the unregulated 30v is intact?
Resistor R406 2R7 roasted completely, TR401 BF305 that I replaced and
tested OK is now open between base and emitter.

That is expected in the case of a short or over-current in the 105V line. TR401 and TR404 form the over-current protection on that line as a bistable. It is designed so that on triggering, you have to power down the unit first before it will work again. Extra safety feature for 105V! The current for flipping the bistable goes over R406. So in case of a dead short: boom!

Quote:

Originally Posted by pixeltwisted

As previously posted PC 149 worked fine out of the case. Am also pretty sure I saw some smoke coming from behind PC150 but can find no burn marks or obvious failed components all transistors on the board all test fine with a DMM.

Hmmm, I would expect the fault causing the short on PC148 or PC128. Also check, if you made a fault during putting everything together again. If there is something wrong on PC150, I would check around TR8, TR9, D6 and D7.

Denis

pixeltwisted · 20th Jul 2020, 10:46 pm

Again a very quick response thankyou Denis.

Triple checked my soldering work on PC148 after install but maybe I missed
something.

Did measure SK603 connections 1,3,5 and 7 to ground all before connection
and all were over 2kohm. My question is what should I consider low resistance?

Also the second I turned the unit on my brain screamed why did you put the
plugins in! I can see the funny side now.

Will spend time tomorrow checking my work on PC148 to see if I missed something. As you know it is not so easy working on these ****** units.

7th Jul 2020, 5:21 pm	#1
pixeltwisted Triode Join Date: Jul 2020 Location: Redhill, Surrey, UK. Posts: 28	Attempting the repair of Telequipment D75 Hello folks this is my first post on the forums. Stupidly I recently purchased (cheaply) a mostly dead Telequipment D75 scope as a project. The reset neon lamp V61 was lit in the picture offered by the seller. Considering the age and unknown previous repair attempts decided not to power on. Upon opening on PC 149 I was greeted with leaky filter caps C404, C405 and C406 (image attached) and following further testing of components cap C408 47uF read less than 10nF. Also it appears diodes D401-D413 have previously been replaced. Cleaned PC 149 (image attached) and replaced C404, C405, C406 and C408 and applied power with the board outside the chassis (chassis ground connected). R406 then started smoking luckily I switched off before R406 was destroyed, further diagnosis found TR401 BF305 was shorted between emitter and collector! Am pretty sure TR401 tested good and the fact that the seller provided a picture with V61 lit and no smoke has me worried. Removed TR401 while I await replacement to be delivered, no smoke and outputs read: Unregulated 30v = 33.7v +24 = 26.44v +105 = 105.1v -24 = 23.93v So a small success, before I destroy the replacement any ideas as to why TR401 might have shorted? It makes me laugh that TR401, TR404 and R406 form the short cicuit protection. Another issue is with the unobtainable EHT oscillator transformer, it appears that the Primary and S1 secondary are shorted. S2 and S3 check out OK. I removed it and tested by injecting a 1khz sine wave at 0.5v. S2 and S3 pass it between each other in both directions but P and S1 are dead. Have attached schematic excerpt (thanks to Pinörkel's amazing work) and picture of the Quintupler melted wire and crispy C320 capacitor, before spending more money repairing PC 148 does anybody know where I might source a replacement transformer? Apologies for the length of the post. Any help from you folks is greatly appreciated. Kindest Regards, Pixeltwisted. Attached Thumbnails

7th Jul 2020, 8:44 pm	#2
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Attempting the repair of Telequipment D75 You say that primary winding of EHT transformer and Sec 1 are shorted, but you can inject a sine wave into one secondary and see it in another. But surely if you have any wires shorted, the result will be a heavily damped transformer that would not see an injected wave in one winding appearing in another? Most EHT faults are in the high resistance chain, which on early production were carbon resistors. Only VR37 types should be used as replacements, with fewer resistors of higher values being used. If you search, you should find where I have detailed all these changes previously. Les.

7th Jul 2020, 9:01 pm	#3
pixeltwisted Triode Join Date: Jul 2020 Location: Redhill, Surrey, UK. Posts: 28	Re: Attempting the repair of Telequipment D75 Many thanks for your reply MotorBikeLes, my understanding of transformers is clearly lacking. My mistaken thought was that if I inject a sine wave into the primary I would get something on all three secondaries. Also If I did the same on a secondary it would appear on the other windings as occured between S2 and S3. As suggested I will search for and read all of your previous posts before attempting repair of PC 148 and returning everything into the case. Again thankyou for your time. Regards, P.

