Yamaha RA-100 Transistors - fuses blowing

[AmpFix]

I am in the process of restoring an older Yamaha RA-100 revolving speaker amp combo from the mid-70's.
I was left in a barn for a few years and the 12" speakers were seized when I got it. Not good.

I cut the combo in half to have a top part with the amp, crossover, preamp and revolving speakers and separate bottom 2x12 cab. I changed all electrolytics and found that it had been previously repaired while going through the amp. At least one bass pot in the preamp and the main output transistors were previsouly changed.

I plugged it in and it kinda worked. Sound was coming through the top speakers with the 2x12 disconnected, albeit a little distorted. As soon as I plugged in the 12" speakers, F2 and F3 blew on the power amp board.

I did a little more testing and F3 blows every single time now, even with no preamp connected. F2 blows only if I try to use the top speakers with the preamp connected. I think that the main TR6 & 7 transistors are bad. They've been replaced by some previous owner with 2N3773 but they are supposed to be 2SC1080.

When I check resistance on the 2N3773 transistors, I get:
B+ > E- : 1.7MΩ / 1161Ω
B+ > C- : 19.4kΩ / 1161Ω

B- > E+ : OL / 1206Ω
B- > E+ : OL / 1206Ω

Something seems not right. Based on this, I think I need to check all transistors but since they are no longer in production, what do you guys suggest?

The ones I think I need to replace are 2SC1080, 2SC783 and 2SA483.

[Enzo]

Mighty helpful if you can post the schematic.

An amp that has been around 40 years? Not surprised it has had some work done in the past.

If you are checking for shorted transistors, measure directly from emitter to collector. If you get 1100 ohms to ground from both ends, that sounds like zero ohms E to C to me, but that is why you have to check. On the other hand, your other reading - I assume you reversed the meter leads or something - shows a difference. In circuit, they measure very different one way or the other, so if the thing is suspect, remove it and test it out of circuit.

Stop blowing fuses, every time you do, it stresses the circuit more. Look up light bulb limiter, make one and use it.

Until you know the amp is stable and does not make DC on the output to speakers, do not connect a speaker. With no speaker connected but amp running, is there DC voltage across the speaker wires? I suspect your amp output is "blown" with DC on it. The woofers connect directly to this, and draw excess current. The smaller speakers are fed through capacitors, which block DC, so no fuses blow.

2N3773 is a reasonable substitute. I'd probably use MJ15024 in my shop. I am sure the driver transistors can be subbed. In fact I may have some TO66 that would be perfect, I'll check next shop visit. Probably can use TO220 in their place. Make sure those 0.22 ohm and 330 ohm resistors are not open. And see those two 10 ohm resistors in parallel nearby? Make sure those are not open.

[Jazz P Bass]

Here is the SM:

Yamaha-RA100 guit amp.pdf

[AmpFix]

Mighty helpful if you can post the schematic.

An amp that has been around 40 years? Not surprised it has had some work done in the past.

If you are checking for shorted transistors, measure directly from emitter to collector. If you get 1100 ohms to ground from both ends, that sounds like zero ohms E to C to me, but that is why you have to check. On the other hand, your other reading - I assume you reversed the meter leads or something - shows a difference. In circuit, they measure very different one way or the other, so if the thing is suspect, remove it and test it out of circuit.

I checked the transistors out of circuit.
B+ > E- means positive lead on B and negative lead on E.
B- > E+ means negative lead on B and positive lead on E.

For C > E, I get 0Ω in both directions (C to E and E to C).

Stop blowing fuses, every time you do, it stresses the circuit more. Look up light bulb limiter, make one and use it.

Definitely no more fuses going in until I find out the root cause(s).

Until you know the amp is stable and does not make DC on the output to speakers, do not connect a speaker. With no speaker connected but amp running, is there DC voltage across the speaker wires? I suspect your amp output is "blown" with DC on it. The woofers connect directly to this, and draw excess current. The smaller speakers are fed through capacitors, which block DC, so no fuses blow.

Yeah, when I have this thing re-assembled and hopefully fixed, I'll check the output on a scope before hooking up the speakers. I suspect that the seized speakers have something to do with the fact that the power amp is shot.

F2 definitely blows when F3 is out and when the small speakers are used. I tried following what F2 protects but its unclear to me right now. I still have to dig in to that. Perhaps its a side effect of the problem on the power amp.

