Fender Bassman 200 with DC on output

[Enzo]

HV rails, yes, but of course that is the entire power amp.

C3 measures shorted but only one direction? If the cap was shorted, it would measure so both directions. But you already pulled the caps anyway. SHorted main filter caps are pretty unusual.

By the way, the diode test is nice in that it is a shorted, non-shorted sort of test, but when it gives you a shorted indication, then it is time to use resistance rather than diode test. The diode test will beep at zero ohms as well as at maybe 200 ohms, yet those two conditions are usually from far different things. SO it is a useful distinction.

On the far right, see D31,32? they are wired reverse bias across the output and rails. With a speaker load, then they are essentially across the supplies too. SO no power but loaded, do either of those diodes test short? Or no load and measure right on the parts.

When powering up, even on the bulb, always work with NO speaker load until the amp is stable and not making DC. In fact, if powering up on the bulb lights it bright WITH a load but not WITHOUT a load, that is a VERY valuable clue.

So: is C3 just shorted (whether itself or the circuit it is in) or does it measuer like a diode? If it does measure like a diode - "shorted" only one direction - that sounds more like something in the circuit is shorted putting a junction across the rail.

ANy time transistors blow up, ALWAYS check all the resistors associated with them.

Q10. You removed it and left it out? The amp won't work that way. You can leave out 12 and 13 for testing, but not 10. is that 47 ohm above Q10 open? How about the one under Q11?

You checked for EC shorts on the outputs? Good. Did you check for E-B shorts on them? If Q14 is shorted EC, then with a load, the base of one of the positive side outputs acts as a junction to ground, maybe what you are seeing. And BC shorts too, always check all three potential short paths on a transistor.

Were you checking transistors in circuit? remember all these drivers are wired directly to each other, so for example the EC of Q14 is parallel the BC of Q16. SO if you pull a shorted transistor, before putting something new in, go back and t=check the remaining transistors to make sure another bad one was not hiding under the short of the first one, so to speak.

[ThisLifeILead]

Q10. You removed it and left it out? The amp won't work that way. You can leave out 12 and 13 for testing, but not 10. is that 47 ohm above Q10 open? How about the one under Q11?

You checked for EC shorts on the outputs? Good. Did you check for E-B shorts on them? If Q14 is shorted EC, then with a load, the base of one of the positive side outputs acts as a junction to ground, maybe what you are seeing. And BC shorts too, always check all three potential short paths on a transistor.

Were you checking transistors in circuit? remember all these drivers are wired directly to each other, so for example the EC of Q14 is parallel the BC of Q16. SO if you pull a shorted transistor, before putting something new in, go back and t=check the remaining transistors to make sure another bad one was not hiding under the short of the first one, so to speak.

Thank you Enzo for the nice detailed response! I am testing the amp with NO load. like you said, it has to be stable with no load before a load is plugged in.
D31 and D32 with no load test fine. i pulled one leg up on D31 to test as they conduct both directions.
i checked all the resistors directly coupled to each transistor i replaced. I found no resistors out of spec. It must be a transistor i missed.
Yes C3 measures like a diode. something is pulling the power to ground.

i will address these last items when i get home from work tonight.

[ThisLifeILead]

So: is C3 just shorted (whether itself or the circuit it is in) or does it measuer like a diode? If it does measure like a diode - "shorted" only one direction - that sounds more like something in the circuit is shorted putting a junction across the rail.

When pulled, the caps tested at 2K an rising.

Q10. You removed it and left it out? The amp won't work that way. You can leave out 12 and 13 for testing, but not 10. is that 47 ohm above Q10 open? How about the one under Q11?

all those resistors all test good (in circuit) But why cant i fire it up to look for shorts without Q10?

You checked for EC shorts on the outputs? Good. Did you check for E-B shorts on them? If Q14 is shorted EC, then with a load, the base of one of the positive side outputs acts as a junction to ground, maybe what you are seeing. And BC shorts too, always check all three potential short paths on a transistor.

Were you checking transistors in circuit? remember all these drivers are wired directly to each other, so for example the EC of Q14 is parallel the BC of Q16. SO if you pull a shorted transistor, before putting something new in, go back and t=check the remaining transistors to make sure another bad one was not hiding under the short of the first one, so to speak.

Q14 is indeed shorted.

I put it back together with, Q14 removed now also and im still getting a short.
sure enough Q18 and 19 are shorted B-C. Wow im learning alot here. Thank you Enzo! I will pull those two transistors and fire it back up and see if its stable.
whats weird is Q19 and 18 are part of the (-) but that makes sense because im seeing a negative resistance across the (+) rail cap. C3.

[Enzo]

remember, I said C3 or the circuit it is in. If the cap is out and tests OK< go back and measure the solder pads it came from, is there still a shorted condition between the pads?

