Sansui 9090DB Protection Board Fused Resistor

[danrclem]

I bought a 9090db a couple days ago that wouldn't come out of protection. The guy that I bought it from said that it had been in storage for years and was working before but when he tried to turn it on after taking it out of storage it wouldn't come out of protection.

After some searching I found that the protection board (2657) has two resistors (R13 and R14) that if bad won't let it come out of protection. I also read that if you take the fuses out and that is the problem that it will come out of protection. I didn't unhook these resistors but both of them read open. I took the fuses out and it came out of protection so I guess I found my problem and hopefully there aren't others. My dbt went dim very quick. So far so good.

The service manual says that they are 4.7 ohm 1/2 watt fused resistors. The colors don't look like 4.7 ohm colors to me but maybe they got hot and that changed the color. Now for my questions. Is a fused resistor a special kind of resistor that I have to buy or just a normal resistor that is used as a fuse? I'm thinking that's it's a regular resistor but I just want to make sure. Is a metal film ok for this application? Assuming that this fixes it should I bring it up slowly with a variac and using a dbt or just leave it on the dbt for awhile to make sure nothing goes haywire.

Thanks, Danny

 

[The Fuxtor]

look for flame proof resistors of the same wattage . do not increase the wattage by a whole lot or you will negate the fuse action the resistors were meant for. I would try the dbt on first power up, who knows right? I am not sure about the 9090db, but i have a qrx5500 that wouldnt relay engage while under a dbt. Needed the full juice to make it click...
While you have the board out you really should check the rest of those fusibles, they tend to drift and/or go open after a while ( 35 years! )
Charles

 

[danrclem]

This should be a good fit, correct? I couldn't find anything at Mouser that said flameproof.

Those two resistors are the only ones on the protection board. There are some on the driver board and a few elsewhere. I guess I should hunt them all down and get a reading just to be safe.

Thanks Fuxtor.

https://www.digikey.com/product-det...onents/NFR25H0004708JR500/PPC4.7BCT-ND/614264

 

[The Fuxtor]

Look good, nothing bigger then 1/2 watt. Should do the trick! I had some 9090/8080 units that wouldnt come out of protection mode because of faulty fusibles on the amp board. Get them all replaced, they will all be out of spec.

 

[LBPete]

Have you changed out the fusable resistors on the driver board? That's the first place to start with a 9090DB in protection?

Please post the serial number for the database. Click on the link in my signature block.

- Pete

 

[danrclem]

Have you changed out the fusable resistors on the driver board? That's the first place to start with a 9090DB in protection?

Please post the serial number for the database. Click on the link in my signature block.

- Pete

I haven't changed out anything yet but I just got through checking all of them and there were several of them in the driver board that was very high. I'm going to order all of the fused resistors for the power supply board, driver board and the protection board. There are several of them that would probably be ok but I might as well do it all while I'm there.

I'll post the serial number tomorrow. Thanks for the info.

 

[danrclem]

Assuming that the problem is the driver board and I take the fuses out of the relay board then will it still come out of protection?

 

[sKiZo]

Replace ALL the fusible resistors. I used metal film here with good results. DO make sure you stand them off from the board as they get HOT. Putting little kinks in the legs so they sit proud of the board helps to keep things tidy.

 

[LBPete]

Assuming that the problem is the driver board and I take the fuses out of the relay board then will it still come out of protection?

I don't know. Never tried it. Besides, what would that get you? You won't get sound output without those fuses and you could still have bad fusable resistors in the driver board. The fusable resistors on the driver board are way more of a problem than anything on the relay board. For that matter, Sansui deleted the fuses on the relay board at some point during production.

- Pete

 

[danrclem]

Thanks for the advice sKiZo.

LBPete, I know what you're saying but I was just wanting to know if that would help in diagnosing a unit but odds are I'll never work on another 9090db again.

I'm very far from being a seasoned tech and most of my soldering has been on point to point. How hot should I get my soldering iron when de-soldering and soldering on this small board.

Thanks again to all.

edit; I almost forgot to send my SN to the database but it's done now.
I have a couple more Sansui pieces in storage and I'll dig them out and add them. That is if I don't forget.

 

[LBPete]

It takes a little trial and error. The short answer is hot enough to melt the solder quickly but not so hot as to lift the copper trace.

How are you removing the solder? Are you using a solder sucker or solder wick?

- Pete

 

[danrclem]

I'll probably use a solder sucker and if that doesn't get it all I'll then use the wick. Not lifting the trace was my main objective. I did that one time and don't want to do it again but my soldering iron then wasn't too swift. I have a Hakko 888D now so maybe things will go better with it. I'll try to hunt down an old board and practice.

One more question. If I have a 1/4 watt resistor that is fusible and one that isn't what's the difference? Will the fusible one open quicker?

Now I need to get off my hump and order some resistors.

 

[LBPete]

The fusible resistors are designed to open if too much current passes through them. Unfortunately the fusible resistors Sansui used have become unstable over time. The are not reliable and frequently increase in resistance. Sansui eventually phased them out. Replace them with metal film resistors of the same resistance and watt rating. Don't use ordinary carbon resistors. They can catch fire if overloaded.

The F-2624 driver board has the largest concentration of fusible resistors of any board I've seen. The values and locations are listed in the service manual. They are all 1/2 watt except R33/R34. Sansui lists them as 1/4 watt but replace them with 1/2 watt. They are the ones that fail most often and are typically the ones that cause it to stay in protection mode.

- Pete

 

[danrclem]

On Digi-Key's site they list some metal film resistors description as fusible but on Mouser's site I couldn't find a description of fusible. Not saying it isn't there but I just couldn't find it. Are all metal film resistors fusible or just the ones with fusible in the description?

I measured in circuit so I don't know how accurate this is but R33 read 302 k ohms and R34 read 140 k ohms. R41 and R42 read 232 and 211 ohms and R44 read 695 ohms. Most of the rest of them were in tolerance or not much over but they'll be changed out along with the three in the power supply and the two in the relay board.

 

[Hyperion]

Are all metal film resistors fusible or just the ones with fusible in the description?

Don't buy fusible resistors as replacements - they are a liability - just observe Pete's suggestions in the post above and you'll be good.

Resistors described only as 'Metal Film' are not fusible.

 

[danrclem]

Thanks Hyperion. I was just confused as to whether metal film and metal film fusible were the same thing with different descriptions. There's a lot to learn and I've barely scratched the surface.

 

[LBPete]

Be careful buying resistors from Mouser or Digikey. They stock very modern replacements that are sometime teeny tiny and look stupid on a vintage circuit board. Either look carefully at the spec sheet to see the physical size or get them from Parts Express or another supplier that caterers more to hobbyists.

For the driver board you need:
6 - 4.7 ohm - R35/36, R47/48, R49/50
4 - 150 ohm - R41/42, R43/44,
2 - 10 ohm - R39/40
2 - 180 ohm- R33/34

- Pete

 

[danrclem]

Thanks LBPete. I'm going to get those, 2 - 4.7 ohm for the relay board and 3 - 1.0 ohm for the power supply board.

You're right about the size. The ones that I've ordered from Mouser before were all very small. Some bigger resistors would be easier for these old eyes to see. I have my order picked out on Mouser but I haven't ordered yet because I need to pick some caps for another project. I haven't ever bought anything from Parts Express but I'll go there and see what they have.

 

[Skywatcher]

Thanks LBPete. I'm going to get those, 2 - 4.7 ohm for the relay board and 3 - 1.0 ohm for the power supply board.

You're right about the size. The ones that I've ordered from Mouser before were all very small. Some bigger resistors would be easier for these old eyes to see. I have my order picked out on Mouser but I haven't ordered yet because I need to pick some caps for another project. I haven't ever bought anything from Parts Express but I'll go there and see what they have.

It sounds like you have the earlier version of the 9090db. On the power supply board there would be the 1 ohm fusible resistors. They were "upgraded" in the later models to actual fuses instead of the resistors. They are a 1.5A 250V pigtail fuse that are covered with plastic tubing to insulate them, and soldered in "upright". This pair is right next to F01 and F02. Plus another single one that is a 1A 250V fuse next to F03.

 

[danrclem]

Mine doesn't have those three fuses in it but does have the resistors in what appears to be the same spots. Do they physically solder in to the same exact holes with no other changes?