I picked up an inexpensive Zenith 'suitcase' style stereo record player yesterday. Model KPS-80L. It was smelly and in generally poor condition, so it won't be restored, but I did carefully remove the amplifier and record changer.
Schematic for the amp is here:
http://www.rsp-italy.it/Electronics/Radio Schematics/Zenith/_contents/Zenith KPS80.pdf
The amp is model KPS-80 and it consists of a power transformer putting out around 260 vct and 6.3 vct into a 6X4 rectifier. B+ is around 250 volts after rectification, with a lower-voltage B at around 185 volts. Audio tubes consist of very common 12AX7A and 6AQ5 tubes. Nicely, all appear to be original Zenith branded.
Unfortunately, this particular record player was set up to have three output transformers, as you can see in the linked PDF file. The OPT for the center channel (8 inch speaker in the main unit) is reasonably sized, but the two OPTs for left and right channel outboard speakers are itty-bitty little things. The design of the entire output is not really something I'm familiar with.
The tone controls are separate and easily removed from the circuit, for which I am grateful.
So since I am stuck at work today waiting for other people to do their jobs so that I can do mine, I redrew one of the channels to see what I'd have to do to make this work with a proper input (volume control only) and a normal 2-channel output.
My work is enclosed. Would you mind having a look at it and offering your comments? My plan is to leave the power section basically alone, with the exception of recapping it. It's fine - simple 6X4 tube rectifier into a CRCRC set of smoothing caps and resistors. I will of course replace the old electrolytic caps and observe modern safety precautions with regard to grounding, fuses, power switches, and such.
EDIT: Oh, I see that I also failed to take into account that the cathode resistor and bypass cap on the 6AQ5 is shared on the original schematic. If I want to use individual resistors/caps on the cathodes, I guess I need to reduce the values. Maybe 68 Ohm each channel?
Schematic for the amp is here:
http://www.rsp-italy.it/Electronics/Radio Schematics/Zenith/_contents/Zenith KPS80.pdf
The amp is model KPS-80 and it consists of a power transformer putting out around 260 vct and 6.3 vct into a 6X4 rectifier. B+ is around 250 volts after rectification, with a lower-voltage B at around 185 volts. Audio tubes consist of very common 12AX7A and 6AQ5 tubes. Nicely, all appear to be original Zenith branded.
Unfortunately, this particular record player was set up to have three output transformers, as you can see in the linked PDF file. The OPT for the center channel (8 inch speaker in the main unit) is reasonably sized, but the two OPTs for left and right channel outboard speakers are itty-bitty little things. The design of the entire output is not really something I'm familiar with.
The tone controls are separate and easily removed from the circuit, for which I am grateful.
So since I am stuck at work today waiting for other people to do their jobs so that I can do mine, I redrew one of the channels to see what I'd have to do to make this work with a proper input (volume control only) and a normal 2-channel output.
My work is enclosed. Would you mind having a look at it and offering your comments? My plan is to leave the power section basically alone, with the exception of recapping it. It's fine - simple 6X4 tube rectifier into a CRCRC set of smoothing caps and resistors. I will of course replace the old electrolytic caps and observe modern safety precautions with regard to grounding, fuses, power switches, and such.
EDIT: Oh, I see that I also failed to take into account that the cathode resistor and bypass cap on the 6AQ5 is shared on the original schematic. If I want to use individual resistors/caps on the cathodes, I guess I need to reduce the values. Maybe 68 Ohm each channel?
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Double the cathode resistor if two are used. I'd start with 330 Ohms each - reduce them if idle dissipation allows.
The original circuit used the phono cartridge channels in opposite polarity. Which gives push-pull for bass center channel, SE for R/L channels. Note that R/L speakers are wired in opposite polarity to return them to in-phase.
The original circuit used the phono cartridge channels in opposite polarity. Which gives push-pull for bass center channel, SE for R/L channels. Note that R/L speakers are wired in opposite polarity to return them to in-phase.
Thanks as always, Tom! I missed that phase inversion thing. Rather clever of the original designer, eh? Quite different from a Motorola 3-channel amp I have, which has actual tubes in push-pull for the center channel and SE tubes for left and right. This is economical, if not hi fidelity.
I will double the cathode resistors as you suggest.
I know this is a very old thread, but if you're still around, did you actually perform this modification? I couldn't tell if it was purely hypothetical to begin with.
I just acquired a KPS-80 that's in very nice working order, but I'm interested in making some modifications along the lines of what you propose here, so I'm curious.
I just acquired a KPS-80 that's in very nice working order, but I'm interested in making some modifications along the lines of what you propose here, so I'm curious.
I'll try this thread. I just picked up one of these guys tonight for $30. It powers up, but the turntable doesn't spin. No surprise there.
Have any of you guys rebuilt one of these or used it for anything? I'm not sure what to do with it, but the price was right, it's in good physical condition, and I figured "why not?" Any input or ideas?
Thanks,
Mike
Have any of you guys rebuilt one of these or used it for anything? I'm not sure what to do with it, but the price was right, it's in good physical condition, and I figured "why not?" Any input or ideas?
Thanks,
Mike
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