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preiter  
#1 Posted : Monday, May 14, 2007 8:28:25 PM(UTC)
preiter

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Well I finally powered up my rev C to test it and neither channel is working. It's difficult to debug. There isn't a full schematic on the site and the PCB info doesn't match the rev of the board that I have.

The only documentation I have to go on is the pinouts for the two chips. Both boards show the same symptoms.

The output on the 3886 is close to the negative rail (about 7V above the -35V negative rail).
V+ and V- are about 70V apart (seems correct)
V+ and V- on the op-amp are about 25V apart (seems correct)

The 2 1k 3watt resistors each have about 24.5V across them (they get really hot, I don't leave the board powered up for long).

The ground pin on the 3886 doesn't seem to be connected to the output ground connector, should it be?

Also, I fried one of the 3886 chips by shorting the output to one of the rails with my voltmeter probe d'oh! I pulled it out and probed the board without the chip and the output pin is centered between the two rails. So it's the chip itself forcing the output low. Strangely enough, the 3886 doesn't get hot at all.

Any advice on debugging this thing? Brick wall

P.S. which of the input terminals is signal and which is ground?
Russ White  
#2 Posted : Tuesday, May 15, 2007 5:01:18 AM(UTC)
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Hello,

Its hard to say whats wrong, I am thinking it over, but for now these might help you.

Russ
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Russ White  
#3 Posted : Tuesday, May 15, 2007 5:14:49 AM(UTC)
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IN_GND is the pad closest to the heat sink and the input pad is the pad closest to the input cap(C13) as indicated on the silk screen.

Do you have any pictures you can attach?

What sort of transformer are you using?

Also how is your transformer connected?

Cheers!
Russ
preiter  
#4 Posted : Tuesday, May 15, 2007 9:02:43 AM(UTC)
preiter

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Transformer is Avel Transformer

Wired as Brian suggested...

Quote:
No, you form a center tap by tying the two center secondaries together (RED+ORANGE), then provide three connections to each boards power connections (AC1: BLACK, AC2: YELLOW, PGND: combined RED+ORANGE).


I'll try to post a pic tonight.

Thanks for the quick replies.

Edited by user Tuesday, May 15, 2007 11:34:23 AM(UTC)  | Reason: Not specified

preiter  
#5 Posted : Tuesday, May 15, 2007 12:10:21 PM(UTC)
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Well, looking at the schematic, the 1k resistors have V-(V+) on one side of them and -12V (12V) on the other side, so I guess it's not incorrect to have 24.5V across them. They do get mighty warm, though.

I'll look at it some more tonight, having the schematic will help a lot.
Brian Donegan  
#6 Posted : Tuesday, May 15, 2007 12:37:02 PM(UTC)
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It is normal for those resistors to get hot, so not a problem in and of itself.
preiter  
#7 Posted : Tuesday, May 15, 2007 11:32:50 PM(UTC)
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I poked at it some more tonight.

The relay is disconnecting the output ground.
The output of the 3886 is still close to the negative rail.
The input signal is making it through the lm318 and to the lm3886.

What's controlling that relay?
preiter  
#8 Posted : Friday, May 18, 2007 12:25:32 AM(UTC)
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I found that there is only 2V across ZD2.

Probing right around the big resistors and the two zeners I get. V+ is 35V, V- is -35V, -12V is -12V and +12V is +2V.

Does this mean that ZD2 is the bad component? Or could something else cause that?
Russ White  
#9 Posted : Friday, May 18, 2007 5:11:46 AM(UTC)
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preiter wrote:
I found that there is only 2V across ZD2.

Probing right around the big resistors and the two zeners I get. V+ is 35V, V- is -35V, -12V is -12V and +12V is +2V.

Does this mean that ZD2 is the bad component? Or could something else cause that?


Could be, or possibly installed backward?
preiter  
#10 Posted : Friday, May 18, 2007 7:08:06 AM(UTC)
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It definitely is installed in the direction indicated on the silk screen. Also installed in the same direction as the other channel, which measures correctly.
Brian Donegan  
#11 Posted : Friday, May 18, 2007 7:21:23 AM(UTC)
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A pair of replacement zeners and a new LM318 (just in case) are on the way.

You should probably also (for kicks) check the resistance of C6 (and C11 for comparison) to be sure C6 is not shorting. I'll send you another one of those as well.
Russ White  
#12 Posted : Friday, May 18, 2007 7:24:28 AM(UTC)
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preiter wrote:
It definitely is installed in the direction indicated on the silk screen. Also installed in the same direction as the other channel, which measures correctly.


Ok so just to be clear, the two zeners are oriented opposite each other when viewed from the top?
preiter  
#13 Posted : Friday, May 18, 2007 9:25:57 AM(UTC)
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Yes
preiter  
#14 Posted : Friday, May 18, 2007 9:26:35 AM(UTC)
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Brian Donegan wrote:
A pair of replacement zeners and a new LM318 (just in case) are on the way.

You should probably also (for kicks) check the resistance of C6 (and C11 for comparison) to be sure C6 is not shorting. I'll send you another one of those as well.


Thanks!
preiter  
#15 Posted : Thursday, May 24, 2007 12:26:06 AM(UTC)
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Well I pulled out ZD1 and tested it, it works fine.
Then I pulled out C11.
There's still a bunch of voltage across R1 (like 30V).

So it looks like -12V is shorted to ground somewhere.
Brian Donegan  
#16 Posted : Thursday, May 24, 2007 4:28:01 AM(UTC)
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Did you use the socket for the LM318? If so, pull the LM out and check voltages again.
preiter  
#17 Posted : Thursday, May 24, 2007 1:16:54 PM(UTC)
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Yep, pulled the lm318 and now there's no voltage across R1.

So the LM318 was pulling lots of current.

I wonder if that's because the chip itself is bad, or because the inputs are messed up and driving the output to one extreme.
Brian Donegan  
#18 Posted : Thursday, May 24, 2007 1:36:38 PM(UTC)
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Quote:
Probing right around the big resistors and the two zeners I get. V+ is 35V, V- is -35V, -12V is -12V and +12V is +2V.


With ZD1 pulled, you shoudl see 30V across R1, as it is no longer shunted to GND. I think you shoudl have pulled and tested ZD2 (+ rail) as it was the one not measuring correctly. Are you seeing a good +12V there now?

I am thinking it is a bad LM318 at this point, unless +12 is still +2.
preiter  
#19 Posted : Thursday, May 24, 2007 3:39:46 PM(UTC)
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Sorry, I'm mixing results from the two channels here. On this channel the -12V rail was measuring close to 0 and the 12V rail was measuring 4V.

So I pulled ZD2 and the -12V rail was still close to 0.
So I pulled the capacitor and the rail was still 0.
Then I pulled out the LM318 and it measured -35V as expected.

So the LM318 was sucking enough current to pull that rail down to 0, which takes 35 mA. Assuming something similar is going on with the +12V rail (forgot to measure it with the chip out), it's pulling 31mA off of that rail.
Russ White  
#20 Posted : Thursday, May 24, 2007 3:55:15 PM(UTC)
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Not that it matters much now, but pulling the zeners should never be done with the LM318 installed.
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