AB763 Bandmaster Sounds Fizzy and Ratty

[Jake1524]

(MORE INFO A FEW POSTS DOWN)

Hi All,



Starting to troubleshoot my 1968 Fender Bandmaster. The amp seems to function fine at lower volumes, but when hit around 5 on the volume knob, I start to get this god awful fizzy, ratty, almost vibration like distortion. It almost seems that is independent of the core note/notes I am playing.



The amp has been recapped, the pots have been cleaned, etc and this is still happening. My guess is the issue lies after the preamps as this is happening on both the channels, albeit a little worst on the Vibrato channel.



Where would you start with something like this? I will check voltages when I get home later, but would love any insight or help that will get me pointed in the right direction with this.

[bepone]

in short? literally anything can cause this, every component, wire, socket, tube, solder point, capacitor, all
you need to start to check all

[romberg]

Is it a "bandmaster reverb"? If so, I have a theory on what you are hearing.

Mike

[pdf64]

It’s not the reverb version https://www.thegearpage.net/board/index ... t-36864414

[bepone]

i would measure all cathode voltages to see where is the problem.. and that feedback wire over everything is just wrong

[Jake1524]

i would measure all cathode voltages to see where is the problem.. and that feedback wire over everything is just wrong

My fault.. let me follow up with a bit more information!

I have attached a gut shot of the amp, voltages as well a sound clip.

So far I have:

-Pulled the entire board to make sure nothing was shorted underneath
-Checked every resistor value
-Double checked and reflowed every solder connection, ground etc.
-Chopsticked all connections - nothing obvious here
-Performed Lead dress

Worth noting:

-The amp has an aftermarket Allen TM40 OT installed
-Mid Pots added to both channels

Going to address a couple of the lead dress issues when I get home later, but the amp is still behaving improperly.
Not sure where to go from here!

Audio link: https://soundcloud.com/user-47032081/ba ... al_sharing

[Reeltarded]

Have you checked for stray voltage on board around plates and such? Fishpaper breathes in moisture and can create havoc with no visible cause.

Your description is apt. Sounds like facey dirt riding on the clean signal.. hmm... thinking.

[Reeltarded]

Ok. I have a plan. Check all PI and output resistors for drift.

Do you have a can of compressed air? Have yoi ever done a freeze test?

[Jake1524]

Ok. I have a plan. Check all PI and output resistors for drift.

Do you have a can of compressed air? Have yoi ever done a freeze test?

Hmm, i have checked all resistors for drift. Nothing seemed more than 5% of at a max... I don't think enough to cause this issue(though I may be wrong. My tube amp knowledge is very rudimentary).. Any specific culprits I should double check this evening?

Have not done a freeze test yet, but open to trying anything!

I also have not checked board for stray voltages... do I just probe around on the board and see if I get any measurements?

[bepone]

technically this is not sounding so bad.. i have one 5E3 which sounds worst than that and still you can manage great sounds from it

lets move this nfb wire, to the right side of phase inverter tube, not left

[bepone]

one pic with better light , with great detail, pls

[Jake1524]

technically this is not sounding so bad.. i have one 5E3 which sounds worst than that and still you can manage great sounds from it

lets move this nfb wire, to the right side of phase inverter tube, not left

I will move the wire and post some better pictures this evening.

Thanks for all your help guys!!

[Reeltarded]

For stray voltage check you ground the meter and probe the board at short distances from eyelets especially between plates and cathodes. That can be trouble.

bepone is the master of board restorations. ^^^^^^

[bepone]

another UP just under total re-assembly and boards and wires after 50 years ARE a problem just working on it now

[Jake1524]

For stray voltage check you ground the meter and probe the board at short distances from eyelets especially between plates and cathodes. That can be trouble.

bepone is the master of board restorations. ^^^^^^

Ok so i have some updates. I have a feeling you guys are on to something with this.. poking around on the board yielded some spots as high of 150mv. Attached pics to make sure im not being a total idiot hah.