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Old 13th July 2010, 04:24 PM   #31
P.L.F. is offline P.L.F.  Poland
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Default PL36 ver.full complementary transistors equivalents

Had more time with 90VDC on PS caps - the PL36 PCB gets warm. Is that normal? Measured LVPS for IC across zeners - too low (1.8-2.4V) - why?
Doesn't look good... I think about 'en block' replacements of all transistors and diodes on the driver board...

Help needed, please!

In the meantime a question: I see MPSA18 on the PCB - is MPSA43 a good replacement? Or BC639?

Tx a lot...
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Old 13th July 2010, 07:00 PM   #32
THD+N is offline THD+N  United States
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Przem,
Yes, it is normal for certain parts of the PL-36 to get warm. I noticed in one of your pictures that the original Class-A transistor w/ steel heat sink has been replaced with a substitute transistor w/ a much smaller heat sink. This heatsink is too small and the transistor will run fairly hot. I would recommend a larger heatsink with more area. At the moment, this is not contributing to a problem your are having, but it may in the long run.

In order to troubleshoot the Zener circuit, I would take out the 4 output rail fuses. If you do not have sockets installed for the input op-amps, I would do that next. This makes taking the op-amps out very easy. If they are good sockets, you do not have to worry about any kind of degradation from the socket. I use sockets all the time and have never had a problem. With the output rail fuses and op-amps out, you can now troubleshoot your Zener power supply problem.

You say you have replaced both Zener diodes? 7.5k dropping resistors are good? At what location is the MPSA18 (Q103)? I don't recall this transistor in the original amp.
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Old 13th July 2010, 07:47 PM   #33
THD+N is offline THD+N  United States
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While troubleshooting, you may also want to disable the protection circuit temporarily by unsoldering one lead of D101 / 105. This will be one less area to be concerned with until you get the amp stable and working correctly.

It is also very easy to chase your tail when repairing amps... if the amp has a failure and you know it has been repaired, most times it is best to replace all the diodes and transistors with new ones (and the correct ones!) since there are not that many of them. Some people would argue that this is not true troubleshooting, but in my experience, when bad components are left in the circuit, and new components are installed, the new parts may be damaged. All it takes is a weak diode or transistor, and these components may check OK with a meter, but turn on the power and they show their symptoms.
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Old 13th July 2010, 08:23 PM   #34
P.L.F. is offline P.L.F.  Poland
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Quote:
Originally Posted by THD+N View Post
Przem,
Yes, it is normal for certain parts of the PL-36 to get warm. I noticed in one of your pictures that the original Class-A transistor w/ steel heat sink has been replaced with a substitute transistor w/ a much smaller heat sink. This heatsink is too small and the transistor will run fairly hot. I would recommend a larger heatsink with more area. At the moment, this is not contributing to a problem your are having, but it may in the long run.

In order to troubleshoot the Zener circuit, I would take out the 4 output rail fuses. If you do not have sockets installed for the input op-amps, I would do that next. This makes taking the op-amps out very easy. If they are good sockets, you do not have to worry about any kind of degradation from the socket. I use sockets all the time and have never had a problem. With the output rail fuses and op-amps out, you can now troubleshoot your Zener power supply problem.

You say you have replaced both Zener diodes? 7.5k dropping resistors are good? At what location is the MPSA18 (Q103)? I don't recall this transistor in the original amp.
THD, many tx for responding!

Yes, the heatsinks are not the same as in earlier models, still it is factory mod in fully comp models. Such heatsinks are obsolete now, but I agree - need to find a better cooling stuff for the mirror transistor.

Great idea - i will pursue the problem in line with your guidelines!

Yes, both zeners are new now and so are the dropping resistors (old were indeed drifted).

Here is the location attached (the picture is taken before repairs started). I think Q102...
Attached Images
File Type: jpg PL36 comp version.jpg (158.6 KB, 194 views)
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Old 13th July 2010, 08:57 PM   #35
P.L.F. is offline P.L.F.  Poland
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Quote:
Originally Posted by THD+N View Post
All it takes is a weak diode or transistor, and these components may check OK with a meter, but turn on the power and they show their symptoms.
I believe that's exactly the case here... Writing my shopping list then, still I have some new parts for replacement for PL400/700s2 amp, but indeed for instance those MPSA18 surprised me as well...

