Ampeg B5R - Not switching channels

[garytoosweet]

Back again with another transistor snafu.

Amp is Ampeg B5R. It is stuck in 'Clean' channel. Unable to change into 'Overdrive' channel.

I've attached both full schematics as well as screenshots of what I believe to be the problematic/relevant sections as the schematics are huge.

Neither front panel switch nor foot switch are able to change channels.

Originally I suspected Q1:B. It is one quarter of a MPQ222A, a 14-pin TO-116 with 4 NPN transistors inside, responsible for the complex channel switching in this amp.

I took the MPQ222A out of circuit and tested it on a signature tracer - and it seemed to check out fine.

I installed the chip back in the amp and powered on to do further testing - and the channel switching now worked! For a few switches anyways, before ceasing to function again.

I'm not sure why it worked for a few switches intermittently. I pulled out the board again to inspect for bad joints but all looks fine.

I'm having trouble understanding how this switching system works, thus having trouble effectively troubleshooting it. \

Voltages of Q1:B:
Clean Channel mode (Channel Select Switch Out)
B: .011VDC
E: .007VDC
C: 14.3VDC

Overdrive Channel mode (Channel Select switch pushed in)
Q1:B
B: .020VDC
E: .003VDC
C: 14.3VDC

If I manually bridge collector and base, the amp switches into Overdrive channel and seems to be fine.

I think I should try substituting the MPQ222A with discrete NPN's. I'm having trouble figuring out what will be a suitable replacement for switching purposes.

Any help with a method of troubleshooting this or just a nudge in the right direction is very appreciated.

Thank you in advance for any help.

[g1]

Was that IC socketed, if so, did you clean the socket?

[garytoosweet]

Was that IC socketed, if so, did you clean the socket?

No socket on the IC. Directly soldered to board.

EDIT: I also went back and checked continuity between the IC and next component on the PCB trace, all seems fine.

[The Dude]

FWIW: That IC is just 4 transistors in one package. You should be able to check the IC by checking each internal transistor the same as you would 4 separate discreet packages. For pinout, see the datasheet you linked in post #1.

[garytoosweet]

FWIW: That IC is just 4 transistors in one package. You should be able to check the IC by checking each internal transistor the same as you would 4 separate discreet packages. For pinout, see the datasheet you linked in post #1.

Yes I pulled it out of circuit and tested each transistor individually and all tested ok. Then when I put it back in, it worked briefly before stopping again. I may pull it to test again. Maybe it has an intermittent fault.

[The Dude]

Regarding your voltage readings in post #1 on Q1:B, the transistor is not biased on in either mode (channel A or channel B). It should change in one mode or the other. Voltage on the base of the transistor should go to around .7V when the channel is switched. I can't find on the schematic where the base voltage comes from past pin 4 of J11. Does the voltage on that pin change when you switch channels?

[Enzo]

I would suggest resisting the urge to pull parts out to test them. A lot of unnecessary soldering and unsoldering makes opportunities for new problems to arise.

So if you short Q1b E-C the amp switches and works? OK, but that is just the very end of the chain. When you switch what happens at the bas e of that transistor, it is not being turned on.


Pin 4 of J11 ribbons to pin 4 of J12 on the power amp. And all that does is loop through the FS jack, and the zener sets 3.3v on it.

My first suggestion? Well, I guess second now, Your panel switch runs THROUGH the footswitch jack. Did you try a footswitch? If not, check that jack, and see if there is less than half an ohm across th cutout contacts on that jack.

For that matter look on the P/A drawing near the FS jack, see zener D2? 3.3v zener, does it have about 3v across it?

[Enzo]

Oh, and why does it work sometimes and not others? The fact it sometimes wakes up and works tells me all teh parts are likely OK. I'd suspect a poor contact on the FS jack or bad solder on either J11 or J12, or even just poor contact on the ribbon ends.

[garytoosweet]

= The Dude]Regarding your voltage readings in post #1 on Q1:B, the transistor is not biased on in either mode (channel A or channel B). It should change in one mode or the other. Voltage on the base of the transistor should go to around .7V when the channel is switched. I can't find on the schematic where the base voltage comes from past pin 4 of J11. Does the voltage on that pin change when you switch channels?

The voltage on pin 4 did not change, nor did it change on Pin 4 of J12 where it connects to the power board.

Pin 4 of J11 ribbons to pin 4 of J12 on the power amp. And all that does is loop through the FS jack, and the zener sets 3.3v on it.

My first suggestion? Well, I guess second now, Your panel switch runs THROUGH the footswitch jack. Did you try a footswitch? If not, check that jack, and see if there is less than half an ohm across th cutout contacts on that jack.

For that matter look on the P/A drawing near the FS jack, see zener D2? 3.3v zener, does it have about 3v across it?

Tried foot switch, not working. Tested jack itself and it's connections on board, tests OK.

