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Old 12th April 2009, 01:59 PM   #1
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Default One Channel quiet - any bight ideas??

Hi all,

I have reached the final testing stage of my 845 amp based on this design -

http://www.audiodesignguide.com/New845/New845v2.html

I fired it up tonight for the first time and great result - through one channel but the other channel very quiet!

The good channel is good, no problems. The quiet channel is just that - quiet. I can hear the music, it sounds fine, no crackles, buzzes or hisses but just very quiet - maybe 10-20% the volume of the good channel.

Here is what I have done -

1. Swapped speaker output - bad channel still bad
2. Swapped inputs - bad channel still bad
3. Swapped all tubes - no change
4. Checked all heater voltages - OK and equal both sides
5. Checked all bias voltages - OK and equal both sides
6. Checked all B+ - all good both sides (common B+ voltage supply)
7. Checked input resistance through attenuator from input - equal and appropriate both sides
8. Double checked all wiring - spent 2 hrs tonight following all signal and output pathways - nothing obviously wrong

The power supply and bias supply obviously OK. The tubes are fine. The input signal is OK as far as the driver grid.

All parts are new and good quality.

So, where to next?

I am not keen to pull it all apart yet as it has been very carefully built. I have obviously stuffed something up but what?

Any one have any obvious answers? Am I missing a no brainer?''

Any ideas welcome.

Rob
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Old 12th April 2009, 02:13 PM   #2
SY is offline SY  United States
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How did you verify that the signal was getting full strength to the input tube grid?

I'd first check plate voltages to the input and driver tube, cathode voltages of those tubes, and the input stage cathode bypass caps. If any of the transformers have windings that are split and need to be connected together, I'd double check the phasing.
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Old 12th April 2009, 02:47 PM   #3
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Rob,

A likely cause is a bad solder joint. I've sometimes been able to trace faults such as this by tapping the wiring in various places with a plastic knitting needle.

Only you can find the problem. Good luck.
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Old 12th April 2009, 02:54 PM   #4
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Sy,

Thanks for the quick reply. All phases checked during construction.

However, hang on a second - admission of stupidity coming up.

Do you think substituting the 1K resistor (as circled on the schematic) with a 332K resistor could cause the problem?

Damn those caddock 132 model resistors - they have too many little numbers written on the side. I have just discovered that I put a 332K resistor in here instead of the 1K as required.

It was of course in the most inaccessible spot. However, even though it is 1 am and my wife is going to kill me in the morning, I had to replace it - problem solved

It sounds great but boy does it generate some heat!!

I will post a report when I finally tidy it all up. In the meantime, it works and it is going to be a beauty when finished.

Cheers,

Rob
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Old 12th April 2009, 02:57 PM   #5
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Quote:
Originally posted by ray_moth
Rob,

A likely cause is a bad solder joint. I've sometimes been able to trace faults such as this by tapping the wiring in various places with a plastic knitting needle.

Only you can find the problem. Good luck.

Thanks Ray_Moth,

Your wishes obviously helped, and you were correct as well, only I could have uncovered such a stupid mistake!- almost sabotage!

Cheers,

Rob
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