Attempting the repair of Telequipment D75

[pixeltwisted]

Some progress to report. Following the arrival of replacement resistors and transistors
got PC149 repaired and working again outside the case.

Further resistance measurements made me suspect the S2A plugin to be the culprit
of my previous issues.

With both V4, S2A plugins removed I desoldered PC147 16,17,18 105v wires and measured
resistances to ground again, all seemed good. To be sure I also disconnected the 105v
connection to PC128 from PC148.
Resoldered PC147/18 back to PC148 and tested with power no magic smoke, then
re-connected PC128 back to PC148 again tested OK.

To cut a long story short everything has been progressivley reinstalled apart from S2A
with no problems.

So opened up S2A to be presented with electrolytic capacitor C202 with a burn
mark on the negative lead and a bulged positive end (photo attached)
Will order a replacement capacitor and in the meantime will continue to test S2A.
Have not seen this problem with a capacitor before so any diagnosis appreciated.

Another question regarding normal operation of the Reset neon lamp on S2A. Is it
always lit during normal operation? It seems to be perilously close to the case
(photo attached) I have not had S2A plugin working it is only that the previous owner
showed a photo with the reset lamp lit?

As always thankyou all for your time.

Gavin

[Pinörkel]

Originally Posted by pixeltwisted

Some progress to report. Following the arrival of replacement resistors and transistors
got PC149 repaired and working again outside the case.

Further resistance measurements made me suspect the S2A plugin to be the culprit of my previous issues.

With both V4, S2A plugins removed I desoldered PC147 16,17,18 105v wires and measured resistances to ground again, all seemed good. To be sure I also disconnected the 105v connection to PC128 from PC148. Resoldered PC147/18 back to PC148 and tested with power no magic smoke, then re-connected PC128 back to PC148 again tested OK.

To cut a long story short everything has been progressivley reinstalled apart from S2A with no problems.

Good work there.

Originally Posted by pixeltwisted

So opened up S2A to be presented with electrolytic capacitor C202 with a burn mark on the negative lead and a bulged positive end (photo attached)
Will order a replacement capacitor and in the meantime will continue to test S2A. Have not seen this problem with a capacitor before so any diagnosis appreciated.

I would not worry about the burn mark. I have a bunch of brand new capacitors here that all look exactly the same. I think it is from spot welding the wire to the can. However, a bulging positive end on the other side is not good and not related to the burn mark.

Originally Posted by pixeltwisted

Another question regarding normal operation of the Reset neon lamp on S2A. Is it always lit during normal operation? It seems to be perilously close to the case (photo attached) I have not had S2A plugin working it is only that the previous owner showed a photo with the reset lamp lit?

The lamp is on as long as the sweep is running from left to right. Just switch the sweep to "single". Then it will go off. Then set Time/Div to 50 ms and push the sweep reset button. The lamp will light up as long as the trace moves from left to right and then go off again. The lamp mini-pcb being close to the case is normal. Of course you could apply some insulating tape.

[pixeltwisted]

Hi thanks for your reply.

Thankyou for your description of how the reset lamp operates. Have not installed
S2A back into the unit as nothing has been changed to remedy the resistor
burning/transistor opening problem. I need to investigate further as to what is
wrong with S2A or SK601 before I damage anything further.

Asked the question only because the person who sold it to me showed a picture
with the reset lamp lit and I wanted to know if it was normal and not something
I should investigate on the S2A plugin. Now I know it is normal.

Managed to get an out of focus dot on the CRT without S2A installed, when I tried
to change focus the CRT flashed in the middle. Have only managed to get flashes
on the CRT since. So at least PC148 & 128 are doing something and my work
earlier appears not be the cause of the short circuit.

With PC149 installed, V4 plugin removed and S2A plugin installed the resistance
measurements on PC149 pins:
1=964 Ohm
3=872 Ohm
5=6.56 kOhm
7=912 Ohm

With PC149 installed and both V4 & S2A removed resistances are:
1=2.185 kOhm
3=2.219 kOhm
5=8.95 kOhm
7=5.193 kOhm
This led me to believe wrong or not that S2A was a problem.

Strange the 105v pin has a higher resistance even though that is where the short
circuit appears to be hiding.

Another question have attached two images showing two wires twisted together
one of which is insulated on two boards in S2A, looking at your schematics cannot
figure out what they do?

