Ampeg B-100R Melting Bridge Recto

[R.G.]

Pulling the output transistors helped. The amp didn't want to blow a fuse with or without the bulb limiter.

OK, good step in the right direction. Pulling output transistors means the output stage simply can't pull current. So either the transistors are bad, or the opamp/bias setup is telling them to pull current. Could be either one. Or one of them is bad and the others are trying to pull the output back to where it should be. Lots of choices.

There's higher than normal voltage at IC2 (-22VDC @ P4?) and -6V on J6. The voltages are dropping as I'm testing... why is that?

I'm guessing that the voltage at IC2P4 and IC2P8 is not centered on ground.The output transistors holding J6 at nearly 0V in operation is what centers the power supply for IC2 at ground. With the outputs removed, there isn't anything there to center it. I take that back. R63 and R64 center the opamp power supply, but slowly and possibly imperfectly.

It is *critical* to the life of IC2 that the difference from pin 8 to pin 4 not be over 44V. It really shouldn't be over 40V, and that should be clamped by zener D9. Those wobble up and down in operation, following the output signal, but if the difference gets bigger than 44V, the IC is in danger of sudden death syndrome.

This amp will produce the correct output voltage without a load on J6 if you put a single pair of the output devices back in. If you can separate out one or two that are shorted, that may be an OK thing to do.

If it were me, I would order a new set of output devices and all other semiconductors back through IC2. Mouser lists the TIP147 for $1.45, TIP142 for $1.28, NE5532 is $1.00, MPSA06/MPSA56 are $0.25 eachm MPSA13 is $0.10. 1N5366B is $0.36, 1N914 is under a nickel. Less than $8 gets a full replacement set. (prices today, Mouser.com)

[SYSDOA]

RG -

Thanks for the clarification. I've gone ahead and swapped the semi's as requested. Fortunately, this time I actually have everything for this amp on the shelf so I can get to it pretty quickly. I'll reinstall the newbies, bring power up slow and report back.

[R.G.]

Even better~! Everything in stock. I nearly always have to order something!

[SYSDOA]

Yes, one of the few times second guessing the public has paid off! Ok the following were replaced:
IC1
IC2
Q3-8
C35-38
R65 & R66 as well as R55-58
One filter cap and the bridge

First power up smoke rose off 58 at about 50V from the variac. Swapped again and second power up sounded like the fuse was burning up in the variac. Im looking it over now, would D21 and it's counter part be a culprit?

[R.G.]

Yes, one of the few times second guessing the public has paid off! Ok the following were replaced:
IC1
IC2
Q3-8
C35-38
R65 & R66 as well as R55-58
One filter cap and the bridge

OK, you have a fairly new set of parts to work with.

First power up smoke rose off 58 at about 50V from the variac.

Are you limiting current any way other than riding the variac and looking for smoke?

If not, you are prone to burning up the parts you just put in. In this case, smoke from R58 indicates that Q8 was conducting more than 4, and probably more like 8A RMS. Not good for R58 or Q8, or for the rest of the Qs that were involved in it.

You need to either get a meter on the AC power line current and keep it there so you can see current start to shoot up, or make and use a light bulb limiter so you don't have to ride the variac. The LBL lets you bring it up with the limiter keeping things from burning out while you can probe to find out what the voltages and currents are. You can't do that with a meter on the AC current, and you can't even watch the current if you don't have a meter on the AC current. And you have to do these things safely. We don't want to lose you to electrocution.

Failing that, you have no other good options except to begin isolating parts by preventing current from getting to them. For instance, pulling out at least one end of R32 prevents any signal from the preamp from getting to the power amp; if the preamp has gone mad and is feeding the power amp ultrasonics, this can be burning out the output transistors and there's nothing the power amp can do about it. Pulling up one end of each of R67 and R68 means that everything connected to +/-15V is not powered, and cannot be sucking down the current. Likewise pulling up one end of each of R65 and R66 means that nothing connected to +/-16 can be having any effect on the power supply. These last you have to do carefully, with your eyes open, as +/-15 powers IC4, which feeds the power amp input after the master volume, and +/-16 feeds IC2's input, which MUST be held at 0V to keep odd things from becoming likely in the power amp circuit.

So - to proceed, you need (a) a current meter on the incoming AC power line so you can manually shut it down when things go off the deep end or, and preferably, a light bulb limiter which will let you bring it up a bit and debug without killing your new parts while you do; (b) and oscilloscope, to check for ultrasonic oscillations that can be killing your parts while you look for the cause; and ( c) some careful divide-and-conquer to start eliminating things that can be eating current or providing high frequency oscillation.

[velo-hobo]

Hello, I realize this is a bit of a zombie post but I have a similar issue with my B100R that I hope to get some assistance with here. Already this thread has been very helpful so I thought I would bring it back from the dead to try to get some more answers.

