Yamaha EM-150ii power amp repair

[Rhod]

Great, thank you.
On good channel with lamp limiter, bias measured 7.9mV.
0.47R just above TR12, voltage drop of 6.9mV
0.47R above TR11: 7.8mV

On bad channel with lamp limiter and TR12 removed, when bias raised slightly, lamps glows brightly and reading shoots straight from ~4mV to 40-odd. At highest stable bias reading possible (4mV) the .47R just above TR12 had zero drop, the one above TR11 read 5.3mV.

I’m looking forward to what this tells us! Thank you.

[g1]

How about the .47R's at TR13 & TR14?

[Rhod]

.47Rs

Good side (Bias 7.9mV)
TR11: 7.8mV
TR12: 6.9mV
TR13: 7.8mV
TR14 7.2mV


Bad side (Bias 4.0 mV)
TR11:5.3mV
TR12:0mV
TR13: 1.7mV
TR14 1.5mV

[g1]

So it looks like it's not just TR12 (though it still may be bad), but something is turning on TR11 too hard (big difference compared to TR13 & 14).
Post your B,C, & E voltages for TR6,7,8. Also compare them to what the ones on the good amp read.

[Rhod]

Transistor leg voltages wrt Gnd (V):

TR6
Good amp B = 1.18 C = 47.1 E = 0.58
Bad amp B = 1.13 C = 47.7 E = 0.54

TR7
Good amp B = -45.9 C = -1.21 E = -46.5
Bad amp B = -47.1 C = -1.14 E = -46.9

TR8
Good amp B = -1.19 C = -47.0 E = -0.59
Bad amp B = -1.13 C = -47.9 E = -0.53

[g1]

Those numbers all look good. You have replacements coming for TR11 & TR12?
It is also possible that TR6 is breaking down under working conditions when the bias is turned up. Or there is some kind of fault regarding the insulators or screw bushings for the output transistors. Are those output transistors socketed or soldered in?

[Rhod]

That’s what I thought. At least, nothing leaped out at me! Output transistors are socketed, so as you say, it could be something to do with the insulation/screws. If the insulators were at fault I would expect to be able to measure a short between C and ground though.

Maybe I should swap all the output transistors from the good side over to the bad side, with or without their insulators, to see what happens.

Or I could just order replacements for TR11,12, 6 and see what that does for us...

Thanks!

[g1]

If it were me, yes I'd try swapping parts between sides. Either suspect parts into good side or good side parts into bad side. Your limiter lamp should protect you if you shut down when you see bright bulb.
Be careful to keep track of what's what, taking notes as required.

[Rhod]

She lives!!!
Starting swapping things around as encouraged. First tried the whole good amp on the bad side, just swapped it heatsink and all. Was surprised to hear .... nothing. Well, apart from a very faint signal if you really turned it up. Then tried the ‘bad’ amp on the good side, and it came through loud and clear. Lamp limiter nice and dim, bias easily adjustable. I knew the preamp was working as we’d quite happily been using the desk with the line outs, but I checked that again just to be sure. Line outs still ok. The only thing that lay between the preamp and the power amps was the eq section, so inspected that. After some continuity testing I noticed that one of the output connections was hanging off! Soldered it back and bingo! Both channels working fine...

Thank you so much to you for enabling me to get this thing working. Enzo and Jazz P for starting me off and g1 for sticking with me! You are awesome, thank you. Not easy with multiple faults, and I needed the expertise, thank you.

Now I just need to get ALL the eq faders to work, up AND down! Also, any tips on resurrecting VU meters? I bet there’s a thread or two about that on here somewhere.

Thank you again...