Hi all,
I found an old Decca A3000 amplifier in the local bulk collection, and decided to take it in and see if it still worked.
The power cord was wrecked, and the amplifier had unusual speaker output-plugs, so i had to make my own cables to suit my speakers, and replaced the mains plug.
Upon powering up, the speakers began to make all kinds of noise and i immediately shut the amp down (mind you this was LOUD! i feared my test-speakers were going to explode...).
I proceeded to to work on the discrete output stages, and in fact completely reverse-engineered them
(I looked for information already, but the DeccaSound A3000 doesn't exist on the web.)
After replacing the electrolytics and checking the transistors, i concluded the amplifiers themselves were fine.
I put it back together and powered up, only to get the same noise.
HOWEVER, i did a test where i override the amplifier's original input set, and connect a decoupled volume knob directly to the amplifier modules.
May have been a dumb idea, but it worked, and those amplifiers are perfectly fine! (No overheating or distortion whatsoever)
So i figured the input board (Tone controls, volume/balance, input select, etc.) was shot, and proceeded checking the components, replacing any that were out of tolerance, and replacing the electrolytics. I also restored the pots as those were quite 'gritty'.
Once again i powered it back up, and now it half works. The right channel is producing sound, but the left channel is unusually quiet
If i turn Balance all the way to left, and then turn the volume way up, the two channels are the same volume.
And even when both amplifiers are passing the same signal volume in this manner, the right channel output transistors get freaking hot! (This also happens when there is no input and volumes are minimal for all controls.)
All transistors checked up OK on the input board, and I have not reverse-engineered the input board, but i'll get around to it at some point.
And so now i ask: What could be the issue on that board? Why does it turn the right-channel output into a heater? Are those transistors really OK or should i replace them all (BC153's)? Or should i design my own discrete input board, or even make one using OP-AMPS?
I'm really keen on getting this working again, i'm not lying when i say this amplifier actually made s#!t speakers sound good.
Cheers
I found an old Decca A3000 amplifier in the local bulk collection, and decided to take it in and see if it still worked.
The power cord was wrecked, and the amplifier had unusual speaker output-plugs, so i had to make my own cables to suit my speakers, and replaced the mains plug.
Upon powering up, the speakers began to make all kinds of noise and i immediately shut the amp down (mind you this was LOUD! i feared my test-speakers were going to explode...).
I proceeded to to work on the discrete output stages, and in fact completely reverse-engineered them
After replacing the electrolytics and checking the transistors, i concluded the amplifiers themselves were fine.
I put it back together and powered up, only to get the same noise.
HOWEVER, i did a test where i override the amplifier's original input set, and connect a decoupled volume knob directly to the amplifier modules.
May have been a dumb idea, but it worked, and those amplifiers are perfectly fine! (No overheating or distortion whatsoever)
So i figured the input board (Tone controls, volume/balance, input select, etc.) was shot, and proceeded checking the components, replacing any that were out of tolerance, and replacing the electrolytics. I also restored the pots as those were quite 'gritty'.
Once again i powered it back up, and now it half works. The right channel is producing sound, but the left channel is unusually quiet
If i turn Balance all the way to left, and then turn the volume way up, the two channels are the same volume.
And even when both amplifiers are passing the same signal volume in this manner, the right channel output transistors get freaking hot! (This also happens when there is no input and volumes are minimal for all controls.)
All transistors checked up OK on the input board, and I have not reverse-engineered the input board, but i'll get around to it at some point.
And so now i ask: What could be the issue on that board? Why does it turn the right-channel output into a heater? Are those transistors really OK or should i replace them all (BC153's)? Or should i design my own discrete input board, or even make one using OP-AMPS?
I'm really keen on getting this working again, i'm not lying when i say this amplifier actually made s#!t speakers sound good.
Cheers