D2 was located just under the big heat sink for the power transistors. In order to access it I needed to remove the power board, then desolder all 8 power transistors to remove them attached to the heatsink.

Pulled it out and D2 tested ok. Put the power board back in the amp to test without the transistors/heatsink attached. Now, CHANNEL SWITCHING WORKS!

I had previously tested J11/12 ribbon cable for continuity and it all checked ok. I cleaned the pins for good measure. I re-soldered both of the connectors at the PCB.

NOW I began re-assembling the amp. At each step I would power ON, check channel switching.

Put power board with re-attached transistors back in, channel switching OK.

Re-attach effects loop board, channel switching OK.

Re-attach all nuts on pots, channel switching ok.

Finally, re-attach input breakout board, channel switching doesn't work

Take off nuts from input jacks and remove from front panel, channel switching works again.

When I desoldered in the switching IC, I would have been testing with the input breakout board removed from the panel, explaining why it worked briefly before I put the Input jacks back on.

SO the channel switching works until the INPUT jacks are attached to the panel. Once they touch the panel amp is stuck on the clean channel.

Trying to wrap my head around what's happened here.

According to the schematic, the sleeve is connected to BLUE wire going to ground on the preamp board. This is true but on my amp it's also connected to chassis at the panel. Inside of the chassis is finished but they have intentionally left a small bare metal area around the jacks, so it was like this from the factory. Looking up photos of this amp online I see half of them have the same jacks as mine, and the other half have insulated jacks.

[garytoosweet]

Also if I have a speaker plugged into the output the channels do not switch.

Channels only switch with load/speaker attached and the inputs not connected to chassis.

As soon as you connect a speaker/load, or connect inputs to chassis the amp turns to Clean channel and is not able to switch.

If I just insert a plug into the speaker out jack, not connected to a speaker/load, you can still switch channels.

[garytoosweet]

This one never got resolved and I am looking at it again today.

When nothing is plugged into the amp, and input jacks not connected to chassis, channel switching is working as intended.

When a plug is inserted to ANY jack the switching circuit is tied to ground (as it would typically be in Clean mode) and does not switch .

With nothing plugged in and the input jacks are not attached to chassis, the circuit works as intended. Voltages are all right, cathode of D2 shows 2.6k to ground. With anything plugged in or inputs attached to chassis, cathode of D2 is a short to ground and no switching works.

All the components in the circuit test OK. All jacks are functioning correctly. I'm testing everything I can think of but am scratching my head a bit.

If anyone has any suggestions, it is much appreciated.

EDIT: The channels are switching with a foot switch (as Enzo suggested to try a few posts ago...).

Jack tests ok, 0 ohm continuity between the cut outs. I know the panel switch is working...

EDIT2: While measuring resistance from Q1B base to D-GND, I noticed by pushing on the base leg of the IC, I was able to continuously switch channels. The connection on the board is good, so I'm suspecting the multi-transistor IC thing. Going to try replacing the 4 discreet transistors.

EDIT3: Replaced the MPQ package with 4 discreet 2N4401's. NO change, still the same symptoms.

EDIT 4: It wasn't actually pushing on the base leg of the IC that cause the issue. I've noticed now when putting the slightest bit of pressure on the board just behind the area where the switching transistors are, the switching works. So it must be a bad connection. Possible Pin 5 of the molex, as it's the only thing the panel button switch uses that the FS doesn't, and the FS works consistently.

[garytoosweet]

The B5R is one of the rack mount Ampegs. Comes with Rubber feet, I guess you remove them if you put it in the rack.

This one had the rubber feet removed, and then the screws that hold the rubber feet were reinstalled back into the threaded holes. The screws are about the perfect length to touch the bottom of the PCB(!), actually they are a few fractions of a mm longer than that.

In this case the screw was just long enough the remove the solder mask on the PCB, when the board was inserted the screw would short these three traces in the attached photo, which was the cause of my problem here. Removed the screw and all's right with the world.

I did notice the wear on the PCB, thought it was weird. Tested it for continuity but it all tested fine. I would pull the board, test the switching circuit, it all checked out, then when I reinstalled the board to test with power, the thing wouldn't switch. The screw is black and located almost under the black chassis lip, easy to miss i guess... for me at least.

The owners manual mentions "If the feet are reinstalled, never use screws which will protrude farther into the amplifier than the original hardware." But doesn't really mention not to reinstall the stock screws if you remove the feet. Using the next size down wouldn't be a bad idea.

Thanks for all help.

[g1]

This one had the rubber feet removed, and then the screws that hold the rubber feet were reinstalled back into the threaded holes. The screws are about the perfect length to touch the bottom of the PCB(!), actually they are a few fractions of a mm longer than that.

Some of the early GK800RB's would short the hot of the speaker jack to chassis when people did this. The power amp would blow up the next time it turned on.