Thank you again, Przem

P.S.
on the previous picture I forgot to mark electrolytic caps - all replaced too...
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Last edited by P.L.F.; 13th July 2010 at 09:06 PM. Reason: adding P.S. comment
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Old 13th July 2010, 09:49 PM   #36
THD+N is offline THD+N  United States
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The original protection transistors can be replaced with MPSA92 / 93 and MPSA42 / 43 transistors. I have used these in all my PL (Series II) repairs and they work fine, no issues.
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Old 15th July 2010, 09:28 PM   #37
P.L.F. is offline P.L.F.  Poland
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Default PL36 refurbished

To continue my 'progress report':
  • output rail fuses out and OPA IC out too -> across each Zener diode: +/- 15V;
  • left channel rail fuses in and no OPA IC -> LVPS network with +/- 15V;
  • both rail fuses in and no OPA IC -> as above.
Consequently, my focus was PL36 components' problem. I decided to follow THD+N advice (thank you!) and replaced all diodes and transistors (picture attached). Q106/206 exchanged into MJE340 with better/equal heatsinks. One diode was soldered wrongly before I put the new one. I found one resistor with cold solder and slightly burned (2.7K) and two resistors with 4.4K instead of 3,9K (for the moment I've put two .25W in parallel 7,5K+9,1K as could find the proper value in my drawer). Also had to beef up some PCB traces - someone who repaired the amp before used too hot solder gun for sure...

Didn't have time to re-connect the PCB to output stage - tomorrow will do...

Tx for all suggestions!
Attached Images
File Type: jpg PL36 after all semi's replaced.JPG (317.2 KB, 176 views)
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Old 20th July 2010, 12:18 PM   #38
P.L.F. is offline P.L.F.  Poland
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Not much progress: after reconnecting the driver board almost all symptoms remained with the key one being:
- when OPA IC's are in the voltage across Zener diodes is less than 2V
- when OPA IC's are out the voltage level goes up correctly.

I only increased PS to 40V measured on the big caps and rail fuses were out. All semi's are new including bias transistors. Several more replacements as stated in earlier posts. What's left are resistors, but those remained look rather good.

Without getting the right level of voltages for OPA IC's cannot move forward. Honestly I am puzzled - a big dark hole...

Every suggestion is appreciated!
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Old 20th July 2010, 03:43 PM   #39
wg_ski is offline wg_ski  United States
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You've replaced the zener bias resistors with a higher value, right? The opamp's current draw may be too high to bias through 7.5k at 40V depending on what type you're using. Temporarily put back the original value and see if you get more across the zeners. You can go back to the higher value when you get done troubleshooting and know that you can run safely at full voltage.

You may still need to go to 60V rails before it "snaps" on. I would be using the light bulb limiter *without* the varaic to make sure there's enough to get the shunt regulators started. With the rail fuses out, the outputs and drivers are effectively disconnected and the only thing you could really hurt are the predrivers, which are cheap in the grand scheme of things.
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Old 20th July 2010, 08:06 PM   #40
P.L.F. is offline P.L.F.  Poland
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Quote:
Originally Posted by wg_ski View Post
You've replaced the zener bias resistors with a higher value, right?
Hi Wg_ski,

Rail fuses are out until fixing the PL36 driver board issues. The dropper 7,5k resistors are new and with proper unchanged values. In the meantime replaced C113, C102, C103 and C106 (and shadows for the left channel) in case having a shorted capacitor at nominal voltage. Finally changed IC's and .... bingo! I have realized that actually the IC's were replaced before changing all the semi's, some resistors and caps (just mentioned). So clearly they both were damaged by the origin cause of wrong zener PS voltage levels. I've put new ones and the readings across both zeners at 100VDC measured on main PS caps was +/- 15.07V. Success!

I am ready to proceed with further stage of starting up/ testing the amp.

Tx a lot for your contribution!

/Przem
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