Pioneer SX-990 Trimmers

[mustang1102]

Love this forum, really appreciate the wealth of information I have gained from being a member for just the short time I have been on........I originally posted this in the General Discussion forum, and was advised that I may get more responses from this one. So, here goes.

I'm turning to all you experienced folks for a problem I currently am having. I have a Pioneer SX-990 receiver with no left channel output, blows the 2 amp left channel fuse as soon as it is turned on. Actually, all the output transistors are checking good, emitter resistors, drivers as well. Only thing I can find wrong so far is that the 50 ohm trimmer pot for the left side is OPEN, no continuity between the two outer legs on the pot. So I was wondering if you know of any suitable source and replacement for it, as well as the other trimmer on the same channel which is a 30k pot. Started looking at Digi-Key and Mouser, but have come up blank for replacement so far. Also not sure how high of wattage would be required for the two pots. Any suggestions would be greatly appreciated.

 

[Watthour]

½W is sufficient. All the supporting resistors are ½ or ¼W.

This should do your 50Ω application: https://www.mouser.com/ProductDetai...=sGAEpiMZZMvygUB3GLcD7rufck0TIBKyvMXhsw0qkjY=

This should do your 30KΩ application: https://www.mouser.com/ProductDetai...=sGAEpiMZZMvygUB3GLcD7swPUEOakrxnhFNZm3EDxuM=

 

[mustang1102]

Thank you Watthour.......(great name by the way!) I was looking at those originally, but was hesitant because the style is so much different than what is currently being used, which look like a regular potentiometer, three lug, with a short plastic slotted stem for adjusting. I appreciate your looking them up, I will order them tonight !!! I'm hoping when the trimmer opened, the fuse blew quick enough to keep the output transistors for cooking....they are testing good with a Sencore Cricket tester, so we'll see. Thank you again, great people on AK !

 

[Watthour]

Before placing your parts order, it may be good to verify a few other items in the set, and avoid paying multiple shipping charges. There are probably 2SC871s in the control amp, and there is also another half-dozen or so in the phono/head amp (W15-049). They can be noisy little sons of biscuits. BC550Cs can handle that nicely, as well as 2SC2240 or KSC1845Fs. Both are lower-noise AF amps. You might as well clean out the 2SC870s while you are in there, too.

Then there are the capacitors.

 

[mustang1102]

OK, so I thought about what you said and held off ordering the trim pot for now. Your right, I need to look at everything before I order, I was anxious to see if the trim pot was going to solve the fuse blowing problem for the left channel. Again, appreciate your guidance and input. Caps for sure, I didn't know about the issues with the 2SC871's, so I'll check those out too.

 

[Watthour]

If when you replace the components you suspect are the root of the problem, it would be VERY prudent to power the unit through a DBT for initial testing. One can usually be made for under $15, and can easily save that in burned components and frustration.

http://audiokarma.org/forums/index....udio-output-just-stopped.699663/#post-9371559

 

[john stumpf]

im presently working on a sx626,which in reality seems to be a warmed over 990, the easiest way to mount the bornes trimmers of your choice is to remove the old ones and gut them. the actual resistor is a 3/4 ring that is easily separated from the phenolic plate. its then quite simple to bend the bornes leads as required and solder to the contact eyelets a dab of jb weld to secure the new resistor to the phenolic plate and then re-install. while your at it you can solder two wires to each bias resistor to serve as test points to adjust the 15mv drop across the bias resistor. it wont be much fun to access otherwise

 

[mustang1102]

If when you replace the components you suspect are the root of the problem, it would be VERY prudent to power the unit through a DBT for initial testing. One can usually be made for under $15, and can easily save that in burned components and frustration.

http://audiokarma.org/forums/index....udio-output-just-stopped.699663/#post-9371559

I have spent countless hours reading tons of threads that deal with troubleshooting and repair of receivers, and the DBT seems to be a common denominator when troubleshooting. I have been lucky up till now, and have not had any issues. But, you're right, and I know I'm pressing my luck. I will take the time now to build one, as I hope to continue to making repairs to equipment and keep the history of some of this stuff going. Thanks for the push.

