Ampeg B22X with multiple problems

[Randall]

I'm sure going to need help with this one.

This 70s Ampeg chassis came to me last May. It had sat in a barn or something for 12 years, and was dirty as hell. The boards were covered in some kind of film, and there was gobs of old brown flux everywhere. I hit it with a stuff brush and some acetone and then hosed it down. Then I left it in my driveway in hot sunny Florida for the afternoon, and let it set in my shop until parts arrived.

I recapped it, put in a new power cord and fuse holder, sprayed it all out and put it on the bulb limiter. Short story, dead shorted OT. So now we need a replacement. Fliptops has them, but they were on backorder during a pandemic.

Three months later, it arrives. I install it and fire it up again. It powers up OK, and it passes signal, but not nearly enough, with channel two being the strongest. None of the funky echo/afterbeat/tremolo functions work. I don't know yet about the reverb. It is also doing some loud throaty noises when I plug into the ext. speaker jack, which I wanted to do instead of dealing with the XLR plug. And further it either already has, or goes into oscillation when I probe one of the power tube grids or the side of the PI that feeds it. With all these problems, I thought I would just try to get signal flow sorted out with this thread, and start another for other issues for the sake of confusion.

In channel two I get a nice strong wave up to the circled S after the first premp tube V4. (What is that circle S for anyway?) Then it heads off on a wire to the tone control board on the front panel. The signal passes, but is about 8 times less at the treble pot wiper than it is arriving on that wire coming circled S. I have snipped the wire from 470K R61 going to echo switch, no change. I am unsure what to do next. I was hoping I might get advice before I pull the tone control board, which means pulling three rocker switches as well. These are just a bear to service, almost nothing is labeled on the board.

And what is PEC 6470000 in the tone stack?

https://ampeg.com/support/files/Sche...matics%201.pdf

[glebert]

I'm sure going to need help with this one.


And what is PEC 6470000 in the tone stack?

https://ampeg.com/support/files/Sche...matics%201.pdf

It's this do-dad: https://www.fliptops.net/catalog/p-1...ge-ampegs-used

If you have good signal at S then you aren't very far away from Z, which corresponds to the external amp input. I think most tone stacks are quite lossy, don't know if 1/8th the amplitude is appropriate or not.

If it was as dirty as you say, I wonder if all the tube sockets need to be cleaned and tightened, and maybe bypass the external speaker switchjack to make sure corrosion there doesn't screw up your connection to the speakers.

[Enzo]

And what is PEC 6470000 in the tone stack?

The "printed electronic circuit" looks like a square ceramic thing made of the same stuff as ceramic caps. Looks like seven wires come out. The internal circuit is on your schematic, complete with values. so if you ever get a bad one, you can just replace it with parts. it is just four caps and two resistors.

The other channel works? the circuit around the PEC is the same, so compare signals. You suspect the PEC? Short pins 1 and 2 together. ANy help?

[Randall]

AY Yi Yi, this thing is kicking my butt! So while concentrating on the partially working channel 2, it has developed a loud audible squeal at maybe 3K when the volume is turned up past around half, or the treble pot is turned up and/or the ultra Hi switch is engaged. In fact, I can get three different notes with different combinations. I tried switching OT primary leads, but this gave me a loud machine gun sound, which I have never seen before, so I switched them back.

Frustrated with this, I decided to focus on tracing the unworking channel 1 to see what clue I might find. I get a clean signal up to pin 1 of the tone module, then I lose some. I jumpered pins 1 and 2 to bypass it, and I now get clean signal to pin 2, but that's where I get lost. Channel 2 works (albeit with problems) with V6, 7 and 8 pulled, but I don't see a path for channel 1 except thru the tremolo tube V8, except it goes thru a 5.6M resistor to circled Y, which I am not finding what that is about. Also, out of nowhere while I was pondering, the amp went into loud machine gun mode with no input signal, surely threatening to toast my shop speaker. What is that about? Man, this is a complicated amp.

I currently have hijacked J5 ext. amp jack to use as a speaker jack, to rule out the dual impedence switching jack. I have the yellow 8 ohm OT wire connected to it.

