Heathkit MA12 project

[Whaam68]

Over the last couple of years or so I was lucky enough to pick up a couple of basket case MA12’s fairly cheaply from different sellers on gumtree that looked like loft/shed finds. Luckily most of the valves were present but one baseplate is AWOL Which I’ll have to make. I finally got round to working on the first of these a couple of weeks ago. I like Heathkits like these as the documentation is so good which helps for the theoretically challenged like me!

I eventually figured out from the schematic that if not using the “control amp” I’d have to bridge between pins 4&5 on the pre amp accessory socket to get it to power up. On a 15 ohm dummy load the lamp limiter bulb dimmed but didn’t quite go out which didn’t surprise me (in real life it’s much dimmer than in the pic) What was a bit concerning was once I took the lamp out of circuit the main smoothing cans (which are new production) were thrumming audibly....and vibrating (but not heating up...I checked). I put the lamp back in circuit and left it to soak for another 45 minutes. After this all seems normal (I have yet to check voltages but will). Is it likely this was just caps that presumably been in storage reforming?

A lot more still to do on these as the wiring on the 2nd amp is a bit of a shocker!
Mike

[PJL]

You are talking about the large F&T caps? I would have thought it more likely the noise was originating from the mains or output transformer. Check the voltages are all OK and it is not taking excessive current as instability might cause this to happen.

[Whaam68]

Hi. It definitely seemed like the large above chassis cans I could feel a vibration to the touch but only on these caps. Mains transformer and output seemed OK. But yes next job is checking voltages.

[Whaam68]

Just to update this project I replaced the paper caps with new production film caps on the amp I completed which resolved the power supply issues. I then applied the gain reducing mod mentioned in the hifi world Heathkit amplifier article replacing the ecc83 with an ecc82 http://www.audio4maniacs.co.uk/article-heathkit.html
With reference to the schematic attached earlier in the thread the gain mods were replacing r9 with a shorting link, changing r10 to 22k and r13 to 100k matching r14....both feeding the anode of the ecc82. Anyhow it seemed to work well on the one amp I completed playing a line level source from my squeezebox. I do have a recollection from an article I read some time ago about the Mullard 5-10 circuit that there was a specific reason r13 and r14 weren’t exactly the same value but I emailed the author of the article who confirmed they should both be 100k. I’d be interested if anyone has any insight on that.

It turned out the second amp was in much worse shape than I thought with some serious rust. Lifting the grommets the metal was flaking away. I’ve had to strip it down completely and re spray the chassis. A much bigger job than intended but it’s looking good. Problem is I’m going to end up with one new looking amp and one slightly tatty one...hey ho! The first two pics are the one I’m rebuilding from scratch and the last pic is the tatty amp now with film caps.

[Whaam68]

Still moving slowly forward with this ....finally got the amp I stripped down to a chassis completed....powered it up on the lamp limiter after carefully working through the Heathkit build manual which was a great experience in itself...why don’t they still have such well documented kits! (Ok I know back in the day these cost an arm & a leg!) anyway all seems fine. Still to do are fabricate a baseplate as I only have one for two amps and re build the matching preamp. Here’s a before and after.