Lenco A-50 Vintage amplifier repair question

[donny007x]

I'm restoring a 1977 Lenco A-50 amplifier (that had a big oops-moment in its life). The guy I got it from told me that he tried to repair it, but he couldn't figure out the problem.

He provided me with a service manual and with a picture of the board with annotations. I immediately noticed a mistake in his notes, a NPN transistor was installed where a PNP part was expected (Q506 in the left channel, part of the protection circuit).

When I first powered the unit on, R507 started glowing red and sparks emitted from Q509. Power transistor Q512 was shorted out, along with most other transistors (and diode D502) on the left channel.

The power amplifier board was severely damaged during previous repairs, traces are gone and components were bodged in. I also found a 30 Amp fuse installed where a 1.5 Amp one should go... 

I replaced every single transistor on both channels (including the TO-3 power transistors) along with the faulty diode and charred resistor. The traces were patched up and all wire-wrap was replaced with soldered connections.

Now both channels work again, except for one thing...

One particular transistor in each channel (Q503 L and Q515 R) runs incredibly hot (finger burning hot), even if the amp is just idling at minimum volume. I want to know what I can do about that.

Attached is a schematic of the power amp section (with the offending transistors marked).

The parts that I used meet or exceed the spec of the ones listed in the service manual, the following transistors are installed now:

TO-92:
   Q501, Q502, Q504, Q505, Q507: BC639 (NPN)
   Q503, Q506, Q508: BC640 (PNP)

SOT32:
   Q509: BD139 (NPN)
   Q510: BD140 (PNP)

TO-3 Power transistors:
   Q511: MJ15003 (NPN)
   Q512: MJ15004 (PNP)

Both channels have identical components installed.

[SeanB]

Check D504 is not either shorted or more likely open circuit. If it is open then the input stage bias will be way too high, and will make the transistors run excessive current.

[donny007x]

D504 measured fine, 0.67 V drop.

I found another major issue with this amp: high DC offset. I'm reading -400 mV at the speaker terminals on both channels.

[AF6LJ]

Do you have any information on how VR-501 / 502 are suppose to be adjusted?
As for your offset, you could have one or more damaged resistors.

[Jasper]

Given that d504 is marked on the schematic as a zener, how is 0.67v drop ok? It sounds like a zener was replaced with a regular diode...

(HZ14B doesn't show up on google, but possible 14V?)

[AF6LJ]

Given that d504 is marked on the schematic as a zener, how is 0.67v drop ok? It sounds like a zener was replaced with a regular diode...

(HZ14B doesn't show up on google, but possible 14V?)

That wouldn't surprise me.

[donny007x]

The service manual doesn't mention any adjustment procedures, both pots are set in the center for the moment.

 Thanks for the tips, D504 should indeed be a Zener diode...  But I'm not entirely sure what type exactly, the part number is a dead end.

[cvanc]

Thanks for the tips, D504 should indeed be a Zener diode...  But I'm not entirely sure what type exactly, the part number is a dead end.

'HZ14B' is a no-show on Google but 'Z14B' has a lot of hits - a 14 volt Zener, which sounds like it might fit the situation?  Good luck with your repair.

[cvanc]

Is it possible your Zener is actually the correct HZ14B part but it's just installed backwards?  That would give you the ~0.7V you are measuring (and possibly overstress R527 in the process)...

[AF6LJ]

The service manual doesn't mention any adjustment procedures, both pots are set in the center for the moment.

 Thanks for the tips, D504 should indeed be a Zener diode...  But I'm not entirely sure what type exactly, the part number is a dead end.

I am thinking the pots are to be adjusted for minimum crossover distortion.
Just be mindful of the dissipation on that transistor.

[cvanc]

The pots look to be typical idle current adjustments.

Given the current overheating issues it might be good to set them for minimum overall current draw for the time being.  Later, once the circuit is working right, adjust the pots for the sweet spot where both distortion and current draw are reasonably low.