A few months ago my B100R, which I have had for 12 years, blew a fuse for the first time. I wasn't doing anything unusual with it, playing at moderate volume at home. The amp has been gigged heavily (on and off) for a long time so it was not altogether surprising.

I replaced the fuse and it blew right away so I knew there was something drawing too much current. I checked the transformer and bridge recto and they are working fine. I also checked the large filter caps, they measure appropriately with a cap meter but I am aware that this is not a definitive test. Couldn't get the mains voltage much past 20V without my light bulb limiter lighting up.

So I pulled the power transistors and it seems to operate stably without them. I tested the power transistors, and Q5 has an emitter-collector short. I am wondering if the component failed on its own, and maybe took out some other parts, or if something else took it out. With regard to the preceding posts, it seems there are many possible answers.

I checked IC2 with the PTs pulled, pins 4 and 8 have about 39V between them, the inputs measure around 6V and the outputs measure around 18V. The cement resistors seem to measure ok but I did not pull them from the board for testing. R40 and R45 check out fine.

I should note that I did find a suspiciously cold-looking solder joint on the emitter of Q3 - could this have caused a problem further downstream? I reflowed the joint upon finding it.

Since the amp seemed stable with the PTs pulled, I tried putting Q6 and Q7 back in the circuit but the bulb lit up at around 70-80V on the mains. So either one of those is also bad in a way I have not discovered, or something else is fishy. I ordered two whole sets of the power section semiconductors (Q3-Q9, IC2, D9-D15) from Mouser, so they will be arriving by this weekend, but I want to see what else I can investigate before then. Could anyone shed some light on this for me?

[Enzo]

Hi, welcome to the forum.


May I suggest you read and make use of the thread here, but really, start a new thread for your amp. You will get better response than tacking on the end of someone else's repair from a year ago. And there will be less potential for confusion between the two repairs.

When solid state amps blow fuses, by far the most likely cause is shorted output transistors. In any case, until you know the amp is stable, do not connect a load. If opposing outputs are shorted, the amp will blow fuses. If the amp is simply putting out DC, it will blow fuses if a load is connected, but without a load it just puts DC on the output jack and stays cool.

I HIGHLY recommend never running a solid state amp without its output transistors. The amps are highly fed back on themselves, and they are counting on the presence of that part of the circuit. Without them, the feedback has nothing to feed back. And less so in this amp, but inmany, operating without the outputs makes the drivers try to drive the output, and they can be easily damaged that way, they are not designed for heavy current.

I am wondering if the component failed on its own, and maybe took out some other parts, or if something else took it out.

While it is fun to solve this mystery, ultimately it doesn't help. We have to find all the bad parts and other damage and repair it. Whether the output transistor failed and took out a driver, or if a driver failed and took out an output is kinda irrelevant.

[velo-hobo]

Thanks for the welcome! I have read this thread quite a bit while trying to figure out my specific situation, already it has been very helpful. There seemed to be enough relevant info here already that I thought it would be appropriate to tag onto the end of this discussion, but if a new thread makes more sense perhaps I will start one (unless a moderator would like to move the most recent posts to a new thread instead).

I should note that I have been working on this amp with no load connected. I am aware that SS amps are unlike tube amps in this regard, but I didn't realize that pulling the output transistors could cause trouble in other parts of the amp though, thanks for informing me.

Since I have a new complement of SS components on the way, where else should I look for problem spots? Nothing seems obviously burnt up, although I have only tested the components I mentioned above, including the emitter resistors, which test ok.

I should mention that R69 (for the pilot lamp supply) looks a little toasty, but measures fine. It is right next to R65 (or R66 - I don't have the PCB in front of me right now) so maybe it just took some heat externally? In any case all of these resistors seem to be fine.

[52 Bill]

I'm with Enzo, I hate scrolling through the old posts to get to what is current, but do whatever you want to do.

Did you actually remove all 4 outputs transistors? Did you test them all out of circuit before you put them back in? I can't say for sure, but as Enzo hinted, you may have caused some additional damage by running the amp that way. I try and remove as little as possible to test and repair anything that is in front of me.

It is very easy to test the outputs for shorts while in circuit, finding which one of two or more paralleled devices is bad takes a little more work, but in only pulling the bad devices, it allows you to test the rest of the circuitry before ordering parts. If you had just found the short and pulled Q5 you could then power up the amp through your limiter to see if the amp would have powered up normally. If it still drew too much current then you could look at the drivers, etc.

Check all of the transistors and diodes in the power amp in circuit. If anything doesn't check right, then pull something out to test it. If the power transistors have already been pulled then test them all out of circuit and then put them back in until you complete diagnosing the other problems. Try and limit the variables that you add to the mix until you know what is wrong with the amp.

[Enzo]

RUnning an amp without its outputs installed is like running a car engine with the pistons removed. Yep, exactly like that. Sorta...