Music Man RP100

[lowell]

This amp is blowing the fuse. It's chassis #2100rp and is the page that says PC BOARD GP2.

Tried new power tubes which made the problem worse. With no power tubes the fuse doesn't blow. Also checked the following which all tested fine w/ the meter:

bipolar transistor drivers
all diodes in power amp and supply
resistors surrounding bipolars
1.5k screen resistor
there's no AC on the B+

If anyone has any ideas they're much welcome.2165-rp_&_2100-rp.pdf

[Enzo]

No power tubes. Got B+ on pins 3 and 4 of each socket? What is on pin 5? D9 looks like a 22v zener, so do you get POSITIVE 22v or so on the grid pins? Look at TR1 the bias control. What voltage is on it? I expect between a half a volt and a volt or so. Certainly under 2v.

[lowell]

Pin3:690
pin4:347
Pin5:22v
TR1:1.1v

also, only one power tube is redplating. No signs of arcing on sockets. Have tried 2 new pairs of tubes.

[Enzo]

Your meter can't test transistors under load or under operating conditions. MJE15032 or MJE15034 should work fine in place of the original cathode drivers. They cost less than $1.50 each. I'd try a different pair. Those are useful transistors to have. You might keep an extra or two on hand, and the PNP version too. I been using those in blown up MAckie subwoofers lately.

[lowell]

Enzo,
Ok I'll try that. What brings you to this conclusion? Are you just eliminating them as a possibility since all else seems fine?

[lowell]

The transistor seemed to fix the power amp section. Only replaced the one from the red plating tube. Volume is low and distorted, and +/-16 voltage to preamp supply is only +/-13 (measured at R62/63)

[Enzo]

What brings me to that conclusion is that you have a red plating tube - that means excess current flow. You checked the voltages at the tubes, seemed OK. The voltage at TR1 tells me the bias control is not open. That leaves only one place for excess current to be happening - the drive transistors. And all you can check with a meter is the basic transistor function, not leakage or current handling. SO we change out the part.

I highly recommend changing those drivers as pairs.

Now you have stopped the red plating, but it still sounds bad. So what voltages are at each power tube cathode now? And you have those two 3.9 ohm cathode resistors, what voltage is dropping across each one? Low and distorted volume could still be one power tube not working.

And applu a signal to the amp, now scope pins 1 and 7 of the op amp that drives those cathode transistors, IC7. Got the same signal both places?

[lowell]

Replaced second drive transistor. Cathodes are 60v. Cathode resistors are 23-24v. Scoped IC-7 - Both outputs are the same, but hard clipped. Scoped IC-3 - input was good, but output was clipped. Measured R24 - good. Replaced IC-3 - nothing changed.

[Enzo]

So go into that HI jack after IC3. Does that come out the speakers OK? And does that 10k volume control right after teh jack function as it should?

If the signal leaving IC3 is all clipped and funky, no way in the world the powr amp can clean it up.

[lowell]

The 10k volume pot is not working right when checked w/ my ohm meter. I replaced it w/ a working one, the amp is still low volume and distorted when going into the input. When going into the HI jack there is nothing, no sound.

[Enzo]

So find out why. That high return jack is the input to your power amp.

[lowell]

Ok this was weird. I could not get any signal on my scope after the volume pot, then after powering down and removing IC7 it was there. I guess it's possible IC7 was loading it down a lot. I put IC7 back in and the signal was fine up to the input of it. There was NO signal on the output this time...? Anyway I replaced it since it's in a socket and voila, amp works great. Thanks Enzo.