The 1K resistors on the FEB of my Threshold 4000 went up in smoke and did a nice job on the PC board while at it. I’ll be replacing the 4 resistors and also wanted to change a few of the caps while in there. Unfortunately, 3 of the 4 caps I want to replace are wrapped with a black Threshold label. I removed it hoping to find the values but no luck. I have 2 schematics for the 4000 but neither is a match to this FEB.
Looking at the “sort of close” schematics I have, I’m guessing the 2 smaller caps are 47 uF. They look to be on opposite sides of the schematic and the polarity is switched (I think). They measure .001 uF and 20.5 uF. I labeled what I thought was + and – on the board, but I’m not even sure of that.
The larger cap is labeled + on the board. It measures 400 uF. I’m thinking 470 uF?
There is a board number: Threshold 053-0308.
Anyone work on this particular Threshold 4000 FEB and have an idea of these 3 caps?
Thanks
Looking at the “sort of close” schematics I have, I’m guessing the 2 smaller caps are 47 uF. They look to be on opposite sides of the schematic and the polarity is switched (I think). They measure .001 uF and 20.5 uF. I labeled what I thought was + and – on the board, but I’m not even sure of that.
The larger cap is labeled + on the board. It measures 400 uF. I’m thinking 470 uF?
There is a board number: Threshold 053-0308.
Anyone work on this particular Threshold 4000 FEB and have an idea of these 3 caps?
Thanks
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See if there are details available for the FT317 transistor near the caps.
390uF may be a value, unlikely.
470 +/- 20% is within range.
Put a newer material small cap if the seniors advise it for the one reading 0.001, check the app notes first.
22 may work...
390uF may be a value, unlikely.
470 +/- 20% is within range.
Put a newer material small cap if the seniors advise it for the one reading 0.001, check the app notes first.
22 may work...
There is a board number: Threshold 053-0308.
I think both the smaller caps need to be replaced. I don't think either is close to the value it should be.
I think both the smaller caps need to be replaced. I don't think either is close to the value it should be.
Thanks, That makes sense.
Any ideas about the values of the smaller caps? 47 uF, 100V?
Generic electrolytics, two for bootstrapping the drive to series outputs and one across the bias circuit. I will dig up the schematic later.
Thank you for that. The schematic is slightly different than what I have. At the series output I have two 1K resistor where there is a 1K and 1.5K, also the position of the 2.2K resistor and the 47 uF cap is swapped around (If mine is 47uF), I think I have it figured out.
The caps on my board in question are:
2 at 47 uF, 50 V
1 at 470 uF, 16 V
Would you mind taking a quick look at the marked schematic just to make sure I don't end up burning the rest of this board!
Thanks,
Eric
The caps on my board in question are:
2 at 47 uF, 50 V
1 at 470 uF, 16 V
Would you mind taking a quick look at the marked schematic just to make sure I don't end up burning the rest of this board!
Thanks,
Eric
Thank you all. It truly is amazing how fast I get help on this site every time I get stuck.
Thanks!
Thanks!
Why??
Either they were under-designed and would have failed young, or something else took them out.
I'd be thinking what else went bad.
I think age and heat did the resistors in. These resistors run extremely hot. This amp belongs to a friend, I was not aware that he had not turned it off in a year, he was of the school that you should leave your amplifier on 24/7. I have pulled and tested the electrolytic caps. None are in very good condition. Being originals from 1980 I wouldn't expect them to be in tip, top condition. Leaving the amp on for a year straight certainly didn't help. They may have contributed to the resistor failure.
When he got the amp a year ago, the same resistors on the other channel were burned up. In that case, it was just the resistors that failed. That side has been running fine ... for a year .... non-stop now! I'm guessing these resistors failing by themselves, without any other component failure, is a possibility.
I had rebuilt the output board on the same side as we're discussing today, a year ago, everything new. I don't think anything on that board failed this time but I will do a bit of testing once I get further into this. I'm not an expert but if an output transistor fails, I usually see the .68 (or 1) ohm resistor associated with it, burned fairly noticeable. All looks good on the output side.
I'll post up once things are back together.
When he got the amp a year ago, the same resistors on the other channel were burned up. In that case, it was just the resistors that failed. That side has been running fine ... for a year .... non-stop now! I'm guessing these resistors failing by themselves, without any other component failure, is a possibility.
I had rebuilt the output board on the same side as we're discussing today, a year ago, everything new. I don't think anything on that board failed this time but I will do a bit of testing once I get further into this. I'm not an expert but if an output transistor fails, I usually see the .68 (or 1) ohm resistor associated with it, burned fairly noticeable. All looks good on the output side.
I'll post up once things are back together.
Thanks all. That did. I swapped 4 resistors and those 3 caps in question above and all is good. I set the bias and did a few hours of endurance testing and it looks like the 4000 will live to see another day.
Thanks for the help,
Eric M.
Thanks for the help,
Eric M.