[Enzo]

WHich channel now? I see Y on channel 1, but channel 1 DOES go through the trem tube. Signal goes to grid of V8 pin 2, then out the plate over to pin 7 , and out that plate and on up to point Z at the 7199 tube.

Channel 2 leaves the treble control through R21 and off to point Z which is that same 7199 grid as channel 1 ends at. SO 470k R21, 470k R22, and 150k R57 are mixing resistors combining Channel 1, channel 2, and the reverb, into the phase inverter circuit. Channel 2 is just two stages in V4.

Are you calling the channels 1 and 2 as per the schematic? Or maybe by position on the panel which might be opposite the schematic? As it sits you have me confused. Your descriptions seem to fit opposite channels.

[Randall]

I am referring to the channels per the schematic, but I did type a mistake, once calling Ch 1 as Ch 2. Sorry about the confusion, I have corrected that. I understand channel 2's path, it's the squealing I am struggling with, so I switched over to Ch1 to see if I could make some progress there and determine if the squealing is isolated in Ch 2.

I was questioning whether the CH 1 signal path had to go thru V8 to get to the 7199, which I had pulled during working on the other channel to try to eliminate variables. I wasn't seeing very clearly the path, but your description opened my eyes to it now, so thank you for that. It goes from V8 second plate thru C42 and R69, and then it's mixing resistor R22. Makes sense now.

Why does Ch 1 go thru V8, the tremolo tube, but not Ch2? Is trem only one Ch 1? I can't tell because the front panel is nearly unreadable.

And what could this loud machine gun noise be? It will be sitting there at idle and suddenly, BAM, BAM, BAM, BAM!

[Enzo]

Channel 2 does not go through the trem.

It sounds from description like you are going into intense motorboating

[Helmholtz]

Motorboating is a very low frequency (relaxation) oscillation. Such low frequency means that large time constants must be involved as found in the power supply. Typically the supply voltage is pulsating during the effect. If so, you should check the filter caps and their (ground) connection. Temporarily paralleling a good cap can help to identify the bad cap/section/ground contact.

[Randall]

I have replaced all filter caps previous to even powering it up. I have paid close attention to all grounds. Could it be possible I have a faulty new can cap?

[Helmholtz]

I have paid close attention to all grounds.

Did you verify zero Ohm to ground? If you use a (multisection) cap with negative on can don't forget the can to chassis contact.

Could it be possible I have a faulty new can cap?

Could be.
Please verify if you have pulsating voltage at least at one of the PS nodes.
And try paralleling a good cap between the PS nodes and a good ground.

[Jazz P Bass]

"Could it be possible I have a faulty new can cap?"
Yes.

[Enzo]

Could it be possible I have a faulty new can cap?

ANY part can be bad at ANY tome - new, old, in between.

[Randall]

Sigh. What an albatross this has become.

[Randall]

I have double checked my power supply grounds, and they good. I always draw a map before clipping any wires, so I am sure it is wired the way it came to me. One thing tho, ground wires from the power tube cathodes, standby switch (switches PT center tap), OT, isolated speaker jack and main board all go to the can cap ground lugs, but I don't see where it is connected to chassis. I know it is because I get ~.03 ohms between the lugs and chassis, when I always get.02 ohms between shorted probes. Should it be this way?

Another thing is as I have mentioned, using the line out jack as a temporary speaker jack wired to the yellow 8 ohm wire, This one is not isolated like the other one, it is getting it's ground thru the toothed washer. Would this be a problem?

[DrGonz78]

I get ~.03 ohms between the lugs and chassis, when I always get.02 ohms between shorted probes. Should it be this way?

Another thing is as I have mentioned, using the line out jack as a temporary speaker jack wired to the yellow 8 ohm wire, This one is not isolated like the other one, it is getting it's ground thru the toothed washer. Would this be a problem?

The ohms reading is fine .03, as you deduct the meter resistance. Did you test resistance from chassis ground to the soldered connection on the output jack? That will confirm your question if the toothed washer is making a good connection or not.