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Old 23rd Jul 2019, 6:20 pm   #1
snowman_al
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Default Heathkit MA-12 stereo pair of amplifiers

This story started 12 months or so ago when I acquired a nice example for a very good price.
No valves but in very good cosmetic condition overall.

It had been nicely built originally and someone had repaired it in the past, with replacement cathode resistors (5 watters!) and EL84 coupling caps. The rest was original - nice black Hunts caps and bulging reservoir electrolytic...
I guess it just faded away and was replaced rather than repaired again?

So it was a fairly easy restoration trying to keep as much originality as I went. Re-stuff the big 60 + 250uF can, replace the 0.1uF Hunts caps with Russian pio caps (the 0.47uF Moldseal caps measured impeccably so were left), replace the cathode bypass caps and swap out a couple of out of spec resistors etc. The 32 + 32uF can was also left as it measured well and reformed nicely. The coax input socket was dismantled and an RCA socket grafted in its place.

Worked first time and all was well, so one down and I was now looking out for another to keep it company.

Pictures are as it arrived 1 and 2, then first power up 3 and 4.
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Old 24th Jul 2019, 7:57 am   #2
snowman_al
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Default Re: Heathkit MA-12 stereo pair of amplifiers

So late November another MA-12 comes up for auction. It looked a little 'scabby' in the pictures and obviously had seen some 'repairs', but did have some valves left.
I nearly did not bid thinking another would come along soon. (As it happens I have not seen another 'single' since then.) But in the end I did bid and had to pay slightly more than the first one went for.

I was rather disappointed when it did arrive.
It was not as well made as the first one and had seen several repairs. All the coupling caps had been replaced, as were the cathode bypass caps and one resistor. The large 60 + 250uF cap had gone, replaced with a 32uF dangler and 400uF can.
Worse though was the general rusty state that had not been evident from its pictures. Only one thing to do, strip it and rebuild it.
So I set about cleaning the chassis, treating the rust spots, cleaning or replacing the rusty hardware and checking the components.

Then I discovered why this one had stopped working, the output transformer was high resistance on one primary winding.

First 3 pictures are as it arrived. 4th is chassis stripped and 5th is cosmetic re-assembly (new valve holders, screw heads polished, coax socket replaced, etc.
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Old 25th Jul 2019, 5:57 pm   #3
snowman_al
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Default Re: Heathkit MA-12 stereo pair of amplifiers

Having discovered the faulty output transformer, the whole project had to be put on hold for a few months, for Christmas and family reasons.
This gave me time to decide how to move it forward. I wanted to keep them as original as possible, within reason, so 2 new output transformer (one for each) was a last resort.

In May I was able to get started again and Terry (Valvepower) donated the shell of a dead 60 + 250uF capacitor to the build. Now with no more responses to a replacement transformer, it was time to bite the bullet and have Ed Dinning rewind my faulty one.

Well that turned up on Monday and it was not long before I had a pair of working MA-12s playing tunes for the first time in Stereo.

So a success for about 40 minutes, when the first amplifier started making light popping and rustling noises... Swap the EF86 and ECC83s round, no difference. Ok must be the carbon anode resistor of the EF86. No a new metal film made no difference. Change the phase network resistor then, no not that, but leaving it disconnected 'cured' the problem. So the 510pF domino cap is faulty... reconnecting it brings the popping back once the chassis is at temperature or 30 odd minutes playing time. So very nearly there.

Many thanks to everyone will has helped, especially Terry and Ed.
Alan

For information, the black caps are Russian PIO 0.1uF caps and the 0.47uF
caps are 'yellow' poly caps wrapped in masking tape and painted grey to match the Moldseals. At the moment all the valves are Russian too.

Pictures speak for themselves. The last picture is their first time in Stereo.
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Old 25th Jul 2019, 7:31 pm   #4
Ed_Dinning
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Default Re: Heathkit MA-12 stereo pair of amplifiers

Hi Al, hopefully it is finally beaten and will not come back to bite you !

Ed
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