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TTi EX354T Power supply - CRACKING!
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weejag:
Well, this has been interesting.
I put 18v between the two diodes D6 and D58 to power IC 8 and the 15v regulator.  This worked well and I got appropriate signals on the output to drive Q5.  IC1 put out an 83kHz (supposed to be 100kHz).

Measuring the HT line nothing was seen until the suggested voltage was applied to Vrect then the circuit worked.  I limited the current on this line and as this was increased so did the HT voltage up to about 180v. The Q6 / Q10 detection circuit shuts down IC8 if the supply voltage falls below 85v so this was disabled by removing Q1.

As I increased the current limit on the Vrect supply (and the supplied voltage also increased) I thought I was on to a winner as the HT voltage increased. I noticed that two resistors R120 and R121 suddenly became white hot and blew. I had forgotten to current limit the low voltage supply which was could put out 4 amps.  This was quite fortuitous as it made me realise that the only way that both these would blow is if the the thyristor SC1 kicked in.  This suggested the problem was on the secondary side and also explained the hiccups.  I replaced the resistors and current limited the line to 500mA.  Later I realised that having attained a certain HT voltage this supply could be switched off as the supply from TX2-F was able to sustain IC8.

FB8, FB5, FS2, and MA1 and MA2 were removed to try and remove any problem in the secondary side.  Unfortunately the hiccup still persisted so I looked to the transformer. I thought that TX2-G may be short circuit as it measured only 0.0005ohms ( in circuit).  I removed the transformer, cleaned it up and retested. This time it was 0.01ohms but the other ranged from 0.17ohms to 0.05ohms and considering that TX2-G seemed to have the thickest wires I thought this was probably reasonable.  If it had really been 0.0005 and was now 0.01ohm it may have resolved the problem.  It seemed much better when I took the HT up to 340v before it tripped.  Stupidly I decided to give it a proper go and replaced the missing components and plug in the mains supply to see if anything had changed.  The big 470microF 400v capacitor went bang!  I have a replacement on order.  Unfortunately, I still don’t know the answer but thanks to NCTNICO I have had a great deal of fun trying to get there.  What a day!
I’m now thinking that the OPTE must still be working despite the removal of the MAGAMP coils (and that It was not really logical to look to the transformer).  Next move is to remove Q14,15,16 and Q11,12,13 and test them individually but that’s for another day.

Any further suggestions?
asis:
Due to the fact that during the experiments you used your own Lab PSU and powered the circuit with V+ (20~30V), even with the FS2 fuse removed, voltage on optocoupler IC14 (pin1) could arise through the circuit:
V+ ->R60; D35; Q16; D10; R30.
Keep in mind that Q16 - STP60NE06, has an internal diode between S(3) & D(2), connected in the forward direction and taking into account the total component of the switching thresholds of the zener diodes D35(18V) + D10(6.8V), the voltage level could enough to turn on optocoupler IC14.
weejag:
Thanks ASIS.
I'd missed that V+ was there at all so I guess that I need to look at those components first.
Getting there!
weejag:
Actually looking at V+ on the secondary side it is derived from TX2-G rectified by D8 and smoothed by C29/39 so in fact this is disabled by removing FS2.
However, I only realised this after having checked every component in this part of the circuit.  I was only left with the LM358N so again I socketed this chip and replaced it (as I had one available).
I received my new 470uF 450v capacitor and stuck it in.  Having not really discovered any obvious faults I thought that I would have another go with NCTNICO's powering up option.  I got the HT up to 380v and there were no hiccups.  I guessed I might be there. I replaced Q1 to monitor undervoltage, and powered up with the variac.
Eureka!  At 120v I got an HT of about 390v which stayed steady when I went to full 240v AC input.  Sorted!
As expected, when I connected the front panel all went well and it now sits beautifully on a shelf in the workshop.

I didn't expect such a journey but I have learned loads from the experience.  As it was, the main culprit seems to have been the big capacitor C88.  I had gone down many rabbit holes having believed that testing this cap with an LCR meter and getting a good capacitance value and good ESR meant that it was OK.  Clearly this was not the case and I am thinking of revisiting Paul Carlsons Capacitance Leakage Tester project.

Thanks for all those willing to help.  TTFN
Hydron:
Congrats and enjoy the supply - I also resurrected one of these in the past (free from work due to being faulty) and it served me well for years. In my case one of the logic chips went bad so a reasonably easy fix given the service manual availability (allegedly it was damaged by something falling down into the supply through the vents, though I'm dubious it would have caused that failure mode).
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