Kindest Regards,

G.

[pixeltwisted]

Following a bit more testing with S2A removed. Applying a signal to CH1 or CH2 and using
Trace Locate am able to get a faint trace with vertical deflection, obviously no horizontal
deflection. Then after releasing Trace Locate get a bright flash towards the centre of the
CRT and until a power off am unable to get a trace back.

Could this be the 105v bistable protection kicking in?

Apologies for so many dumb questions.

[Pinörkel]

Originally Posted by pixeltwisted

Managed to get an out of focus dot on the CRT without S2A installed, when I tried to change focus the CRT flashed in the middle. Have only managed to get flashes on the CRT since. So at least PC148 & 128 are doing something and my work earlier appears not be the cause of the short circuit.

If S2A is not installed, you can turn the scope on and set (important!) a not too high brightness (power knob indicator pointing straight up is about right on my scopes). Then center the focus knob (if it is not in a focused position) and press the trace locate button. A small dot should now appear somewhere on the left side of the screen center. The trace locate button overrides some circuits that control the trace position and may have brought it off screen.

Something to be aware of: If the D75 is switched on and you switch it off and immediately on again, then you will get some VERY bright and fancy stuff going on the screen. Due to some capacitors still being charged and the heating filament in the tube still being hot you will see some stuff that usually is invisible because the tube has not heated up yet. When this happens, it can be extremely irritating and you might think that something blew up. Usually this is nothing to worry about, but I would avoid it. Extremely bright electron beams are not good for the CRT.

Originally Posted by pixeltwisted

With PC149 installed, V4 plugin removed and S2A plugin installed the resistance
measurements on PC149 pins:
1=964 Ohm
3=872 Ohm
5=6.56 kOhm
7=912 Ohm

With PC149 installed and both V4 & S2A removed resistances are:
1=2.185 kOhm
3=2.219 kOhm
5=8.95 kOhm
7=5.193 kOhm
This led me to believe wrong or not that S2A was a problem.

Strange the 105v pin has a higher resistance even though that is where the short circuit appears to be hiding.

Maybe I have some time to do comparative measurements on my scope for you tomorrow, but I can not promise.

[Pinörkel]

Originally Posted by pixeltwisted

Another question have attached two images showing two wires twisted together
one of which is insulated on two boards in S2A, looking at your schematics cannot figure out what they do?

I may be wrong, but I think, this is called a "gimmick". It a cheapo-style capacitor with a value of a few pico farads that is there to couple some high frequency stuff from one side to the other. They are not mentioned in the schematics, but in the PCB layouts their positions are marked yellow.

[pixeltwisted]

Thanks for your advice and the offer of measuring your resistances. Please do not
spend time on this.

It has been a long day and I just made a very stupid decision, test the voltages on
SK603.... as I tested pin 1 - 30v unregulated small spark.... I hate myself sometimes.

Fuse FS402 had popped. Replaced it now trace appears on the right hand side of the
CRT without pressing trace locate. Also intensity is now working in reverse!? just after
turn on the trace is very bright (actual improvement on intensity compared to before)
and gets dimmer as the pot is turned clockwise.

Anyway with 12.5kV floating around inside this am putting everything away and come
back to it with a clear head.

Yes I know I am an idiot. Apologies for wasting your precious time.

[pixeltwisted]

An update from the resident idiot.

Am not sure how just by measuring DC voltage a spark occurred but it did.

Destroyed Transistors TR403 & TR414 on PC149 power supply. Also TR2 on PC150
died. Causing PC149 voltages to read 30v=32v 24v=19v, 105v=89v and -24=0v. (Rounded the values)

By swapping transistor BC207 TR1 for TR403 and MPS6518 TR153 from S2A
now have all power rails reading correctly outside the case.


Am not going to try anything else until replacement transistors arrive.

Before my stupid mistake yesterday am sure both CH1 and Ch2 showed equal deflection,
then afterwards CH1 had less deflection and CH2 had half the deflection of CH1.
All transistors in V4 tested good, left the dual Fet's TR601 and TR602 alone until I
replace the other components, as desoldering the resistor and the difficulty in replacing
them has put me off the idea.

Anyway when I replace TR1 & TR2 on PC150 I will test everything again and report back.

Unless it is safe to leave TR1 & TR2 off of PC150 just for testing?