7th Jul 2020, 9:59 pm	#4
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,867	Re: Attempting the repair of Telequipment D75 Hi! The step–up ratio between P/S1 and S2/S3 is very large, and also 1kHz is much too low a frequency to test this transformer with, hence you're not getting anything measurable on P/S1! Turn up your generator's frequency to 25 kHz and the sine–wave amplitude to maximum (about 5V RMS with most of the cheaper generators), then connect the generator's main 50 ohm output to winding P with TR304 disconnected, this time you should get some output from S2 and S3 which you can measure at the quintupler's output. If you get little or nothing with your generator's maximum output across winding P, the quintupler module may have a duffy diode or capacitor in it, in which case you'll have to try and dig into it to attempt a repair (a heat–gun will soften the epoxy filling) or get one from another Member! These e.h.t. oscillator transformers do sometimes develop short–circuited turns in the windings but not as commonly as generally imagined – it's more generally the paxolin insulation panel for the tags of the transformer that breaks down, and the effects of this are usually self–evident as black burn/arc–tracks along the material itself! Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!

7th Jul 2020, 10:29 pm	#5
pixeltwisted Triode Join Date: Jul 2020 Location: Redhill, Surrey, UK. Posts: 28	Re: Attempting the repair of Telequipment D75 Hello Chris55000 your reply gives me some hope that it is my ignorance that makes me think the transformer is at fault. My test gear is minimal and the 1khZ sine wave was being generated by an op-amp bubba oscillator I put together for troubleshooting an audio mixer earlier this year. I have not powered the scope up since I have owned it so I think once I replace the faulty electrolytic caps on PC 148 I will install it and the transformer and test in circuit. The paxolin board definitely has some burn marks, not tracks though, it has clearly been extremely hot at some point. I will take a photograph tomorrow when I can light it properly. MotorBikeLes mentioned about resistors in EHT section, one of his previous posts lists resistors for the D83. Here it seems I have lucked out I seem to have a later revision of the board as all are VR37 and measure spot on, apart from R323 which is a 3M9 not 5M6. Am still trying to read and understand all previous posts regarding this scope. Must take a rest from this machine and come back to it tomorrow. Many thanks to you both for your help. This forum is amazing.

8th Jul 2020, 4:38 pm	#6
pixeltwisted Triode Join Date: Jul 2020 Location: Redhill, Surrey, UK. Posts: 28	Re: Attempting the repair of Telequipment D75 Have taken some images of the state of the EHT transformer and two of PC 148, second image of PC 148 shows R315 which I think is a cermet pot? Also R318 which is a VR37 resistor like the others in the high resistance chain as recommended by MotorBikeLes in another post. Does anybody have any ideas as to why TR401 BF305 on PC 149 shorted? Would rather not destroy the new one coming. The 20nF capacitors C307 and C310 have burnt/crispy ends would you suggest I replace them before testing the board in circuit, they measure OK for capacitance but I do not have the ability to test for leakage. All transistors resistors and other capacitors seem to be OK. Have now replaced electrolytic capacitors C202 and C304. Will be a few days before I receive replacement for TR401 so will update when everything is re-installed and tested. Kindest Regards, P. Attached Thumbnails

8th Jul 2020, 5:58 pm	#7
pixeltwisted Triode Join Date: Jul 2020 Location: Redhill, Surrey, UK. Posts: 28	Re: Attempting the repair of Telequipment D75 Following your advice Chris55000 I knocked up a breadboard amplifier to test the EHT transformer. (image attached) Primary P passes a 20kHz sine wave to S1, S2 and S3 secondaries. Am very relieved that I do not have to get a replacement just to test the rest of the unit. Much cheaper to learn something new than blindly buy replacement parts. Once again many thanks for your advice. Attached Thumbnails

8th Jul 2020, 9:55 pm	#8
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Attempting the repair of Telequipment D75 Those caps on the EHT board all look OK to me. Yes, a later board with VR37 resistors. Not likely to have problems with those. The values were quite different to the early carbon types, as well as type. Upright cermet pot also correct for later models. I always replaced them on D75 and D82 if carbon. Les.

8th Jul 2020, 10:16 pm	#9
pixeltwisted Triode Join Date: Jul 2020 Location: Redhill, Surrey, UK. Posts: 28	Re: Attempting the repair of Telequipment D75 Thanks Les for looking at the images, the help already given is very much appreciated. Am very glad this forum exists and people like yourself are willing to help learners like myself. As mentioned earlier I am awaiting replacement TR401 transistor, have actually two in the post just in case. Am going to go over PC 149 with a fine tooth comb looking for potential problems before reinstating.

