Just did a little research...I took Q421 and Q425 out of circuit.

Q415 tested OK.

Q421 did not test OK !
Could this be the culprit? hmm

The obvious test, replace it and test the results.

 

Yes that could be the cause since D415 is on the collector of Q421.

 

Moreover, during the voltage readings, a difference appeared between Q421 on the left and Q422 on the right.

On the first collector there was -0.35v and on the second collector there was -2.45v.

Besides the base and the collector of Q421 indicated the same value, so if it is the junction was short-circuited.

EDIT: After replacing Q421 and before turning the device back on, remember to reset VR401 to zero (counterclockwise).

 

Besides the base and the collector of Q421 indicated the same value, so if it is the junction was short-circuited.

That did not hold true for Q429 where the emitter and the base were the same (within the count error of the meter).


Remote troubleshooting does indeed have some difficulties.

on edit: however, with kirchoff's voltage laws, and hindsight, Q421 collector voltage plus one diode drop of 0.7V does equal the base voltage on Q425.

 

Good evening "JoeJester",

I did not speak of the transistor Q429, but of the Q421 which had its base and its collector at the same value.
Probably a shorted junction.

 

I did not speak of the transistor Q429, but of the Q421 which had its base and its collector at the same value.
Probably a shorted junction.

I know, and I mentioned the same condition (where the base voltage and another voltage, e or c, was the same) did not hold TRUE for Q429.

All suspicious readings need to be checked. For the lack of a standard, I chose 10 percent of the published reading to indicate the errors in my spreadsheet.

 

Nothing to contribute to the troubleshooting here. I am new to the forum and was amazed at how helpful everyone was during my troubleshooting process on my sweep/function generator issue.

I just spent 6 hours reading through this entire thread and I must say bravo! This thread is an incredible troubleshooting journey. I applaud the tenacity of everyone involved. I was an electronics technician in a former life, although I have not been practicing as often as I should, I love a good troubleshooting task and this one is outstanding.

MrChips, JoeJester: You guys rock. I haven't seen this level of troubleshooting since I left Radar troubleshooting in A-school.

Marius - This post will alert me for any new posts on the thread. Good luck and keep at it. It's now become legend and must be completed. I have trained many technicians in industrial maintenance and most lack your abilities. You might have started a novice, but you are really gaining some impressive skills.

 

@CBTech_Joe you will find this type of troubleshooting challenging. Reminds me of when the only troubleshooting I did was in my head relying on my technicians to provide what they were doing when they were troubleshooting, or in some cases, learning to troubleshoot and expand their experience base.

As a case study, this one, once completed, would require a review on all the steps and mis-steps, to try and mitigate the mis-steps. The best learned experience comes from the mis-steps.

I have every page printed as a pdf for future review.

 

@CBTech_Joe you will find this type of troubleshooting challenging. Reminds me of when the only troubleshooting I did was in my head relying on my technicians to provide what they were doing when they were troubleshooting, or in some cases, learning to troubleshoot and expand their experience base.

This is exactly how I have been troubleshooting for the past 15 years. It is very difficult. I have had to be on-site for problems that were too complex for timely remote troubleshooting, but as time passed and my techs developed their skill level, my on-site time was reduced.

As a case study, this one, once completed, would require a review on all the steps and mis-steps, to try and mitigate the mis-steps. The best learned experience comes from the mis-steps.

Every problem that caused us to have production interruptions was thoroughly reviewed and debriefed to find out what we did wrong as well as what we did correctly and how we could have avoided those mis-steps. Afterwards the best methods we could have used were documented and put into our equipment notes. This was not done at the management level (where I would give a very basic review) but with all the technicians in the shop.

Sorry, didn't mean to hijack the thread.

 

If we did everything right the first time then we didn't learn anything.
Making mistakes is a learning opportunity not to be wasted.

 

This was not done at the management level (where I would give a very basic review) but with all the technicians in the shop.

Out-briefs are always written by the technician, the others hear the story and hopefully gain some insight. And this thread here is one story.

 

@CBTech_Joe: Thank you for kind words, i'm glad you find it interesting, i think so too


So guys, i got the replacement for Q421 today.
Checked the new one with my tester just to see, and it was ok.

