Yamaha EM-150ii power amp repair

[Rhod]

Sounds like a sound idea. Don’t you love it when one project leads to another?! ;-)
Might be tricky tracking down a 100w incandescent - I could pull a 60w from the attic!

[Enzo]

They don't sell regular bulbs any longer, but they still sell them in "special service" or "rough duty". Plus special duty lamps like heat lamps and spotlights can be had. The bulb limiter won't care if the bulb you use is finny looking.

[Rhod]

I’m back after making a lamp limiter. The bulb in the attic turned out to be 100w - result!!

Ok, turned on the unit through the limiter but with only the good power amp connected. Limiter lamp glowed bright then dimmed immediately as you would hope. DC offset on good channel settled at 1.1 mV. Repeated the exercise with both power amps connected. Limiter lamp glows bright and stays bright. No voltage at speaker outputs (registered 0.2 mV or so then settled on zero.

Does this help?!

[Rhod]

I went back to Jazz P’s original reply and took a look at TR6 and TR8 transistors. I didn’t pull them from the board but all seemed to test ok B-C, B-E in both directions and matched the readings on the good side.

Also, when you said about checking the power rails did you mean the power supply to the power amp boards? They seem to be a pretty reliable +/-50v on the good and bad sides (B+ and B- on the schematic).

If I’m looking for something ‘fried’ before the output transistors any thoughts where I should be looking next?

Thank you!

[Enzo]

Checking B-E and B-C is swell, but you are forgetting E-C. Imagine E and C shorted together. The two virtual diodes would still test OK.

[Rhod]

You’re spot on, Enzo, thank you, and we may be getting somewhere:

C-E on TR6 (in circuit)
good side = 763 mV
bad side = 53 mV (so looks shorted)

E-C on TR6
good side = 510 mV
bad side = 53 mV

So my next step is to pull out TR6, test it properly out of circuit, and look for a replacement...

[Rhod]

Hmmm....TR6 tests ok out of circuit.
B-E = 648
B-C = 639
E-B,C-B,C-E,E-C all OL.

Any thoughts?

[Rhod]

Ok, so I figured that if that transistor was ok then something else in circuit was amiss to make it look as if it was shorting when in circuit.

The collector and emitter of TR6 connect to +B and Ba on the schematic which were shorted. In turn these are connected to the base and collector of TR12, which was ok before but now tests as faulty (voltage drop of ~900mv instead of OL across C-E) This was one of the old transistors, but looking at my notes, in one of the positions where there was a bad transistor before.

In a bit of a twist I checked to see if the short across +B and Ba was still there without the bad transistor in circuit - it wasn’t - but then when I reassembled the circuit with the faulty transistor but without the heat sink I couldn’t replicate the short at all.

So is the problem with that transistor, or, more likely, something else is wrong that’s causing these transistor failures. I guess I should have built the lamp limiter before firing up after my first replacements.

I think I may be at what I see Jazz P has in the past called the GRRR stage! Any support would be greatly appreciated. Thank you.

[g1]

What do you mean by "Ba" (TR6 connection) ?
And "without the heatsink", do you have the screws installed mounting the output transistors? That is often how they get their collector connection.

[Rhod]

Thanks, g1.

Ba as marked in yellow on the attached pic - I notice this isn't marked on the schematic posted by Jazz P earlier up the thread. It refers to a connection to a different board with the output transistors on. Please let me know if the size/resolution is not appropriate.
Good thinking about the collector connection - I did reattach the transistors with the screws and the connection seemed OK, but without the heatsink these weren't tightened so this may have been the reason - I will check this.

[Rhod]

Checked with everything bolted back to the heat sink. Short not apparent now.

[Rhod]

Progress report: amp now stable i.e. operates with lamp limiter at a low glow. Whatever that short was it’s no longer with us. Still nothing from the right channel, but no surprise there if one of the other output transistors is now bad.
DC offset approx 2mv across both channels.

I can get a replacement transistor, which would have the happy by-product of matching the set as Jazz P recommended, but my question is - what else should I check before putting in my order given that I don’t want to pay postage twice?

Many thanks.

[g1]

It's possible one of the mica spacers has a crack or hole and was allowing the short before, and realignment cured it?

I've lost the plot a bit, you now have a dim bulb and no DC on the output? Which transistor are you suspecting and why?

edit: never mind, I see the transistor is for matching output purposes. You should be ok for the time being (for test purposes) if you don't run at higher power levels. Find the source of your 'no sound' for the bad side and then order your parts needed.

[Rhod]

Thank you, g1:

One of the NPN output transistors, TR12, has a ~900mv drop across C-E which led to suspect that it’s not good.

Setting the bias from an indicated 0mv up towards the correct value of 23mv causes the limiter lamp to glow brightly.

Can someone advise me on how to check for other potentially problematic components? How do I go about finding the source of no sound as g1 suggests? Thanks!

[g1]

One of the NPN output transistors, TR12, has a ~900mv drop across C-E which led to suspect that it's not good.

Setting the bias from an indicated 0mv up towards the correct value of 23mv causes the limiter lamp to glow brightly.

Remove that suspect transistor at TR12 for now. Check the bias on the good channel. Do not adjust if it's not 23mV as the bulb limiter may alter the reading. What does it measure?
Now check each of the .47R5W resistors on the good channel (DC mV across each). What do they measure?

Now go to the bad amp (with TR12 removed) and try to bring up the bias so the readings across the .47R5W are similar to the good side. Still bright bulb?
If so, is the reading across one of the .47R higher than the rest?