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Old 31st January 2013, 10:29 PM   #11
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D108 and D109 according to my documents they attach to the base terminals of the pass transistors Q118 is2SB649A and Q117 is 2SD669A. look at the lower right hand corner of the print in your document. They are located right there on the print. these are your lower rail regulators circuits.
Perry wanted you to verify the +&-15 volts DC out of these two regulators. If its not there then ohm out the zener diodes they tend to short out by my experience on several of these....hope this helps some...
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Old 2nd February 2013, 05:16 AM   #12
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Quote:
Originally Posted by 1moreamp View Post
D108 and D109 according to my documents they attach to the base terminals of the pass transistors Q118 is2SB649A and Q117 is 2SD669A. look at the lower right hand corner of the print in your document. They are located right there on the print. these are your lower rail regulators circuits.
Perry wanted you to verify the +&-15 volts DC out of these two regulators. If its not there then ohm out the zener diodes they tend to short out by my experience on several of these....hope this helps some...
Interesting... all these elements are actually on a smaller board that connects to the main board by a set of 14 pins and a set of 10 pins plus a whole bunch of goopy glue.

Ohming out the diodes is frustrating with my DMM because it will start out at a random number for resistance, sometimes negative, and count up incrementally until it hones in on a number. It seemed like the DMM would keep counting up indefinitely in some cases - not sure what to make of that.

That said, only some of the diodes on the board would measure a resistance in both directions whereas some would show the intended ~infinite resistance in the "wrong" direction.

D108, D109, D112, D114, and D159 allowed measurement of resistance in the "wrong" direction. :/

D111 and D113, on the other hand, immediately showed infinite resistance in the "wrong" direction.
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Old 2nd February 2013, 05:35 AM   #13
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This may not be easy to diagnose without power applied.

What sort of goop is on the board?

Is the noise a constant tone or an intermittent/random sort of a noise?
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Old 2nd February 2013, 06:08 AM   #14
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Quote:
Originally Posted by Perry Babin View Post
This may not be easy to diagnose without power applied.

What sort of goop is on the board?

Is the noise a constant tone or an intermittent/random sort of a noise?
Some sort of yellow-ish glue, old and hardened.

The noise is a constant and loud tone, as though the unit is receiving a constant signal. It does attenuate/chirp to the beat and starts making some interesting noises if the RCAs are plugged in.

I have access to any number of oscilloscopes at work - not sure about a DC power source...
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Old 2nd February 2013, 06:11 AM   #15
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If it's a yellow/beige fixative that's turned brown and hard, it needs to be removed completely from the board.
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Old 2nd February 2013, 06:53 AM   #16
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Quote:
Originally Posted by IDiggity View Post
Interesting... all these elements are actually on a smaller board that connects to the main board by a set of 14 pins and a set of 10 pins plus a whole bunch of goopy glue.

Ohming out the diodes is frustrating with my DMM because it will start out at a random number for resistance, sometimes negative, and count up incrementally until it hones in on a number. It seemed like the DMM would keep counting up indefinitely in some cases - not sure what to make of that.

That said, only some of the diodes on the board would measure a resistance in both directions whereas some would show the intended ~infinite resistance in the "wrong" direction.

D108, D109, D112, D114, and D159 allowed measurement of resistance in the "wrong" direction. :/

D111 and D113, on the other hand, immediately showed infinite resistance in the "wrong" direction.
The diodes in question will most likely be shorted and read a dead short on your meter or something near zero ohms. Bouncing readings are normal when there are capacitors near by charging and discharging as you reverse the probes. On dead shorted diodes they will read dead short or some very low readings like 20 ohms or less, usually zero in most cases. On the rare situation where the diodes go open they also sometimes are broken or exploded on their case visibly. You might want to test with live voltage each pass transistors output to see of +&- 15 volts are being delivered like Perry suggested if the readings are causing confusion.

And perry is correct that fixative they use should be removed carefully with dental tools. It tends to harden and turn colors as its useful lifespan is used up. It can in some cases cause problems like shorts it it carbonizes too badly.
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Old 2nd February 2013, 01:38 PM   #17
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I don't know as much as these other guys on here but I'd say it may be the diodes. You've at least isolated it the power side of things. to be sure you could use some rca silencers to short the input make sure it's producing noise regardless of signal.
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Old 6th February 2013, 12:18 AM   #18
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Thanks, all, for the replies.

I'm told I might have access to a suitable power supply at work. I'll see if I can test voltages in a live power setup.

In the meantime, I thought you all might get a kick out of the response from tech support:

Quote:
Originally Posted by TechSupport
Before I refer you to a service center, being that you already replaced the volume pot and you obviously know what you are doing, let me make a suggestions. Judging by what you have written, it almost sounds like the problem is a bad ground or cold solder joint. Please try tightening everything inside the amp, and go over all solder terminals to let the solder flow again. This may fix your problem.
Quote:
Originally Posted by Me
I am actually quite a novice at soldering on PCB boards. Do I understand you correctly that I should re-flow every solder joint on the circuit boards, or are there specific joints I should be looking for?

I checked the resistance on the internal trace that connects with the exterior ground connection - after my fix job, the resistance is less than 1 Ohm (about 0.75 Ohm).


A car audio expert on an online forum suggested that the diodes tend to short out in these units. Could that possibly be the cause?
Quote:
Originally Posted by TechSupport
Yes, a good idea is to let the solder flow on every solder joint. Of course you donít want two joints to flow together unless they were already that way. While I have not heard of a diode issue on these amps, to be honest, anything is possible. But the two suggestions I made is always a great starting point when you have a problem like the one you have described.
There's gotta be maybe hundreds of solder joints on multiple boards that are soldered and glued together.
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Old 6th February 2013, 02:30 AM   #19
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I don't agree that resoldering all of the joints is a good idea. I've seen too many instances where someone did this and bridged a joint. That makes it virtually impossible to troubleshoot via an email or forum post.
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Old 11th February 2013, 01:01 AM   #20
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Update: I've poked around enough now to find out that I have access to a bunch of benchtop electrical equipment at work.

The easiest items to access:
HP 54600A oscilloscope
RSR FG-32 sweep generator
Tenma 72-2085 DC power supply (up to 24V, up to 6A)

Will the above power supply provide enough current to do the kind of testing I will likely need to do? I'm sure something that can provide the amps that a car battery can would be better, but I'm just looking for "sufficient for testing" here.

Experts please weigh in. Many thanks.
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