How important is this cap's value (Sony 8-301W)? - Videokarma.org TV - Video

Phil Nelson · #1 10-01-2011, 05:01 PM

After replacing about two dozen electrolytics, my Sony 8-301W portable works great, except that I can't bring in the horizontal frequency (using frequency adjuster R4 and the horiz. hold control).

Capacitor C34 is 1mfd/12v. I couldn't find a 1-mfd electrolytic, so I used the closest I could find, a 3.3mfd/16v. Just wondering if the value of that cap is that critical?

Here's the horizontal portion of the schematic:

http://antiqueradio.org/art/temp/Son...lSchematic.jpg

I highlighted in yellow five caps that I replaced in this section, including C34.

Here's the full schematic:

http://antiqueradio.org/art/temp/Son...WSchematic.jpg

If it's important to use that value, perhaps someone could tell me where to find a 1mfd/12v electrolytic. Mouser didn't seem to have any.

If 3.3 mfd is close enough for C34, then I'll start looking for other causes.

Thanks!

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

radiotvnut · #2 10-01-2011, 05:17 PM

Did you look for a cap with a rating of 25V or 50V? Iuf is a common value; but, probably not at 12V. A more common 25V cap will work fine. It's quite possible that the 3.3uf cap could be throwing the circuit off enough to cause your problem.

Phil Nelson · #3 10-01-2011, 07:05 PM

Sure enough, they have 1mfd/25v. May as well order one and give it a whirl.

Regards,

Phil Nelson

kx250rider · #4 10-02-2011, 11:18 AM

That's a funny circuit (compared to the typical horizontal oscillator-driver-output in most sets I've worked on), so I'm not sure why it seems to want feedback from the collector of the output. But definitely the 300% increase in the cap, would flatten the feedback pulse, and that would cause a stability problem if the circuit is looking to that feedback for some reason for syncing or stability. It's my guess, that it's something to do with the AGC keying, but still....

Go with the 1uF @ any voltage 12v or > .

Charles

old_tv_nut · #5 10-02-2011, 02:32 PM

Without knowing the value of L13, it's hard to guess if the 1 uF is supposed to be tuning to a certain frequency or is just decoupling. In case it is tuned, I vote for keeping the 1 uF value correct.

By the way, not sure how much to trust this schematic, since it shows two different waveforms both on point #60. (The waveform on point #61 seems to have a false copy pointing to #60.)

Phil Nelson · #6 10-03-2011, 02:11 PM

Yes, I noticed several differences from the schematic, including one electrolytic not shown at all in the photo of the board it's on. Where values didn't match, I stuck with what was in the TV, figuring it must have worked when it left the factory. Everything looked original -- no signs of previous service.

The parts list describes L13 as "Peaking (500uh)."

I'll have a new cap in a day or two, so we'll learn then whether 1 mfd makes a difference.

Thanks!

Phil Nelson

old_tv_nut · #7 10-03-2011, 07:55 PM

Unless I'm pushing the wrong buttons on my calculator, that coil and capacitor resonate at about 7 kHz, about 1/2 the horiz frequency, so changing to 3.3 uF definitely could change things.

Phil Nelson · #8 10-03-2011, 08:34 PM

My fingers are crossed. Here's the best I could get (the TV is lying on its side):

A familiar scenario. Fewer and fewer slanty bars as you turn the adjuster. You feel like you're only one more turn away from a good picture, and then the adjuster can't turn any farther.

The TV may still have a multitude of other problems, of course, but it can't hurt to replace that cap with one of the right value.

Phil Nelson

Electronic M · #9 10-03-2011, 08:57 PM

Silly me I thought this thread was about the 80's color set you fixed, and when I read the schematic I thought "Wow they were still using HV rect tubes in the 80's....who'd of thunk it".

Sandy G · #10 10-03-2011, 10:23 PM

You're 85-90% there, don't give up !

Phil Nelson · #11 10-03-2011, 10:35 PM

Fixing the 1980s set inspired me to haul this one off the shelf. When I got it last year, I didn't have an ESR meter. Now I have no excuse for ignoring it. With that meter, you can check every electrolytic in the TV in no time.

The construction is interesting. Tightly packed, of course. The boards are surprisingly heavy and everything is hand-soldered (not very neatly). I'm not comfortable with solid-state things in general, but this is more my speed than the 1980s Sony, with all of those black-box ICs and components the size of a grain of rice.

Phil Nelson

Mr Hoover · #12 10-04-2011, 05:04 AM

Hi
I restored 3 of these a few years ago,I remember one had problems with the horizontal oscillator running off frequency and took a bit of adjustment with cap values to get it back.

From the picture the oscillator is running only just a little bit high though,from memory mine was very low.

I was told that with electrolytic caps at that time the stated value would be the minimum and could in practice be higher,never lower.

I managed to make some pcb connector extenders which would plug into the main chassis clear of the set and plug any of the boards into them enabling work on an individual board while the TV is running to be carried out which helped a lot.

Hugh

ctc17 · #13 10-04-2011, 11:20 AM

Check those diodes, maybe one is a tad leaky.
Check or just change C98/99
C100/101 could defiantly cause it.

Phil Nelson · #14 10-04-2011, 02:19 PM

Thanks for the tips. I will look at those smaller components if the new 1-mfd cap doesn't cure it.

Sure would be handy to have extender cables to work on this. Very few of the interesting test points are accessible when it's all put together.

Regards,

Phil Nelson

Phil Nelson · #15 10-06-2011, 12:30 AM

Didn't get far today, but I found that installing a 1-mfd cap for C34 didn't magically cure everything.

Say that one of those AFC diodes (X28, X29) is bad -- what type would I order to replace it? The parts list only gives a Sony part number (1T261), which didn't turn up anything in a quick search. These diodes are teeny-weeny, not the type of thing I have in the parts drawers.

I'm starting to suspect there's more going on than simply being off frequency. Watching it for a longer time today, the vertical isn't as stable as I thought, suggesting maybe a problem in the sync section or something else that affects both vertical & horizontal sweep.

Maybe I was too generous when I checked all the electrolytics and passed about half of them as good. A failure rate of 50% (at best) is pretty awful. If I had the right parts on hand, I'd be tempted to shotgun the rest of 'em. That's a crude approach, but you can't reach anything with a scope probe when the TV's assembled, and I can't see building custom board extender cables that I'll never use again.

Phil Nelson