Music Man HD 150

[xtian]

Friend gave me an ailing Music Man HD 150 head -- this has a solid state pre and a quad 6L6 power section. It suffers from a severe hum that does not change with the controls. Guitar can only be heard faintly over the hum.

Caps have not been changed in 20+ years, according to owner. There are two 100uF filter caps under a can on the outside of the chassis. They look OK visually. Sounds like building a rig to test DC leakage is a project, so I haven't done that.

I swapped a set of 6L6 tubes from my Mark IV, and got the same hum. I noticed, after only 20 seconds of operation, that the first two tubes were cool to the touch, but the third was very hot. Fourth was hot but not quite as much.

There may be some heat damage near these pairs of 5w resistors. Will have to lift the board to find out. Does this look like a repair made by someone who didn't have the right resistors on hand?

Should I go ahead and replace the filter caps?

[xtian]

And here is the schematic.

[TheGimp]

I would replace all the Al Electrolytic caps, including those on the preamp and driver boards.

The blue film cap from the power switch to the chassis is a holdover from a two wire design and may be an issue as well. I've seen them trip GFIs as well as causing hum problems.

[Structo]

I don't know but that would make a hella Dumble amp.

Nice chassis and transformers!

[tubesinside]

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.

c

[David Root]

Those 150R resistors in series appear to be R88 and R89, which are shown on the schematic as 350R 3W. It's clear there has been some overheating and failure of R88 and 89 in the past, so this is a weak spot which you have picked out.

This is a +/- 16V supply which would be for the transistors somewhere, not familiar with that part of these amps myself.

[xtian]

Those 150R resistors in series appear to be R88 and R89, which are shown on the schematic as 350R 3W. It's clear there has been some overheating and failure of R88 and 89 in the past, so this is a weak spot which you have picked out.

This is a +/- 16V supply which would be for the transistors somewhere, not familiar with that part of these amps myself.

Had a closer look: these are two pairs of 150 + 470 = 620 ohms. I'll definitely replace those. Funny, this is the SAME place the Fender HRD has high failure rate, in the 16v supply!

[ToneMerc]

Those 150R resistors in series appear to be R88 and R89, which are shown on the schematic as 350R 3W. It's clear there has been some overheating and failure of R88 and 89 in the past, so this is a weak spot which you have picked out.

This is a +/- 16V supply which would be for the transistors somewhere, not familiar with that part of these amps myself.

Had a closer look: these are two pairs of 150 + 470 = 620 ohms. I'll definitely replace those. Funny, this is the SAME place the Fender HRD has high failure rate, in the 16v supply!

I used the MM style HV and bridge bias supply supply in my last 100W build. In that location I used a 2W 620, they get hot too.

TM

[David Root]

Why 620 when the schematic is 350? Anyway if it were mine I would put in 5W sandbox types.

[xtian]

Why 620 when the schematic is 350? Anyway if it were mine I would put in 5W sandbox types.

My question exactly. "Sandbox" = cement resistors?

[ToneMerc]

Why 620 when the schematic is 350? Anyway if it were mine I would put in 5W sandbox types.

The MM schematic I used has 620/2W

[David Root]

I didn't know that, just working off the provided schematic.

Sandbox=Cement, yes.

[rdjones]

Why 620 when the schematic is 350? Anyway if it were mine I would put in 5W sandbox types.

That looks like a 2100 series chassis (21 inches wide), which has the 620 ohm resistors in the 16V supply.

The 6L6s should run fairly cool in that amp, they are biased nearly class B.
That's if you even want to call it "bias". There's a positive voltage on the grids.
The 2 tubes running hot indicates something not right on that side, and the difference between those 2 tubes is probably just normal variance between tubes that seems to be more pronounced with such high voltage.
You might leave it on low power until things get sorted out.

rd

[rdjones]

2100, includes 150 and 75 watt versions.
Possible mistake, the 75 shows the 150 watt output transformer, which I think had it's own OPT.

[Reeltarded]

I can't help you with technical stuff, but let me just say SCORE!

Those things are GREAT for power. I used to have a pair of them for just power. Best rig EVER! Love those!