Funny Sound in BFSR

TheAmpNerd

Member
Messages
1,056
I've been tracking this down for the last couple of weeks.

An intermittant sh*it sound coming from this vintage amp.
You'd play then after awhile it would get all weak and distorted.

SO

After taking my time and undoing all kinds of mods done to the amp
it still did this...intermittantly.

AND

Replacing tubes and finally choke and OT it sounded fine for a bit.


THEN

Got to the sh*tty intermittant sound again.
It sounded like spitting....And I kept thinking
some sort of short somewhere in the amp.

FINALLY

I found it. I took the amp back to the bench when it
started makeing the spit tone again. I started sticking
it with one of those STICKs and get what I figured out?

On the B+ from the standby switch,
it had been arching to the middle board
of the amp. The middle board is the board
that insulate the main eyelet board from the
chassis.

When I lifted the eyelet board and pulled the
middle board out from under the eyelet board,
the middle board had about a silver dollar sized
circle of carbon on it. Right where it would cause
both channels to should like sh*t...underneith
the final coupling caps and at all the b+ junctions.

Three other techs had gone through this amp
and tried to fix it. None did. They had tried
replacing trannies, wiring, and components.

I got lucky and happend to start poking the
eyelet board when it started making noise again.
It happened over in the PI area of the board,
so as I tapped it, it started arching.

A lot of times you have to be patient
and wait for the problem to crop up again.
 

brad347

Member
Messages
1,260
i commend you. I would've thrown the amp off a third floor balcony LONG before that point came.
 

mbratch

Member
Messages
2,381
That's an interesting discovery. Maybe time for me to get under the eyelet board on my amp.
 

hasserl

Member
Messages
4,711
Intermittent problems can be very hard to find, and very frustrating. It feels good to figure one out and correct it.
 

TheAmpNerd

Member
Messages
1,056
donnyjaguar said:
I live for these types of repairs. The ones the other blokes can't find. :)
They are only great when you find them.

Until that time, they are an utter PITA (Pain In the Arsh).


There was another one earlier in the year. It was a
1964 or 1965 Princeton Reverb. It had some rather
unusual former "tech" problems, including:

One of the resistors in the PI had a cold solder joint,
screwing up the tone.

Who ever replaced the output tranny ran the green wire
right over the speaker jacks ground bus wire...(it arched)

A replacement cap (thinking it was the .02 right by the
reverb B+) it's lead was overly shuved into the eyelet,
thereby shorting it with the reverb B+.
Another lift the board and see what is going on
underneith it.

A replacement three foot power cord was cold soldered.

And finally, the reverb pot carbon trace was crumbling internally.
you'd clean it out really well and it would work fine until you took
the amp out to gig and a new piece would break off again and get
caught in the wiper.
 




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