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Old 25th April 2013, 09:24 AM   #1
Komiyan is offline Komiyan  Australia
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Default Resurrecting a defunct A3000

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
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Old 26th April 2013, 02:20 PM   #2
Komiyan is offline Komiyan  Australia
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Join Date: Apr 2013
Ok...
Well i found my problem with the 'dead' left channel. When i was replacing the components on the input board, i first checked all of the transistors.
The problem was, there was no markings on the PCB, and the transistors are almost completely round, making it difficult to tell the polarity: it turns out i popped the final output transistor in backwards on the left channel! (I have now learned to mark the board and transistor should i be unsure in the future)

Now that made me feel dumb, but i fixed one of my problems.

The other problem, the right channel's output stage overheating.
I just performed a quick check and found 1V of DC on the right-side output, and at the same time i checked the schematics i extracted from the amplifier's physical topology, and adjusted a trimpot which i figured controls the amplifier's input bias. Turns out, this also hits the output, so i adjusted the DC away, and i'm now testing the amplifier to see if all of the issues are solved so i can put the cover back on, and put this baby on my main speakers!

Tonight i feel so dumb yet intelligent at the same time
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Old 26th April 2013, 06:21 PM   #3
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Congratulations. The more you do something the better you get at it. Drawing your own schematic shows some initiative rare in 2013. I did that for my first transistor amp I repaired in 1990; no internet then, no hifi repair clubs in this town.
As DC is deadly to speakers, yet speakers can highlight problems when they occur, I use resistors to load a broken amp speaker jacks while I test it. I have two five ohm 225 w resistors on in series on each channel. then I put a 4 ohm car radio speaker in series with 3300 uf (back to back, minus to minus) across one 5 ohm resistor. I find when DC occurs, you don't have to continually monitor it with a meter, the strange sound would tip me off to when I had DC. I found a bad solder connection that way, one installed by the factory in 1994 and never caught by previous repairmen. The previous repairman had tied of the power supply to the channel with the bad op amp input and labeled the input "do not use".
Have fun on your next project.
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Old 13th July 2013, 06:11 AM   #4
Komiyan is offline Komiyan  Australia
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Join Date: Apr 2013
I have been using the A3000 for some time now, and i am modding the original A3000 tone-control schematics to suit my own "A2400A"...
It's going to be pretty cool when i'm done with it
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