Musicman HD-500 Blows Fuses

[DGstrat]

I've seen a lot of good advice given on this forum, so I thought I'd share my current troubleshooting effort and see if there's anything that I'm missing.

I have a musicman HD-500 bass amp on the bench that blows both of its power line fuses. I isolated this to the power amp section (big surprise) and ultimately found one power transistor shorted (I2).
I received replacements from ernie ball and replaced both of the transistors on the V+ rail. (I1 and I2). Unfortunately my dim bulb tester is still showing a large current draw. When I check V+/V- I see about 4 volts (vs the 82 on the schematic). It appears that both output sections are being turned on. I have done diode checks on all of the transistors and diodes. I have also disconnected the V+/V- supply from the circuitry (the PC board design made it easy to do so) and verified that all the other logic on that board powers up fine with no excessive current when that is done.

At this point my plan is to replace the 4 zeners (D3, 4, 5, 6) because the soldering on those looks sub-part compared to the rest of the board, making me think this has been repaired before. The 2 zeners that are supposed to be 1W don't look like the other 1watters on the board. I am also suspicious of the bias transistor (T9, BC549).

Any other thoughts before I send off a parts order?

musicman_bass_hd500_pg8.pdf

thanks for your consideration.

Dave

[J M Fahey]

Hi Dave.
Sorry but "visual" inspection tells little or nothing.
1) start by checking that output transistors are not shorted to chassis through poor insulation (bad micas, improperly set hardware, etc)
What are they? Metallic TO3? Plastic TO218/247? or some other package?
2) measure voltage across all of them. (drain to source).
3) measure turn-on voltage (gate to source).
4) measure bias voltage: (T10 to T11 base to base)
Post results before we can go on.
Visual is nothing.
Multimeter rules .
This is starting to sound like a soccer team fan war song
Good luck.

[R.G.]

The bias voltage is a likely suspect as JMF says. When I replace outputs, I like to temporarily short the bias voltage to zero with a jumper till I verify operation with the new devices. This will make for crossover distortion, but still allow you to run it at full power into a dummy load and check it out. When it's all proven with zero bias, you can remove the jumper and dink with bias. If it works fine with shorted (zero) bias and blows fuses with the bias normalized, you have an important clue to the problem.

[J M Fahey]

COOL trick.
Thanks for sharing.

[DGstrat]

JM -- You're right, I should have put my readings into the first post.

No shorts on the output transistors (per the meter).

output is sitting at 0

82VS rail is at 4V
-82VS rail is at -4V

HIGHG (gate voltage on I1 and I2) is 2.6V
LOWG (gate voltage on I3 and I4) is -2.4V

T10 Base is at 3.2V
T11 Base is at -3V

T7 Base is at 3.4V
T8 Base is at -3.5V

RG -- are you suggesting jumpering across T9's collector and emitter to get it out of the picture?

thanks!

Dave

[J M Fahey]

Answering for RG: not only short across T9, but even better short "T10 to T11 base to base" and follow his instructions.
What he suggests is the most important test in this case: you either recover your rail voltages (or at least a significant value, say +/-40V or more) which means you have a bias problem, or not (either stays low or you recover very unsymmetrical, say +25/-4 or some other extreme mismatch), in which case beyond bias you still have some bad device shorting things up.
*Always* test without speaker or any load attached.

[Enzo]

If your 82v rails really were LOADED down to 4v, at the very least, your outputs would be hot enough to grille steaks. If that is not the case, then find out why the voltage is going away.

[J M Fahey]

Dear Enzo, maybe you missed his earlier post, he's using a "dim bulb tester" (sic) meaning a lightbulb current limiter.
The voltage fall is safely across it.(thanks God)

[Enzo]

Oh.

It worked.

I'm a dim bulb.

It detected me.

[DGstrat]

Ok. Thanks for clarifying RG's instructions, just wanted to make sure that I understood correctly. I'll report back.

Dave

[DGstrat]

Ok, now we're getting somewhere...

Jumpering out the bias transistor does allow the amp to power on with the dim-bulb staying dim. While still running through the dim bulb, I measured the rails at 50V.
I then went to unsolder T9 to test it out of circuit and discovered that the leads literally fell off the part as I removed it...
The replacement is already on order, will update the thread when I receive it.

Thanks for the support.

Dave

[DGstrat]

Replaced T9 with a new BC549 and observed a large bias voltage difference between T10 to T11. I was able to dial that out with the bias trimmer.
After that all supply voltages looked close to normal and the dim bulb stayed very dim. Verified no large DC on the output and tried a speaker. Finally, some sound!
I then ran a sinewave through it, watched it on the scope and adjusted the trimmer to remove the crossover distortion kink.

I was a little surprised that the trimmer was that far off.

thanks again,

Dave

[Steve Conner]

Well, if you replaced the bias transistor, that would have affected the bias trimmer setting. You can't really compare to what the setting was with the old bias transistor, and say that it was "off".

[DGstrat]

Point taken. Probably a poor choice of words on my part as well.

Dave

[J M Fahey]

Agree and add: REPLACE THAT TRIMMER !!!! Whatever burnt the bias transistor certainly overloaded it.
I don't trust it any more.
After replacement, re-start biasing again no load and with the bulb limiter, in case by chance the new trimmer is set for HUGE idle current.
Once itīs roughly "right" you remove the bulb limiter and fine tune.
Congratulations.