Sorry Citation 16

I’m a sucker for a hard luck case. I picked up a beat-up HK Citation 16 this past April thinking I could restore it and use it in my own personal system. In retrospect, I don’t know what I was thinking. Perhaps, “It couldn’t be that bad… could it?” Well, it was. Worse, in fact. Rusty, rained on, bent, dirty, chipped – generally beat to crap. And that was just cosmetics. After getting it on the bench, I discovered 60 VDC on the left speaker output, and no signal present on the right. Turns out it had evaporated traces, a few dead caps and several open resistors on the driver boards, two bad emitter resistors, burned out display LEDs, dead neon pilots, and apparently, an arcing problem with one of the TO-3 driver transistors, where under high current conditions the transistor case had burned a hole through the mica insulator and had arced to the grounded heat sink! (see arc-welded tranny, below)

Okay, fixed all that, hooked up an input source and gradually brought up the volume. Sounded great – that is, until I hit about 5 watts output at which point the amp started distorting horribly, with sharp, staccato transients – it sounded exactly like a crummy woofer sounds when you bottom out the voice coil. I put a scope on the left output and gradually brought up a 200 Hz sine wave with no load – perfect. As soon as I connected a load and brought the input signal up a little bit, however, I started getting a low frequency parasitic oscillation on the upper part of the sine wave (see photo). I connected the scope and load to the right output and got exactly the same oscillation, just in reverse phase, spiking up on the lower part of the waveform.

What the hell? The Citation 16 is a true dual mono-design – each channel is completely independent of the other. The output signals for both channels travel through a relay board, but no way that could cause equal but opposite phase oscillation under load in both channels. Guess I’m not as smart as I thought I was, because it took me almost a week to finally figure out what was happening. Seems pretty obvious to me now, but it gave me fits for a while. Anyone here care to guess what the final bitch-kitty turned out to be?? :scratch2:

Too many possibilities, but I'll generalize by saying a ground (short) was being made somewhere when you put a few volts through it.

EchoWars said:

Too many possibilities, but I'll generalize by saying a ground (short) was being made somewhere when you put a few volts through it.

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Yeah, and I exhausted most of those possibilities before I finally figured it out. I noticed a tiny hint of hum in one channel - something you never hear in a Citation 16. To rule out power supply hum, I scoped across the big filter caps, and it took me a while to believe what I was seeing. Two of the four caps had nice flat DC across them, as expected. The other two? 34V P-P one one and about 49 V P-P on the other! I thought it had to be the rectifiers, but they all checked good.

Turned out to be two of the four monster filter caps, one dead in each channel. I pulled them and they were so badly dried out that they rattled when I shook them! Apparently, the amp was "motorboating" - something I haven't heard since the filter cap went on my Dad's Dynakit Mk2 30 years ago. The dead cans were diagonally oriented to each other which is why the oscillation appeared to be reversed between left and right channels.

So now I get to hunt down four new 10,000 85V beer cans for this sorry beast, on top of everything else I've sunk into her. Oh well - she'll probably never be much to look at, but I can't wait to hear her purr once I get that brand new four-barrel under her hood!! :thmbsp:

Wow...

Well, they don't have to be 10,000µf, if that helps. There are a lot of choices that will work.

Rattled huh?

I bought some filter caps for a Citation 19 once from Mouser - these are just a bit taller. Seem to remember they were about $30.00 or so apiece, though - ouch! Think I'll do a little shopping around first...

Geez EW, don't you ever sleep?

skibjr said:

Geez EW, don't you ever sleep?

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Here lately, seems like the answer would be 'no'.

Well, my caps are on the way!
Closest replacements I could find at Mouser (my usual parts source) were some 10,000uF 100WVDC Cornell Dubiliers @ $29.00 apiece - about what I expected. Before I placed the order though, I thought I'd check the 'Bay and see if I might just get lucky... and I did! Aaron at Electrum Designs was auctioning 14,000uF 100V caps in the exact size I needed at $25.00 each, a good deal already, but accepted my offer of $80.00 for four, including shipping! I still have quite a bit of restoration work to do while I wait for the replacement caps, but it's nice to know she'll be singin' soon! :thmbsp: