• 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.

Roberts/Akai M series amp problems

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wboyd

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Joined 2004
#1
I recently acquired a set of Roberts/Akai 6BQ5/EL84 amps from an old worn out reel to reel machine (M5 or M7?). I would like to use these amps as is, as mono blocks.

After making a few minor adjustments (wire hacking and tracing the input), I have the right amp sounding great (it’s almost dead quiet).

My problem seems to be the left side amp. It’s very loud and noisy. I have schematics and a parts list for the amp, but unfortunately I am not an electronics guru and really don’t know a whole lot about the amp’s topology. Currently I suspect either caps or tubes. I have been looking for suitable replacement caps, but am not having any luck with finding the correct values needed. Can someone help me source them?

Also, how can I test the caps with my multi-meter, I was not sure if they should be tested while installed (while I assume they should be under load). I don’t want to blow anything up, or kill myself for that matter.

Or if anyone has any experience with these amps, you input is greatly appreciated.

Thanks,

Wayne
 
#2
You can't test the caps in-circuit - and you really can't make a valid test with a DMM anyway - the problem with old caps is leakage at high voltage, and they're probably bad... a voltage test in-circuit would tell you, but if they're REALLY bad, you'd be risking the output transformer and power transformer. Do they have AC line fuses?

Tubes can be swapped between amps to check them.
 
#3
Tom,

Thanks for the reply. I am slowly inching along as far as tube equipment goes and every little piece of info helps. Are there other things on the amp that I can test, such as filament or plate voltage? Will this help me diagnose problems as well? And what about measuring the output…can that be done with a DMM?

Also, I have been doing some research on capacitors, but I am confused as to what are suitable replacements for that old, leaky caps. I would eventually like to replace all caps on both amps. I am trying to source them, but have not been able to find exact replacements for the ones used. What type of tolerances are acceptable as far as uf, pf and voltages as well and do they have to be exact?

Thanks,

Wayne Boyd
 
#4
Polypropylene are the best, thought polyester (Mylar) are smaller, and might be the only ones that fit. Mouser.com and Digikey.com have some, haven't looked lately.

I mostly work on older radios, and I get the Mylar caps from Justradio.com. 20% is fine, 630V are still small, and will work anywhere.
 
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