Fender Bassman 200 with DC on output

[ThisLifeILead]

Hello folks, a friend brought me his Bassman 200 to try and repair. One day it blew a fuse so he replaced the fuse and after that a loud nasty DC hum was present on the speaker output. So we know there is DC on the output. I am a total SS newb. Ive spent the last 8yrs repairing Tube related analog gear. to me those are just easier.
anyways, he dissassembled the ouput and power supply board before dropping the amp off. Me being an idiot I ordered all new output transistors along with Q14 and Q15 and Q9 on the output board. The only transistors i didnt replaced were U1 and U2 on the output board.
i replaced Q16, Q17, Q18, Q19. Q9, Q14 and Q15. power up the amp and blammo. its fried again but this time R21 (100ohm) blew like a lightning storm.
I am just trying to wrap my head around this SS stuff. R21 is between Q14 and Q15 emitters.

look to page 13 of the attached PDF.


Obv Q14 and Q15 will need to be replaced again as one must be shorted to send that much current to burn out the 100ohm R21.

So measuring the resistance between the collector and emmiter on the output transistors one half are shorted again and the other half measure around 35ohms.

From what im gathering i should remove the shorted output transistors, replace R21 and then fire it back up. See if i can get it to stabilize. I do not believe its the power supply as i feel i would continuously be seeing problems in the power supply itself. but once stable i should be checking voltages of all sections.

Im looking to learn from this amp so please push me in the right direction.

[Leo_Gnardo]

Obv Q14 and Q15 will nee to be replaced as one must be shorted to send that much current to burn out the 100ohm R21.

Limited experience on these, but one I fixed a couple years ago, Q14 & 15 were the problem. I used MJE 340 and 350, plus added a little heat sink to each.

[ThisLifeILead]

unfort I replaced Q14 and Q15 when i replaced the 4 output transistors.....

[Dave H]

To prevent more shorted output transistors I'd connect 100R 5W resistors in series with the +57V and -57V supplies to the power amp and disconnect the speaker until you have corrected the DC offset on the output.

[ThisLifeILead]

To prevent more shorted output transistors I'd connect 100R 5W resistors in series with the +57V and -57V supplies to the power amp and disconnect the speaker until you have corrected the DC offset on the output.

Thanks Dave.
So i guess the tricky part is to find a break in the supply to insert the series 100ohm resistors.
Pardon my newbness and silly Q's with SS. This is as i see it. the only power coming into the output power supply is through the Collectors. +/- 57Vdc so in order for R21 to see over current situations is if one side shorts from Collector to emitter or if one of the output transistor base pins short?

There is little ground connection in these SS devices so i doubt its a component thats shorted to ground but i guess its worth looking into.

[52 Bill]

Or use a light bulb limiter until you find all of the problems with the amp.

[ThisLifeILead]

Or use a light bulb limiter until you find all of the problems with the amp.

i did but it still blew the output transistors. i could see the bulb light up as the short occured again and heard a buzz afterwords. I then fired it up without the limiter and that burnt up R21.

[Tom Phillips]

I don't see verification that you are doing the initial testing WITHOUT a load on the output as Dave H suggested. Would you clarify?

[ThisLifeILead]

I don't see verification that you are doing the initial testing WITHOUT a load on the output as Dave H suggested. Would you clarify?

Yes, that was all done yesterday. tonight i will disconnect the Load and report back :-)
Thank you everyone for the fast responses.

[Jazz P Bass]

The reason for the 'blowing up' is most probably the fact that the high side & the low side output transistors both turned on at once.

The speaker being hooked up simply gave it a path to ground. (Ouch)

I see no mention of Q12 & Q13 being checked or replaced.
These two transistors are 'limiters' for the output.
If they fail short they can ruin your day.
They can safely be removed for initial testing.

[g1]

And for safety's sake, what wattage bulb are you using in your limiter?

[Enzo]

When DC on output, ALWAYS remove any load from that output. No load until the amp is stable and not making DC.

If you measure E-C for shorts on output transistors, remember that they are all essentially in parallel on each side. SO check them again once they are removed.

[ThisLifeILead]

And for safety's sake, what wattage bulb are you using in your limiter?

60Watter :-)

[ThisLifeILead]

When DC on output, ALWAYS remove any load from that output. No load until the amp is stable and not making DC.

If you measure E-C for shorts on output transistors, remember that they are all essentially in parallel on each side. SO check them again once they are removed.

I did measure them after i pulled them and found most to measure around 600ohms from E-C. 2 measured open from E-C. Today I fired it up with no load and no limiter (FACK). Burnt up Q14 or Q15. Amps not infront of me at the moment. Okay so then i pulled the board. Replaced R21 and found the +/-50Vdc power supply caps had faulty solder joints! I resoldered the caps to find they now measured 2K and rising across their leads went up to 12K before i lost interest. (yesterday they measured ALOT higher). So now tomorrow i plan to remove and test them out of circuit. we might have a low restive load pulling somewhere to add to the fun. Again thanx everyone!

[ThisLifeILead]

Allrighty, measured for any E-C shorts on the board and pulled to test. I found 5 transistors that test bad. Q13, Q10, Q9, Q15, Q16. Swapped Q9, Q15, Q16. Pulled and left out Q13, Q10.
Using current limiter, it still shorts. pulled 56v Transformer taps. Stable. So the problem is in the High voltage rail. Pulled Filter caps. Still shorts. (put back) Found when measuring filter caps in circuit with the diode check function the +rail cap is shorted in one direction. The - rail cap does not give me a short.
So correct me if im wrong, we have a diode short in the (+)50v rail somewhere. C3 is the cap im gettin the short when measuring across.
Any help is appreciated! and the current limiter actually has a 100W bulb not a 60W bulb.
Cheers!