Yamaha EM-150ii power amp repair

[Rhod]

Dear all, I’m wondering if any of you good folks In this forum might be able to help. I’m repairing an old Yamaha powered mixer for my son to use. The story so far:
Cleaned all pots, repaired or replaced broken pots, repaired broken connections, replaced main on/off switch. At this stage the preamp worked fine, good clean signal coming from line out jacks, both channels, but blowing fuse so only one channel working on speaker jacks/headphone output. Tested output transistors and found two bad ones: 1 2SC1403B and 1 2SA745B (MA circuit board, one of Tr11/12 and Tr13/14 on the schematic). I got equivalents to replace: MJ21194G and MJ21193G and put them on the board with their original mica film and new thermal paste. When I fired it up, still no output on the duff channel, but the fuse didn’t blow (so progress of a kind!). After a short time, some loud crackling on the bad channel on the phones output (no speakers connected) then joined by a loud hum, as that side heated up significantly. The good channel stayed good. I turned it off before things started smoking!

I checked the 0.47ohm cement resistors as per this thread: https://music-electronics-forum.com/...ad.php?t=32209
and they all measure about an ohm in circuit.

Questions: would the mismatch between the old and new transistors cause the overheating/lack of output and should I have replaced all of them?

If not, what do you think I should check next?

I am reasonably experienced in repairing stuff, can use a multimeter at a basic level and can follow instructions but I am no electronics expert! I have a copy of the schematic downloaded from https://elektrotanya.com/yamaha_em-1.../download.html

Hoping someone will be able to point me in the right direction- thanks in advance!

[Jazz P Bass]

I would suggest that when replacing 'paired' output transistors, that you replace the pair.
Being in parallel, you want a tight set.

There are test voltages on the schematic that I would check.
To me, the most relevant, after power supply rails, is the base connection of the output transistors (+0.6Vdc /-0.6Vdc)
Also, when confronted with shorted outputs, it is best in the long run to also replace the driver transistors (TR6 & TR8)

[Enzo]

DId you check for DC on the speaker? DId you adjust the bias?

[Rhod]

Thank you, Jazz P, appreciate your response. I thought that might be the case with the replacements. I guess I was investing the minimum into what is an interesting piece of kit but worth very little, until I was convinced that it could work!

I’ll have a look at those other transistors first and also see if I can get those voltage readings - any tips as to how to take them while the amp is rapidly rising in temperature?!

[Rhod]

Oh, another question: if the new and old transistors are causing a problem. Would this be enough to prevent any amplified signal AND the crackling/hum AND the overheating?
Thanks!

[Rhod]

I checked for dc on the other channel when starting this work and all was well as you would guess. Couldn’t check on other channel as fuse was blowing immediately. I could do that now but would have to do it quickly before the temperature gets critical! Haven’t checked the bias - I sort of assumed that doing that would be a fine tuning thing rather than one to make it amplify at all! I will take a look, and return here if I can’t work out how to do this (and if it needs e.g. an oscilloscope!!).

Btw after posting this everywhere in error I think the right place for it is here, where Jazz P Bass has chimed in:

https://music-electronics-forum.com/...ad.php?t=51223

[Enzo]

Don't worry about where the thread is, focus on the amp.

First off disconnect any speaker or load. A load turns any voltage offset into a current draw.

A grossly underbiased amp will run REAL hot.

[Rhod]

Thanks!
I can see that the bias can lead to the overheating problem, but would it be responsible for no amplified signal at all on that channel?

[Enzo]

Never think up reasons not to test something. I don't know what is going on in your amp, but until you can get it stable enough to let it run, we can't very well be doing more advanced troubleshooting of the signal chain.

[Rhod]

Good point, well made.

[Rhod]

Ok, checked bias - on good channel with bad channel disconnected first to practise! Read high: ~38mv so reduced it to ~23mv as per the manual. Then tried the bad channel. Read really high: around 400-500mv, even with bias adjustment pot fully counter clockwise.

Turned it off as I could smell it getting hot.

I also checked the voltage at the base of the output transistors as Jazz P Bass suggested in the parallel thread.

Read +/-0.6v on the good channel - all good but way higher on the bad: beyond the 20v setting on the multimeter: ~36v! No wonder they’re getting hot.

Where does this leave me?

Thank you!

[Rhod]

Bless the moderator who sorted out my posts and united these threads! Thank you.

[Enzo]

Are we operating with NO load on the amp? And do we have a large DC offset on the "bad" output?

[Rhod]

Yes, no load. Haven’t checked DC offset - I will check that in the morning (I’ve headed to bed now!). I’m guessing voltage across speaker output terminals, all volumes at minimum?
Thanks for your help.

[Jazz P Bass]

I would recommend at this point that you build a 'Lamp Limiter'.
What it does is runs one leg of the Vac input through an incandescent lamp thus limiting the current draw.
Something is fried in the section before the output transistors and is pulling current.