protection circuit on a 9090db question

Rich Harkoff

New Member
Hello,
This is my first post I hope I get things right. I've been using AK for about a year and a half, this is an amazing place. Last fall I completed a restoration of a 9090db, it's up and running and sounds great. This web site has been a great help in bringing the unit back to life. A past owner apparently had some problems with the protection board and decided to outboard the relay using this huge relay in an octal socket and lamp cord to wire it in to the PCB. Relay was glued to the side of the chassis, the PCB was also broken in two places. I found a board on ebay, after changing the caps I installed it in the unit. Here's where we get to the question, finally. When power is applied it takes just shy of 9 seconds to come out of protection, this seems excessive since it takes just under 4 seconds for my 9090. I initially built up tnsilvers new driver board, and I also found an original on ebay and shotgunned nearly all the parts. No matter which driver I use the 9 sec timeout stays the same. It should be a lot quicker, right? I'm not educated enough in electronics to understand how the protection circuit works so I'm asking the experts here for some thoughts.
Thanks in advance for any help,
Rich
Green Bay
 
The protect circuit uses a RC time constant to develop the time delay. RC is a combination of a resistor and a capacitor. If either one is out of tolerance or the wrong capacitance/resistance value were installed. It would alter the time delay.
 
I could also be that it takes approx. 9 sec. for the amplifiers DC offset to stabilize.
 
But also, if the DC offset hasn't settled to below the threshold for triggering the protection circuit, it will remain engaged until such time as the DC offset falls below that threshold trigger voltage.

Posts crossed a bit - avionic said pretty much the same thing with different words. ;)
 
But also, if the DC offset hasn't settled to below the threshold for triggering the protection circuit, it will remain engaged until such time as the DC offset falls below that threshold trigger voltage.

Posts crossed a bit - avionic said pretty much the same thing with different words. ;)
:p
 
Thanks for the info guys. So other than the fact that it takes about 9 seconds, the delay is nothing to be concerned about?
Rich
 
Thanks for the info guys. So other than the fact that it takes about 9 seconds, the delay is nothing to be concerned about?
Rich

Three seconds is typical. If the timing circuit is off then 9 seconds is only an issue if you want it to be. If the offset is really taking 9 secs to get below threshold I'd be curious about that. It can be checked upstream of the relay to see what the problem actually is...
 
Hi Doug,
I just checked it again with my stop watch, 8.6 sec from the time I push power button till the relay clicks on. I have a concern that this might cause some problem at some point which is why I posted. I haven't fully buttoned up the amp at this point and haven't really cranked it up for too long. Still a bit cautious. The offset as well as the bias adjusted without any problem on both driver boards. I changed the EC's out on the relay board with same spec as the originals, I checked the board over initially and it didn't appear that any components were replaced. I'd like to get to the bottom of this if I can, where and what would I be looking for "upstream" to try and identify the cause?
Thanks,
Rich
 
Hello again. I'm a bit out of my league here, I do have some experience with electronics but haven't a clue on this. I did check the AC coming in, it was at 23.9v and it dropped to 23.2v when relay activated. You mentioned check the output of each amp before the relay, are you referring to the 945's at TR 1 & 2? Check voltages or ?
Rich
 
You need to check DC volts to ground/chassis (from power on) at the blue wire and the grey wire (Left CH and Right CH) from the bottom of the power supply board. This will tell you what the DC offset is prior to the protection relay closing so that you can determine if the offset is actually taking 9 secs to settle.
 
Hi Doug,
Well apparently the 9090db decided to fix itself. Since I got the unit up and aligned last November it's been sitting on a table in my studio, the only thing I have done to it was remove tnsilvers new driver for a stock driver I found on ebay and restored, that was in January. That driver is at the QRX guys for the "differential mod" and tnsilvers is back in the unit. The amp has been in use since November, all this while taking the approx 9 sec to come out of protection. Last night I set about hooking up my 2 meters to the blue & gray wires as you suggested, to do this I put the amp on its side. With meters hooked up, stopwatch in hand I hit the power switch and..........relay clicks in just over 3 sec, WTH? I shut everything down waited about 5 min and powered up, again it took about 3 sec. Next attempt was about an hour or so later, same thing, about 3 sec. I have no explanation for this, today I put the amp back down horizontally on the table and fired it up, and again it comes right out of protection in about 3 sec. The voltage drops quick, starts about 3.2v and relay clicks at about .02vdc, both left and right channels track pretty close voltage wise. So I guess it's fixed, but it's pretty weird. I did check the underside, all wires are bundled as the should be, no wires got pinched between chassis bottom and table. Any thoughts?
Thanks,
Rich
 
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