Bassman 250 help

[Randall]

I have a Fender Bassman 250 that has the mains fuse blown that went out with a loud thunk. Upon initial inspection I find the four A1294 on the neg rail to read in circuit, C to E = 0.5 ohms on all four. C to B and B to E reads 10.3 ohms on all four. D 222 reads 0.2 ohms. The C3263 on the positive rail have no such short readings and are all relatively consistent with each other.

Before I call the fellow with an estimate, I seek advice as to how to go about this. Lift D222 and see if the outputs still read shorts? Pull the outputs and see if the rail is there? If so, then replace outputs, driver and D222 for good measure? What if pulling the outputs does not restore the rail? How much time would you foresee spending on this, and what if you got hours and hours into it, and it still had issues?

These type of meltdowns on power amps are not my strength, so fire away. And thank you.

http://support.fender.com/schematics...atic_Rev-A.pdf

[52 Bill]

All of the outputs are basically wired in parallel with each other, so when one shorts, they will all read as shorted. I will try and unsolder each Collector one at a time until I find the bad one. And it often is only one that is bad.

Have you meter tested the other transistors as well?

For estimating, once I have found a shorted transistor, I will remove it and as long as the other transistors test okay, I will power it up on a limiter with the one removed transistor. That will show the basic condition of the power supply and the rest of the power amp. If it powers up fine and passes signal I will then know that there will probably be less than an hour in time to remove and replace the outputs and or drivers.

If it doesn't power up correctly, then I will assume that there will be a couple of hours of time finding all of the problems and repairing them.

Just my $0.02

[Enzo]

D222 is almost certainly not the problem. But go ahead and check.

Agree with Bill the outputs are in parallel, so one shorts they all appear to be shorted.

Here is a trick, rather than unsoldering anything, use your meter. measure resistance right on the transistor legs, E to C. Look at the reading. There is a good chance they all read shorted, but if you look closer one will measure like 0.2 ohms while the other three measure like 0.5 ohms. If only one is shorted (usually the case), right on its legs you get the low resistance of the short. But on the other ones, they essentially read as open, so you wind up reading the resistance down through the 0.15 ohm ballast resistor, and back up through the same resistor on the shorted one. So you get then 0.3 ohms more than at the shorted one. Does that description make sense?

Assume Q214 is shorted E-C. measuring right at Q214 you get close to zero ohms E-C. Now move to Q216. Remembering that Q214 is a short, your meter now reads from collector of Q216, through shorted Q214, then on through R232, then back through R236 to the emitter of Q216.

So I go down the row, if one checks out a half ohm lower than the rest, I unsolder just that one, and recheck the rest. it won't be a large difference.

Your 10 ohm reading makes me think one is shorted E-B, so you are really reading two 4.7 ohmers in series on side by side outputs.

Q212 is highly suspect.


And if Q212 is not shorted, and one or more outputs are shorted B-C, then all four probably are shot.

[Randall]

Wow, you guys never cease to amaze me. At a closer look, indeed Q214 reads 0.1 ohms C-E, and Q212 reads 5.5 ohms C-E. From this can we wager that these two are the bad ones? I will lift them and see.

[Enzo]

So Q214 is bad. I would change Q212 just because it probably saw stress. But is it bad? Look at the circuit, Q214 is now a wire. Do you see how a C_B short in Q214 places the 4.7 ohm resistor across Q212 E-C?

Pull the bad Q214 and then remeasure Q212, I bet the 5 ohms is gone. I'd still replace Q212, but I bet it is no longer showing as shorted.

[Randall]

Right again Enzo. with Q214 out it measures E-C 0.1 ohms, E-B 0.6 ohms and C-B 0.6 ohms. Q212 no longer measures the 5 ohms, in fact it matches the other driver pretty close. I put it on a limiter as Bill suggested, and it looked good with both rails fully there. Off the limiter it passes signal. You even answered my next question as to whether or not to change Q212 due to possible stress. Do you think it is completed with replacing Q212 and Q214 if it is working after this? No reason to worry about stress to the other outputs on that side?

[Enzo]

Well, my thinking on parallel outputs is this, and others may disagree:

When you have a row of parallel outputs, and one shorts E-C, then it takes all the current, leaving the others to do nothing. After all, when Q214 shorts, the others now have zero volts across them. SO I really don;t worry about them if they seem to be working.

Q212 on the other hand had to drive that shorted mess, and may seem OK, but could easily have suffered in the process. False economy to save a dollar or two hoping it is OK.

[Randall]

Thank you Enzo, and thank you Bill52. I have ordered the parts, and I have learned a good lesson in the process. It makes perfect sense. I knew I had a weak spot in my understanding of this, and am so happy that once again this great forum helped me to get a clearer idea of how things work. I hope to someday be able to contribute as much as I receive.

[Enzo]

If there is a good lesson to be learned here, it is to focus on the circuit rather than the parts. It is easy to just go down the rows checking parts, but all parts work in the context of a circuit. And by widening our attention to look at that context, we see things like those parallel paths for current.

I hope you followed my discussion of that path down through one resistor and back up through another and the shorted part to make the neighbor part appear shorted when it was not. This is exactly the same as someone putting their ohm meter across the heater fuse in an amp and it seems to be OK, yet no heaters come on. They had an open fuse, but their meter was measuring the path from one end of that open fuse, back through the VERY low resistance of th power transformer winding, then through the very low resistance of the tube heaters, and back to the other end of the open fuse. That is why we remove the fuse from the holder to check it, otherwise parallel paths confuse us.

Another thought, look at Q212, it may be OK for now, but if excess current flowed through it, say it was shorted after all, that current had to come from somewhere and go somewhere - a circuit. Current doesn;t flow without a complete circuit. So if Q212 were bad, we might want to look up at R228 and even Q211 as possibly damaged.

[Jazz P Bass]

Ordering parts does not constitute a repaired amp.

Use your limiter when you first fire it up.

Check the base & emitter voltages and verify that they agree with the circuit.

And good luck.

SS amps can be a bugger bear to get up & going after a failure.

[Randall]

Yes Enzo, I did indeed follow your discussion of signal path readings, thank you for that. Q212 measures good with Q214 out of circuit, but I will replace it anyway. A quick check of Emitter and base and collector voltages looks good, so I am hoping this one will be OK once the new transistors come in.

[Afropaddy]

Fascinating read guys, by the way where can I access an electriclal schematic for Bassman 250 aa the one from my manual's "gone missing"