Amp Repair -- DC detector? Amp only works right at high volume? Help? - diyAudio
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Old 30th December 2007, 09:15 PM   #1
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Default Amp Repair -- DC detector? Amp only works right at high volume? Help?

So I bought the service manual. . . Still chasing the static on the right channel that goes away under load and isn't in the preamp or tone sections.

And, the headphone section feeds off the speaker terminals. Of course its very weird that you can shut off the speakers and the headphones play clean. Turn the speakers back on and the right channel gets scratchy at moderate to low volume.
The change appears with-out touching the volume pot.

I was wondering if there could be a goofup somehow in the DC detector circuit, pre drive, diff amp, or power amp?

Any clues? This is a lot bigger amplifier than I'd normally fool with.

Lightly loaded running headphones works perfectly; but, just punch the speaker button "main" and the right side goes staticy. Technics SA5370 receiver

Thanks for your help!!
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Old 30th December 2007, 09:28 PM   #2
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Pull the speaker off the side that has static. Put your meter on it and see if you are getting DC on the output. See also if the static changes with temperature.

My 1st guess is maybe the diff pair need changed.
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Old 30th December 2007, 11:37 PM   #3
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I'm not quite sure how to do that.
Is there any way to turn it into a nice preamp and then load a couple of gainclones onto the big heatsinks?
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Old 31st December 2007, 01:10 AM   #4
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Default Dump it or fix it?

Quote:
Originally posted by burnedfingers
Pull the speaker off the side that has static. Put your meter on it and see if you are getting DC on the output. See also if the static changes with temperature.

My 1st guess is maybe the diff pair need changed.
Okay, I made the adjustment recommended by the service manual--cut the two jumpers opens up a small bank of resistors to help balance the voltage (less volts on the right).
Yes, that was slightly nicer. So was cleaning the volume control and replacing a foul-looking cap (1 rusty leg).

I guess this is over my head.

Do you think I should dump it, or is there some way to save it? I mean, some OTHER way besides playing it at near-top volume (which it does quite well).
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Old 31st December 2007, 07:04 PM   #5
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Default Fixed it!

I finally found the power amp stage input filter cap. Got the rectifier disconnected and the preamp + radio still runs fine on the #2 power source. I disconnected the input filter cap and fixed L-pad (2.2 ohm against 100k ohm with 4.7uf input cap--removed). Now, I have disconnected power and audio from the old power amp stage.

There's plentiful available power amps to use. . .
Then I put some leads on the board and plugged in a different power amp for testing. OH my gosh!! One of the best preamps ever made combined with a brand new power amp. Yeah!!

Guess what fits an old Technics amp? hint: matshusta, National Semiconductor, Panasonic, Technics. . . The National LM3886tF (famous gainclone) is plug-n-play with the 30 year older Technics amp. Temporarily, the bass and treble controls are a loss because they were part of the old power amp stage. However, the rest works like brand new. Actually, I remember it new, and this is better.

Now I've got to figure out how to get the bass and treble controls back. I was thinking of disconnecting the "Loudness" button to turn it into an equalizer bypass switch. Its just a dpdt switch that turns on a horrible sounding circuit that I never want to turn on. Re-employed, it could make sense to cycle equalization on or off.

Do you have any suggestions on a little kit form stereo bass and treble? I'd sure like to be able to turn up low bass without causing a boom (higher bass). Sometimes I do need the treble control on a movie
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Old 31st December 2007, 08:32 PM   #6
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"Still chasing the static on the right channel that goes away under load and isn't in the preamp or tone sections. "

Speaker relay.

"And, the headphone section feeds off the speaker terminals. "

No, it feeds directly off the amplifier output (usually through a 330 ohm resistor).

"Of course its very weird that you can shut off the speakers and the headphones play clean. Turn the speakers back on and the right channel gets scratchy at moderate to low volume.
The change appears with-out touching the volume pot."

Speaker relay.

"I was wondering if there could be a goofup somehow in the DC detector circuit, pre drive, diff amp, or power amp? "

No.

Try putting the speakers on 'B', if the noise stays on the right channel it's the relay.
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Old 1st January 2008, 06:17 AM   #7
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Default No need to worry about it. ;)

There's no speaker relay. That went out when the original power amp was desoldered and removed. The problem. . . very leaky, rusty, and smelly problem. . . is no more.

In its place is a new problem.

The preamp is made to drive a 253k to 102k load. The amp is made to drive a 50k to 10k load.

How do I match this up?

The new amp getts pretty hyperactive when its fixed l-pad is 100k. . . preamp's volume control hits 50 watts by #3 on the dial.

Can I just add an additional resistor from input to ground on the "amp side" of the decoupling cap?
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Old 1st January 2008, 09:12 AM   #8
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ok, no speaker relay?..... speaker switch then..... clean it with deoxit.
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Old 1st January 2008, 10:09 PM   #9
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Default different problem now

That first problem is gone--old power amp stage is gone, so there are no rusty contacts because there are no contacts whatsoever.
Now, both speakers play perfectly.
And, much too LOUD!

The replacement power amp section, gainclone/overture by National Semiconductor (same parent company as the Technics SA5370), was patched in at the input cap spot (located after several hours with the service manual).

Now the 28+28 power goes to the new power amp. This works well, except. . . There's been a change in the last 30 years. . .

The old onboard passive preamp (with radio) expects 100k load (specifically 100k resistors to ground and 2.2k resistors to hot).

The new power amp section is expecting 22k. Oh darn.

On 100k, the amp is very loud at #3 on the volume dial!
On 22k, the preamp sounds werid, so ............ ?
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Old 2nd January 2008, 06:05 PM   #10
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Which chipamp schematic did you use?

You could add a non-inverting unity-gain opamp buffer, with a resistive divider (with 22k lower leg) from its output to ground, to act as a high-input-impedance attenuator for your power amp's input.
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