Thursday, 21 January 2021

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! 

Saturday, 16 January 2021

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" 


Saturday, 2 January 2021

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.