Ampeg B2-RE bass amp DC on output

[lespaul924]

Hello, I have a ampeg B2-re bass amp that with no signal or load will have 1.5 volts dc on the speaker output. With a signal present it will rise up to 10volts where it will trip the relay on the output.
The main voltage rails seem a little high, should be +-60, but they are +-73.

There are 8 power mosfets, 4 on each side IRFP240 and IRFP9240. I don't believe they are the problem though. I have 3 new of each that test the same with the diode charge/finger discharge method. However; will only putting 3 on each side cause significant problems just to troubleshoot?

The DC shows up on the Sources and a little on the gates of the mosfets, but also amongst the drivers and the limiter section. lowering or raising the signal level will lower or raise the dc on the speaker, or until the relay trips.

does there tend to be a common cause for dc on the output, or what is the best method to start troubleshooting?
Should I start checking transistors, diodes, voltages or something else. The 16 volt rails all seem fine just the main one is high. pre out works fine.

Feel free to ask questions or demand answers
Thanks

[Steve L]

Ampeg B2-RE bass amp DC on output

Hello,

I just finished repairing one of those a few weeks ago. The one that I had was completely dead. Several components burned, shorted mosfets, etc., so it is not the same issue as yours. However…

In my experience, even with what looks like it might be a small issue with your amp, I would not mess around with that, nor any other mosfet power amp.
Those mosfets are not real cheap.
If there is anything slightly wrong in the power amp circuit, it will take a new set of the mosfets out real quick.
There are not that many active components in that power amp.
Take out all of the power amp transistors (MPSA06, MPSA56, etc.) and check them all out with a DVM. You can then check most of the resistors and diodes on the board without removing them.

I know that might seem to be the slow way at first, however it is not worth messing around with replacing parts and then putting everything all back together, powering it up to see if you have got if fixed and possibly taking out other parts.

By the way, if you need them, the best price I found on the mosfets was from Digi-Key.
Also, don’t replace the 1N3070 with 1N4148 diodes if you find any bad. The PIV is not high enough.

Steve

[lespaul924]

Thanks for the response and a quick update

I was getting a little antsy from checking nearly every component and not finding any bad, but Q13 2n3440 was blown. after replacing it the amp powered up and no dc on the output, but after about 15 seconds the fuse blew. looked over my work to see if i put the amp together correctly and pulled the new 2n3440 and it was still good. So, back to checking components to see if something else was blown or just blew on this startup.

[lespaul924]

Can anybody confirm that I am using the proper schematic for a B2RE, provide a better one, or even just help determine appropriate component values.

I have a B1re that I am using as a guide and it matches this scheme.
The b2re on the other hand has 2 extra mosfets and a bigger power supply(extra filtercaps and larger value resistors). The supply voltage also is 72 volts instead of the 60 on this scheme. is it intended to be this high by design or is it because some components were blown and not dragging it down?

My main question however is the gate and source resistors. The B1 uses the 47ohm half watt gate and .47 ohm 5watt source as on the schematic. The B2re on the other hand has 1 watt 47ohm gates and also 47 ohm5watt source, not .47 Can anybody confirm that this is correct? this amp has been worked on before.

I had intended to use the resistors and mosfets out of the B1re as a fix before ordering more parts. Can I get away with the .47 5 watt source resistors instead of 47 ohm. and also only 3 mosfets on each side instead of the 4. 300 watts instead of 450, but could it damage anything? kind of temporarily convert the b2 to a b1 power amp section, I'm just worried about the bigger power supply.
also on initial startup can I get away with just Q10 and Q14 in place or only one on each side?

Thanks

[lespaul924]

oops, here it is

[52 Bill]

I don't have any definitive answers for you here, but the last one of these that I worked on had the 47/0.47 combination of resistors.

In testing it, it ran fine with one set of missing output Mosfets (I just pulled the shorted ones out).

As for the overvoltage question, has the primary been wired for 100 volts instead of 120?

[Steve L]

Ampeg B2-RE bass amp DC on output

The B-2 that I recently repaired had 1 watt / 47 ohm resistors on the gates and 0.47 ohm / 5 watt resistors on the sources. 47 ohm resistors on the source leads would not be proper in the amp, nor most others for that matter.

I can't remember what the supply measured. As was mentioned, check the AC input connections. 73V unloaded sounds about right for a 300 watt power amp.

