Ampeg B-100 problems

[Jetbat]

Hi. I'm having a problem with a Ampeg B-100 amp. I had originally wanted to put a fuse holder in the back panel because the fuse was soldered inside the amp and to install a grounded power cord. I also wanted to upgrade some of the resistors to low noise ones and replace old electrolytic capacitors. I switched out the parts and installed the fuse holder and power cord.

When I turned the amp on a buzz and hiss came from the speaker. This was not there when I played through the amp right before I worked on it. The buzz was the same level with the volume turned up or down so the problem was coming from after the volume pot. Poking around inside I found out the junction of the base of Q2, Highlighted on the schematic, had a charge. If I touched any part of that junction with a wooden stick I would get a louder buzz. Any other part of the circuit would not buzz when touched. The capacitor C11 and C13 would buzz if I put my finger near, but not touching them. If I touched the plastic case of C11, it would buzz like I was touching the end of an instrument cable that was plugged into an amp. I found that Q4, a TO220, would get very hot in less then a minute like it needed a heat sink. I took measurements of DC voltages and labeled them in red next to the schematic voltages. I also included the base voltage of Q6.

I replace the components I changed with the originals one at a time to figure out what was causing the buzz. They were all replaced with the buzz remaining the same. I ordered a replacement for Q2 thinking that was causing the problem. I found out the part number is incorrect on the schematic. The 610083-1 should be a 610083-5. The list of parts below the schematic has the Q2 listed correctly. Replaced that with no change. I replaced Q3 and Q4. No change. I replace Q5 and Q6. Still buzzes the same. I re-soldered all the connections on the PCB. No change.

I am confused to what could be causing this. Anyone have any ideas?

Thanks
Scott

[The Dude]

If you can cause a change with a non-conductive wooden stick, I'm betting you have a connection issue somewhere.
Try tapping lighter and narrow down the issue.
Check solder joints in that area.
Check component legs in that area. Sometimes something looks soldered from the solder side, but the component leg didn't make it through the hole or is broken.
If you removed a board, possibly some wire connections that went to the board broke loose.
Check continuity between traces that are supposed to be at ground potential to the chassis. Possibly something isn't grounded properly.
Have you reassembled it after work was done? Sometimes ground connections are made through potentiometer bodies, jacks, etc. If you haven't tightened everything back up, you could be missing a ground somewhere.

Maybe post some pictures if you can and we will see something.

[Jetbat]

Hi.
With the wooden stick I also do not have to touch anything to increase the buzz. If I bring the stick close to C11 the buzz will increase with closer proximity. It is very weird. I thought the highlighted section was microphonic until I discovered I did not have to touch something, just interrupt the field that is being generated around the parts.

I re soldered all the connections and checked wires. Did a continuity check form the ground on the PCB to the ground prong on the power cord and that was good. The first time I reassembled and tested it I put all nuts back on so the pots were all grounded.

The next thing I was thinking was to start removing components one by one and check them with a multimeter then make sure they were soldered back correctly. I wanted to check and see if anyone had any ideas before I did that.

[Jazz P Bass]

Have you checked the B+ voltage for AC ripple?

I think that you are possibly chasing a ghost with the "hey if I put this here is does this' thing.

[loudthud]

What is the voltage drop across R28 and R29?

[Jetbat]

The B+ ripple is 125mV at C18. The ripple at the output capacitor C17 is 25mV.

The voltage drop for R28 is .112V and R29 is .116V.

There is definitely is something wrong with the amp. Before it was quiet with no hiss or buzz. The main thing I don't like it the fact that Q4 gets so hot after one minute that I can not keep my finger on it. There is 60mA passing through R24.

[Jetbat]

Here are some pictures.

[The Dude]

The base of Q2 appears to have a cracked joint (seen in the 3rd picture). Resolder it.

[Jazz P Bass]

Hey Dude.
What, you have Superman vision?

Nice catch!

If I may add: all of the joints seem to be lacking in that it appears that the pads themselves where not properly heated.
The pad itself & the component lead have to be above the melting point of the solder or you will not get a proper solder joint.
Moving the iron tip back & forth between the pad & the component lead, while adding in the solder, will go a long way to ensuring a good solder joint.
2 cents.

[Jetbat]

It does look like that in the photo but upon closer inspection it is not a crack. I must have re melted the joint and on the photo side it only melted at the top. The other side looks fine. That is not the original transistor and it has been removed for testing too. So it has been installed twice with no change in the problem.

Tonight I am going to remove all the wires running to the PCB, trim, strip, tin, and solder them back in place.

[Jazz P Bass]

It makes no sense that Q4 is getting hot.

With 0.47 volts on the base, it is not even 'ON'.

I would 'diode check' it out of circuit to make sure the leads are properly oriented & that it is indeed an NPN transistor.

[Jetbat]

The original transistors were germanium which is why the BE voltage is low. The voltage readings numbers in black are are printed on the schematic from Ampeg. The original transistors have been replaced with silicon. I have not taken voltage readings at that point since the switch but they both get too hot. The original Q4 transistor diode checked as good.

[52 Bill]

The original transistors were germanium which is why the BE voltage is low.

I can't believe that the original transistors were Germanium, not in an amp of that era.

[Jazz P Bass]

The original Q4 transistor diode checked as good.

If they check good, maybe put them back in?

[Jetbat]

I did. Then I moved on to try replacing the output transistors. That did not do anything. Then I decided to try switching all the transistors out and that did not change anything. That is where I'm at now.