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Old 27th January 2014, 07:34 PM   #3741
bronc is offline bronc  Bulgaria
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Quote:
Originally Posted by Mooly View Post
Is Q1 bust... not if everything else is OK

Do you have +12 volts on pin 8 of U1 ? If not then measure the voltages on Q1. The gate should be pulled "low" toward -12 volts to turn it on.
I found the problem. Stupid me had put the op amp for U1 in the socket of U2. I measured the voltages on the U1 socket and nothing exceeds the voltages that are meant for U1. I put in the correct place and measured everything all over again - all the voltages were the correct ones within a margin of error (0.01V which is most likely due to my cheapish DMM. I finished the build and the amp is now working. I need to put it in the case and I'll be done. Thanks!
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Old 28th January 2014, 01:26 AM   #3742
agdr is offline agdr  United States
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Quote:
Originally Posted by marcus1 View Post
The two pins on the U6 regulator (nearest the filter caps - numbers 1 and 2 on the reg = input and ground?) had a solder bridge hence a dead short. Looking at the schematic, I checked the D4 diode for damage and found it was kaput (it was short circuited) so my next step is to replace that.
D4 would be the only casualty, other than possibly a PC trace. Since you shorted the U6 input to ground you protected the two filter caps (C3, C5) since the short resulted in zero volts across them. Once your remove D4, measure ohms on your meter between the middle pin of U6 and the side of D4's pads without the band. Should be nearly zero ohms if no PC board traces have fused. If they have you can just use some hookup wire to patch the trace. There is a trace on top of the PC board from the middle pin of U6 to C5, then it goes under the board from C5 to C3 and then D4.

Last edited by agdr; 28th January 2014 at 01:32 AM.
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Old 28th January 2014, 06:24 AM   #3743
Mooly is offline Mooly  United Kingdom
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Nothing else will be damaged in all probability.

D4 failed because of overload, the print depending on how thick it is may have suffered (gone open circuit) if the current flow was very large and its conceivable that J1 may have suffered (contacts may not be rated for that kind of current flow... all unlikely. No other components will have been damaged with a short between pins 1 and 2.
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Old 28th January 2014, 06:25 AM   #3744
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by bronc View Post
I found the problem. Stupid me had put the op amp for U1 in the socket of U2. I measured the voltages on the U1 socket and nothing exceeds the voltages that are meant for U1. I put in the correct place and measured everything all over again - all the voltages were the correct ones within a margin of error (0.01V which is most likely due to my cheapish DMM. I finished the build and the amp is now working. I need to put it in the case and I'll be done. Thanks!
That's good
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Old 28th January 2014, 08:56 AM   #3745
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Thanks for your replies.

I already replaced the diode IN4002 with another (a IN4001 - all I had) and there doesn't appear to be any burnt traces, but I've still got problems...
After replacing the IN4002 diode I connected the power supply but this time with a 40W bulb in series with the transformer, then switched it on and measured the supply voltage at the battery terminals as per the instructions (U2 not inserted).

The reading was 31vdc not the 24vdc it should have been. Also, the U6 reg was getting hot so I quickly turned off the power supply.
So, even with the solder bridge removed, I've still got problems with U6 getting hot (presumably by excess current or another short?) and the excessive voltage at the battery and that's with the 40W bulb in the cct - I wonder what would have happened if it wasn't there - more smoke?

One other test I've done (but without U2 inserted) is measure the DC voltages at D3 and D4 as described in "Check the raw DC Voltages" in the Blog.
It's getting late here but I think I did it correctly with really weird readings of D3 @ 41.75v and D4 @ 0.51v!! Both of these readings were to ground at the battery -ve terminal.
I'm confused - any suggestions? Perhaps the regulator is faulty; can I test it while in place?