Again, make sure that you check everything out real well before powering it up. Mosfet outputs don't last real long if there is a problem.

By the way, you might want to check the bias pot. When I repaired the one that I had, I found that it was open. Even with the way the rest of the amp was burned/shorted, that should not have happened. It may be possible that it opened and led to the down fall of the one that I had.

Steve

[lespaul924]

well I went through and basically checked and or replaced everything. For an initial startup I powered it up with only Q10 and Q14 in place. They were part of the initial failure; however tested ok out of the circuit, were not shorted and discharged. After a few seconds some smoke and they were both toast. I'm going through it again now and it looks like the only failures are the two mosfets. ballast resistors grid resistors and driver/limiter parts ok. I did not replace the caps, but they were not open or noticeably burned.

Does anybody have any input if they failed because it is improper to only be running one on each side, or because they may have been stressed or something in the driver stage that may not be operating correctly despite all the components testing fine. voltages to look for or matching issues?

I really don't want to stick $30 worth of mosfets in and have them all blow up. I did not have the heatsink on when powering up with only two.

Any advice is much appreciated
Thanks again

[Enzo]

They have to have a working bias circuit behind them, or they can both turn on, shorting the + and - rails together through themselves.

[lespaul924]

Is it ok if I have no mosfets in at all to test the voltages and bias?

I keep sticking in good pairs Q14 and Q10 and after 10-15 seconds they blow. I've gone through every component in the power section and they test fine. I've been sticking to a 5 amp fuse and using a variac and it seems to be saving the mosfets from taking other things out.

So, the story so far is the amp was letting dc through to the relay. initial inspect showed Q13 2n3440 was bad. It was without a heatsink that should have been there and had a small brown spot on the board, so figured once that was replaced it would be good to go. With a new one in, and after about 30 seconds, the power mosfets blew taking out all of the source resistors, but the damage was more on the N channel side. all the gate resistors were blown along with the limiter section R45,Q3,and D15/16. Now after replacing everything and checking every component trying to get by with only Q14 and Q10 they blow after about 10 seconds.

I have also checked over all the pads and traces for shorts or bad contacts.
What would or could make the mosfets blow when all the components test fine? I have not been using the heatsink, but they blow pretty quick.
When the only thing wrong was the initial 2n3440 the voltages tested fine minus the dc on the output. Now the mosfets blow before I can check them.

Any suggestions on the best course of action?

Thanks again

[52 Bill]

...I have not been using the heatsink, but they blow pretty quick.

Again, I don't know specifics of your amp, but operating the outputs without a heatsink is probably part of your problem.

I can remember thinking and doing the same thing years ago with a Fender something or another. I didn't want to take the time to remount the entire board to test the new transistors. Without the heatsink, in a few seconds the outputs would short from overheating. With the outputs bolted down to the heatsink everything was fine.

Later I found a Fender tech note saying the same thing, don't try and run the amp without a heatsink as the outputs will overheat and short.

[Enzo]

A variac is only half of the setup. The whole point of a variac is you can bring up the mains voltage slowly and back off if excess current starts to flow. That way full voltage is never applied to parts that want to draw excessively.

You must monitor mains current. If the amp powers up and draws 4 amps from the mains at idle, something is wrong. it might take a few minutes for that to burn out transistors, but it is readily apparent on your ammeter.

And what Bill said. You have to heat sink the power transistors. otherwise the thermal regulation can;t work - the transistors can;t cool themselves, and the circuits that monitor temperature won't have anything to monitor.

[J M Fahey]

Hi Les Paul. I assume you are not using a series lamp fixture, at least you don´t seem to mention it.
Please don´t ever hook a suspect amp straight into the mains without one.
Of course the suggestion of using a Variac still holds.
You *can* use only one pair of MosFets for testing, at least until you get DC balance in the output (proper DC offset < 100mV, proper bias) , but with *no* load (speaker or power resistor) applied, only your test equipment.
They should be bolted to a proper heatsink.
You are close to success when you get no DC on the output, the series lamp glows dimly, you measure somewhat around 3VDC gate-source and your +B and -B is reasonably symmetrical and between 25 and 60 Volts, depending on the serial lamp used.
Then you can mount remaining MosFets; if all looks well apply a load and some audio and if everything looks and sounds well, obviously power-limited by the lamp, only then you can test it with full line power. Even then, start gradually using the Variac, as suggested earlier by other posters.
Good luck.