Sony SL-C9 Betamax DC-DC converter

[Freya]

I have been having a go at this converter from my betamax, it has a MK2 type converter fitted that has failed.
Using information already available online i proceded to replace the capacitors one at a time and testing each, they all proved bad from open circuit to >40 ohms on the peak ESR.
At this point i felt confident to put it back and try it , no such luck !

This time due to the very confined space on the pcb i removed a transistor at a time and tested, all OK except Q2 a 2SD789 inverter transistor that has an open emitter. (on order)

My question is looking at the picture of the inverter transformer does it have green spot corrosion or is this a process of manufacture.

[MALC SCOTT]

It looks to me no more than light corrosion on the lead out pins. They need a good clean and resolder, Malc.

[Sanbeta]

Originally Posted by freya2871

I have been having a go at this converter from my betamax, it has a MK2 type converter fitted that has failed.
Using information already available online i proceded to replace the capacitors one at a time and testing each, they all proved bad from open circuit to >40 ohms on the peak ESR.
At this point i felt confident to put it back and try it , no such luck !

This time due to the very confined space on the pcb i removed a transistor at a time and tested, all OK except Q2 a 2SD789 inverter transistor that has an open emitter. (on order)

My question is looking at the picture of the inverter transformer does it have green spot corrosion or is this a process of manufacture.

Ah yes the good ole DC to DC.
I have had five of these repaired and successfully managed two myself.
ALL caps will be bad and the Q2 usually.
Occasionally a resistor will be bad too.
What method did you use for testing the transistor? I have yet to master that.It seems analogue testing meters seem better for that.
I do have the Peak ESR/Capacitance and transistor testing meters.

Cheers.................Kev

[Nickthedentist]

I agree with Malc. Remove it from the PCB, clean the pins and wires with a fibreglass brush, resolder the wires to the pins, clean up the PCB if required, and resolder the transformer to the board.

Presumably caused by some flux which wasn't washed away, or by electrolyte leaking from caps.

Check the PCB's traces aren't similarly affected.

N.

[Freya]

As yes Nick, that idea of the caps causing the green slipped me by. Already removed, cleaned and resoldered.

Kev,
The transistor was obviously wrong when i check with the Fluke DMM, but i did put it on the Taylor transistor tester which proved the same defect with the emitter junction being faulty, not checked the resistors but will do.

Thanks for the input.

[af024]

Watch out for C3 going s/c too Stephen!

See also here:-
https://www.vintage-radio.net/forum/...ad.php?t=95103

Regards,

Andy
PS I've had one transformer go s/c primary to secondary too (not been able to fix that one!)

[Freya]

Thanks Andy, did not think to check the polyesters. C3 is 0.0068uF ?

[Freya]

Well it seems i have been unlucky with this one, the transistor is in but still not playing ball. Checking the transformer again now shows up an internal breakdown

If anyone has a possible transformer or dc-dc spare maybe. otherwise its going to live with no display or tuner (not needed anyway)

[Freya]

Pin 10 on the transformer is open circuit which is a primary, carefully removing the tape over the coil reveals only that the primary is the first wound on the former (typical)
If nothing turns up i may have a go at de-constructing it,

[af024]

Hello Stephen,

I am sorry to hear that it's the transformer. I too have a similar problem (althought mine is primary to secondary short) and also wondered about pulling it apart to try a re-wind, but it looks really daunting (very tiny indeed). Goodness knows what the wire gauge is. I bet I'd hardly see it.

Best of luck if you try it. You might give me inspiration and encouragement if you manage it.

All the best,

Andy

[Freya]

I have been thinking that sony would not have made the transformer just for this model so it must be elsewhere on other bit of equipment. Trying to match the part number is the key.
My idea if i strip it is to colour code the wire ends with nail varnish, hopefully making it easier for rewinding.
The other thing found on examination is the binder tape looks melted onto some of the outer turns.

[Maarten]

Sony absolutely loved using custom parts and had a habit of designing a circuit first that matched all specifications and requirements, then searching for the suitable parts and transistors and if the latter didn't exist actually start manufacturing such transistors instead of adopting the circuit design to compromise.

Chances are, an easily customised part such as a transformer, won't be found in other models.

[vidjoman]

The green pins are reminiscent of the use of acid flux (Bakers fluid) which is very corrosive if not washed away properly. Perhaps the T/X pins were dipped in a shallow bath to solder the wires and liquid flux, containing acid, was used.

[Freya]

Bumping this post in the hope someone may have a scrap machine or a faulty dc - dc convertor knocking around, only the transformer is needed.
Testing again today has shown that the winding between pins 1 &2 and pins 3 &4 are open. The remaining windings are ok.

[HamishBoxer]

Sadly a common failing,see Betamax website.

[Freya]

As I had nothing to loose I thought I would have a go at unwinding the transformer, so after making detailed notes I started with the outer most coils. These happen to be the good ones.
After the third very long one was off it was very obvious that I had uncovered some green decomposed wire, examining using a Lupe showed it was only about 6mm away from pin 1. I decided to unwind at the break, one turn which at that diameter was only 10mm and remake to pin 1.

more tommorow

[HamishBoxer]

Sounds good.

[Freya]

So this morning I have powered up the little dc-dc with its reinstated windings. All is good now with the +38 and -27 volts DC outputs but the 3.8 volts AC is missing.
Could this have been my mistake in winding the AC coil in the wrong direction ? very confusing, I thought AC would have been the easiest to achieve. Coil resistance is good.