A "negative resistance" on the meter means there is some residual voltage in the part you are measuring. Ther is no such thing as negative resistance really. If you get negative ohms, try reversing your probe leads and see if now you get a very high resistance.

If Q18.19 are shorted BC, make sure Q15 isn't shorted EC, because that EC is parallel the others BC.

Why can't you leave out Q10? The whole power amp is a circuit between V+ and V-. The circuit stretches across that electrical space, with the output in the center. The center point between V+ and V- will be zero, if both voltages are the same. So imagine you have a rubber band, with a mark on the middle. Now grasp the rubber band between thumb and forefinger on each side, mark centered, and stretch the band by moving your hands apart. A nice stretched band. That represents your amp. Current flows from side to side to maintain that stretch. Now open your fingers on one side and let the band go. It all snaps over to the other hand.

Q10 is part of the circuit through Q10, Q9, Q11, plus the two 47 ohm resistors. That circuit is stretched between the two 57v rails, about 114v of stretch so to speak. Now if you remove Q10, (or if it is an open transistor) that lets go the top end of that string of transistors. And just like the rubber band, the rest of the circuit snaps down to the bottom power rail.

Q9 wants to be centered near zero, but without Q10, there is no current path to bring Q9 up from V-.

A very similar argument explains why you can't operate without Q14 either, or the outputs will all fall down to V-.

Why don't you just remove Q14--19, and test them individually while out of the circuit. I think some of the in-circuit parallel current paths are confusing your efforts.

And again, if you remove Q18,19, then another similar argument is presented. If those are gone, the current path through Q16,17 and those 0.15 ohm resistors is open, so the circuit there can snap up to V+

In a solid state amp like this, you really need to have a complete circuit. You can't run a car motor with some valves removed or a piston, and you can't pull gears from a clock expecting it to work. Just so, you need all your transistors here. Q12,13 are the only ones you can remove and the amp still work.

[ThisLifeILead]

Clearly i have no idea how solid state amps work ha!
All my electronic skills have been oriented to analog circuitry but this is a good spot to learn :-)
I will pull those transistors and retest them all. will update with my findings.
The fractional resistors 0.15ohms are too low to be accurately measured by my DMM. it only goes to 0.3ohms. Should i just replace the fractional resistors while im at it anyways?
Thanks again Enzo!

[52 Bill]

The fractional resistors 0.15ohms are too low to be accurately measured by my DMM. it only goes to 0.3ohms. Should i just replace the fractional resistors while im at it anyways?

By only goes to 0.3 ohms, do you mean when you touch the two meter leads together you get a reading of 0.3 ohms?

If that is the case, then you are reading the resistance of the leads themselves. If you test a 0.15 resistor do you get a reading of 0.45 ohms?

In most cases I find that the ballast/emitter resistors are either open or okay.

[ThisLifeILead]

By only goes to 0.3 ohms, do you mean when you touch the two meter leads together you get a reading of 0.3 ohms?

If that is the case, then you are reading the resistance of the leads themselves. If you test a 0.15 resistor do you get a reading of 0.45 ohms?

In most cases I find that the ballast/emitter resistors are either open or okay.

measures 0.3 ohms across the resistors. starts as 0.4 then drops to 0.3ohms. the leads measure 0.2ohms. So i guess they are ok

[Enzo]

I am with Bill, they are usually either open or they are OK.

[ThisLifeILead]

I cannot find MPSW92 anywhere except on fleabay from china which takes 2+ weeks to arrive. I cannot find any equivalents that are carried by mouser or digikey.
its suprising to see parts made in 2001 now obsolete.

[Leo_Gnardo]

I cannot find MPSW92 anywhere except on fleabay from china which takes 2+ weeks to arrive. I cannot find any equivalents that are carried by mouser or digikey.
its suprising to see parts made in 2001 now obsolete.

Might consider MPSA92, a much more common part. So common even I keep some in stock. If it heats up attach a clip on heat sink. The D version seems to only handle higher voltage, something you don't need in this circuit. And a little more heat, that's why I recommend the heatsink.

[ThisLifeILead]

thnx bud!

[Enzo]

15 years is forever in electronics. Floppy disc drives? CD players?


Try here:

Electronic Goldmine - MPSW92 Small Signal High Voltage PNP Transistor (Motorola)

[ThisLifeILead]

i saw that place. but they didnt carry any of the other parts i needed. I landed up ordering MPSA92 from digikey along with all the other parts.
thnx guys! once i get the parts ill be back to annoying the hell outta you with stupid questions :-)

[Enzo]

The difference is in power dissipation rating, so DO pay attention to heat.

[ThisLifeILead]

The difference is in power dissipation rating, so DO pay attention to heat.

I am going to make a small heatsink for the MPSA92. kinda like this DIY Heatsink for Small Transistors

also the parts arrived from Digikey today! less then 24hrs from ordering it was delivered. Very nice.