UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Heathkit MA-12 stereo pair of amplifiers

snowman_al · 25th Jul 2019, 5:57 pm

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.

25th Jul 2019, 5:57 pm	#3
snowman_al Octode Join Date: Dec 2018 Location: Llandeilo, West Wales, UK. Posts: 1,092	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. Attached Thumbnails __________________ Never Leave Well Enough Alone...