I found this old thing at the Tip Shop, it was $10. When I plugged it in it fired up and it seemed to mostly work – I could listen to the radio with it. The balance control was noisy and there seemed to be some weird issue with the tuner. Anyway I intend to gut it and make use of the power supply etc. Unfortunately upon measuring the rails I find +/- 33 volts which is a little high, also a max of about 125mv of hum which I am not sure is good or bad. In any case I think I need to use some regulators to drop from 33 to 25. I think I could use a set of LM317 and LM337 chips but I need to find out how much power the LM1875 uses at 20W output.
The inside of the receiver is shown. Most of this will go apart from the power supply (top right) and the heatsink (which I will use to shield the FRDM KL25Z). I also not that there is a 4 way ganged switch at the front which I will make use of as an input selector (this doesn’t appear to be soldered to a circuit board but is point to point wired – which make it a lot easier to utilize)
To the left is the heatsink I intend to use for the power amps, it is currently mounted on some kind of air-conditioning control circuit. This is about 20cm long, 6cm high and 10cm wide. I think it should be enough.
As for the front panel I think I might try to completely cover the front panel in a sheet of aluminium just leaving input selectors, volume (the largest knob), LCD display. I might even forgo a balance control.
Anyway the main task is to design the power supply modifications.