Fender PA 100 Noisy Mixer Circuit

[Smitty]

The mixer circuit in a Fender PA 100 contributes a lot of noise. Even with the four volume controls turned all the way down, setting the master above three results in audible noise and hash. Setting on ten makes the amp unusable. After bypassing the mixer (which necessitated disabling channels two through four) and wiring V5A grid directly to the wiper of the first channel volume control, I was struck by how quiet the amp became. Even with the first channel turned up.

Has anyone successfully tamed this mixer? Four 270K resistors grounded through the volume controls. See attachment:

PA 100 Mixer.JPG

Thanks, in advance...

[B Ingram]

... After bypassing the mixer ... I was struck by how quiet the amp became. ...

Has anyone successfully tamed this mixer? Four 270K resistors grounded through the volume controls.

The input jacks appear to be Switchcraft 13E jacks. But we only see that with a peek at the whole schematic. Funny, but someone was just asking about those on another forum.

The switched-contact of these jacks are being used to mute unused channels, so we should first look to ensure each of those is scrupulously clean. Much of your noise issue could be one or more of those jacks failing to ground the unused-channel(s) and the associated 270kΩ resistor(s).
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Separately, resistors are sometimes noisy. More so if they have DC Volts across them, as shown by the "Noise Test" of this Signal Tracer (which applies about 130vdc across the leads of the test probe).

I'm not saying you have DC Volts across any of your resistors, but the individual resistors could be noisy.

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I would first clean all the input jacks, because they probably all need it. If noise remains, an alligator-clip jumper wire can be used to ground the wiper of each Volume pot. Disconnect only 1 jumper from a Volume-pot wiper at a time, so you know you're hearing the noise-contribution of only that one channel. Rinse & repeat for each channel, so you know whether one is more-noisy than others.

It could be that one channel's plate load resistor is noisy with applied DC Volts. Maybe you swap just that one channel's plate resistor.
By now, perhaps you see why some techs just "shotgun" stuff by replacing a bunch of parts of a given category. They're trying to save their customer labor-cost, which greatly exceeds the parts-cost of the replaced items. But if you're repairing your own gear, you get to decide whether you swap many thing, or find/replace only the bad one.

[Smitty]

Thanks for your advice. I'll take a look...

In my original post, I noted that the noise is present with all of the individual channel volumes turned all the way down. This results in all four of the 270K resistors being grounded. In this scenario how would noise from the input jacks find their way to the V5A grid?

Although I didn't mention it in my original post, as I was diagnosing the noise, I pulled V1 and V2 to isolate the problem to the channel controls and mixer network.

[B Ingram]

In my original post, I noted that the noise is present with all of the individual channel volumes turned all the way down. This results in all four of the 270K resistors being grounded. In this scenario how would noise from the input jacks find their way to the V5A grid

If "everything works like it should," then there would be no noise from the jacks, only the 270kΩ resistors. Of course, if everything worked like it should, then you wouldn't be chasing noise. '

Two items that comes to mind & could contribute to noise:

• Volume pot wiper lifted from the resistive track. Setting of the Volume pot never grounds the channel output; this should also be a "dead channel" when tested by itself (because there's no connection from the Input of the pot to the Output.

list]Volume pot has a missing or high-resistance ground. If the grounded-lug of the pot is soldered to its case, the ground connection relies on clean contact from the pot body to chassis (perhaps via a brass plate). Decades on, there is often corrosion between the brass plate & the chassis, such that some folks remove all the knobs/pots and clean the chassis metal & both faces of the brass plate.[/list]

Regardless, using the alligator-clip jumper wires should help reduce the number of parts in the circuit (and so variables to consider). Alternatively, you could unsolder one leg of the 270kΩ resistors and add them back one-at-a-time to verify quiet-resistor/quiet-channel.

[Smitty]

Thanks for your suggestions.

Before initial troubleshooting, I did some disassembly for cleaning which included removing the faceplate and knobs. Reinstallation included cranking down all the nuts for the pots and jacks. This is a very clean example. No real corrosion anywhere to be found. Brass plate looks clean.

The first thing I tried was grounding the grid lead at the bottom of the mixer which resulted in near silence. The 270K resistors run from the volume pot wiper to a three terminal standoff soldered to each volume pot. So ground is made through the pot case. From the top of the resistor, it's about a tenth of an ohm measured with a Fluke 177. Who knows what it really is. I don't have a low ohm meter.

I'll remind you that single channel operation is very quiet with the channel volume all the way up. So 250K above ground. The four 270K resistors in parallel look like 68K to the grid with all the pots turned down. Noise at this setting is what I'm chasing. I suspect the leads are picking up power supply hash. At least that's what it looks like on the scope.

Next time I have this on the bench I'm going to try swapping in some coax. I was hoping someone had done this before with success, or had figured out the fix to what seems more like a design flaw and less like a service issue.

[Stevem]

Don’t waste your time with shielded wire as that will not help with hiss.

Your issue is resistor(s) based.