Sansui 9090 (non DB) bias adjustment question

hcruz

Looking for that NLA mosfet that fell off th bench
After my AU-517 successful recap I decided to put my 9090 receiver on the bench for an overhaul.

After taking off the casing and faceplate the first thing did was check the offset and bias.

Offset was 7 mV on the left channel and 12mV on the right channel.
Not too bad according to EW sticky.

Now, with the bias I'm confused.
The manual states that readings should be 50mA across fuses (disconnected) F07 and F08 for left and right channels.

My readings of the right channel are 12mA. When I adjust VR04 I can only go as far as 21mA. going the other way around gets it to 8mA and cannot go any further either way.

Am I measuring the wrong thing or is there a serious issue with the receiver?

See Pictures below

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From Service Manual
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you are supposed to take the fuse out for the channel you are setting and use your meter in place of the fuse ,
it looks like you are doing it correctly but there is a problem somewhere .
does the other channel check out ok ?
 
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you are supposed to take the fuse out for the channel you are setting and use your meter in place of the fuse ,
turn whatever you turned up back down and start again

LOOK at the picture and then adjust your comment accordingly......Its exactly what he is doing.
Fuse out, meter across terminals......set to mA......
 
LOOK at the picture and then adjust your comment accordingly......Its exactly what he is doing.
Fuse out, meter across terminals......set to mA......

i was editing as you were reading and writing as it happens .
it first looked to me like the red probe was on a different fuse .
 
I did take the F08 fuse out.
I'll check the other channel and will post back.

Please do, you may have some drifted fuse resistors interfering with the bias setting. I am no kind of an 'expert' on this model so I'll let the more knowledgeable ones lead you on this.
 
Sansui 9090 Bias Problem

Please do, you may have some drifted fuse resistors interfering with the bias setting.

I do see a bunch of the light colored fuse resistor on the board.
I'll take the board out and measure the resistors.

39mA on the Left channel. Did not try adjusting.



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When I first went to do this on a 9090DB I was working on, the SM had the channels reversed. So if you are turning one VR it does effect the other channel, so please check that out. Also of the 14 fuse resistors on the board I would say 4-5 were bad/open.
 
9090 driver board fuse resistors

A question before I start removing parts.

R47 through R50 are 47Ω according to the manual but I think I have 470Ω on my board.
Here is a picture of R47.


47Ω or 470Ω?

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Just read that there are various versions of the board so here are a couple of pictures of the board I have.

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i see yellow violet gold 4.7 ohms .. 47ohms would be yellow violet black
last band sure doesn't look brown to me .
what does it measure ?
 
thats the earlier version with the VBE multiplier instead of the STV-3H diode for thermal bias regulation.....
 
i see yellow violet gold 4.7 ohms .. 47ohms would be yellow violet black
last band sure doesn't look brown to me .
what does it measure ?

You're right :thmbsp:. It does look gold comparing to another one with a brown band.
I also made a mistake reading the manual as it states 4.7Ω and not 47Ω.


With this project I'm going to try being more organized so this is the list of what I plan to replace on this board. Hopefully the parts chosen are the right ones.
The resistors would be metal film 1/2 W as specified in other threads.

(edit: changed Part# 555-RFS16V101MH3#5 for 647-UEP1C101MPD. 100μf 6.3V caps are non polar!)
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When I first went to do this on a 9090DB I was working on, the SM had the channels reversed. So if you are turning one VR it does effect the other channel, so please check that out. Also of the 14 fuse resistors on the board I would say 4-5 were bad/open.

Good spot patfont

Note in your first picture you have what appears to be the fuse for one side removed - but you are adjusting the trimmer for the other channel !
 
I do see a bunch of the light colored fuse resistor on the board.
I'll take the board out and measure the resistors.

39mA on the Left channel. Did not try adjusting.



8593784582_99d8f161a6_c.jpg

Your meter is reading .39 ma AC ! Should be DC! Need to manually change the function with the orange button on the fluke

Before making any more adjustments. Open up the back of your meter and check fuse F1 ( 440ma fast acting fuse).
 
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When measuring Current in this manner.Always start with your test lead plugged into the highest current(A) socket and highest current(A) range selected. Once you know whats actually there then you can safely down range manually.. Saves replacing those damn fuses.
 
Your meter is reading .39 ma AC ! Should be DC! Need to manually change the function with the orange button on the fluke

Before making any more adjustments. Open up the back of your meter and check fuse F1 ( 440ma fast acting fuse).

Newbie mistake :sigh:.
Checked the fuse on the meter and it's still ok. Thanks for catching that one :thmbsp:
I started removing the resistors last night so I guess I'll re-check after the parts are replaced.
 
Help on Protector Circuit Board 9090

So going through the other boards I've come to another fuse resistor dilemma :dunno:.
In the Protection circuit board the manual calls for 10Ω 1W resistors on R13 and R14 but the ones on my board are 4.7Ω

My first guess is that is not a big deal and I should replace with identical 4.7Ω.
Any thoughts?

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