Amcron DC300a overhaul.

Good evening guys,hope everyone is keeping well,and safe.

Whilst sitting here stuck at home, took the opportunity to overhaul this lovely old amp....

Crown DC300a from 1973/74.  Bear in mind this is 47 years old.

I'm getting 0.01 and 0.02v DC  both channels at output, so all ok there, but sound is VERY weak and muffled (max output is about half a watt,on both channels.) Output PCB rails read fine, both channels giving + - 51v. OK. All output transistors tested,all fine. All preamp board diodes and transistors test fine.....i'm getting the correct + - 9.5v power supply at the ic pins....must be something on that main centre preamp PCB, as is affecting both channels.....any ideas? As i'm a bit stuck now.

Ps,i have to say,this is beautifully made,lots of nice shiny copper on the PCB's, really nicely built back then.






Edited by jbl_man - 24 April 2020 at 7:13pm

Maybe you need to use an oscilloscope and signal generator to check signals at different points.

Cleaned those four presets, now have 0.00v DC at output both channels, but output very weak and distorted. Fitted new ua739 ic, still the same. 

Check C4 and C5 (the two caps closest to the Red Switch) as they are part of a diode pump (in conjunction with D1,D2, R1,R2 and D3) that generates about 70volts that feeds both channels driver circuit. Check you have about 100volts with respect to ground on one end of the 2k2 resistor R2…. R2 should have about 30Volts across it.

Edited by APW - 25 April 2020 at 2:24pm

On anything like this, I would change all the electrolytic capacitors on the circuit board. 50 years is well over their expected service life and they will all have gone leaky or acquired a high ESR (equivalent series resistance). Their service life depends (inversely) on temperature, so it's best to replace them with 105 degree rated types instead of the standard 80 degree version.

I'm surprised that you have found resistors gone low in value - carbon types normally drift high as they age.

I must get around to servicing my vintage Turners...

And I’m sure with a few new components this amp will live on for another 50 years.
I can’t see any of these new amps surviving that long.
Any evidence of previous repairs or does it all seem original?