2N3773 is a reasonable substitute. I'd probably use MJ15024 in my shop. I am sure the driver transistors can be subbed. In fact I may have some TO66 that would be perfect, I'll check next shop visit. Probably can use TO220 in their place. Make sure those 0.22 ohm and 330 ohm resistors are not open. And see those two 10 ohm resistors in parallel nearby? Make sure those are not open.

MJ15024 is available through Mouser, so that's good. I'll check the specs out later. For the TO66s, they're a lot harder to find, although Mouser does have a small selection.

I checked the resistors and they're good but I might replace them anyways since I'm in there already. They may have suffered a little when the fuses blew.

Thanks for the response.

[Jazz P Bass]

Yup.
Collector to Emitter at zero ohms means a dead short.

Quote:"I suspect that the seized speakers have something to do with the fact that the power amp is shot."

It may be the other way around.

[AmpFix]

Yup.
Collector to Emitter at zero ohms means a dead short.

Quote:"I suspect that the seized speakers have something to do with the fact that the power amp is shot."

It may be the other way around.

The whole thing looked like it lived in a barn for a few years. Who knows...

I need to understand the whole F2 fuse thing. I'm really scratching my head on that one.

[g1]

Jazz, that schematic didn't work so I'm re-attaching.
Agree that the power amp defect likely took out the speakers.

Fuse F2 is before the bridge rectifier, F3 is after it. If a diode in the rectifier goes bad (or either 2200uf cap off the bridge), F2 protects by disconnecting that part of the transformer winding from ground. If something after the rectifier goes bad, then F3 protects that.

[AmpFix]

Jazz, that schematic didn't work so I'm re-attaching.
Agree that the power amp defect likely took out the speakers.

Fuse F2 is before the bridge rectifier, F3 is after it. If a diode in the rectifier goes bad (or either 2200uf cap off the bridge), F2 protects by disconnecting that part of the transformer winding from ground. If something after the rectifier goes bad, then F3 protects that.

Ah. I've never seen that before, but why would it only blow the fuse with the "voice" button engaged (bottom + top speakers as opposed to just the bottom speakers)?

The switching must be engaging something that causes the current draw to go up.

Attached is a better view of the power amp:
Yamaha_RA100_PowerAmp.pdf

Bottom left, you'll see Pin 7 going to TR6.e

The round (female) connector on the power amp goes to the US-P (male) connector to the preamp. Pin 7 was probably used for bridging the power amps together on the RA-200 and its unused on the RA-100. On the male connector, there's no wire in it.

Bottom line is that engaging the top speakers blows Fuse F2. Sounds like something off of the 23v or 12v supply could be causing this.

[Jazz P Bass]

The top speaker amp is faulty (distorted).

Engaging it takes the fuse out possibly because it takes the fuse over the edge.

[AmpFix]

The top speaker amp is faulty (distorted).

Engaging it takes the fuse out possibly because it takes the fuse over the edge.

Its the same power amp: shared between the bottom 2x12 and top rotating speakers. Its was distorted at first, now the power amp is completely out due to TR6/7. The power amp circuitry is protected by F3. With F3 out, F2 would only protect the preamp (anything on 12 or 23 vdc).

[g1]

Engaging extra speakers in parallel lowers the load impedance which increases current from the power amp. So that would increase current through F2 (assuming at this time F3 is in place).
I can't see any connection between F2 and the 12V or 23V supplies in that schematic, they are completely independent of F2.

[AmpFix]

Engaging extra speakers in parallel lowers the load impedance which increases current from the power amp. So that would increase current through F2 (assuming at this time F3 is in place).
I can't see any connection between F2 and the 12V or 23V supplies in that schematic, they are completely independent of F2.

You're right. When originally looking at the scheme, I thought the secondary was one single winding with different taps. Now I see that the transformer's secondary winding for the power amp 44v is completely independent from the 12/23 v taps. I don't know why I missed that the 1st time around. Too much staring at the scheme without a break, I assume.

I think what is happening is that F2 is blowing (even without F3) when the 2nd speaker is added (I diagnosed a speaker output short in the mechanical assembly of the cabinetry) via the negative -47vdc supply, through TR5 & TR7 (imagine a short @ TR7's collector).

That explains why TR7 is partially damaged but not quite a short like TR6

[Jazz P Bass]

An 'open short'??????

[AmpFix]

I'm really tempted to replace the TO-66 driver transistors with these (NOS?)
http://www.amazon.ca/2SC783-2SA483-T.../dp/B00LQSFF64

Any of you have experience with this seller? The case material finish looks legit, as does the lettering and logos.

[Jazz P Bass]

Go for it.