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..