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Borbely DC 100 Amp: Left Channel Troubleshooting
Borbely DC 100 Amp: Left Channel Troubleshooting
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Old 17th June 2018, 06:59 PM   #21
dbxdx5 is offline dbxdx5
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Sounds good. I'll check the voltages across the four 0.22 ohm resistors a little later and report back.

Funny you ask about a relay. That's one thing that I haven't looked very closely at. I don't have the amp in front of me now, but here's a so-so pic (below the top fuse holder in the photo) from the inside. Again, I'll take a look later to see how it's wired in, confirm continuity, etc.
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Old 17th June 2018, 07:19 PM   #22
Mooly is offline Mooly  United Kingdom
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Borbely DC 100 Amp: Left Channel Troubleshooting
Its a bit hard to decipher that from a picture tbh.

A pretty definitive check is to switch the amp OFF and then check for continuity from the positive speaker post back to the 0.22 ohms. With no relay present, both channels should have continuity. If one does and one doesn't then look for fuses and breaks in the wiring.

If there is a relay then the good channel (and the bad) will read as no continuity back to the 0.22 ohms.

The negative speaker post for both channels should connect to the amps main power supply ground or zero volt line.
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Old 18th June 2018, 02:40 AM   #23
dbxdx5 is offline dbxdx5
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You couldn't tell anything about that relay from my fantastic, detailed, close-up photo?


Well, it appears to be for power-on/off only, with no connection to the speakers. I verified by doing as you suggested and, with power off, testing for (and getting) continuity between the positive speaker terminals and the junctions of the 0.22 ohm resistors for both channels. So no speaker protection relay and no hidden fuses.



Here are my measurements across the 0.22 ohm resistors:


LEFT
K135 (top): +0.3mV
K135 (bottom): +0.2mV


J50 (top): +0.5mV
J50 (bottom): 0mV

RIGHT
K135 (top): +41.2mV
K135 (bottom): +3.0mV


J50 (top): +39.6mV
J50 (bottom): +3.3mV
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Old 18th June 2018, 07:12 AM   #24
Mooly is offline Mooly  United Kingdom
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Borbely DC 100 Amp: Left Channel Troubleshooting
I honestly couldn't get a relay of any kind out of the piccy Working from pictures is always difficult because you don't get any perspective or depth. Once you have seen it for real the picture would make sense... if you see what I mean.

I see the transformer, the red speaker post that may not even have a wire connected to it and two fuseholders, the upper one having a blue wire that has been snipped and a white wire that loops back on itself.

The readings...

The working channel appears to be biased to just under 200ma but the FET's are not sharing the current equally. That's another issue and tbh FET's from the same batch should be a lot closer than that. Nothing you can do at this time about that though.

The faulty channel is not drawing current and it should be.

The big question is what is going on here, because on a working amplifier (any amp FET or normal transistor) no bias current does not stop the amp working in any way.

Lets stick with this issue as it is something definite to work with. Also, we don't assume anything when fault finding, we look at every possibility.

So...

1/ Can you visually look at the FET's and confirm they are all marked correctly. Do any look to have been replaced. Ideally all batch numbers on like for like devices should be the same.

2/ The voltage between points A and B (which is effectively across collector and emitter of the transistor) should vary as you turn the bias preset.

3/ The voltage across the 150 ohm (points C and D) should also vary as you turn the preset. Points C and D go to FET gates (via the 220 ohm 'gate stoppers)

Setting the voltage across the 150 ohm in the bad channel to equal that of the good channel should give similar bias currents. Adjust slowly and keep feeling the FET's to make sure they do not start to get to hot.

Lets see what happens when you try and adjust it.
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Old 18th June 2018, 08:25 AM   #25
Trollet is offline Trollet  Sweden
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Borbely DC 100 Amp: Left Channel Troubleshooting
Hi.
Seems to be something strange going on here.
The outputs should draw about the same current among the pairs and I won't belive that matching would be that bad, or has someone swapped them around?
Probably they're OK because they usually go shorted when faulty.

Do we have a offset measurement ?
It's possible that only one half is on followed by big offset.
Their Vgs determine the voltages on both sides of R38 referenced to ground so it's normal that they differ.
But voltage across should be about the same for a given bias.

I saw that there were sockets for the mosfets.
Are they OK ? I had problems with contact on my Altec 9440.

Figge.
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Old 18th June 2018, 08:43 AM   #26
Mooly is offline Mooly  United Kingdom
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Borbely DC 100 Amp: Left Channel Troubleshooting
We do have a DC offset measurement but I also agree that something very odd is going on. The gate bias voltage we have now (for the bad channel) should still be allowing significant current to flow.

Although no bias will not affect the amplifier operation (apart from a rise in distortion) it does seem to be a 'real' problem on this amp that we can look at and see why there is no bias. That in turn might lead to seeing why there is no audio output.

One other thing... can you hear any hiss/hum/noise from the bad channel once a speaker is connected ?

At the moment all the pieces to the puzzle aren't adding up... but they will hopefully in the end.

Quote:
Originally Posted by Mooly View Post
One reading that can be inferred but needs checking is that voltage on L1 is zero. The driver voltages suggest it will be but you should check.
Quote:
Originally Posted by dbxdx5 View Post

As for L1, yes, the voltage is zero.
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Old 18th June 2018, 11:11 AM   #27
Trollet is offline Trollet  Sweden
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Borbely DC 100 Amp: Left Channel Troubleshooting
I locked again on the picture of the sockets.
One are of another type so someone have been there fiddling with it.
Bad contact there would definitely cause problems similar to those we got ?

Figge
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Old 18th June 2018, 12:50 PM   #28
dbxdx5 is offline dbxdx5
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Let's see...

1. The FET's all appear to be from the same batch. For the J50's, each has the Hitachi symbol and 0A2. For the K135's, each has the Hitachi symbol and 0C2. That said, Trollet is correct about one of the sockets being different: The top one for the J50 on the left channel is different.
2. The voltage between points A and B does vary when I turn the bias preset.
3. The voltage between points C and D, across the 150 ohm resistor (R38), also varies when I turn the bias preset. When the voltage between A and B was at +2.96V, it measured +1.64V between A and B, fwiw.

Confession time: I've been going by the owner's report regarding there being no sound out of the left channel. Yes, I should not have assumed that to be correct. I'll confirm later on.
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Old 18th June 2018, 01:30 PM   #29
Mooly is offline Mooly  United Kingdom
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Borbely DC 100 Amp: Left Channel Troubleshooting
No problem I've been a service tech far to long in a past life. Never believe the customer

The bias control voltage seems to vary OK and so you should be able to get the FET's to conduct the required bias current (by measuring the tiny voltage dropped across the 0.22 ohms).
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Old 19th June 2018, 12:08 AM   #30
dbxdx5 is offline dbxdx5
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I guess the operative word here is "should." Turning the bias preset has no effect on the voltage across the 0.22 ohm resistors when I checked them at the two K135's.


Here's something else anomalous: When I measure the voltage before the 220 ohm resistor at the FET gate of the top J50 (left channel still), I get the same figure I measure where the wire exits the PCB: 0.991V. But right after that resistor, and before its soldered to the socket terminal for the gate, I get -0.551V. For the corresponding J50 on the other side, the voltages are approximately the same before and after (as I see they are at the other 220 ohm gate resistors). I had earlier noticed this voltage difference at this specific resistor, and went ahead and replaced it, but it seems the resistor isn't the problem. Could this indicate a short of some kind with this J50?
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