When I was a young man I would never have made so many stupid mistakes.

[DMcMahon]

Originally Posted by pixeltwisted

When I was a young man I would never have made so many stupid mistakes.

I can concur with this

David

[pixeltwisted]

Good man David glad it is not just me.

[Pinörkel]

Originally Posted by pixeltwisted

Am not sure how just by measuring DC voltage a spark occurred but it did.

I am pretty sure you shorted pin 1 to the pcb trace going all along the bottom side of SKT603, which is connected to pin 7, the -24V line. Doing that can cause collateral damage, since -24V is used as a reference voltage on many places. A destroyed TR403 (overcurrent protection on the -24 V line) is logical. TR414 too because it is directly connected to that in the 105 V line. Unfortunately, all dual FETs in the V4 are very close to that -24 V line.

[pixeltwisted]

Good afternoon Denis hope you are well.

Looking back at the schematic your explanation is sound. Maybe I need some new glasses.

Also looking at where the -24 connects in V4 collateral damage is almost assured there also....
as you pointed out. Hope it has not damaged the dual Fet's TR601 and TR602 but my gut tells
me they have had it. Have found some American suppliers so some hope at replacing them.

Again thanks for your help.

[Pinörkel]

If you look closely, the destroyed TR1 and TR2 on PC 150 are also directly connected to the -24V line. This part of the circuit on PC150 is also connected to the +24 line (bottom left in the diagram, via 147/15) and seems to be the calibration square wave generator. As I see it, other functions should not be affected if TR1 and TR2 are not there.

Regarding the short circuit: Maybe your probe slipped down the soldering joint while measuring. This does often occur, if the probe tips are not pointy but round. Pointy tips dig into the solder and do not slip away as easy.

[pixeltwisted]

I also thought looking at the schematic TR1 and TR2 would be OK removed. But considering my track record am second guessing myself.

My DMM probes are very sharp but I obvioulsy messed up. Ho Hum.

[pixeltwisted]

Removed TR1 and TR2 and powered up absolutely nothing on CRT no matter what controls are
changed signal applied or not.

Removed PC149 and all voltages check out fine.

Am going to wait until replacement parts arrive and begin testing again. Enjoy your Sunday.

G.

[pixeltwisted]

Put all defective componenets back in their original positions and the CRT now
comes to life as it did last night. So the replacements should get me back
on track for further testing.

Also confirmed CH1 and CH2 have different amplitudes for the same signal applied.
Looks like I will need to save up for replacement TR601 & TR602 dual FET's?

Again enjoy what remains of your weekend.

[MotorBikeLes]

Not sure where you would get them today, but years ago, I bought BFW10 and BFW11 very cheaply. I matched them up (quite a spread as purchased) and fitted them as TR1 and TR2 in a few scopes. I used a tight wrap of silver plated wire to join them as a heat sink. Worked perfectly. Not sure I used them at the 50MHz front end, but certainly at 25MHz.
If nothing else, it is a quick and inexpensive test pending spending big cash on OEM stuff.
Les.

[pixeltwisted]

Many thanks Les will look into doing just that. An inexpensive test is exactly what I need.
Although a quick look on a well known auction site shows they are not that cheap.

Found a place in America that supplies Tektronix part 151-1036-00 dual N-channel FET's
at almost the same price as BFW10's. I will see when I get enough money in the bank.

Interestingly the FET's installed are TFD 1551 7840 for TR601 and TFD 1551 7836 for TR603
strange as the service manual lists them as both 151-1036-00.

Just to prove that they are the offending parts am going to swap them over when I build up
the courage. Grin.


Kindest Regards.

[MotorBikeLes]

Those Dual Fets you just listed sound exactly the same as the ones used in the TQ vertical calibration fixture.
I vaguely recall that I will have had those BFW10/11 from Sendz components down at Southend (or just along from there. I contacted him about 5 years ago, and he was still trading. I needed an audio chip for an LCVD TV, and he supplied that. He has or had a website, with a bright young guy looking after it for him, and it needed 24 hours to get to David (Whitworth) himself.
If you look on the back cover of any 1970's onward "Television" magazine, you will find his phone number. May be worth a try. He never charged silly money.
Les.

[pixeltwisted]

Thanks Les found the phone number for Sendz so will try calling them tomorrow.
Fingers crossed.

Really appeciate you taking the time to help out.