 

[mustang1102]

im presently working on a sx626,which in reality seems to be a warmed over 990, the easiest way to mount the bornes trimmers of your choice is to remove the old ones and gut them. the actual resistor is a 3/4 ring that is easily separated from the phenolic plate. its then quite simple to bend the bornes leads as required and solder to the contact eyelets a dab of jb weld to secure the new resistor to the phenolic plate and then re-install. while your at it you can solder two wires to each bias resistor to serve as test points to adjust the 15mv drop across the bias resistor. it wont be much fun to access otherwise

Interesting idea !!! I'll try it out and see how it works, I especially like the idea of the test points. Time to go order some parts !

 

[john stumpf]

if your trimmers are available in "Full reel" order those as they have much longer leads.this also applies to wima film caps if you plan on using any of those.

 

[mustang1102]

if your trimmers are available in "Full reel" order those as they have much longer leads.this also applies to wima film caps if you plan on using any of those.

Okay, thank you. Something I have been wondering/thinking about is: when the new trimmers are installed, should they be adjusted fully CCW, Midrange, or CW before applying power ?? Where would be a good starting point ?? If I get those 10 turn pots, where would I start ??

 

[mustang1102]

½W is sufficient. All the supporting resistors are ½ or ¼W.

This should do your 50Ω application: https://www.mouser.com/ProductDetail/Bourns/3386P-1-500LF?qs=sGAEpiMZZMvygUB3GLcD7rufck0TIBKyvMXhsw0qkjY=

This should do your 30KΩ application: https://www.mouser.com/ProductDetail/Bourns/3386F-1-303LF?qs=sGAEpiMZZMvygUB3GLcD7swPUEOakrxnhFNZm3EDxuM=

Watthour: Although the schematic I have specifies a 30k timmer for the SX-990 I'm working on, I need it in the configuration as shown in the attached picture, or else I'll never be able to get at it to adjust it. I can get this one, at 25k ohm, not 30K ohm. Would there be any major problem if I substitute this one for the 30k when going to adjust ??

 

[john stumpf]

use a 50k if your selected configuration isnt available in the stated resistance.the reason being that one of the outer legs is electrically tied to the center tap via the circuit board. in other words the extra resistance "hangs off the end" as it were.

 

[Super Noob]

I have spent countless hours reading tons of threads that deal with troubleshooting and repair of receivers, and the DBT seems to be a common denominator when troubleshooting. I have been lucky up till now, and have not had any issues. But, you're right, and I know I'm pressing my luck. I will take the time now to build one, as I hope to continue to making repairs to equipment and keep the history of some of this stuff going. Thanks for the push

Built this DBT... very easy to build.

 

[Watthour]

I would be a chicken, and opt for a 503 instead of a 253, even though the adjustment would be more sensitive.

 

[john stumpf]

your right tho about that style deuce devil to use in a 990.in my method of mounting all will work EXCEPT the side adjust multi turns.just make sure the leads exit on a narrow edge and the adjust is accessible.a multiturn is easier in the bias setting to adjust

 

[mustang1102]

use a 50k if your selected configuration isnt available in the stated resistance.the reason being that one of the outer legs is electrically tied to the center tap via the circuit board. in other words the extra resistance "hangs off the end" as it were.

Thank you. I was looking upwards of the 30k at 33k, 37, and 40k, but none of them were in stock. Looked at 50k and bingo, available in IN STOCK. Appreciate your response greatly, this is only my third attempt at restoring a vintage Pioneer unit, so I'm still learning.

If the middle leg does not line up well with the hole in the PC board, is it acceptable to bend it over and tie it to the outer leg that is common to it ??

 

[mustang1102]

your right tho about that style deuce devil to use in a 990.in my method of mounting all will work EXCEPT the side adjust multi turns.just make sure the leads exit on a narrow edge and the adjust is accessible.a multiturn is easier in the bias setting to adjust

Yup, forgot I was going to try your suggestion for mounting the new trimmer......memory kind of comes and goes, ya know ?? Too much partying in the younger years.

 

[john stumpf]

tell me about it !

 

[mustang1102]

I would be a chicken, and opt for a 503 instead of a 253, even though the adjustment would be more sensitive.

Yup, got it..... the 503's are in stock too !