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! ....

NAD 3020 repairs

The famous George rang.

My NAD's developed a nasty hum, can you have a look?

Yeah, why not...

It's the very excellent NAD 3020.

A particular favourite of mine. These must have been one of the best selling amps throughout it's life...

(There's even a wikipedia article on it here.)

Now, many of these are getting a bit long in the tooth now.... and George's is no exception.

I switch it on, and there's some awful humming which just won't go away. Some of these amps suffer with a hum on switch on, which disappears after a few seconds, after frightening the user. We'll come back to that later.

Remove the screws on either side, slide the lid back slightly and lift it off...

One feature of this amp is the shelf at the back, where all the connections are located. Makes access easy if you're fishing around around the back. It's not without it's issues, however...

Turn the amplifier upside down, and remove the base plate (8 screws, short one's in the middle!) 

This is the underside of the connection "shelf" ... dry and broken joints are normal here, so solder the lot up...

There's some caps showing signs of their age, and have failed. I'm going to change the usual suspects..

... as well as the four main smoothing caps.

You'll need to pull the knobs off, and remove the four countersunk screws securing the front panel, and remove it ... be careful to support it, as we don't want to go to the hassle of removing the power meter's ribbon cable. The power LED will pull out.

Remove the screw securing the brace bar behind the front panel..

... and the other screw securing it on the back 

Two screws attaching the PCB to the brace bar, one through the heatsink.....

and the other towards the rear ...

You can now remove the brace bar to access the whole of the PCB. 

So after a quick cap change (there's 4 2,200 uF 63V caps, two 1,000 uF 6.3V caps (I fit 16V types here , I don't trust those 6.3V elctrolytics!) and two 330uF 63V caps...) 

(The other 330uF 63V is hiding!)

..things are much improved, but there's another fault... I mentioned it earlier. When you initially switch the amp on, there's a very unpleasant hum which decays, leaving the amplifier performing ok.. it's like a "whomp" as it turns on. It's a very common fault...

It's caused by this capacitor (and sometime's its neighbour too). I change them both for 47uF 63v types... whomp gone!

Bias is checked , and is spot on. Just got to button it up, and give it a good testing with "The Lasters" by Fred Deakin.

The guilty parties... 

Rockit 6 RPG2 repair

Mike brought me a speaker in.... 

"This belongs to my mate, he says it's crackling, can you have a look?"

Yeah ... why not?

It's a Rockit monitor. 

Sure enough it's popping and crackling ... off with those 8 screws on the edge of the back panel, and carefully remove...

Usual idea, amplifiers (in this case two, one for the tweeter, one for the sub) and transformer/ active cross over all built onto the back panel...

The fault is immediately obvious... the dreaded black goo ... This is some sort of adhesive applied liberally to the pcb to stop the capacitors and other large components "falling out" because they don't trust their soldering (this is a somewhat sarcastic guess, but I fail to see any real reason for it!)  It has an issue. After a time, if it's got hot, it can start to conduct, causing undesired operation of the device. I see it quite often.. for example here.

I can see it's got very dry and crusty (and most likely conductive) in a couple of places. Most noticably here ... we'll have to clean it all off to make a reliable repair. The dry stuff mostly chips off.... 

The rest needs a soak in a bit of acetone (or nail varnish remover), I add a bit to bits of kitchen roll, and allow it to soak in for 30 mins, and the goo just wipes off... 

Get as much off as possible. 

... and reassemble. Simple fix. 

I warned Mike the other one would need doing as well, and cleaned that one up too...

BMW Diversity antenna amplifier.

On old school pal, a once estate agent and daytime telly presenter, now a famous comedian (so we'll let him off for the first couple of misdemeanors) dropped me a message on Facebook....

"I've followed some of your repairs with astonishment and very little understanding.

Do you think you could look at something for me?"

Yeah .. why not ..

It's a diversity amplifier from a BMW E61 Tourer. 

They're apparently a really common fault on the E61 tourer, as they're situated in the tailgate, and suffer with water ingress, and Nick confirms this one was full of water when removed, and he'd left it to dry out on a radiator for a few days...

I expect it to be full of corrosion ... 

The plastic case just unclips, and reveals the PCB. 

Which is surprisingly clean and lacking in corrosion... 

So. The plan is to first work out what on earth this thing is supposed to be doing. The faults are the remote locking wasn't working, and the radio would reset to Radio 2 with the bass and treble set to maximum whenever the ignition key was removed... hmmm.

So, a spot of reverse engineering is required. 

The antennas (there are two) seem to feed into the red and black sockets on the top edge of the PCB. I suspect these may well be "hidden" antennas in the glass... The signal is filtered and is sent to a TEA6101 diversity switch. This analyses the signal from each antenna, and sends the strongest one on to the receivers. One signal is sent to a conventional TDA1576 FM demodulator, and on out to an amplifier and off-board via the molded socket. 

The second half goes to a TDA5210 UHF receiver/decoder.. what's the betting that's our remote lock/unlock ? Output from that feeds the IC under the heatsink clamp... 

There's also a small switching supply for +5V, and some amplification and filtering of RF along the way. 

Working out where the +12V power goes to the board, brings up the +5V supply... good. Poking round the data lines with an oscilloscope shows no activity ... but is it expecting data from the car? 

A careful visual inspection of the board under the microscope shows no corrosion. Time to remove the heatsink clamp...

I thought this may be hiding some sort of power amplifier, but no, it's a microcontroller. There's also some corrosion around it's 6MHz ceramic oscillator.... Scoping up around the microcontroller shows no activity ... 

Removing the oscillator shows up the corrosion.. See that black line between the two of those long pads? 

It's duly cleaned up ... 

The oscillator put back into place after checking the bottom of it is clean.... It's soldered back in with the hot air tool.

When I removed the oscillator, I also removed a small decoupling capacitor by accident (there can be some collateral damage with a hot air tool!) ... thankfully I didn't lose it, and this is hand soldered back in place.

I don't think the heatsink clamp is actually anything of the sort. I think it's an RF shield. Anyway it's replaced. Powering up now shows some activity on several of the microcontroller's pins..

 So, it's time to reassemble it, and send it back to Nick to see if it works or not ... fingers crossed.

**** UPDATE **** 

 It worked :)