Is your 9090DB stuck in protection mode?

Russ57

New Member
One of your members was kind enough to help me out with some part values for my 9090DB that helped me fix it. In return I promised I would post something about the unit. Well I love the way it sounds and it mates very well with Large Advent speakers.

I have noticed that a lot of people are putting thiers on the shelf or dumping them due to the units being stuck in protection mode. Generally that would indicate some bad transistors but that wasn't what I found at all! Instead I would suggest one replace all electrolytic caps on the driver board upping the voltages some. Also in various places in the unit they used a different type of resistor. Most of the resistors are mounted vertically and look like carbon films. The problem ones are horizontal and look like metal oxides. I encourage you to replace them. I found many open or way way out of tolerance. Next adjust the bias current. On the main power supply board you will see two fuses that run front and back where are the other fuses run right to left. Lift on fuse and put a meter across it that will read milliamps. Then adjust the pots on the driver board (the one that mounts upright and is held in place by a piece of metal). The fuse on the right goes with the pot on the extreme right and the fuse on the left goes with the pot on the extreme left. The book says 30 milliamps +/- 1 milliamps but I found it would trip protection mode at 31 milliamps. So might be wise to set it a little low but make this adjustment on a warmed up unit. There is also a thermal switch that mounts on the heat sink between the output transistors that could be the problem. It should read open. Be sure all fuses are in place, eight at least, and all the proper value and type. The unit is so well fused that they can help point to what transistors if any are bad. In many cases right before the fuse they also have a 1 ohm or some such low value resistor as a back up to the fuse in case some idiot replaces the fuse with a piece of wire so check those too.

Couple of other little points. Power meters don't work? Try and see if they work with the Dolby and if so move the pots on the protection board, accesible after removing bottom cover and flipping unit over. The speaker protection fuses are there too and if blown unit won't go into protection mode but you will have no sound. Unit acts flaky....static sound....radio station bleed through..etc. Clean the dolby selector pot. Everything runs through there so bad channel balance and a host of problems can be solved by a good cleaning. Too bad it isn't easy to get to without a lot of work but do what you can. In fact on all vintage units cleaning pots and selector switches is a good first step.

Thats about all I can think of right now. Hopefully this will help some of my fellow tweakers get their unit up and running. They are nice items and worth the time and effort. Might make some of you more comfortable about buying one stuck in protection mode.



Happy listening,
Russ
 
Any piece of vintage gear is a crap shoot. Who really knows it's history, unless you're the original owner.

It's all about bringing them up to snuff & keeping them there. As I've said before, a lot of people don't seem to understand that.

Now how much time & money you want or need to spend to do that is another story.

So to answer the posted question.... NO... Neither of my 9090DB's is stuck in protection mode.

Just my 2 cents.
Moon
 
Hi Russ:

Thanks for taking the time to post the very informative Info.

Another thing to mention might be the relay, which when its acts up, might cause the stuck-in-protect mode as well, in which case cleaning the contacts or replacing the relay altogether might correct the problem.

Regards, B/F.
 
Thanks Russ for the info. I have a 990 but I'm sure it pertains to my model as well. I have one power meter that does not work. I have a replacement, but have not put it in yet.
The only other problem I have with my 990 is in the selector switch. When set to am or fm, the display will not light up. But if I put it between selection, the display will light up.
I have cleaned the selector switch alot, but I'm thinking I may have to get a very fine file or paper and sand each contact of the switch. It's hard to get to, but I don't know what else it could be. I will post my results here when I do it.
 
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