Output transistor balancing?

[nielsk]

Working on a later EMT 140s, chasing down a spurious noise I notice that the output transistor pairs are not running at the same temperature. On each side one is running much hotter than the other (uncomfortable to touch), but it is the opposite one on either side. The 2n3053 feeding each pair is running very hot as well.
They all have big heat sinks on them.
This looks to be using an early transistor approach where the B+ is fed to them through the output transformer.
Working on this thing is a real PIA, is there a component (resistor?) I should check or easily tweak to at least get them to run at the same temp without removing everything?
Or maybe a use different transistor? I have found in other devices changing to a BD139 not only sounded better but ran very cool...

Thanks!

 

[Voyager10]

It looks like R43 is a trimmer which sets the DC operating point of the power amp section. It will need to be adjusted to get the voltage at T16 base to be the same as T17 base. According to the diagram these should both be 10V.

It's possible that something else in the circuit is bad (e.g. dead capacitors) which will need fixing first, though.

Can you measure the DC voltages around the circuit (e.g. at the points marked on the diagram, or anywhere identifiable) and post the results?

 

[nielsk]

Ha! Not seeing the trees for the forest... thanks for that.
Base of T13 was 9.38 on one side and 9.4 on the other, adjusted to 9.5 as per schematic.
No noticeable change in temperatures.
I will check voltages around the 2n3053s (nothing is marked) and report

 

[nielsk]

T16 A: C=29, E=16.8, B=16.7
T16 B: C=29, E=9.3, B=9.7
T17 A: C=30, E=14.9, B=10.3
T17 B: C=30, E-9.2, B=10
T18 A: C=14.9, E=4.1, B=4.7,
T18 B: C=8.6, E=3.9, B=4.6

I have replaced the A transistors with 2n3053A

 

[Voyager10]

OK, so the 'B' side is looking more or less right. T17B's base and T16B's base are 0.3V different so perhaps the trimmer can be fine-tuned to reduce this.

On the 'A' side, something's very wrong with T16 - there's no way it should have +16.8V on the emitter. (T17 also shouldn't have 14.9V on its emitter, but it might be being pulled up by T16).

Can you remove T16 and check it on a transistor tester?

 

[nielsk]

I have done that, it checks as good.
the 2n3053A (new parts) have higher HFE than the 2n3053, some of the 3053 measure around 30 while the 3053A measure around 100.
Am I correct the the circuit determines the gain and the transistor HFE is the maximum gain that it can deliver, so using a higher HFE is not a problem?

 

[Voyager10]

OK, the main mystery here is still T16A's emitter being more positive than anything else.

Is there a physical fault on the circuit board somewhere, eg. a short on the heatsink?

 

[nielsk]

Well, another "file under assume nothing!"
I removed T16 and still had 15 VDC on the base pad, so I removed T15 and a rusty old set screw popped out from under it!
Must have fallen in the many moons ago.
T15 tested bad, replaced it and T16 and now all voltages are right in line.
Ai Carumba