Model 201 with noisy HFMA2 Amplifier

[mauro]

Hey to Ron, Rob and all the good folks here,
I've got another pesky issue with an amplifier but I'm batting a thousand with the help of this forum!
I'm working on a 201 and have 2 amplifiers for it. Just finished rebuilding both and they sound great.
I'm testing them in the same juke using the same tubes.
First amp has no issues, second one makes a random scratchy noise 2-3 times during a song, it sounds like when the needle of a home record player misses the record and lands on the mat. Crrrrr, then goes away, but the music still is playing and audioable.
My first thought was the 2N109 transistor so I swapped it out for a new replacement but still the same result.
Only other difference in the amps is the offending one has the 30mF filter cap replaced with a 47mF and there is a 5.6 Meg resistor (installed previously) from pin 1 of the 6BJ6 to ground. I left it in as I was curious to see why it would be there.
If someone knows the probable cause of this or has some troubleshooting pointers that would be most welcome.
Thank you,
Mauro

[mauro]

The plot thickens,
I hadn't mentioned that at some point prior to receiving this jukebox someone has replaced the 1.5 HVS choke part number 305446-4 with a physically larger one mounted on top of the chassis. Part number on the replacement choke is 305615-3 and measures about 46 ohms across the leads. The original measures about 96 ohms across the leads (tested on the other amp).
I listened to them at full volume for the first time just now and the amp with the fault is definately lacking in gain as compared to the other amp.
Just thought I'd add this in as it may be significant.

Mauro
Mississauga, Ontario, Canada

[Ron Rich]

Hi Mauro,
Mostly above my pay grade, when one "re-injuneerz audio !-- One thing I have found often on the amps with the "plug in" transistor. The socket fails intermittently--so I now solder all transistor in on the "backside" of the socket, and close the front up with tape. Ron Rich

[mauro]

Ron,
Thanks for the suggestion, I soldered in the transistor but still same results.
I guess I'm going to order a Hammond replacement choke, decrease the value of C35 filter cap from 47 to 33mF and remove the 5.6m resistor from pin 1 of 6BJ6 to ground.
If that doesnt make a difference (and I doubt if it will) then I will just have to work my way through it slowly with suspicions at every component.
Thanks,
Mauro

[Ron Rich]

Mauro,
Change tubes--one at a time, first--may be a "bad" tube,or tube socket ?
Ron Rich

[mauro]

Solved!!
So I dragged out my signal generator, scope, signal tracer, etc and got it all set up. Played with it for hours and still couldn't get decent sound so I went back to the beginning and started checking my work and rechecking resistors. I came across one that had suspicious readings at the 27K resistor at pin 5 of V101 (12AX7), I clipped one side and found that it had increased in value substantially! I replaced it and I'm happy to report that the 201 is belting out the tunes beautifully again.

Ron,
Thank you for always being there to lend a hand and point us in the right direction,
Mauro