Phono Board SX-737

[jmmani2]

First time poster and wishing I had thought to look here first. I’m 39 and got into vintage audio about two years ago. I’ve repaired a couple of turntables and a few sets of speakers. This summer I found a really nice SX-737 for a great price. Everything checked out and seemed to be working except the phono (sound on right channel but only static and pops on left).

I watched YouTube videos for two months before deciding to tackle replacing the capacitors and transistors. I bought a repair kit online from a highly reviewed seller and swapped everything out today. My first test after the repair showed that I actually made everything worse and now I have nothing on either channel.

Here’s the real kicker. Not once in my research did I find a warning against unwrapping the wire on the pins. So, like the newbie I am, to better access the solder joints I unwrapped all the pins to work and then rewrapped them to reinstall the phono board.

Won’t you know that the first post I found on audiokarma while trying to figure out what I might have done wrong was a post warning about the grave mistake of unwrapping wires from the pins. Sigh…

So, here I am unsure of how to proceed. I feel pretty confident that I replaced on the transistors and capacitors correctly. The new transistors were ECB, so I knew that they would go in differently than what I was removing. I don’t think my solder joints are awful for someone that has only soldered a handful of things. And I matched all the capacitors to the originals and the markings on the board.

I believe at least one of my issues is now in the wire wrap since I essentially broke the air lock that originally existed. What are my steps moving forward? Can I solder the rewrapped wires to the pins to create a better connection? Cut, strip, and use the wire wrap hand tool I got off Amazon to do try again? I have no techs nearby that I’m aware of and I’m generally a really handy guy and rarely need to call in professional help for repairs around the house. Admitting defeat here will hurt but I’m not above it if needed. I’d like to think I can solve this.

I’d appreciate any and all advice. I’ve attached some pictures of the board in its current state in case anyone can diagnose my issue from a picture.

 

[painttoad]

what transistor replacements did you use?

if those are ksc1845, they aren't installed correctly

you say the new ones are ECB but you have them installed with collector/ base swapped

 

[jmmani2]

The kit I bought came with two KSA992 transistors and 4 KSC1845. The directions said that the KSC1845s went in backwards compared to the 2SC1313 that were already there. It had the 1845s identified as ECB and said when you’re looking at the writing, the left pin is E, the middle is C, and the right one is B. I positioned the 1845s so that the left pin (E, according to the directions provided) was lined up with the small arrow (don’t know the official term) on the board.

Guess that might be my big mistake…

 

[painttoad]

Guess that might be my big mistake…

insert them like this

note the marking under the transistor in my pic.

you can see the emitter on the left-with the arrow.
the base is the flat line on the right
you can not see the collector-slanted line opposite the emitter

 

[jmmani2]

Thank you! I’ll start there. Should I get new transistors (were these likely made bad by installing them wrong and turning the unit on)? Or, can I just decoder, reposition, and resolder? I feel like they would be shot but maybe not

 

[painttoad]

Thank you! I’ll start there. Should I get new transistors (were these likely made bad by installing them wrong and turning the unit on)? Or, can I just decoder, reposition, and resolder? I feel like they would be shot but maybe not

i have reused them out of curiousity and saw nothing wrong but i replaced them anyway.

i suggest that you just use new ones.

 

[pa_200]

The pins of the 1845 are installed wrong. You may use my picture as help.
On your picture it looks like you made an EBC pin connection.
I strongly recommend, to get transistor tester like the LCR-T4 Mega328 on EVERY replaced transistor to identify the correct pin out?

 

[jmmani2]

Thanks everyone! I’ll take out and replace the 1845s. The others look okay? Also, any advice on the wire wrap?

 

[redk9258]

Always refer to the data sheets for transistors. The Mega 328 testers are great too.
These might help...

From KSA992 datasheet...


Swipped from some rando website..



I would solder the wires to the pins. Make sure the pins are clean and not oxidized. You cannot rewrap wire that has been wrapped. The sharp turns around the pin weaken it. This is what gives it an air tight seal.

 

[painttoad]

Also, any advice on the wire wrap?

if i disrupt a wirewrap, i don't try to recreate it. i solder it but the pins can be tough to get solder to bite. sometimes they need scraped. they always need flux.

if they give you grief, you CAN remove the pin and solder directly to the board. it's not something i do but i have seen people do it. i don't believe that it would be possible in every situation. it'd avoid doing it.

i've never used the wirewrap tool to wrap wire but from what i've seen, there is rarely enough wire to strip back fresh wire and wrap.

 

[redk9258]

Also note the sometimes the silkscreen on the PCB can be wrong. It's always a good idea to take multiple clear pictures before you start.

 

[jmmani2]

Always refer to the data sheets for transistors. The Mega 328 testers are great too.
These might help...

From KSA992 datasheet...
View attachment 3831331

Swipped from some rando website..
View attachment 3831332


I would solder the wires to the pins. Make sure the pins are clean and not oxidized. You cannot rewrap wire that has been wrapped. The sharp turns around the pin weaken it. This is what gives it an air tight seal.

Appreciate the input! Yep—I learned that wire can’t be rewrapped only after trying to rewrap it. Part of learning, I guess.

I’ll add some solder and see what happens.

 

[jmmani2]

Also note the sometimes the silkscreen on the PCB can be wrong. It's always a good idea to take multiple clear pictures before you start.

I was prepared for that but the instructions I got talked about the new transistors going in backward. I think I just misunderstood what backward meant in terms of transistors.

 

[jmmani2]

if i disrupt a wirewrap, i don't try to recreate it. i solder it but the pins can be tough to get solder to bite. sometimes they need scraped. they always need flux.

if they give you grief, you CAN remove the pin and solder directly to the board. it's not something i do but i have seen people do it. i don't believe that it would be possible in every situation. it'd avoid doing it.

i've never used the wirewrap tool to wrap wire but from what i've seen, there is rarely enough wire to strip back fresh wire and wrap.

I definitely think stripping new wire would leave me scrambling for enough length. I’ll try to add some solder and see what happens. Thanks!

 

[painttoad]

I’ll try to add some solder

flux first

 

[jmmani2]

Thanks!

 

[jmmani2]

The pins of the 1845 are installed wrong. You may use my picture as help.
On your picture it looks like you made an EBC pin connection.
I strongly recommend, to get transistor tester like the LCR-T4 Mega328 on EVERY replaced transistor to identify the correct pin out?

These pics were a big help. I definitely see the mistake now with the 1845s. I can’t quite tell about the KSA992 that I used to replace the original two 2SA725… not sure if I’ve got those in right or now

 

[painttoad]

I can’t quite tell about the KSA992 that I used to replace the original two 2SA725… not sure if I’ve got those in right or now

post a pic. we will tell you.

 

[jmmani2]

post a pic. we will tell you.

Oops. Thought I did

 

[painttoad]

Oops. Thought I did
View attachment 3831526View attachment 3831527View attachment 3831527View attachment 3831528View attachment 3831529

oh. sorry.

every transistor in every pic is installed wrong