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Yes, I'm quite mad!
...as long as you know 😂
You are pretty far in now, and it is too late to scan that original PCB...
I wonder if you wouldn't have been better off going for something like an LM1875 (in a pair) solution if the goal is just to get it to make sound again? Fewer components needed and probably an easier solution overall.
You are pretty far in now
Oh yes way in deep over my head <gasp> 😳
I've scanned the PCB layout from the manual received via snail-mail so as to check all tracks against circuit diagram - all seems ok - (notes on diagram)
Checked a few resistances on the connectors to the Power amp inside the case (Power Amp PCB not connected)
Signal GND to Chasis = 90 k Ohms
Signal GND to GND on Power Supply = 0.5 Ohms
I notice on the PCB there are two holes drilled, one on signal GND and the other on Power supply GND... Could the GND issue be the cause of all these components frying? I wander, when I construct the new board should I merely connect signal GND to Power GND to test the new board?
I'll construct the new board with that 'option' over the next week - just makes it a little harder with two separate sources for GND
all good fun 😂
Signal GND to Chasis = 90 k Ohms
Signal GND to GND on Power Supply = 0.5 Ohms
I notice on the PCB there are two holes drilled, one on signal GND and the other on Power supply GND... Could the GND issue be the cause of all these components frying? I wander, when I construct the new board should I merely connect signal GND to Power GND to test the new board?
0.5 ohms between both ground planes at the power supply is normal because that's where they are centrally connected. 90k at the board is possible because of various components that link the power path and signal path.
Those 2 holes between signal GND and power GND look like a precautionary design measure. At the time of board design they were not sure if power GND/signal GND separation was "worth it". But ultimately they found that the separation was benefitial in terms of noise rejection, so they kept the planes separated, which forced them to run the extra wires from power supply to amplifier.
When you construct the new boards, you should retain that separation, or else you'd create a ground loop when connecting the preamp and that might cause a hum or other noise. For offline testing of this new board you can - actually must (!!) - connect those 2 GND points at the amplifier board, but when you connect the preamp, the jumper between power GND and signal GND must be removed.
In the original design the startup circuit R1/C1 acts on both channels at the same time, but in your new design you should add the same components to each channel. Make it part of the power path (and not the signal path as done in the original!!). This way you have 2 identical, interchangeable boards for the 2 channels and you don't have to worry about modifications for offline tests!
<snip> For offline testing of this new board you can - actually must (!!) - connect those 2 GND points at the amplifier board, but when you connect the preamp, the jumper between power GND and signal GND must be removed.
In the original design the startup circuit R1/C1 acts on both channels at the same time, but in your new design you should add the same components to each channel. Make it part of the power path (and not the signal path as done in the original!!). This way you have 2 identical, interchangeable boards for the 2 channels and you don't have to worry about modifications for offline tests!
Very valid points, will be taken on board - literally - Thank you (-:
Nearly there - in principle!
Latest draft of many before soldering components onto vero board:
Login to view image...might just check the circuit against the manual a few more times tho😁
(Click photos to enlarge).
Tested this afternoon prior to dismounting. Plays fine.
All original apart from my fingermarks in the dust.
Interested? 👍
I also have the power supply.
Martin
Behold a thing of beauty ! 🙂 - brings back happy memories of what my board looked like a few months ago before i started 'repair'
Very interested... but have I gotten too deep into the replacement project to afford such a solution?
You'll know my first question..
please PM me a cost inc UK delivery
Thanks for thinking of me and posting such fab photos😐
Pete
a few more snaps to amuse you all Login to view image Login to view image Login to view imageOk...
One channel completely rebuilt / circuit checked and wired up with untested original board containing new parts.
Now to start from scratch (again) and follow Manfy's kind instructions on how to unwire and rewire power supply and various GND points to test voltages whilst having a 40w bulb in serial with live 240v. A 40w bulb in serial with mains power is tested (without amp board connected) and hey-presto bulb lit briefly and then went very dim - just need to understand wiring (signal and power GND conundrum) for each channel test as I've incorporated the 'startup circuit R1/C1' on the new board! - before power on to everything
Hopefully next week we'll have sound 👍
I'm glad you've got it up and running again. Hopefully it holds up and doesn't cause you any more troubles.
Wow! I'm impressed! Congratulations!
To be honest, I wasn't very hopeful after having seen what you allowed your soldering iron do to the original board 😎 and after seeing some blatant mistakes in your "latest draft" in the post above (sorry for not posting when I saw it;I had 2 hectic weeks).
I was actually hoping that you'd take up Martin's offer of getting your hands on a proven working board..
But now that you've managed to make a new board from scratch, all I can say is: Chapeau!
Blood, sweat and tears at its best! 😁.
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