Quote:
Originally Posted by agdr View Post
D4 would be the only casualty, other than possibly a PC trace. Since you shorted the U6 input to ground you protected the two filter caps (C3, C5) since the short resulted in zero volts across them. Once your remove D4, measure ohms on your meter between the middle pin of U6 and the side of D4's pads without the band. Should be nearly zero ohms if no PC board traces have fused. If they have you can just use some hookup wire to patch the trace. There is a trace on top of the PC board from the middle pin of U6 to C5, then it goes under the board from C5 to C3 and then D4.
Quote:
Originally Posted by Mooly View Post
Nothing else will be damaged in all probability.

D4 failed because of overload, the print depending on how thick it is may have suffered (gone open circuit) if the current flow was very large and its conceivable that J1 may have suffered (contacts may not be rated for that kind of current flow... all unlikely. No other components will have been damaged with a short between pins 1 and 2.
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Old 28th January 2014, 09:08 AM   #3746
Mooly is offline Mooly  United Kingdom
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Start at the beginning and measure and record these voltages. Firstly I would remove all the opamps U1 to U4

With your meter on DC volts and the black lead on ground,

1) What is the voltage on the striped end of D3 ?
2) What is the voltage on the NON striped end of D4
3) What is the voltage on the NON striped end of D1
4) What is the voltage on the striped end of D5

If anything gets hot or smokes with the opamps out then there is a fundamental problem at the front end.
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Old 28th January 2014, 04:34 PM   #3747
bronc is offline bronc  Bulgaria
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Ah, damn it! Now I'm having issues with the grounding of the RCAs and it's really annoying. When I turn up the volume I can hear a loud (especially at high gain) and distinctive high pitched sound. t's similar to buzzing but its not a constant sound although I think there is some kind of a pattern in it. This happens every time I connect the amplifier using the RCA jacks to another device. I tried all different combinations of connecting the ground to the RCA but none of them solves the problem. First I had the two grounding rings connected to each other and soldered a single ground wire to them with the two RCAs and the rings touching the panel directly. Then I tried having again a single wire to both rings but this time with one RCA isolated from the panel using the plastic ring. I tried having both RCAs isolated again with the two rings touching. I tried running two separate ground wires with the RCAs touching the panel. Nope, still doesn't solve the problem. I've cut both traces on J2 and I'm soldering the ground as per NwAvGuy's guide for connecting an ODAC to the amplifier. If I disconnect the RCAs from the external device (a DAC) and touch them with my fingers I can hear a very loud buzzing sound. I've wasted over 3 hours trying to find the problem but I can't.. Any ideas?

P.S. I've sanded the area around the RCA holes even though the panels don't have any paint on them as they are brushed steel.
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Old 28th January 2014, 04:48 PM   #3748
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Quote:
Originally Posted by Mooly View Post
Start at the beginning and measure and record these voltages. Firstly I would remove all the opamps U1 to U4
Thanks; Aside from measuring the resistances when U2 was in, all the opamps have been removed during testing and the switch S1 has been off.

Below are the readings across the diodes


With your meter on DC volts and the black lead on ground,

1) What is the voltage on the striped end of D3 ?
41.3v
2) What is the voltage on the NON striped end of D4
1.5v
3) What is the voltage on the NON striped end of D1
30.9v
4) What is the voltage on the striped end of D5
1.7v

If anything gets hot or smokes with the opamps out then there is a fundamental problem at the front end.
I agree

Here is a photo of the board if you can see anything obvious (Replacement diode for D4 is temporary).
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Old 28th January 2014, 04:56 PM   #3749
Mooly is offline Mooly  United Kingdom
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Something is seriously amiss here. If you really have 41 volts DC on D3 then that points to the input voltage from your adapter being sky high.

Firstly I would suggest you measure on AC VOLTS the voltage from ground to the Non striped end of D3 (the AC input to the board) What have you got ?

And another thought, is your adapter putting out AC ? which it should and not DC which it should not.
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Old 28th January 2014, 05:00 PM   #3750
Mooly is offline Mooly  United Kingdom
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I'll be back in a little while... also check your meter for accuracy... something doesn't add up here. 41 volts should have the caps going pop too unless they are rated at 63 volt. Try your meter across a battery or something to confirm its OK.
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