Music Man HD 150

[xtian]

Mouser caps and resistors are on the way. I'll let you know what happens.

[Cliff Schecht]

Sounds like building a rig to test DC leakage is a project, so I haven't done that.


Should I go ahead and replace the filter caps?

Sorry I just read this but I will point out that it's incredibly easy to test for leakage. The downside is the inherent danger in this testing because you'll be working with high voltage, but you of course already know this. But all you have to do is hook up a high voltage source through a large resistor (I use 68k 10W because the beefy leads stay put, 100k makes the math easy, 2-3W minimum for safety in case of a really leaky cap) and measure the voltage drop across the resistor with your DVM. The voltage drop across the resistor divided by the measured resistance will give you the leakage. With caps in the 100uF range, anything over maybe a few hundred uAmps (microAmps, just to be clear here) is probably bad.

With all of that said, I'd replace those caps.

[xtian]

I replaced 10 caps and 2 resistors. No change. Hum is very loud.

Check the two transistors on the heatsinks. They are connected to the cathodes of the tubes and are usually the culprits. The driver IC may also be at fault.

Any test for these failed parts? Or just order replacements and hope?

[tubesinside]

Very easy to check the transistors assuming that you have a meter.
1 Remove the power tubes and be sure that there is no voltage on the caps.
2 Switch the meter to 'continuity' or resistance measurement.
3 Place the red lead on the centre pin (collector) of the transistor (no need to remove it from the circuit at this stage)
3 Place the black lead on the right hand pin of the transistor.
4 A low reading indicated that the transistor is faulty.

If you find that this is indeed the problem ...be sure to replace both transistors.
Cyril

[tubesinside]

Oh ! I notice from one of the pictures that the 'death cap' is still connected.
Remove the blue capacitor that's connected to the centre pin of the toggle switch next to the mains fuse.
Looks like you already have a 3 pin lead.

Cyril

[xtian]

Good call, Cyril! One transistor reads low resistance (600 and 150 ohms, from center pin to outside pins), and the other transistor reads very high resistance. It's nice to have two transistors to compare.

1. So the one with low resistance is burnt out, right? I'll order a pair of replacements.

2. You mentioned that the driver IC might also be bad. Part number reads GL4558 / N06R0K. Should I replace that, too?

3. I've been testing with the ground switch in center position, which leaves the death cap out of the circuit. You're suggestion I can just remove it entirely?

Thanks!

[tubesinside]

Don't worry about the driver IC if you have found that one of the transistors is bad. It's decoupled from the transistors by a cap. Sometimes when an op-amp fails and the output goes to the psu rail you will get hum.
The bad news is that one of your power tubes may be faulty. Unlessnyou have a way of testing these (out of that amp) I would recommend changing those also.
I've been repairing tube amps for 30 years. Not a bad call after all. . C

[tubesinside]

Sorry forgot to mention. Remove the cap.

[xtian]

Cheers for Mr. Tubesinside! I replaced the two power transistors as suggested, and the problem is solved. No hum. Nice loud, clean signal. Fairly crappy solid state preamp distortion, but you can't have everything.

Thank you!

[xtian]

Have a similar Music Man on the bench. This one has low output, but no hum. It's not the tubes. Voltages all look good for the power tubes and the op-amp ICs. The two power transistors test OK, using the tricks I learned from tubesinside earlier in this thread.

You can see in the schematic, there is a clean channel and a "limiter" channel. The low volume issue is common to both. All other controls and reverb work fine, just low volume.

Suggestions for further troubleshooting?

[tubesinside]

OK Xtian, let's see if the drive to the output stages is OK. I assume that you have a meter with AC scale. (a scope would be a bonus).
Connect a test load to the output and drive the preamp with a sine-wave.
Measure the ac voltage on pin 1 and pin 7 of IC4 with respect to earth. You should read up to 10V AC. If this is OK then measure the ac on pin 8 on the power tubes. This should be up to approx 25V AC at full power. Let me know how you get on.

[xtian]

If I really crank the input sine wave, I can get up to 10 vac on IC4.

With signal turned up to create 1.5vac on IC4, the AC at pin 8 on power tubes is 10vac.

[xtian]

…and you're thinking, "well, that sounds OK, actually." And you'd be right, because now the amp is working!

I guess I knocked something into place. No idea what.

Fixed by accident!

[Reeltarded]

This reminds me of The Lfe of Brian.

[tubesinside]

Ha...I have that effect on gear....
Could have been a bad connection on the IC (is it in a socket?)