9090DB Can't get correct Bias

Although you responded above that R47-R50 are 4.7ohms, in the picture, the resistor color code still looks incorrect: the 4th band should be silver but it looks to be the same color as the the first band, i.e., yellow.
 
Hi Lee
Well you may well be right about those resistors. I ran out of Vishay and found these in my pile of resistors.They said 4.7 Ohm but I just took them out of circuit and note they range from 5 to 5.5 ohms.
I have 4.7s in my Mouser cart but ,tomorrow I will look to see if I have other so called 4.7 ohm resistors. I hope that a better replacement will do the trick.
 
I have not read the whole thread, but a difference between 4.7 and 5.5 ohms is not enough to have that much effect on the bias.
There's something else preventing the current flowing .... or they are 55ohm resistors or something along those lines...
 
Whatever it is - it's on those driver boards - keep looking, as it must be something you have changed. Try putting the old bias diodes back in?

How far out is the adjustment?
 
Last edited:
as measured one one of my units the bases of TR11/12 and TR13/TR14 should be at = 1.1V and -1.1V respectively and adjustable from the bias pot. If they are the issue is downstream. If they are not the issue is upstream..
 
Well I haven't had my morning coffee,yet.But you guys are keen. So I found some 1 watt 4.7 ohm and replaced R47-R50. Now I can dial in the correct bias. Not sure why exactly. Those other resistors measured from 5.1 ohm to 5.7ohm. But the new temporary resistors I put in have done the job.As I mentioned I ran out of Vishay and not sure of these others I used.
Will do some more testing this am. I am happy but not sure why such a modest change would make the difference.
 
Hmm,well dc offset and bias dialed right in. So can't complain.
Those resistors I used were the only change from the last 9090DB I did. They measured so close and are supposed to be 4.7 ohm I didn't think they were an issue.
I have two more 9090Db's to restore but I think I will take a break from that series until my hair grows back.
Appreciate all the help and advice.
 
Well I haven't had my morning coffee,yet.But you guys are keen. So I found some 1 watt 4.7 ohm and replaced R47-R50. Now I can dial in the correct bias. Not sure why exactly. Those other resistors measured from 5.1 ohm to 5.7ohm. But the new temporary resistors I put in have done the job.As I mentioned I ran out of Vishay and not sure of these others I used.
Will do some more testing this am. I am happy but not sure why such a modest change would make the difference.

Very odd behavior from the original replacement resistors, 5.1-5.7ohm should work, what is the exact color code on those resistors? In the picture, it looked like: yellow, violet, green, yellow, brown, i.e., 4.75Mohm 1%.
 
Lee you are correct. They are 4.7Mohm. Can't believe I missed that(yes I can). I never seem to deal with M ohm resistors and if I paid more attention to the colour code and looked more closely at my meter to see the M.
Sorry for the tail chasing. The color code is yellow,purple,black,yellow,brown but reads about 5Mohm.
 
Back
Top Bottom