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Old 15th Jan 2021, 7:44 am   #1
P-Sussex
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Default Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

Sony SL-C7 Rebuild Alive for 20 Seconds Then ‘POP” & Smoke.

I have been rebuilding a Sony SL-C7 and I fear I may have taken on more than I can chew, with my experience.

The machine initially was Red light only with dead clock, no Green light when switched on. No other life. I began in the PSU replacing C110 & C101 (4.7 350v), heat sink transistors Q101, Q102 [2SC23351, surge resistor R101, ICI TL494CN (with heat sink protection when install). The main suspect diodes were removed from the board and tested as OK and simply replaced. I also replaced many of the other E. Capacitors for good measure as I had them to hand.

I then worked on the Timer boards, removing the very badly corroded battery. I had to remove the ribbon cables on the battery board and give it a good clean. On reattaching the cables I checked the paths of the connectivity from the ribbon cable wires. Many just seem to pass through to the other side of the board, and pass directly where the battery was. So many of these lines on the PCB were gone due to corrosion. Anyway I decided to bridge these wires from one end of the board to the other, crude but at least I had connectivity between the three boards.

The dead push switches where replaced on the timer board and tested for continuity for each one. So far so good.

So when it cam to testing: In Stand By mode Red light on, I was surprised to see the clock worked, I could even press the Clock push switch and the clock started its count from 00:00, so clock OK.

Next came the On switch. Green light came one and there was life. At this stage I was feeling really pleased with myself. I decided to put a tape in and pressed the eject, yes eject worked, marvellous. Then the sound of a crack, pop from the PSU area, Then some smoke. So the green light was on for about 20 seconds. It was hard to tell where the smoke came from but coming from the area where the timer meets PSU. The glass fuse F006 has gone, seriously blacked out. (Do fuses like this cause smoke?)

I changed the fuse and tried again, yes I didn’t appreciate at the time I could cause more damage, but totally dead now not even life to the timer/clock from the ‘Ever Power Supply’. No Red light.

I realise the dangers of working on switching PSUs and don’t want to poke around unsafely with it switched on, unless I fully understand the areas I am checking.

I do have a second C7 machine also purchased in a similar state, my thinking was to make a good machine out of two. So I have access to second set of parts.

So my question is will the replacement/checking of the initial parts that I replaced be futile? What parts should I be looking at? The crack/pop sound is this sound typical of a particular part that blows which I can easily check.

I notice from my PSU checking that one of the Q101/102 2335 transistors has gone so far. The replacement diodes appear OK. Have not checked the E. Capacitors yet.

I am also in the process of double checking all my work on the timer/clock boards in case I have created a short somewhere which caused this.

Any thoughts and pointers would be most welcome.

My experience is beginer level.

Thanks in advance.
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Old 15th Jan 2021, 8:45 am   #2
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

Just to add to the above from examining the PSU unit I can't see where smoke could have come from. There is no evidence on any of the boards of any burn out or other obvious physical damage. Apart from the fuse F006 which was really blacked out. Thanks
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Old 15th Jan 2021, 9:15 am   #3
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

Don't worry it is not you, the C7 and C5 are possibly the worst of all VCRs from a service point of view you could take on.
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Old 15th Jan 2021, 9:36 am   #4
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

First all those electrolytic caps are suspect in C7 power supplies and no, fuses do not smoke.

Double check the replacement caps are the correct way round (polarity).

Also I seem to remember those transistors have to be genuine Sony ones.
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Old 16th Jan 2021, 10:51 am   #5
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

Besides the obviously damaged parts, you need to replace the two small electroytics in the base circuit of the chopper transistors using new 105 degree parts by Panasonic or ones of similar quality. These will more than likely have caused the failure. The smoke will have come from the surge limiter resistor that usually survives.
As stated above, you'll need to source (and save up for!) genuine parts for both transistors - change them as a pair. Sonys (Sonies?) don't take kindly to pattern or equivalent parts.
If you're lucky and your spares machine has the fuse intact, then change those capacitors and any others that need it before fitting it. That might be your best option.
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Old 16th Jan 2021, 3:37 pm   #6
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

