Polytone minibrute IV humming

[JeffGGG]

I own a couple of older MB IVs. One of them is kind of quiet as long as you don't turn on the reverb and the other one hums quite noticeably as you turn up the volume pass 3 and especially if you turn the reverb on or set it to the BRIGHT switch setting.

My question to the Polytone gurus is, when faced with this type of situation are there standard components you would replace that would typically quiet the hum? Can someone give me a list of here are the most typical components that should be replaced to quiet these guys down?

Thanks for any and all comments.

[52 Bill]

Welcome to the place. The two things to check, are the reverb tank mounting and the preamp chips.

Be sure that the output side of the tank is mounted as far away from the power transformer as possible. I know that this is impossibly close quarters, but if you are sure the output transducer in the tank is as far away from the transformer you will get the least amount of hum from reverb as possible.

The preamp hum could be caused by a shorted IC. First check to see that the chips are seated in the sockets, by pressing them in. Do any of them feel warmer than the others? Also make sure that all of the cables that connect the two chassis' together are fully seated and dressed away from the speaker.

[Megastar]

Hi I have had a couple of these amps in for repair I have all the schematics for old and new I even have a mains trasforfer for the pa378 if anybody wants one.
But I do need some help as I have just had mini brute 3 in without a reverb tray and the schmatics dont say which tray it is can anyone send me the long number that describes the impedences thanks Dave.

[Megastar]

Also I would like to add these amps are an absolute nightmare to get exact they are getting old now so all of the caps should be changed.
I have replaced the pa 378 with a G28 mosfet module 150 watt and it is a far better amp than the stock one.
saves alot of of time and hassle.

[Alex R]

...a G28 mosfet module 150 watt...

Could you say a little more - where are these available?

[rf7]

But I do need some help as I have just had mini brute 3 in without a reverb tray and the schmatics dont say which tray it is can anyone send me the long number that describes the impedences thanks Dave.

The tank measures 30 ohms on the input and 220 ohms on the output with my meter.