So i installed it.

Power ON.

All seemed good, idling with no problems.
Started to adjust at the VR's, now i got the correct value (25mV) at about half a turn at LEFT channel.. Progress(!)
See attached picture, here's both are at correct value.
Still, RIGHT reaches the value almost too easy compared to LEFT as you can see at the picture.
But i guess it doesn't need to be a problem.

Anyways.... i let it idle at set value for a 10-15 minutes.. all seemed fine.

So i went further to adjust Vr403 and VR404.
Adjusted LEFT (VR403) at first, put it at 3mV as in the manual, all good.
Then i adjusted RIGHT at the same value, seemed fine.

Only about a minute later, i could hear the power supply had a big draw, a little smoke came and it blew the 2A main fuses..

The smoke came from R513, it's a little swollen

This isn't an easy fix

 

This isn't an easy fix

Yeah, it's looking like that.

Q503, Q505, Q507, Q509, Q515, Q517 need to be checked ... they are the active devices.

Should all prove to be good, we are down to checking all the passive components. If one is bad ... that whole circuit needs to be fully checked.

The same component may have caused both failures, but that remains to be proven.

 

As I was thinking more about it this morning, you just replaced a transistor whose emitter attaches to that resistor. make sure you didn't accidentally bridge a solder connection between two runs.

Whatever happened, it may have caused other components to fail.

On edit ...

These are the likely suspects:

E-C short in Q503 or Q505 will cause an excess current in R513.
C-B short in Q507, Q509, Q515, or Q517 will cause an excessive current in R513

Any one of those conditions will cause 8+ amps on the 60V power supplies. More than enough to cause that fuse to blow, and a short enough time for R513 to smoke.

 

I checked, i did not make a solder bridge when replacing Q421
I will remove the transistors and test them later on.
It idled just fine, but last time i adjusted VR403, Q515 blew... and it looks like something similar happened now even with the earlier faulty Q421 replaced.
Seems like something that’s hard to find is defective...

 

Seems like something that’s hard to find is defective...

Still, we start somewhere. Remember, we had a number of components out of the circuit before the failure. Did you happen to check them before re-installing them?

We just have to be extra caution and not contribute more to the problem.

 

I do not install any component without testing at this point, even new ones.
After the last incident with blowing Q515, i did remove all power transistors, installed two new ones (515/517)and the two others tested ok.

I took out the PCB now, and removed the transistors.

About the smoke and R513 i mentioned, it looks like i was wrong, R513 measures ok out of circuit and it looks like it hasn't been hot, the paint on i fooled me,sorry, that was a faulty observation from my side...
I actually don't know were that little smoke cloud came from right now, but maybe from a power transistor... don't seem to find any other damaged component..

But, the transistors tested like this:

Q503: OK
Q505: OK
Q507: Blown
Q509: Blown
Q515: Blown
Q517: Blown

I need to order a bigger batch of power transisors at the next order..

What could cause all four to blow.. just by adjusting VR403......

 

Hello,

I don't know where you bought the power transistors, but unfortunately there are a lot of fakes components found on the Toshiba 2SC3182 and 2SA1265.
Ensure the integrity of the seller.

 

What does BLOWN mean? I realize it means NO GOOD, but let's get down to how they failed. SHORTED (b-c, b-e, e-c) OPEN (b-c, b-e, e-c)
In lieu of waiting for those components, try to get the substitutes for them that are locally.


You described a blown fuse AND R513 smoking. A circuit oscillating could have overdriven those four components. The challenge is to explain all those conditions. So, please inform us of how each transistor failed.

Don't forget to get a replacement for R513 if you measure it out of specifications.

 

What does BLOWN mean? I realize it means NO GOOD, but let's get down to how they failed. SHORTED (b-c, b-e, e-c) OPEN (b-c, b-e, e-c)
In lieu of waiting for those components, try to get the substitutes for them that are locally.

yes..my tester says no good
I will measure them to check how they have failed.

@Technics44 i have just bought them from ebay... don’t know if they are genuine Toshiba or not..