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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Cyrus Power Amp Flashes Yellow
Cyrus Power Amp Flashes Yellow
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Old 13th August 2018, 12:40 PM   #1
Tenson is offline Tenson  United Kingdom
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Default Cyrus Power Amp Flashes Yellow

Hello All,

I have an original Cyrus Power and Pre combo (the one that's 23 years old!).

Three times now I have been listening to music just fine for a couple of hours then all of a sudden the Power flashes the LED yellow. The manual says this means DC was detected at the outputs.

I have already tried going over all the joints to refresh possible dry solder. No help.

Any tips on where to go next? On the corner of the PCB there are some very hot looking (blackened PCB) regulators and bits. Maybe they need replacing but I'd have expected ICs to either work or not.
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Old 13th August 2018, 01:57 PM   #2
lineup is offline lineup  Sweden
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There should be a potentiometer to adjust for DC.
More to the beginning/input of the circuit.
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Old 13th August 2018, 02:17 PM   #3
Mooly is offline Mooly  United Kingdom
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Cyrus Power Amp Flashes Yellow
Parts that run hot are primary suspects, and yes regulators can and do fail more often than might be realised. Almost always this is down to the way they have been used rather than an inherent issue with the devices.

If you are 100% certain no dries are the cause of the issue then I would look to replacing the VAS transistor in the power amp on the grounds that it probably runs hot and is probably of a type/package/manufacturer that is known to suffer various intermittent failure modes. If you suspect the regs and they are in a part of the circuit that could affect DC offset then just change them.
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Old 13th August 2018, 07:53 PM   #4
Tenson is offline Tenson  United Kingdom
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I checked the DC on the output and it does seem much higher on the left channel. I watched it get as high as 240mV. I bet it switches the amp off at 250mV.

There is one small pot per channel, but it doesn't seem to affect DC offset. There is also a bias test point and I get about 3mV on there. Here is a photo.

Any tips what I need to check or adjust?

Click the image to open in full size.
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Old 13th August 2018, 08:04 PM   #5
EmiliNickson is offline EmiliNickson  United States
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That's pretty cool. Where did you get it from?
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Old 13th August 2018, 09:17 PM   #6
Tenson is offline Tenson  United Kingdom
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Sorry I meant to post a bigger image.

It's just a Cyrus 'Power', nothing that rare. I bought it on eBay about a year ago to match the Cyrus 'Pre' I had got many years before from a forum.

Click the image to open in full size.
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Old 14th August 2018, 07:25 AM   #7
Mooly is offline Mooly  United Kingdom
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Cyrus Power Amp Flashes Yellow
We would really need to see a circuit diagram to be able to advise further. A DC offset that alters slightly isn't a common fault generally. Most such faults give a definite swing to one of the rails rather than a small indeterminate offset.

One possible approach could be to let the offset trip out and then very carefully use freezer spray to see if any of the small transistors respond to cooling. You can use a can of cheapo air duster (poundland) as freezer by inverting the can. You need to practice dispensing the stuff literally drop by drop. If you blast a whole area then you have no idea what you actually cooled.
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Old 14th August 2018, 02:21 PM   #8
Tenson is offline Tenson  United Kingdom
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I went over all the through hole parts with fresh solder, but I didn't touch the SMD parts. I will try sticking some flux and hot air on the SMD components and see what happens.
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Old 19th August 2018, 01:31 PM   #9
Tenson is offline Tenson  United Kingdom
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Right.. I found that dropping a bit of alcohol on T101 made the DC bias go all over the place. Blowing on it makes it reduce. I also found just poking it with a stick makes it fluctuate a little. So I was thinking I found the culprit, but actually poking T201 on the other channel also makes the DC bias vary, so I dunno.

I suppose I'll try swapping the part. It is a SOT23 with 3B on it and a Z (or sideways N) in the top left corner. Anybody able to identify this? Looks like BC856 of some kind?

Click the image to open in full size.
Click the image to open in full size.

If it helps, the circuit may be similar (but not identical) to the Cyrus 3 from the same time period.

Click the image to open in full size.
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Old 19th August 2018, 01:41 PM   #10
Mooly is offline Mooly  United Kingdom
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Cyrus Power Amp Flashes Yellow
Hmmm... not so sure on that. Changes in bias current shouldn't trip anything. Also moisture (conductive) could be a factor when dripping ISO onto things.

Poking the board definitely shouldn't alter things to any degree. Preset resistors might be influenced a little but that all.

Things like this are suspect. Notice how the solder appears cracked around the joint. You need to look closely at all components to check the soldering.
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