We’ve been scrapping an old analogue transmitter at work, and I decided to recycle an old rack-mount chassis, as well as some other tit and tat, as an electrical box for the high-current DC parts of my solar system.
The fun part about giving this guy a first look-through was that it did actually turn on, and it did actually play on both channels … barely. The right channel was heavily distorted and about half of the time when the power switch was flicked, it’d only flicker and buzz for a while before setting the breaker off.
The turn-on issue was caused by a severely worn-down power switch. The contact surfaces had literally vaporised and disappeared from switching that 1,3 kVA transformer for thirty years. There’s no remedy able to get that into shape for switching big loads again. At least not without giving it a helping hand.
It workes very well, however there should be two relays used if you’re to be picky; the original switch is configured to turn the two primary windings on at slightly different times. This is in order to reduce the start-up current somewhat.
(I was later informed that switches such as these, commonly found in AT computer chassis’ would probably work as replacements.)
With the power switch issue out of the way, let’s move on to some intensive maintenance of the actual electronics. Starting with a re-cap.
Worth noting is that C101 and C201 (100 µF/10 V) were actually completely dried-out; they didn’t even register on the meter. It’s odd that the amplifier worked at all without them, as they seem to be in the signal path.
While new caps are always fresh and nice, it didn’t remedy the right-channel bias issue. The cause for that was two-fold and somewhat unexpected. For starters, the bias adjustment potentiometer looked like this on the inside:
The less obvious issue, however, was that a small (0,47 µF) tantalum capacitor, C113, was shorted. It’s connected across the base and collector of one of the transistors in the bias circuit, as well as across the bias adjustment potentiometer. Short-circuiting C113 is equivalent to turning the bias down to zero.
The affected channel had been serviced before and a lot has been replaced. I’d wager that C113 failed at that time and that the service technician didn’t bother actually listening to the amplifier before giving it back to the customer. Nevertheless, replacing C113 (with a 2,2 µF electrolytic; C213 was also replaced to avoid any channel imbalance) and VR101 (the bias pot) remedied the problem. The channel still has a slight DC offset of 20 mV or so, but I’ll live with that for the moment; there’s still a few transistors that should be renewed.
The same service technician also installed MJ15015 and MJ15016 transistors for the output of the channel – transistors with a maximum VCEO of 120 V! The power supply rails in the P-2200 add up to 160 V; 120 V transistors have no place in such an amplifier. It may work, but any safety margins intended by the designer are lost. If the output terminals are shorted or over-loaded, these transistors will be up in smoke before you can even think “blown outputs”! The proper MJ series transistors to use would be MJ15022 and MJ15023.
Anyhow, with that bitter-sweet story out of the way, only the cleaning-up remains. I’ll spare you the narration, enjoy!
And that’s it! The only things remaining to do is to give the incompetently serviced channel another overview in the future, replacing driver transistors and of course outputs. One of the thermal indication thermostats has also failed, so I might replace that some time in the future. That’s a minor issue though.
Thank you for visiting, I hope you enjoyed reading about this project as much as I enjoy listening to it!
Beaten up and left for dead, this guy was saved into my workshop, moments before being condemned to the scrap heap. Despite looking like (and probably having) it’s spent the last decade in a barn, it does power up and play. Sure, the power switch is broken and the right channel is stuck in pure class B … And the thermal overload LED is permanently lit, and a few bulbs are out. And some screws and the “rack feet” are missing. But it plays!
I use a camera remote for most of my pictures. It is of course a DIY, and it’s made out of an old PS/2 mouse. It’s mighty handy, but I always disconnect it from the camera when I’m done, since it’d just be dangling off of the connector if I left it there.
The result of this is that I all to often can swear that I put it over there . . .
The solution is duct tape.
It’s madly practical, and it’s literally made out of four pieces of duct tape! What more could one ask for?
It’s about pressing a button and making things happen!
Pictured above is my IBM Model N2 keyboard, mere moments after getting disassembled in a rather unprofessional manner. It had been acting up, so I figured that I’d take it apart and try to fix it. I should have looked for some instructions first.
The small SMD electrolytic caps had gone bad. The 2.2µF unit got replaced by a 4.7µF one without any obvious problems arising from it. I added some “China goo” to keep them in place afterwards.
I’m leaving that for the future, though. When the spring is place, it triggers all the time. It sort of works without the spring, so I’ll fix it when I’ve got the patience.
The curse of using nice keyboards is that when accidents like these come around, you can’t just toss’em away and buy new ones. No, sir. You need to painstakingly take them apart and manually scrub every last drop of dried-up coffee out of there, and just pray to a higher power that the smell will go away.
I didn’t bother taking more pictures after that. Lots of hot water cleaned everything right up. Except it still smells like coffee.