Music Man 2475-130 Repair

[RWood]

This one has me stumped, mainly because I don’t have experience with the solid state phase splitter arrangement.

Symptom - a prominent hum that appears when the power tubes conduct. Volume and settings have no effect. Measurement across the 3.9R quickly approaches 400mV and climbing, when I shut it down.

I have replaced all 11 electrolytics including the two main caps, 100uF 500V in series. I have double checked their polarity. No change in the symptoms from before the cap job.

The HT line seems to be working, applying low voltage from the variac I see voltage on all four plates and half voltage in the screens.

I suspect a problem in the grid drive solid state business, maybe the op amp?

What the owner told me was that it had been working fine, then one day it had the hum right at startup. He noticed a white-top EL34, lost vacuum, replaced all four tubes with new EH. But even with the new tubes it still had the hum so he gave me a call.

Any ideas what to check next? I am on my phone now but i will link a schematic later and some pics.

Thanks, guys

[g1]

Monitor the voltage between grid and cathode. (DC volts, one probe to each)
Either the zener that sets the voltage at the grids, or the transistor at the cathode is bad.
I don't recall ever having to replace the zener in these, but the transistors do fail, and I would suspect that may be your problem. Have you checked the transistors with the diode function on your meter?

[RWood]

Thanks, g1. No, I have not checked the transistors.

The grids are 3 volts positive wrt the cathodes.

Are these the transistors?



It is a DB-2 board



Here is the schematic, on pg 2:

http://el34world.com/charts/Schemati...0_2275_130.pdf

[g1]

Yes those transistors. It looks like someone has installed Tip31C as replacements. I'd go with 2N6488 if they are bad.
(see attached)

[Enzo]

MY first suspect would be a failing tube. That would certainly ramp the current. Look at the notes in the corner of the schematic, how do your voltages line up?

[RWood]

Or something that’s making a tube fail. Remember it was retubed after the first event. I still have the original three surviving EL34s.

Enzo, to which corner and what voltages are you referring? My amp iis on pg 2 of that link.

g1 thanks for the replacement part number, nice to see availabilty. Should I check the zener diodes too? Which ones are they on the schem?

One other thing, the 3.9R sensing resistors both measured 4.5R ...... pretty far off for setting the bias, eh? I will look into 1% replacements.

[RWood]

The first TIP31C I tested as a diode and showed conductivity in all six combinations.

[g1]

Sounds like it's bad then.

[Enzo]

3.9 ohm resistor. First short your ohm meter probes together to see what your meter says about zero ohms. Most probes have a half ohm resistance or so. That doesn't matter when you measure 220k or something, but when you measure 3 ohms, you have to subtract the probe resistance from your reading. The resistors are likely fine. If they are off but the same, it still balances out. Remember, nothing precision here.

Sorry, you are right, this drawing has no notes, but many do. For an extremely similar amp look at Peavey Heritage VTX, the notes lower right there would be informative for your case as well.

Shorted power tubes often take out the cathode transistors.

What zeners? D17 is a 16v zener, it sets the +16v for the grids. DO you have 16v on the grids? If so the zener is working. I don't spot another offhand.

[RWood]

Thank you g1 and Enzo! I am stoked and have parts coming.

The probe resistance was .1Ω (Fluke 115) so that 3.9 resistor is still 9% higher than it should be. It might get me in the ballpark, current-flow-wise but my OCD will be doing backflips, and for 76¢ I can get a 1% 2W part and rest easy. Especially since I am getting the transistors anyway.

Not getting anywhere near 16V on the grids; I ordered some 1N4745 just in case those went bad too.

Here is the offensive tube. Not that it matters a whole lot, but I am wondering if a mechanical failure resulting in the tube losing vacuum caused the transistors to fail, or vice-versa.

[g1]

Here's the bias info for the DB-2 board. While you're in there I'd check to make sure the IC in the power amp section is 1458.

[RWood]

Thanks for the service bulletin and instructions.

The driver board does appear to have a 1458 and I ordered a couple more just in case.

I think this is the one you're referring to:



One new question. This amp has a non-original reverb tank, and I don't know yet if it works with the amp. Do you have a tank # for one that is compatible? The schematic does not show a p/n.

Thanks for all your help.

[g1]

Is there a number on the tank?

[RWood]

4FB2B1A

[Enzo]

That is what I would select. The circuit drives the pan directly from the IC.