Heathkit MA-12 Hifi amplifier restoration.

Neil called ...

"Got a load of old 'eathkit gear here , could use the once-over... Care to have a look?"

Yeah... why not?

There's four MA-12 amplifiers, two USC-1 "Stereo control units" (that's a pre-amp to you and I), and an AMF1 tuner ...

The amps are in various states of repair ...

Including one with a valve down to air ...

It's a simple push-pull amp, with an EF86 front end, an ECC83 (12AX7) phase splitter and a pair of EL84's wired ultra-linear. Hmmm , we've seen that before, and looking at the schematic, it's a Mullard 5-10 in disguise..

Off with the bottom ... 

Nice tag-strip construction, and, as these were sold as kits, the assembly looks OK.. some of these can be a disaster area.

Obvious faults... it's full of Hunts capacitors (evict on sight) , and the electrolytics are, well.. past it.

First things first, get that can out, and re-stuff it with nice modern parts ....

The cap is simply removed by gently prying it out with a screwdriver, and saving it.

Next drop the whole can into a container of boiling water for about 15 mins...

Wind a screw into the capacitor's innards ... 

and extract! Probably best wearing some gloves whilst you're doing this bit, I don't know what chemicals are in there ... 

This can is a dual 250uF, 60uF , 350V device. I've got a modern 220uF and a 68uF to go in there, so this is assembled and wired... 

.... and slid back into the can. The cap is sealed on with a little RTV.

The other can is a dual 50+50uF. As it's rubber cap is visible, we want to leave that intact. 

So the bottom bracket is warmed up slightly, and pried off.. 

and the bottom but off with a small pipe cutter... 

It's stuffed with two 47uF caps. They're slightly different shapes so they fit inside the can. 

The base is then sealed up with a splodge of hot-melt. 

The Hunt's are evicted and replaced with some nice polys. 

The two cathode bypass electrolytics (22uF, 25V) are replaced.

Testing shows the output is weak (our valves may be old), and unacceptable distortion...

Attention is turned to the output stage ... 

DC conditions on the two valves should be closely matched... they're not.

The two cathode resistors R21 and R22 have risen significantly in value, one to more that 3 times it's value, they're swapped out. R23 and R24, the screen gird resistors, are also out of spec. Changing this lot puts matters right. 

Now to do the other three! ....