8th Jul 2020, 11:32 pm	#10
Pinörkel Pentode Join Date: Jun 2019 Location: Dortmund, Germany Posts: 161	Re: Attempting the repair of Telequipment D75 Hello, nice to see my scanned manual was of some use. The diodes D401-D413 in your first post look quite original to me. Your reading on the unregulated 30 V of 33.7 V, and consequently the +24V line, seems to be higher than what I would expect. Have you checked if all the rectifier diodes are working, especially D405-D408? Maybe one is not and you only get half rectified 50 Hz output (best to see with another scope). I had a similar issue on my D755. However, the defective diode was undetectable with a DMM in diode test mode. Also check the large through hole vias on PC149. Those tend to have cold soldering joints. I have some reference values of the PSU ripple in this post. Seeing that you replaced the leaky filter caps C404, C405 and C406. Before they are otherwise thrown in the trash, I would be interested in the defective set. Denis

8th Jul 2020, 11:51 pm	#11
pixeltwisted Triode Join Date: Jul 2020 Location: Redhill, Surrey, UK. Posts: 28	Re: Attempting the repair of Telequipment D75 Denis your manual was one of the main reasons I even began this repair, without it I would have been lost. Your point regarding the diodes makes sense now that I think about it, I have only checked using a DMM so tomorrow I will break out my Rigol scope and take a look. Thanks for the suggestion. With regards the leaky caps I have kept them, I have used cheap no name replacements in the scope until I am sure it is worth using Nichicon/Rubycon eventually. Was even contemplating re-stuffing them but to be honest I won't. You are more than welcome to them as thanks for your manual scanning work. Gavin

16th Jul 2020, 4:04 pm	#13
pixeltwisted Triode Join Date: Jul 2020 Location: Redhill, Surrey, UK. Posts: 28	Re: Attempting the repair of Telequipment D75 Apologies for the delay in updating this thread Life is getting in the way of my hobby. Denis PM me and I can arrange to have the caps sent to you. May take some time but will arrange. Replacement transistor finally arrived and installed tested OK no magic smoke, deep joy. Decided not to put my Rigol scope in the firing line just yet as am unemployed and have no way financially of replacing it, one reason for getting the D75 is it can be sacrificial. So replaced D405-D408 with 1N4007's, now have 33.25V rather than the 33.7V previously, obviously above the 30v required. Tested the 26v winding feeding the full wave bridge and it measures 25.96V? Could it just be the board is not in circuit and not under any load? Any ideas greatly appreciated.

16th Jul 2020, 9:02 pm	#15
pixeltwisted Triode Join Date: Jul 2020 Location: Redhill, Surrey, UK. Posts: 28	Re: Attempting the repair of Telequipment D75 Cannot thank you enough for doing that, removing any of the boards in this scope is akin to contortionism so fair play. You now give me the confidence to reinstate PC 148 & 149 with replaced electrolytics and see what happens. Reading one of your earlier posts you mention that the green power light came back to life inexplicably, mine did too...... Again thankyou for your help.

16th Jul 2020, 10:22 pm	#17
pixeltwisted Triode Join Date: Jul 2020 Location: Redhill, Surrey, UK. Posts: 28	Re: Attempting the repair of Telequipment D75 Thanks to your PDF I was very aware of the 7V winding and the circuit involved, am pretty sure it was just a dry solder joint. Just made me smile that you mentioned it in one of your posts from another thread and it happened to me also. Am quite sure the designers of these scopes had no regard for the assemblers or repairers of their equipment... so possibly demonic. Regards P.

20th Jul 2020, 8:01 pm	#18
pixeltwisted Triode Join Date: Jul 2020 Location: Redhill, Surrey, UK. Posts: 28	Re: Attempting the repair of Telequipment D75 Hello all hope you are safe and well? Denis I hope you are having more luck with your scope than I am with mine. Still waiting for you to PM me with an address to post the three capacitors to. Spent quite a frustrating time resoldering EHT board PC148 back into the case after replacing C302 & C304 electrolytics. Then plugged PC149 into PC147 and with V4 and S2A plugins installed (possibly a big mistake?) powered on the unit and the magic smoke appeared. Rapidly turned off but the damage is done. On PC 149 Fuse FS402 which is on the unregulated 30v is intact? Resistor R406 2R7 roasted completely, TR401 BF305 that I replaced and tested OK is now open between base and emitter. As previously posted PC 149 worked fine out of the case. Am also pretty sure I saw some smoke coming from behind PC150 but can find no burn marks or obvious failed components all transistors on the board all test fine with a DMM. I Should have tested without the plugins installed to reduce the number of possible failure points. Will have to go through everything with a fine tooth comb trying to find shorts as the components that failed form part of the short circuit protection. As always any ideas greatly appreciated. Regards. P. P.s I have replacement resistors and transistors on order.

20th Jul 2020, 10:46 pm	#20
pixeltwisted Triode Join Date: Jul 2020 Location: Redhill, Surrey, UK. Posts: 28	Re: Attempting the repair of Telequipment D75 Again a very quick response thankyou Denis. Triple checked my soldering work on PC148 after install but maybe I missed something. Did measure SK603 connections 1,3,5 and 7 to ground all before connection and all were over 2kohm. My question is what should I consider low resistance? Also the second I turned the unit on my brain screamed why did you put the plugins in! I can see the funny side now. Will spend time tomorrow checking my work on PC148 to see if I missed something. As you know it is not so easy working on these ****** units.