Sausui 9090 Bias issue

[gto127]

Just recapped driver board on sansui 9090(non DB version). I am able to get 50ma on left channel but only 43 on right channel(pots are about as low as they go on both sides-10ohnm and 2.7). I had also replaced 4 shorted outputs on the side that is slightly low. I can find no resistor out of spec on either side. They are supposed to be within 1ma of 50ma. Any suggestions on what could cause this?

 

[JonSnell Electronic]

D05/6 low voltage due to low current flow or just old.
I am presuming you mean the Sansui 9090/8080

 

[gto127]

This diode does not seem to be available. I see where some people are using 2 IN4148 in series. Would you recommend that or is there a better solution?

 

[JonSnell Electronic]

2 x 1N4148 is fine. Check the voltage drop first.

 

[gto127]

Getting a volt drop of 1.25 volt in circuit with unit on with the originals.

 

[nigelwright7557]

It could be due to the new transistors.

I got caught out when I bought some new transistors for an old amp.
The amp would oscillate a bit on the output.
I checked the new transistors and they had a higher gain than the old ones.

 

[gto127]

Jon Snell:Went ahead & put in new diodes. Had no effect. Any other common failures?
NigelWright: My transistor checker is broke. Any other way to check gain on new transistors. They appear to be NOS sankens. I used same part number not a sub.

 

[gto127]

I reversed sides with the driver board & problems remains on that side so guess driver board is OK. Does this mean output transistors I bought off ebay are fake or bad? They certainly look like new old stock. Paid $30 for 4. Is there a modern equivalent? 2SC1116A were originals.

 

[llwhtt]

Since the idle current is so close and you ran out of adjustment you can either increase R33/34 OR decrease R35/36. Basically the base of TR07/08 is too positive. For modern replacements, may or may not fix the problem, you can try MJ21194, they are cheap and available.

Craig

 

[JonSnell Electronic]

Agree, if the replacements have a 0.7volt base emitter voltage and the old ones had 0.55 volts drop. they will have trouble biasing correctly. I thought the problem was on the driver board!

 

[Welcome]

Practically no such thing as NOS transistors, especially on Ebay. Pretty much guaranteed fakes. Use modern equivalents instead - from a reputable source (not Ebay).

Found this thread:

http://www.diyaudio.com/forums/parts/157391-sanken-2sc1116a-original-fake.html

Fakes of this particular transistor seem widespread. Not surprised.

 

[gto127]

If it's fake do I need to replace ? Are they unreliable?

 

[llwhtt]

How thick are the mounting bases of those xsistors? Real SanKens are a tad over 3mm thick. As far as reliability, if they're fake who's what they really are, could be overkill or not even close.

Can you post a pic of them? I have some real ones to compare.

Craig

 

[nigelwright7557]

Always buy from reputable dealers.

 

[gto127]

I will take pics in a few hours.

 

[tsd71]

Make sure you check all the fuse resistors on the power amp board. just had this issue with an 8080DB.

 

[gto127]

Here is pics. Measured .55 drop with diode test out of circuit.

 

[tsd71]

Did you do the 6 way diode test recommended by Echowars? Also check those resistors on the power amp.

 

[gto127]

Checked power resistors-OK. Not familiar with 6 way diode test-do you have link?

 

[llwhtt]

My examples are at work but those look pretty damn good if you ask me. Thick mounting base, lots of little nicks in the base, small diameter glass seals around the pins, and they seem to measure OK. I'll comment about the lettering in the morning.

I would do the resistor change I mentioned, move up OR down to the next value depending on what you have on hand. I usually use a decade resistor so I end up with the pot in the middle of it's travel. I was working on a Marantz 2275 yesterday and they used a 2.2k pot where all the amp wanted was about 50 Ohms, very touchy towards the end of travel. Knowing it only needed 50 Ohms I would use a 100 Ohm pot, puts the wiper close to the middle and not touchy.

Craig