Leak Stereo 20 Repairs and restoration.

Next up, Ian's Stereo 20

It's a slightly later "chocolate" coloured version (earlier ones where gold) and develops 20 watts from a pair of EL84's (6BQ5) driven from an all ECC-83 (12AX7) layout of preamp and phase-splitters. Power is provided from a GZ32 rectifier.

Dating from 1962, it's nicely laid out, and easy to work on. Looks like it's had some repairs in it's life, especially to the left hand channel (top in this picture) ...

Every capacitor is changed out for new, even though the two cathode bypass capacitors in the pre-amp had been changed some time in '97 accroding to the date codes. The main coupling capacitors were dated 1979. 

A good number of resistors were outside of tolerance, or not to the schematic, and were replaced. If something was out on the left channel (of which there were many), the corresponding part was replaced on the right, so as to avoid upsetting the balance between channels. 

The main cans are re-stuffed with modern 32uF components, and are mounted back using a 3D printed plastic cap.

Whilst the actual foil and (dried out) electrolytic matting were very easy to remove, not so the black sealing goo, which I presume is some kind of bitumen. Leaving the empty cans in the freezer for a few hours (whilst Mrs. Doz was absent!) hardened it, and it was easy to chip off. 

The modern components were a tight fit.

Finished of with a new Cap-cap TM ;)

You can get the files from https://www.thingiverse.com/thing:4872632

Once everything is done, the unit is powered up, and gives reasonable results, albeit down on power. HT is good, but one side has mismatched valves, and the other side, well, they are just tired. Ian told me that he'd borrowed a valve out of his other amp, as one had gone to air. It really does deserve a new set of output valves. 

The guilty parties...

Linn Keltik repairs

As well as sending me a lingo (see here), John sent down a Klout amplifier... It had the same issues as this one, only was completely dead. A re-cap fixed it. 

It was also fitted with the Keltik Bass extension module. This is simply a low pass filter to allow the amplifier to drive a woofer as part of an active cross-over system. 

It's really nicely made, and was given a re-cap to ensure reliable service.

The Keltik module is mounted on a single metal spacer which bolts up to one of the four transistor heat spreader cap screws. Remove the spacer before attempting to remove the cap screw, or the whole thing locks up. The other end is supported in a groove on the speaker terminal output insulator.

There's a single connector to remove on the board.

We'll need to remove that screening can, which is simply desoldered. 

Same the other side...

.. to gain access to those hidden electrolytics.

Look at all those precision poly caps... nice!

You'll need 2 off 220uF 16V , 8 off 22uF 50V, 2 off 2.2uF 50V and 1 off 10uF 63V . 

They needed replacement, some were physically leaking, and smelt horrible during desoldering! 

Another saved from landfill! 

Linn Lingo repair - won't change speed.

John from the cold north rang...

"Got a Lingo here, it's been serviced by someone here, but it's now stuck in 33 RPM. Care to take a look?"

Yeah - why not?

It's the first version of the lingo.

Off with the lid, and I can see some work's already been done. It's been recapped. The workmanship is good... or is it?

The original repairer was obviously proud of his work, as there's a signed sticker inside the unit. I've blurred this out to prevent embarrassment. 

Now the lingo is similar to other power supplies of this era from linn, they suffer with capacitor failure. Now you could upgrade your supply to the latest and greatest (don't forget kids, Linn ownership is all about upgrades, right?), or get it repaired. This has all new capacitors, of quality brand, so it should be good. It was only done in August last year (it's now January). Apparently 45 RPM hasn't worked since they had it back... 

So it's just stuck in 33. Pushing and holding the switch just causes the Red led to go from bright to dim repeatedly, and never get to 45. Great ... it's usually either U7 or U8, best to change both to avoid issues. It's a 74LS74. The best way to get them out is to cut off the legs, and remove each leg separately to avoid damaging the (as usual) fragile double-sided print. I get two new IC's and socket them. Powering back up, and switch to 45 , it briefly lights the green 45 RPM light, before dropping back to the original fault. Damn. 

Thankfully having socketed the IC's, I can remove them without damaging them, and test them in the sometimes useful DiagnoSYS IC tester (thanks Norm).

One is duff... double damn... was it a duff one out of the packet? I replace it, and promptly blow another. Triple damn. What's going on?

There's a bit of feedback from the motor drive amplifier on the lingo, so it knows when to turn the voltage down to the motor. This feeds U4, a 74LS221. It's removed , and a socket fitted. A new chip changes the fault slightly. Sadly the DiagnoSYS tester can't do 74LS221's..  Checking U7 again shows it hasn't failed again, so that's something. Out with the scope. There should be a ~5 second timing constant. Checking around, I find it. It's on pin 6 of U4... and is formed by C7 (220uF) and and R33 (56K) (those of you who have just got the calculator out, will inform me that that's 12 seconds, and yes, you're right, but it triggers the IC as it moves over the trigger threshold, of around half of the supply). The cap is obviously brand new, but is removed anyway and tested. It tests fine. But it's only 22uF. C7 and C9 have been reversed!! (now you know why I blurred the label!).

It's reassembled again, and this time works fine! 

"The person that never made a mistake, never made anything" 

Ahem.. 

Heathkit Stereo control unit USC-1

Happy New Year! 

Having sorted the MA-12's, time to move onto to the USC-1 preamp...

It's designed to be mounted into the front of a cabinet of some description. 

They keen-eyed amongst you may notice the absence of mains transformer. The power supply is derived from the MA12 amplifier via a multi-pin lead. 

It's a reasonable layout, but the chassis edges are sharp. There's two PCBs, each containing Two EF86's and an ECC83. The EF86's produce two diferent levels of gain, one setting for TAPE1/2, GRAM and MIC, and the second stage conntecting the inputs from RADIO and AUX. There's no real RIAA equalisation on the GRAM but a little bit of loading. The second EF86 feeds a tone stack, which is then amplified by one half of the ECC83, followed by the other. There's also a low pass filter implemented here. It's full of horrible Hunts... 

The base is removed to gain access to the solder side.

... and a good while spent changing all those nasty caps out...

I chose not to re-stuff the can on this occasion, as there's a nice bit of tag strip that forms the power supply filter, so they could just be mounted there ...

At some stage in the past, the volume control has been replaced, for one without a switch. It's also been wired up backwards. It's now wired up correctly, so as not to deafen the unsuspecting user.

How's it sound? I thought there would be more "blow" due to the EF86's cascaded, but it's reasonably quiet. Obviously the TAPE setting is nothing more than a head amplifier, so not suitable for a line level output from a modern deck.  Pity about the RIAA, or lack thereof...*  Tone controls are actually very nice, and don't offer too much cut or boost. 

The switch-gear is lovely, and has stood the test of time really well. 

*upon reading the manual, it transpires the input stage was designed for a Decca FFSS cartridge, the manual does give some suggestions about use with other carts. 

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...