SX 850 No Relay Click - Replacing Power Supply Board

[vwong688]

Hi,

There are lot of things going on on this SX 850 I am trying to restore.

I have replaced the capacitors and transistors according to the parts lists floating around on AK. I have to admit because it was my first time attempting electronics work like this, mistakes were made.

After initially restoring the power supply board, it was working great for a period of time until I decided to dial the BIAS in. This caused a capacitor that I had installed backwards to explode. With my BIAS in adjustment no mans land now but a new capacitor replaced in, the relay no longer clicks. I have gone ahead and recap the protection board which was very straight forward and I feel very confident with what I did.

Some other work I have done was switch out the old speaker terminals for new Banana ones. They were working fine after the work, before the exploding cap debacle.

Here are the readings for my power supply board right now. Note: I can't keep a fresh fuse in FU4 without it burning out immediately! This started happening after my novice hand shorted some pins while trying to take measurements....

Power Supply

1: 0

2: 0 (5.4v)

3: 0 (5.4v)

4. 4.3mv (13.9v)

5. 4.2mv (13.9v)

6. 65.6dcv (51.5v)

7. 65.6dcv (51.5v)

8 47.64dcv (36.5v)

9 47.64dcv (36.5v)

10 47.5dcv (36.5v)

11. 36.74dcv (28v)

12 .5 mv

13. -13.04dcv (-13.5v)

14 -51.58dcv (-51.5v)

15 -18.75dcv (-19v)

Protection

2. -13.28dcv (-13v)

9. 47.43dcv (35v)

10. 47.45dcv (36.5v)

11. 47.48dcv (36.5v)

12. 7.7acv

I have a spare power supply board at hand that was extracted from a confirmed working unit. It looks to have its Q7 and Q5 replaced before as well as new capacitors.

Given all the bad work I may have already done to the existing board, can I have some advice about attempting to switch the boards. I can see that I will have to resolder a lot of pins. From reading the forum, I have found it it is best to not undo the wire coils where there are.

Any wisdom will be greatly appreciated!

 

[jd3rdcoast]

Was everything working before you started your recap journey? Always best practice to fix ALL issues prior to moving forward with additional rework. Or wind up not sure if problem was resolved or your created more issues that are not harder to pinpoint.

 

[vwong688]

I tested the receiver after every capacitor and transistor I replaced. This has been an arduous crash course in electronics work for me and my lack of adept multimeter skills was definitely a hinderance. I ended up putting the new power supply board in and the relay clicked so all is well for now.

I'm a little skittish about checking voltages on the replacement board given my prior experience.

If i wanted to test capacitance can I just do it on the board. Do i turn the receiver on when testing?

 

[dcmeigs]

Make yourself a DBT, dead bulb tester. With a 100 watt bulb in series with your project, you wont continue to blow fuses. The bulb acts as a current limiter. You need to remove a cap from the board to test it. Glad you got your protection relay working again.

Simple DBT.

 

[vwong688]

Thanks for the tip!

Side question, my power cord is taped up by the previous owner. Only polarized cords are available in most outlets, how would I wire up a polarized power cord to my sx 850? Where does the white/black wire go?

 

[zebulon1]

I use donors from newer equipment. Trim the spade sides down so the plug can be used in either position. The non-polarized cord is actually a benefit but seldom needed.

Your getting the hang of this.
The line cord is important but not so much on transformer powered circuits. Some old tube radios ran without a transformer. A proper line cord wired correctly in this modern world could help in the safety aspect. The transformer-less chassis would become hot depending on which way the plug was connected in the line socket. The modern sets transformer acts as barrier and isolates the line voltage from the chassis.

 

[vwong688]

So i can't replace it with a non polarized cord?

Again pardon my lack of knowledge on the matter! From what I read I could wire the hot to the switched circuit and the neutral to the other one?

Would it make more sense to just cut the cord down the damaged area and buy a non polarized replacement plug like this one?

Plug

 

[zebulon1]

You can do that but its a little cheesy.
Yes you can wire the hot to the switched circuit and the neutral to the other one. This would be the best connection.
You could get this and file the spade down. Its a flat cord, well within specs. The Pioneer strain relief would have to be checked for dimensions making sure it will fit the new cord.
I don't think you can find a smaller gage line cord. Like a 20 gage. That's why I try to keep good vintage cords around.
When I go for a walk on the bulk garbage day I bring my wire cutters.

 

[vwong688]

Just replaced the cord, thanks for the tip!