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Advice on Parabee and failing C4S

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banjopicker:
One of my Parabees started producing scratchy distorted output at a reduced volume level. Once I isolated it to the amp (and ruled out the tubes) I flipped it over and saw that one of the two LEDs on the C4S was not lit.

Obviously the LED should be replaced, but is there anything further upstream that I should be looking at as a cause of the C4s failure? The manual shows 6x 100uf 450v electrolytics in the power supply that Doc has suggested degrade over time. Could old electrolytics on the PSU cause a cascading failure of the C4S? Would it be wise to replace the electrolytics while I have the soldering iron out? I can read a schematic but I in no way understand electronics theory.

As much as I would love to send this in to Doc for a factory refurb, it isn't an option financially, and these units are proudly DIY anyway (with a lot of community help). I am on my way out of town so can't get the amp on the bench with a multimeter before I leave, but I would like to get prophylactic parts order in so I can get right into it when I get back. Would appreciate advice on the most productive way to get the music flowing again.

Part of a complete Bottlehead system: Seduction, Foreplay 2, Parabee, and Straight 8s -- working pretty well after all these years, and a lot of fun.

Thanks!

Paul Birkeland:

--- Quote from: banjopicker on August 05, 2021, 04:30:35 AM ---Obviously the LED should be replaced

--- End quote ---
No, unfortunately that's just not how these things work.   There are three wires entering the C4S board, one is ground, one is B+ in, and one is plate out.  You'd want to post DC voltage measurements of all of those points.  It's possible that you need to debug the board, but it's equally possible that there's something else going on in the amp that's causing the issue.  I definitely would not pull out the board and replace the LED, as that's unlikely to resolve your issue.



--- Quote from: banjopicker on August 05, 2021, 04:30:35 AM ---Could old electrolytics on the PSU cause a cascading failure of the C4S? Would it be wise to replace the electrolytics while I have the soldering iron out? I can read a schematic but I in no way understand electronics theory.

--- End quote ---
This depends a little bit on what's in there now, but generally this is a good idea at the 20 year mark.  If you happen to have black Panasonic snap-in caps, I would leave them alone for another decade.

These are my personal Paraglows that I built out of a few dead amp carcasses I collected over the years and this is a service I offer for legacy product owners.

banjopicker:

--- Quote ---No, unfortunately that's just not how these things work.
--- End quote ---

Thanks for the reality check. That means I will have to wait for two weeks before I can look at it properly or order parts. I still have my manual, so I assume you are suggesting that I check resistances and voltages against the checkout values in the instructions. Will do when I get back and post any anomalies.

Thanks!

banjopicker:
Trip canceled so able to test the Parabee with a multimeter.

All the resistance measurements looked normal for a right side Parabee.

On the voltages I got around 330v for terminals 1 and 2, and then the voltmeter wouldn't settle down to give a reading for 15, A1, A2, A4, and B6. B8 showed .004v

Is there a way to test the C4S in-circuit?

Thanks for your help

Paul Birkeland:

--- Quote from: banjopicker on August 11, 2021, 06:38:30 AM ---On the voltages I got around 330v for terminals 1 and 2, and then the voltmeter wouldn't settle down to give a reading for 15, A1, A2, A4, and B6. B8 showed .004v

--- End quote ---
Do you have a 300B installed?

You'd want to have good voltages on A1/A2/A4 before even considering the possibility that there might be an issue with the C4S board.

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