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Old 10th August 2007, 08:22 PM   #1
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Default Carver M-500t Protection Relay

I've got a dead Carver M-500t that appears to be triggering its protection circuit. The relay triggers a few seconds after power-up. The voltage rail is sitting pretty at 74VDC, just like its supposed to.

I have a copy of the official Carver Repair Manual for this amp, but I have to admit its pretty skinny compared to most of the Carver manuals. It has the schematic, a parts list, and a page that lists the tune-up parameters, but it is completely lacking in troubleshooting information. There isn't a single page in the service manual that discusses what conditions will trigger the protection circuits or how the protection circuits work.

I had presumed that the triggering of the relay indicated that the overcurrent protection circuit had tripped because the amp had blown out a power transistor. In-circuit testing of all of the transistors on the heatsink has failed to show any shorted transistors.

The amp uses a compliment of 2SD845 and 2SB755 transistors by Toshiba in the output section. I've found a data sheet in English for the 2SD845, but I have only been able to find a data sheet in Japanese for the 2SB755. Unfortunately Japanese is Greek to me, and I'm still looking for the pinout information for the 2SB755.

There's no question that the big relay in the center of the amplifier board is tripping a few seconds after power-on. I'm just not sure why:

The voltage supply is normal (74 VDC) at the test point on the main amplifier board.

The bias test points look like the measurements are being taken across the two ballast resistors on each side of the push/pull compliment of power transistors. Although the difference between L/R channels isn't very far off, the offset for the two channels seems to be significant. (See attached diagram)

Diagram

Is this really a problem with the power transistors, or could the protection circuit be being engaged by something else? The PSU rails are at appropriate voltage, which suggests that the PS isn't providing a low resistance path to ground. Unfortunately the documentation on this amp doesn't list the conditions that can activate the protection circuit, which makes troubleshooting a bit more difficult.

Any help would be appreciated.
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Old 10th August 2007, 11:05 PM   #2
anatech is offline anatech  Canada
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Hi solderhead,
You didn't search well. Here is a thread dealing with eactly your problem, see here: Transistor Substitution

One thing I am missing is a clear copy of the schematic and the setup page. Could you do me a huge favour and sent me a full resolution scan of those things? We'll get both of those running in no time.

In case you are curious, I used to do Carver warranty service in Canada. My manuals went with my shop when I sold it.

Please send the scans to bhomester at gmail dot com.

Thank you,
-Chris
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Old 13th August 2007, 07:40 PM   #3
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i found that thread when i was searching. as it turns out my problem doesn't appear to be identical to the one in that thread. at any rate, you now have a copy of the M-500t service manual to help rebuild your library.

the biggest problem that i am having with this amp is isolating the nature of the problem that causes the protection circuit to engage. unfortunately, the M-500t Service Manual fails to list the conditions that are monitored by the protection circuit. it also fails to list any troubleshooting criteria in order to determine which circuit is at fault. unlike the Service Manuals for the M-1.5 type amps, which provide all of that data, this service manual isn't anything more than a Schematic and a Parts List.

Looking at the logic used in other Carver amps like the 1.5, the following circuits will shut down the PSU:

1 - clipping
2 - over temperature
3 - DC fault
4 - over voltage
5 - over current
6 - differential low frequency


I'm assuming that these circuits are also monitored by the M-500t. The question is how to determine which one is responsible for shutting down the amp.

I think its safe to rule-out # 1 and 2 with no signal into a cold amp.

I think I can rule out # 3 as the bias resistors for the output transistors only measure about 50 mV DC referenced to ground. (I'm measuring on the proximal side of the relay rather than at the outputs)

I think I can rule out # 4 as I have:
- normal voltages at C508 - C511 on the PSU board.
- voltages of +/- 75, 42, 75 VDC on the BCE of Q401-Q404
- voltages of +/- 43, 75, 43 VDC on the BCE of Q405-Q408
- normal voltage of 74 VDC on the test point at the amplifier board.

In addressing Condition 5, I have confirmed normal junction gap and resistance measurements on all of the power transistors, bias transistors, and drivers that are mounted on the heatsinks, as well as the predrivers Q111-Q114 as well as Q101-Q108. I have also rebiased the output transistors to 6.0 mV DC.

I'm not yet sure how to test for Condition 6.

At this point I have to admit that I have no idea why the protection circuit is engaged. For the first step, I would be happy if I could isolate the problem to the PSU board vs. the amplifier board.
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Old 13th August 2007, 09:55 PM   #4
anatech is offline anatech  Canada
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Hi solderhead,
Another condition to look for.

AC mains failure. This is the 1 uF cap I told you about. You need an oscilloscope to verify the proper operation of the power supplies.

Just thinking about this, check that you 15 VDC supply rails are up and running okay.

-Chris
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Old 14th August 2007, 01:07 AM   #5
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I'm not sure what you're referring to with respect to the AC mains failure.

The 1uF cap C517 has 20 VDC across it.

At the risk of sounding stupid, I don't see the 15 VDC rail.

I have a scope.
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Old 14th August 2007, 02:01 AM   #6
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Hi solderhead,
The protection network is still the same as the other case. So we check the same points. We are still concerned with Q201 through Q207 and the two thermal switches mounted, one on each heat sink.

Q207 is on the ground side of your relay, Q205 ultimately determines whether Q207 has base current or not.

So please measure (referenced to ground) E, B and C on each transistor.

-Chris
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Old 14th August 2007, 02:54 AM   #7
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I will go take measurements. Am I correct in assuming that Q205 is the transistor that is unlabeled on the schematic, right above D201 ?
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Old 14th August 2007, 03:01 AM   #8
anatech is offline anatech  Canada
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Hi solderhead,
Quote:
Am I correct in assuming that Q205 is the transistor that is unlabeled on the schematic, right above D201 ?
That's what I assumed it was.

-Chris
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Old 14th August 2007, 04:45 AM   #9
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Here are the numbers. I am reading the pinouts from Left to Right, looking at the flat side of the transistor with the numbering. As I understand the situation, a TO-92 is supposed to be E-B-C going left to right while looking at the flat side, though some transistors in the TO-92 case aren't pinned out that way.

Code:
All readings are in VDC
Trans    E       B       C
Q201   43.1    -0.59   43.2
Q202    0.7mV  12.4     6.6mV
Q203   12.4     0.7mV   5.78
Q204    6.8mV   5.78    0.7mV
Q205   11.8    43.4     4.61
Q206    0.1mV   4.55   -0.58
Q207   3.72    35.0     4.29
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Old 14th August 2007, 10:59 AM   #10
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Base and emitter should read a difference of about 650mV. Your numbers don't make any sense (206 and 207 might, if you swap your 'B' and 'C' readings).
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