Sx 939 weak channel

[IBDave]

I just picked up a sx939 got a great deal.One channel sounds weak and I also noticed the filter caps are bulging on top.The receiver is very clean and I don't want to damage it any further.does anyone know a tech in the SF bay area I included some pictures

 

[john stumpf]

the caps you showed have a plastic cover on the top that tends to bulge outward is it firm or can you feel a space? thats a solid aluminum can they are in.its better to look at the bottom which is more revealing.not saying that 45 yr old caps are ok its just that they dont seem to expire as fast as the smaller ones.

 

[john stumpf]

you might want to consider a deoxit treatment including the pre out jumpers on the back and maybe adjust the bias and offset before you shell out for a rebuild. provided you have the means and are comfortable doing that.

 

[IBDave]

you might want to consider a deoxit treatment including the pre out jumpers on the back and maybe adjust the bias and offset before you shell out for a rebuild. provided you have the means and are comfortable doing that.

Thanks for reply John I think I will get some deoxit I checked the dc offset and got 2.8 one side 7.2 on other but I don't know how to adjust I got $30 into it wanted to do some preventive maintenance i have a few vintage receivers that quit working

 

[john stumpf]

the procedure is simple as its in the data base or in hi fi engine in the service manual. there is an ERROR as it tells you to adjust to 30mv its an error it should be 15mv!!!! you will find that syringes work real well shooting d100 into the switches the same for the flush with d100.ive discovered that d100 take's TIME as well as cycling to get the most bang for the buck i let it set for a couple of days before flushing it. as far as resistor pots only use fader lube.ive got a 939 and a 9930 they are lovely sets. if you do decide to set bias and offset use aligator clips and only adjust until its warmed,20-30 min.( DONT move the clips with the set powered up!!!!) .that IS the RIGHT PRICE!!!

 

[IBDave]

the procedure is simple as its in the data base or in hi fi engine in the service manual. there is an ERROR as it tells you to adjust to 30mv its an error it should be 15mv!!!! you will find that syringes work real well shooting d100 into the switches the same for the flush with d100.ive discovered that d100 take's TIME as well as cycling to get the most bang for the buck i let it set for a couple of days before flushing it. as far as resistor pots only use fader lube.ive got a 939 and a 9930 they are lovely sets. if you do decide to set bias and offset use aligator clips and only adjust until its warmed,20-30 min.( DONT move the clips with the set powered up!!!!) .that IS the RIGHT PRICE!!!

Thanks John I bought some deoxit d5 and it worked the stuff is magic.The sx939 sounds amazing hooked up to jbl 4410a.

 

[john stumpf]

looks like the tuning needle "grain of wheat" is burned out.i take it the channel's are even now? be warned that there are 2sa725's that can cause loud popping noises as they age and will need to be replaced. from my experience its just a matter of time. my 939 did just that some years ago and that is how i discovered this forum!!!

 

[IBDave]

looks like the tuning needle "grain of wheat" is burned out.i take it the channel's are even now? be warned that there are 2sa725's that can cause loud popping noises as they age and will need to be replaced. from my experience its just a matter of time. my 939 did just that some years ago and that is how i discovered this forum!!!

Yes needs some lights replaced but both channels sound equal i have a sx1050 that started making loud popping noise.There is a guy on ebay selling restore kit with parts known to fail have you seen it?I have considered giving it a try what you think?
Dave

 

[ivandezande]

Yes needs some lights replaced but both channels sound equal i have a sx1050 that started making loud popping noise.There is a guy on ebay selling restore kit with parts known to fail have you seen it?I have considered giving it a try what you think?
Dave

Just use a BOM list from AK, or go through the service manual and make your own. You'll pay almost double for the components buying it in a kit. Just order it yourself.

 

[john stumpf]

thats a fact jack!!

 

[john stumpf]

that 1050 has 725's and 726's i do believe and some other undesirables as well. google the possibilities like "sx1050 rebuild-audi karma" there is a older rebuild list in the ak data base as well that lists the transistor subs as well as the caps

 

[Mr.tnt]

How do you do a bias adjustment on a SX939

 

[larryderouin]

Get the manual off HIFIEngine.com or the AKDATABASE. PAGE 19. It has directions (with pictures for those who use a visual approach) that tell you how to 1st set the center voltage,(idle current), and then the BIAS. No special tools needed.

John; Just an FYI. Bias per the manual is 50mv, should be 25mv. Same as the SX-1010, 838 and 737.

939 is closest to the 1010, not the 1050. Share a lot of boards, faceplate, etc.

Larry