Hi.
As a former engineer with (SES) Sony UK group of service depots I totally agree with Glyn. Basically on no account use non genuine transistors in the power supply. Non genuine transistors will last half an hour at the most. Replacing the electrolytic caps that have been mentioned with top quality 105c or 135c types (Panasonic or Vishay) will give a long lasting repair. Ensure all resistors in the primary circuit are not showing any signs of distress and not off value.
IMHO Some Sony power supplies are very fussy and can be a pain, the KV27PSI comes to mind as well.
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Old 16th Jan 2021, 3:38 pm   #7
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

Quote:
Originally Posted by Welsh Anorak View Post
Besides the obviously damaged parts, you need to replace the two small electroytics in the base circuit of the chopper transistors using new 105 degree parts by Panasonic or ones of similar quality. These will more than likely have caused the failure. The smoke will have come from the surge limiter resistor that usually survives.
As stated above, you'll need to source (and save up for!) genuine parts for both transistors - change them as a pair. Sonys (Sonies?) don't take kindly to pattern or equivalent parts.
If you're lucky and your spares machine has the fuse intact, then change those capacitors and any others that need it before fitting it. That might be your best option.
I've since realised about the 105 degree value and am replacing all the electroytic capacitors with a working value of 105 degrees. Trying to get the best quality I can. All of the e.caps that were in the machine were rated at 85 degrees, so I was just copying what was already there. They are like this as well on the spares machine. I just so happened to watch a YouTube video of a guy discussing VCR switching PSUs, and showing the reason for failure was due to someone installing 85 degree caps when 105 degrees should have been used.

Regarding the two transistors in this chopping circuit both machines were using '2SC2335' by NEC. Which I have simply replaced. Once machine had a 15 ohm 0.25w resister soldered crudely across the base and the emitter, on both of these transistors. The other machine didn't have this. Was this some sort of hack to prevent the transistor from blowing because it was a non genuine part?

I see on a particular site a Sony Labeled bag for these 2SC2335 parts and they have NEC on them, they look just like the ones I am using. Are these the genuine parts that Sony would have issued? Photo attached.

Not having the genuine Sony ones will mean it will not work at all or have a shorter working life?

Quite a learning curve for me.....Thank again for all the input.
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Old 16th Jan 2021, 3:43 pm   #8
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

Hi.
In answer to the 85c caps originally fitted.
The original caps were top quality 85c types, sadly many modern 105c types that are not top quality don't last even as long as the originals. There are even fake branded Panasonic caps as well, so beware where you buy from.
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Old 16th Jan 2021, 4:02 pm   #9
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

The attached photo shows genuine Sony parts. If a part was for use in more than a few different models (as seen on the label), it likely wasn't specially selected, in which case a genuine NEC part would also suffice. Most problems occur when substituting parts (for example using a BUTxx or an ISC 2SC2335 instead of a NEC 2SC2335). Sony themselves use a (NEC) 2SC2331 as a substitute so that might be okay as well.

On the other hand, if the SL-C7 manual calls for another article number than 8-729-133-53 as seen on the bag, there's a chance only that exact part will work right.

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Old 17th Jan 2021, 9:32 am   #10
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

On the C7 which blew, a 15 ohm 0.25w resister was soldered crudely across the base and the emitter of the pair of '2SC2335'. The other machine which I am using for spare parts and reference does not have this resister attached. Should I be soldering a resister across the base and emitter? The service manual makes no reference to this. From my limited understanding is this a hack to help maintain the transistors life?

