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Old 26th September 2012, 03:55 PM   #1
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Default Armstrong 525 distortion on one channel

Hi, when I am using my 1970ís Armstrong amplifier with a laptop line out connected to tape in, I get distortion in the speakers after about half a hour. If I turn the amplifier off and on again, it works again for a few seconds.
However, when using the ďtape out ďas an audio input, it work perfectly fine. I donít have a turntable handy to test any of the turntable inputs. I do have a multimeter and soldering iron, and I can do small repairs.
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Old 26th September 2012, 06:55 PM   #2
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It seems there are quirks to these designs in their selector switching arrangements which may have something to do with the problem, since evidently it's not in the amplifier section.
The Armstrong 500 Series - Amplifier Circuit Diagrams.
Check this site for interest and come back if you find out some more regarding the behaviour of switches, sockets or even preamp, if possible. I suspect the usual old cap. story in the power supply but let's see what's going on better.
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Last edited by Ian Finch; 26th September 2012 at 07:05 PM.
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Old 26th September 2012, 07:56 PM   #3
Mooly is offline Mooly  United Kingdom
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Like Ian I would suspect a cap too. One question though. Are both channels affected or just one ? If you look at the circuit for the preamp, C201 (as far as I can see) doesn't have a bleed resistor on the input side. Depending on the source component (your laptop) it is possible this cap could end up reverse biased and if so that would then leak current, alter the bias of the preamp stages and so cause distortion.

A simple test is to measure the voltage on each side of C201 and confirm that the cap is never reverse biased. If this is the problem area then even the act of connecting the meter to the cap could "cure" the problem. And again, if this is the problem then a permanent fix is dead easy

Have a measure.
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Old 27th September 2012, 12:08 PM   #4
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There is no distortion when using the tape monitor button, so i'm suspecting it could be something to do with the tone controls. I've cleaned all the opts and switches with contact cleaner, and visually checked all the caps.

How would I go about finding C201? I'm useless at reading schematics!
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Old 27th September 2012, 06:04 PM   #5
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It's no more difficult than finding a road on a map. I must say, though, that this is the first schematic I've seen that refers to some parts (such as resistors and capacitors) with numerals only (201 instead of C201, or 203 instead of R203). Jargon is "reference designator", usually abbreviated to "refdes".

Click on the schematic's image "C.16 Control Unit" in Ian's link, slightly less than far left there's a component designated "201", this is C201. The image to the right of the schematic is of the board layout, C201 is the lower left circle. C201 should be in the same location on the actual pcb.

Last edited by jitter; 27th September 2012 at 06:09 PM.
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Old 27th September 2012, 06:51 PM   #6
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And what could be a big big clue as I mentioned earlier

Are both channels affected or just one ?
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Old 27th September 2012, 07:41 PM   #7
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Quote:
Originally Posted by Mooly View Post
And what could be a big big clue as I mentioned earlier

Are both channels affected or just one ?
Both. I have found C201, and will wait for the distortion to appear before testing.
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Old 28th September 2012, 07:10 AM   #8
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Quote:
Originally Posted by linuxthefish View Post
Both. I have found C201, and will wait for the distortion to appear before testing.
That narrows things down a lot hopefully.

When you measure around C201 remember there will be another identical channel so check both. If it is a bias issue of the type I mentioned earlier then both channels would be affected.

If that test reveals nothing (but I'm hoping it will) then check this.

Do you see on the same circuit a resistor marked 207. Its near the top left. That resistor carries the supply voltage for the preamp. When working measure and record the voltage at each end. When the amp goes faulty compare those voltages and see if they have dropped. The power supply that feeds this resistor will be common to both channels so see what that shows.
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Old 28th September 2012, 07:09 PM   #9
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C201 on both channels is measuring 10.6 volts, and drops to 6.4 volts after a while, then the distortion begins! 207 is 47.7 volts, and 45.2 volts when distorted.

The only problems when I got this amplifier were a blown output fuse on the right channel and the case was on backwards!
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Old 28th September 2012, 07:59 PM   #10
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Quote:
Originally Posted by linuxthefish View Post
C201 on both channels is measuring 10.6 volts, and drops to 6.4 volts after a while, then the distortion begins! 207 is 47.7 volts, and 45.2 volts when distorted.

The only problems when I got this amplifier were a blown output fuse on the right channel and the case was on backwards!
Thats a very interesting result re 201. The 47 and 45 volts sounds OK

Is there voltage on both sides of the cap. As you look at the circuit there should really be zero volts DC on the left side of 201 and some value (determined by the component values) on the right side.

If there is voltage on the left side then you need to add a "bleed" resistor from the left side of 201 to ground (the lower end of resistor 202 would be the ideal place). Something like a 100K resistor would be good.

I'm surprised your laptop allows this to occur though... and there are other possibilities. Have you measured the DC voltage on the laptop output without it being plugged into the amp. It should be zero really.
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