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Jukebox preamp distorting.

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Jukebox preamp distorting.

Post by Rebel Rafter » Sat Apr 09, 2016 10:04 pm

Hi all, RR here. Let's see if anyone can figure this one out. I've got an all transistor, all silicon American jukebox preamp which severely distorts on the right hand channel when cold, for those familiar it's a Rowe-AMI R3758B preamp used in the 64 watt power amp type R-2179A. I've fed it with a 1khz sine wave signal and scoped it and for some reason at first apparently only when first powered and for a few minutes the lower ( negative) part of the waveform is severely clipped and flattened but it then clears. Is this one of those hair dryer and freezer jobs? And no amount of tapping the board with a plastic screwdriver handle affects anything, it just remains fine once it's cleared. The preamp is 40 years old this year, dated 1976. It seems to me one of the transistors must've gone a bit iffy, or one of the diodes, it has a fancy DC 2-wire volume control with a lot of diodes, mostly silicon and one germanium. RR.

 
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Re: Jukebox preamp distorting.

Post by sideband » Sat Apr 09, 2016 10:32 pm

Well at a complete guess and not having any idea of the circuit, I'd guess at a dried out coupling capacitor (I've had something similar in a 70's Hi Fi system). If a cap is leaky, it could be shifting the cut-off point of the affected transistor. Maybe the cap is reforming enough after a few minutes to get things going.

 
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Re: Jukebox preamp distorting.

Post by Katie Bush » Sun Apr 10, 2016 11:05 pm

Like sideband, I'm also thinking capacitors, since these are the most likely components to change value, and/or working characteristics when power is applied.

 
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Re: Jukebox preamp distorting.

Post by Cathovisor » Mon Apr 11, 2016 7:42 am

I'm going to differ here and rather than go for what seems to be an increasingly common knee-jerk response on forums of "it's the capacitors!", actually suggest a logical and scientific approach. You have a 'scope, you have freezer spray and it would seem a circuit, so you know what to do. I've seen transistors do this sort of thing too...

 
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Re: Jukebox preamp distorting.

Post by Rebel Rafter » Wed Apr 13, 2016 8:18 pm

Hi all, RR here. I've replaced all the electrolytics, all six of them with new ones, still no difference. So I started probing around the transistors with the scope and I found a nice clean signal going in at the base of the final stage which is an emitter follower and there was serious clipping of the negative peaks coming out at the emitter and the collector so I thought that's it, just change that one, but it was still the same, and same again with a third one, and then I found it was seriously clipping just the same further back, so what next? suspect the test gear? So I tried feeding the signal generator, which is an old ex-RAF Dawe CT439 straight into my old scope which is an old Roband RO50A and perfect display every time after several tries just to make sure, so now what? It looks like I'll have to get some freezer after all, I've got the circuit diagram which shows both expected DC and signal voltages. It's quite a complex thing with an RIAA preamp first for the cartridge input and then an AVC circuit with bass boost at low volume and then the signal splits and takes two paths both separately amplified which recombine at the final stage. Ever seen anything like that anyone? RR

 
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Re: Jukebox preamp distorting.

Post by Rebel Rafter » Fri Apr 22, 2016 8:49 pm

Hi from RR. It was a transistor after all. I found it with some freezer spray and the scope and a 1 kHz sine wave. When cold it was cropping off the negative peaks giving an effect like a half wave rectifier. RR.

 
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Re: Jukebox preamp distorting.

Post by Cathovisor » Sat Apr 23, 2016 12:41 pm

Rebel Rafter wrote:I found it with some freezer spray and the scope and a 1 kHz sine wave. When cold it was cropping off the negative peaks giving an effect like a half wave rectifier.

Thus illustrating the point I made earlier - fix things with methodical reasoning and logical deduction, not with knee-jerk assumptions about 'bad caps' and the like :aao
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