Here is a photo before, I began any work, showing the resister attached cover by heat protectors
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Old 17th Jan 2021, 10:54 pm   #11
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

Hi.
Something in the back of my mind tells me those resistors were part of a modification in a service bulletin. I left Sony in 1997 so its too long to be exact.
Perhaps Glyn may know.
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Old 17th Jan 2021, 11:34 pm   #12
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

I'm racking my brain here!, i think the extra parts were part of the sr08 repair kit which was basically new transistors, fuse resistors, rubber gaskets and the control chip, oddly they didn't supply the capacitors which were the cause of the blow-up, almost no ventilation to keep it cool, didn't help.
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Old 17th Jan 2021, 11:39 pm   #13
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

Quote:
Originally Posted by cheerfulcharlie View Post
Don't worry it is not you, the C7 and C5 are possibly the worst of all VCRs from a service point of view you could take on.
Sony went back to a conventional big mains transformer with bridge rectifiers etc for the C6 model, guess they reckoned the failure rate was just too high!.
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Last edited by greg_simons; 17th Jan 2021 at 11:39 pm. Reason: typo
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Old 18th Jan 2021, 9:50 am   #14
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

It's not impossible that a service kit was fitted ages ago. As Greg says, service kits didn't include the very parts that were needed - however this was relatively early in the machine's life so they weren't giving trouble then. As Trevor says, the original 85 degree capacitors were of good quality. In fact nowadays when faced with a good quality 85 degree part or a odd named 105 degree part in a non-critical position I choose the better part.
As mentioned in another thread, the mania for 'recapping' has led to fakers rubbing their hands with glee!
NEC made the transistors and would have supplied Sony with ones selected to their specification. I would imagine they will be absolutely fine to use. After all, the power supply isn't all that special design wise and many soldiered on for many years, very often powered up 24 hours a day in a poorly-ventilated cabinet. It was only after a power interruption that the destruction happened, but replacing the kit and the two capacitors would pretty well guarantee a working machine again.
I dread to think of the weight and size of the C7 if it had a conventional transformer!
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Old 18th Jan 2021, 11:48 am   #15
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

Hi.
Funny enough I've to pick up a Sony editing kit that's sitting in Inverness after we can travel. There is a lot of stuff to collect including a SL-C7 in an unopened box..... yes unused all FOC.
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Old 19th Jan 2021, 3:00 am   #16
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

I suspect, unselected 2SC2335 were used in this case, which should be confirmed from the parts number. However if the kit types or even the originals were factory selected, it would have been for something like a higher hFE category. Even with normal designs, Sony sometimes managed to make them quite critical to save a few cents or to make them more compact for example. Often, they first designed the ideal circuit, and then went to find suitable parts to realise it.
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Old 20th Jan 2021, 7:40 pm   #17
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

Thank you all again everyone for your help.

I am a little confused by the replacement R101 surge resister. The service manual simply lists it as a 3W 3.3R wirewound resister. Can all wirewound resisters be utilised as fusable / surge protecting resisters? I notice the term 'Fusable Wirewound' and simply 'wirewound' in descriptions for resisters. Is there a difference? The resister I have purchased is wirewound but simply looks like a regular 3w sized colour coded resister. Is this wrong? As a reminder the original part looks like as shown in the attached photo. If using a resister named simply as 'wirewound' 3w 3.3R that looks like a standard colour coded resister is incorrect can someone perhaps point me to a source to obtain the correct part?

Thank you again for helping me out.
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Old 21st Jan 2021, 7:49 am   #18
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

This is a photo of the resistor I purchased for R101 replacement. Is this suitable as it does not use the term fusible or safety in the spec?
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Old 21st Jan 2021, 7:52 am   #19
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

A wire wound res is not normally classed or used as fusible, the only ones that ever were had a resolderable spring on them, which came off when too hot.

The res in the picture is correct.
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Old 21st Jan 2021, 10:53 am   #20
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Default Re: Sony SL-C7 Rebuild: Alive for 20 Seconds Then ‘POP” & Smoke

Ideally you'd use an identical Sony part, but as this is to the same spec it should be OK. The term 'safety' applies to the wattage - replacing, say, a 3W part with a 5W would no longer be considered safe as it would dissipate too much current before failing.
Fusible resistors are designed specifically to fail within a certain time when excess current flows through them. They are generally low wattage components. A resistor such as the one you're replacing is there as a surge limiter. A short should blow the fuse, though it's likely the resistor will overheat before this happens as in your case.
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