• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Newbie needs Magnavox console troubleshooting tips

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I have an old Mag.6bq5 amp that works but has a problem.when I brought it up on my variac it started to make a high pitched whining noise that changed in frequency,then it started to make a low pitched pumping noise.t 60% it sounded good through my headphones.When i cranked it up to about 70% the noise started and I shut it down.
I would like to use this amp as learning project.I have a couple more 6bq5 vintge amps that I would like to restore and maybe modify.I will post some pics....thanks Robert.


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Does the problem noise show up on one or both channels?
I don't think that I'd use headphones to check out a power amplifier that has problems. Use an old, cheap (read: expendable) speaker if you don't have a resistive dummy load. Humans only are issued one pair of ears per person!
Sams has Magn chassis numbers listed under AMP9304-00. This is as close as they get and is probably very similar.

Sams 586-8 year 1962. Unless you want to buy it from Sams your best bet is a local city library.

I have a similar chassis (8701-00) but not the big OTs. 6EU7s are expensive. They are 12AX7s with a different (supposedly better noise figure) pinout. Probably has a 6CA4 rectifier, not cheap. The tubes are probably OK since it is early 60s. They started using import tubes in later years.

First, you need to get the old caps out of it and check the resistors for tolerance. I can see two paper caps in the picture. They have to go.

I had my similar chassis (overhauled completely) in the garage at one time and it began to squeel one day. I recall tapping the 6EU7s and it was the first one. Seems like it was a dirty socket terminal, not sure.

The low pitched pumping noise is called motorboating. It is caused by bad PS filter caps. Get with Mouser or some large distributor and get some new filter caps. Don't use surplus caps, get brand new ones.

It needs a complete overhaul.

Also, look at the hole under the PT where the wires come out. If they have black tar soaked in them then the PT has been hot enough to melt the potting compound. If they get too hot the windings can fuse and the voltage will go high or low depending on the damage. This is caused by operating it with leaking filter caps. The tube rectifier will restrict the current, but the PT does get very hot. Good Luck, Mark
Thanks Mark,that is he info I have been looking for.I do happen to have some Illinois capacitors in stock.The values are .047k-630,the ones in there now read mvd .047-600vdc.Would these make a good substitute.?The filter caps are in the tall can on top,right?I will try to get the shematic and get the values of the filter caps.thanks Robert
Yes, the metal can has 3 or 4 caps in it. Take a lot of pictures of it before you start. Lots of closeups so you can see detail, may come in handy.

Get a magnifier and look at the bottom of the metal can. You will see some symbols.......triangles, squares, etc. Then match these up with the symbols on the side of the can to get the capacitor values including voltages.

The 630V is fine for the 600V. The higher the better. Take note of the stripe on one end of the axial caps. This is the outer foil and if the caps you replace them with have such a stripe (some do-----it doesn't matter if there is no stripe) then put it in the same position. This sends hum to ground. Only certain types of film/foil caps have this.

If you have never done this before, you should practice soldering. Soldering is not glueing. It is flowing molten metal to make a bond. It takes some practice.

Also, the potentiometer on the chassis is a screwdriver-adjust hum control. Turn it for the least hum. Mark
The Sams folder is online at the Yahoo group "Magnavoxfriends". Join and grab it!.

This one shows a 5U4 rectifier, but they also made versions with a smaller transformer and 6CA4 rectifier for units without tuners. If the balance pot isn't installed, you need to add a pair of 390 Ohm resistors in its place to provide the cathode returns for the first stage amplifiers.
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