So, quite some time ago I bought a PR 2100 EQ off eBay... and it arrived looking all of its age, and not working of course - not a surprise.
The other day I was looking around for a small project and decided to get this thing back to working condition.
Mission accomplished, at least it produces sound now and all of the controls and switches work as they did originally. I ended up replacing two smoked opamps, three transistors (for the LED array), and all of the LED's. Some of them were still working but the yellow was broken in half, and others were either dead or had physical damage plus they were all clouded from age. So, new LED's across the board. Looks a ton better too, though the anodizing has seen better days.
With all of that work done, the LED display also works BUT I cannot get the last red LED to light. The LED and the transistor for that segment are fine, I just can't get enough voltage across the whole array to get above lighting threshold on that last red LED. It's an interesting little circuit where you see a couple volt drop at each LED going right to left, so green LED 1 sees 12V on one leg, 10V on the other, green LED 2 is 10V/8V and so on. By the time the circuit gets to the 3rd (last) red LED, it's sitting at -8V/-8V on both legs and no amount of input will get the voltage across the LED high enough to light. The power LED is tied to the array as well, why I'm not sure but that's how they did it.
My current theory is that this problem with the one segment not lighting is that it's because I'm using a +/- 15V balanced supply (from Audio Alchemy) as a temporary bench power supply for the EQ, which is ok but not quite the same voltage swing this EQ would have had originally which IIRC was +/- 18V. That shortfall would limit the total voltage across the LED array, which starts a bit below rail voltage anyway because there are semiconductor junctions and resistors ahead of the starting voltage at the power LED.
My recollection is the PR 2100 and PR 250 amps generated +/- 18V on the DIN jacks to run the preamp, and that because I have 6V less the array runs short of voltage swing. But, I can't prove that theory easily without the proper supply voltages. I've considered modifying the preamp circuit to try to maximize what I can get from the +/- 15V but a) that's more work than I want to put in at this point, and b) it wouldn't work correctly if used with the original amp if I mod it that way.
So, I am hoping to find someone out there that has a PR 250 or PR 2100 that would be willing to work with me at least temporarily to finish this off, either a tech or someone that has a broken or working Fosgate amp that I can test this against. I'd keep it if I had the matching amp but as a standalone that's not usable with other gear, I'll probably resell it after it's tested. I may also go ahead and recap it before that - not that it absolutely needs it but there aren't that many electrolytics in it and they're all 40+ years old.
I'm also planning to put together a little tech note on how these work, the DIN plug pin-outs, repair tips, parts list etc. as there's an almost total lack of tech info on them on the internet currently. And, if any of you are sitting on one of these preamps (250 or 2100) and need it serviced, I'd consider taking a look at it - not guaranteeing I can fix any/all of them, but it's not too complicated as far as devices go and there is still some parts availability since they were designed around fairly common components from back in the 70's.
Next project: get my ADS 2001 mobile amp working again - another blast from the past.
John
The other day I was looking around for a small project and decided to get this thing back to working condition.
Mission accomplished, at least it produces sound now and all of the controls and switches work as they did originally. I ended up replacing two smoked opamps, three transistors (for the LED array), and all of the LED's. Some of them were still working but the yellow was broken in half, and others were either dead or had physical damage plus they were all clouded from age. So, new LED's across the board. Looks a ton better too, though the anodizing has seen better days.
With all of that work done, the LED display also works BUT I cannot get the last red LED to light. The LED and the transistor for that segment are fine, I just can't get enough voltage across the whole array to get above lighting threshold on that last red LED. It's an interesting little circuit where you see a couple volt drop at each LED going right to left, so green LED 1 sees 12V on one leg, 10V on the other, green LED 2 is 10V/8V and so on. By the time the circuit gets to the 3rd (last) red LED, it's sitting at -8V/-8V on both legs and no amount of input will get the voltage across the LED high enough to light. The power LED is tied to the array as well, why I'm not sure but that's how they did it.
My current theory is that this problem with the one segment not lighting is that it's because I'm using a +/- 15V balanced supply (from Audio Alchemy) as a temporary bench power supply for the EQ, which is ok but not quite the same voltage swing this EQ would have had originally which IIRC was +/- 18V. That shortfall would limit the total voltage across the LED array, which starts a bit below rail voltage anyway because there are semiconductor junctions and resistors ahead of the starting voltage at the power LED.
My recollection is the PR 2100 and PR 250 amps generated +/- 18V on the DIN jacks to run the preamp, and that because I have 6V less the array runs short of voltage swing. But, I can't prove that theory easily without the proper supply voltages. I've considered modifying the preamp circuit to try to maximize what I can get from the +/- 15V but a) that's more work than I want to put in at this point, and b) it wouldn't work correctly if used with the original amp if I mod it that way.
So, I am hoping to find someone out there that has a PR 250 or PR 2100 that would be willing to work with me at least temporarily to finish this off, either a tech or someone that has a broken or working Fosgate amp that I can test this against. I'd keep it if I had the matching amp but as a standalone that's not usable with other gear, I'll probably resell it after it's tested. I may also go ahead and recap it before that - not that it absolutely needs it but there aren't that many electrolytics in it and they're all 40+ years old.
I'm also planning to put together a little tech note on how these work, the DIN plug pin-outs, repair tips, parts list etc. as there's an almost total lack of tech info on them on the internet currently. And, if any of you are sitting on one of these preamps (250 or 2100) and need it serviced, I'd consider taking a look at it - not guaranteeing I can fix any/all of them, but it's not too complicated as far as devices go and there is still some parts availability since they were designed around fairly common components from back in the 70's.
Next project: get my ADS 2001 mobile amp working again - another blast from the past.
John
