So I've had it for a while sitting in the closet, but it's finally time to get it working right. It would cut out sometimes, and seemed quiet on one side. The cover also needed some work so off it goes for a heavy sand and scrape, then a few coats of oil and it's looking nice.
Now that I have the cover off I might as well tear in and deoxit the scratchy volume pot and see whats wrong with the Dolby board. I found the post in the dolby selector had slipped out so turning the knob didn't actually move any switches.
I have pushed this back in, but is there a more permanent solution to this?

After all this it was time to do DC offset, it wasn't horrible, mid 20's on both channels, But I got it down to low teens. But now the fun/trouble starts.

DC Bias Current... :tears: Seems to be an error in the service manual, left channel is VR04 not VR03. After cranking 03 all the way from min to max and the current not changing more then 2-3ma I move to the other side, and blow the fuse in my multimeter. And then the board starts smoking... The 180ohm fusible resistor on the left side (right channel) had popped. I've replaced that, but the system still stays in protection when the driver board is in. All the 150ohm resistors measure +/- 3ohms.

I flipped the system on it's side to measure DC offset before the protection circuit and on the left channel only read a few MV of DC, but on the right side I get 50+ volts on the blue and grey wires.

So did I blow my outputs? And do I need to pull them out to test them? And if they do get replaced, should I re-do the whole unit, just one channel, or just the bad units? (with MJ21193/4)

Thanks, and I'll post some pics soon.

Yes you blew the outputs, possibly drivers, pre-drivers, and maybe more. Replace all 8 is my suggestion along with the drivers. Check the right channel real good before you order parts. You got one good channel to compare to. Replace all the trim pots also with Bourns or the like, and make sure the bias is not cranked all the way up when you power it up again. Welcome!

I'm taking a stab at this from the diagrams, but the drivers would be tr12 and 14, and pre-drivers are tr06 and tr08?

If the damage is only on the outputs and not in the driver board I should be able to flip the driver board around and see if the DC on the output jumps to the other side right? Or do I risk blowing the left channel output transistors?

Also has anyone tested these transistors before, can they be checked in place?

Well, you're in the right place, half the owners of 9090dbs have rebuilt them. the other half is gonna.


And, no, I also am looking for a method of testing the transistors....

Well, you're in the right place, half the owners of 9090dbs have rebuilt them. the other half is gonna.



Isn't THAT the truth...:rolleyes:

I'm taking a stab at this from the diagrams, but the drivers would be tr12 and 14, and pre-drivers are tr06 and tr08?

If the damage is only on the outputs and not in the driver board I should be able to flip the driver board around and see if the DC on the output jumps to the other side right? Or do I risk blowing the left channel output transistors?

Also has anyone tested these transistors before, can they be checked in place?

Do that and blow the other channels outputs and fry the other half of the driver board. DO NOT DO THAT! The drivers are the four largest transistors. If I were you I would check every single component on the bad channel.

Are you getting 50v on both the blue and the grey wires coming out of the bottom of the power supply board? If indeed both show 50v, then you have blown ouputs on both channels.

You need to pull the output transistors to check them properly. Remove the heatsink from the back of the unit and pull the transistors one channel at a time. That will leave you a reference to re-install them.

You also need to remove the drivers. Checking transistors in circuit can give false results.

- Pete

These are the voltages I'm getting, The one side seems like it's fine, but the other side is way out.

http://lh3.ggpht.com/_eehNF0RjToI/SWZUlOuYpdI/AAAAAAAADnY/diB59XaVZaI/s800/IMG_3930.JPG

poof
http://lh6.ggpht.com/_eehNF0RjToI/SWZUrIS3TKI/AAAAAAAADng/ERXFRVlOU_A/s288/IMG_3932.JPG

http://picasaweb.google.com/dsellinger/Sansui

Hmm, also not the first time this has been worked on...
http://lh6.ggpht.com/_eehNF0RjToI/SWZnkBIPC7I/AAAAAAAADpg/Zkyl317HTGc/s288/IMG_3940.JPG

Well the output transistors all measure ok. (nice they they just slide in, no desoldering required)
I guess that leaves the driver board. :(

Well the output transistors all measure ok. (nice they they just slide in, no desoldering required)
I guess that leaves the driver board. :(

Of course you replaced the white heatsink goo, and the mylars are intact...
Rumor has it that some members may rework driver boards for folks, I might PM either LBPete, or CDFixer if I were you.
Casey

and the mylars are intact...Mica.

