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Old 17th February 2013, 11:25 PM   #221
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It could be now that the bias setting is simply wrong and is passing too much current in the output transistors. You need to find a spec for setting it.
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Old 18th February 2013, 12:25 AM   #222
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Default Nigel

Do you have a suggestion on how to find that info?
I've learned how the bias works in a simple configuration but this amp is still largely over my head. I am still having a problem seeing what makes up the bias for these outputs.

BR
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Old 18th February 2013, 12:33 AM   #223
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When I designed my own class ab amp I had to come up wit ha bias setting routine.

Turn bias pot until bias is zero volts.
Input a sine wave to input and monitor output to speaker with a speaker connected.
Turn up the bias slowly until crossover distortion disappears.

You can see if the bias is the problem by turning the bias down to zero volts and see if the heatsink gets hot with no input signal.

Be careful though as giving too much bias could blow your amp up.
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Old 18th February 2013, 12:47 AM   #224
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I don't have an adjustable bias on this amp. Though I do have a trim pot I can install if I knew where to install it.

Quote:
You can see if the bias is the problem by turning the bias down to zero volts and see if the heatsink gets hot with no input signal.
How do I do this?

Quote:
Input a sine wave to input and monitor output to speaker with a speaker connected.
Turn up the bias slowly until crossover distortion disappears.
You're talking about using an oscilloscope aren't you?
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Old 18th February 2013, 12:52 AM   #225
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Quote:
Originally Posted by badraven View Post
I don't have an adjustable bias on this amp. Though I do have a trim pot I can install if I knew where to install it.


How do I do this?



You're talking about using an oscilloscope aren't you?
You need to find out which way the pot works to turn the bias down.

I use a signal generator and scope to set bias.
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Old 18th February 2013, 06:36 AM   #226
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There r no bias setting presets in the amp.

Gajanan Phadte
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Old 18th February 2013, 02:19 PM   #227
JMFahey is offline JMFahey  Argentina
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Quote:
There r no bias setting presets in the amp.

Gajanan Phadte
And to complicate matters further, there are TWO bias setting/sensing systems in series, one for each half of the output stage (Q14/Q15), TWO bootstrapped loads (R91/92/99/100) and so on.

The designer went all the way and beyond trying to make it ultra-symmetrical, overdrive in a certain way, etc.

Obviously it *does* work, and added complexity is not a factor with automated assembly, but when burnt it's a mess, even for experienced Techs.
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Old 18th February 2013, 09:21 PM   #228
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Default R91, 92, 99 & 100

I pulled one leg of R91, 92, 99 & 100 and ohmed each. All four are within 1% of being right on the money.

As I stated yesterday, I had changed Q15 but didn't have a new one for Q14. Today I replaced Q14 with a TIP31C. Its way overkill but its an NPN. I turned the amp on and played it for a couple minutes on the clean channel. The temp of Q21/20 climbed up to around 115 deg. I switched to the drive channel and continued and it didn't take but about two minutes and 20/21 were about 150 deg. I turned off the amp because it shuts down around 157deg.

I removed Q20 and 21 and installed new ones just in case they had been damaged from overheating. Nothing changed. It still heats up just the same as before.

Quote:
Originally Posted by JMFahey View Post
And to complicate matters further, there are TWO bias setting/sensing systems in series, one for each half of the output stage (Q14/Q15), TWO bootstrapped loads (R91/92/99/100) and so on.

The designer went all the way and beyond trying to make it ultra-symmetrical, overdrive in a certain way, etc.

Obviously it *does* work, and added complexity is not a factor with automated assembly, but when burnt it's a mess, even for experienced Techs.
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Old 18th February 2013, 09:29 PM   #229
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I'd be surprised if the power stage ever hit overdrive.

I worked with the FM65R (1x12", claimed 65w) for a couple of years with some very loud drummers, and found it stayed clean at very loud levels.

Problem was that my ears started distorting at the SPLs we were playing at, so the above may not be 100% accurate.

IMO, it'd be best simply to gut the existing amplifier, and replace with some 2nd hand PA amp. Sit it in the bottom of the cabinet or something.
Something like this. - I can vouch for that particular model being very good. If the asking price stays low, snap it up.

Chris
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Old 18th February 2013, 10:34 PM   #230
JMFahey is offline JMFahey  Argentina
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You hit 2 nails in the head.

1) I also think the circuit was designed to, say, "soft clip" or "bend the wave instead of chopping it" or something like that, and that might be the purpose of the added stuff .... which complicates repair, because "everybody is connected to everybody" in a DC coupled power amp.
So an error *anywhere* will throw output DC voltage, bias current or both out of whack.
Good luck trying to find it

2) I was also thinking, but didn't want to complicate badraven's life even further, that there is a "plan B", which I have used myself on some repairs, typically when the PCB is *destroyed* , as when it has a hole in it like somebody used a blowtorch.

That would be to pull everything to the right of C40 , meaning U6, Q6 to Q21, D16 to D39, all parts connected to them which are seen to be part of the power amp, clean the PCB, and add on the outside/bottom of the chassis a 100W/4 ohms generic Power Amp PCB, connected directly to the internal speakers.
It would only have 7 connections to the approppriate pads of the original PCB:
* * Input signal (from C40) and signal ground (from where R56+C41 ground)
* +42V (from top of C57)
* -42V (from bottom of C60)
* Spk+ from P1
* Spk- from P2
* New PCB ground to PSU ground , with a 1mm (#18) wire to the pad joining -C71 to + C72 (main filter caps.

You can mount the new pcb close to the chassis bottom with standoffs, with its own heat sink.
Trying to reuse the original one will complicate matters.
It won't even "look bad".

Almost forgot; you'll need 1 or 2 10mm - 3/8" holes in the chassis to pass those wires , deburr them and use some rubber grommet to avoid damaging wire insulation.

If you are fed up with that amp, it's one way to go.
Or make it even quicker, and mount inside some used cheap power amp that fits.

FWIW I often make such "substitutions", with good results.

Basically "all SS amps sound the same" .... or close enough.
And any power amp is better than a dead one

Last edited by JMFahey; 18th February 2013 at 10:38 PM.
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