Carry on.

Mica.

Carry on.

I stand corrected. Thanks Glenn.
Casey

PS. Packing a big old amp today.
C

Thanks, yes I'll be putting new thermal grease on before it's all back together. But I do need to decide if I want to replace the outputs so they match or not (I am wondering if this is why the right channel always sounded weaker).

After getting back from camping, I worked over the driver board pretty carefully and found everything to be ok. I haven't checked all the little diodes yet, but barring that, all the transistor and resistor values seem to match the other side. (Maybe both sides are blown :scratch2: )

Is there anything else I could check to see whats causing the protection light to come on?

So all the diodes test ok, including the 2 big ones mounted to the transistor heatsinks. I suppose I'll need to start desoldering transistors to measure them out of circuit. And I suppose they should just be replaced if I'm going to go to the trouble of pulling them.

I don't supposes there is anyone in the Bellingham WA or Vancouver BC area with a spare driver board I could pop in just to make sure that I'm on the right path?

So all the diodes test ok, including the 2 big ones mounted to the transistor heatsinks. I suppose I'll need to start desoldering transistors to measure them out of circuit. And I suppose they should just be replaced if I'm going to go to the trouble of pulling them.

I don't supposes there is anyone in the Bellingham WA or Vancouver BC area with a spare driver board I could pop in just to make sure that I'm on the right path?

It comes out of protection with the driver board pulled?
If so, that's the problem, that board. Are you sure you didn't put the board in 1 pin off? It happens. I only have 1 db model, and it awaits work on that same board.
It may be a bad idea to "borrow" a board, what if your receiver blew the loaner?
You could swap the outputs from one side to the other to see if the problem follows, and make sure those output leads are not slightly bent and shorting to the heatsink.
Casey

It comes out of protection with the driver board pulled? Yep
If so, that's the problem, that board. Are you sure you didn't put the board in 1 pin off?Very Sure
It happens. I only have 1 db model, and it awaits work on that same board.
It may be a bad idea to "borrow" a board, what if your receiver blew the loaner?
You could swap the outputs from one side to the other to see if the problem follows, and make sure those output leads are not slightly bent and shorting to the heatsink.
Casey I may try this, or try flipping the board around. The output transistors all check out ok, but I do know that a multimeter is a lot different a load then the amp puts on it. If I flipped the board and the problem jumped from the right channel to the left then i would have some verification that it was for sure the right channel of the driver board.

I started pulling transistors last night and found TR05 was leaking in a few directions. So I've got new pots, new caps new TR05/06's and a set of new output transistors to replace those Moto's.
Hope to have it all up and running by next week if the parts come quick. :banana:

Well thanks for all the help, I got it going this weekend and it sounds great.
And hopefully it will for a few more years.:music:
http://picasaweb.google.com/dsellinger/Sansui#
http://lh4.ggpht.com/_eehNF0RjToI/SYVR_5g_enI/AAAAAAAAD3U/-N5VfrvvQHk/s800/IMG_4305.JPG

Good job!

Awesome!

Great job! Another Sansui singing sweetly for the Sansui Forum!
Casey

Glad to see this one singing again, the 9090 DB is a fine receiver. One of the best ever made. They have their quirks, you now know more about your classic Sansui and have better odds of a long term relationship, my friend. Good Karma has gone your way. :tresbon:

Congrats! I love my 9090DB. My favorite receiver of all I've had.

Bringing it back from the brink should make it that much sweeter. :thmbsp: