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Giid leak voltage on Sears 101.557-1 chassis - Printable Version

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Giid leak voltage on Sears 101.557-1 chassis - leftwing25 - 11-28-2016

Hi all. Been working on this for a while off and on. Re-capped, re-resistored, aligned(at least I think it's right). Now I get one strong station at 1600kc, but the oscillator seems to drop out as I dial toward the lower end of the dial. Grid leak voltage on the LO drops to almost nothing at that point. Why would the tuning cap cause this voltage to drop? All suggestions gratefully accepted. Thanks, Ed

RE: Giid leak voltage on Sears 101.557-1 chassis - Radioroslyn - 11-28-2016

Hi Ed,
So your under winx's tower???
Check voltage on pin's 3 and 4 of the mixer tube if you have the 140v or so replace the 6J8. This is assuming that set has been serviced, tube sockets and tuning cap have been cleaned, and check for bent plates. Also check C-6 for proper tracking at the low end of the bc band, could be that it's adjusted so that it is out of the range of the oscillator ability to oscillate (too low).

Terry

RE: Giid leak voltage on Sears 101.557-1 chassis - leftwing25 - 11-28-2016

(11-28-2016, 09:22 PM)Radioroslyn Wrote:  Hi Ed,
So your under winx's tower???
Check voltage on pin's 3 and 4 of the mixer tube if you have the 140v or so replace the 6J8. This is assuming that set has been serviced, tube sockets and tuning cap have been cleaned, and check for bent plates. Also check C-6 for proper tracking at the low end of the bc band, could be that it's adjusted so that it is out of the range of the oscillator ability to oscillate (too low).

Terry

Ha Ha, yeah Terry you could say that.  About a mile as the proverbial crow flies, near the Burgundy strip mall.  Sadly WINX has gone the way of almost all the old AM rock stations.  Now it's hispanic, and I can't remember what the call sign is.  Only one tower now instead of the 3 or 4 that used to be off of 355 back in the day.  It comes in loud and clear no matter what.  Anyway, changed 6j8, no help, 140v on mixer, -6v at osc grid which drops to .113 or so at about 6 or 700kc.  Adjusted padder c-6.  Passes signal weakly at 600kc, but no stations.  I think I'm gonna go cry in my beer now.  Thanks for the response Terry.  Really appreciate the help all you guys offer here.  Ed

RE: Giid leak voltage on Sears 101.557-1 chassis - Radioroslyn - 11-28-2016

You probably know this but I'll mention it in passing R-1 is listed as a 40M resistor. That = 40,000 or 40K ohms not 40 megohm. Check your ground connections at the coil. Rivets are always suspect.
Was down in ur hood for tday in Gsburg.

RE: Giid leak voltage on Sears 101.557-1 chassis - leftwing25 - 11-29-2016

(11-28-2016, 11:39 PM)Radioroslyn Wrote:  You probably know this but I'll mention it in passing R-1 is listed as a 40M resistor. That = 40,000 or 40K ohms not 40 megohm. Check your ground connections at the coil. Rivets are always suspect.
Was down in ur hood for tday in Gsburg.

Yeah, I replaced R1 with a new 39k I had.  Was reading about 2500 over.  Meant to mention that in the last post.  Did you get to Rockville while you were down?  How far is it from where you live?  I'll check the grounds next time I'm poking around in there.  Didn't think of that. Thanks for the heads up on that.  Ed

RE: Giid leak voltage on Sears 101.557-1 chassis - Radioroslyn - 11-29-2016

Did you get to Rockville while you were down?
nah did quite down to Harvard on the Pike (a euphemism from my misspent youth)
How far is it from where you live?
Abt 3hrs north.

RE: Giid leak voltage on Sears 101.557-1 chassis - leftwing25 - 11-29-2016

Well, checking grounds around the coils and trimmers, they all seem good.  Wonder if I could have a bum replacement tube in there. Still don't know why the grid leak voltage drops out when I turn the tuning cap to the low end of the dial.  The oscillator can't be running with no bias can it?  I also seem to remember reading that the 6j8 can be a finicky oscillator, but there doesn't seem to be any alternate replacement for it.  The one in there is one I ordered as nos.  Don't know where to go from here.  Maybe try re-aligning it.  Ed

RE: Giid leak voltage on Sears 101.557-1 chassis - Radioroslyn - 11-29-2016

I like easy questions.
> Still don't know why the grid leak voltage drops out when I turn the tuning cap to the low end of the dial.
Because there isn't enough feedback to keep the osc running. This can be caused the tube not having enough gain (weak or voltages too low), Bad coil, or out of tolerance micas/ tuning cap shorted. Just for grins check the heater voltage on the 6J8 (needs to be 6.0vac or more).

> The oscillator can't be running with no bias can it?
The control grid needs or will be negative when it is oscillating. Not oscillating the grid will be at 0 or positive using a high Z meter, no VOM.

For a drop in replacement try a 12A8. None the the 6J8's 6v friends have a 150ma heater there all 300ma. Electrically the 6A8/12A8 has more gain than the 'J8 which is good.

RE: Giid leak voltage on Sears 101.557-1 chassis - Radioroslyn - 11-30-2016

Sorry misread the specs, use 6A8. All the other tube have a 300ma heaters.

RE: Giid leak voltage on Sears 101.557-1 chassis - leftwing25 - 11-30-2016

Have to dig through my tubes to see if I have one. Meanwhile there is a cap on pin 4 of the 6u7 that doesn't look like it belongs there. Other end goes to ground. could this be a problem?

RE: Giid leak voltage on Sears 101.557-1 chassis - Radioroslyn - 11-30-2016

(11-30-2016, 02:14 PM)leftwing25 Wrote:  Have to dig through my tubes to see if I have one.  Meanwhile there is a cap on pin 4 of the 6u7 that doesn't look like it belongs there.  Other end goes to ground.  could this be a problem?

Don't think so. Just bypasses the SG and SG of the 6J8.

RE: Giid leak voltage on Sears 101.557-1 chassis - leftwing25 - 11-30-2016

Ok, so tried another 6j8 I found, same results. Just noticed that I replaced R-8 with a .1mfd instead of the .22 I had intended to use. Would that affect anything badly?

RE: Giid leak voltage on Sears 101.557-1 chassis - Radioroslyn - 11-30-2016

Methinks you mean C-8, possible maybe may alter the cathode voltage a bit. also check R-2.

RE: Giid leak voltage on Sears 101.557-1 chassis - leftwing25 - 12-01-2016

(11-30-2016, 09:46 PM)Radioroslyn Wrote:  Methinks you mean C-8, possible maybe may alter the cathode voltage a bit. also check R-2.

Yeah, kinda thought that might be the case. Yep, c-8.  Resistors don't use mfds.  Also noticed B+ is about 14 volts low at the rectifier cathode, but plate voltages only about 5 volts low, i.e., 140 reading 135-138.  Possible problem?  Unfortunately don't have any 25z6's to replace it with.  Anyway, will replace c-8 and see what happens.  Thanks, Terry.  Ed

RE: Giid leak voltage on Sears 101.557-1 chassis - Radioroslyn - 12-01-2016

(12-01-2016, 06:45 PM)leftwing25 Wrote:  
(11-30-2016, 09:46 PM)Radioroslyn Wrote:  Methinks you mean C-8, possible maybe may alter the cathode voltage a bit. also check R-2.

Yeah, kinda thought that might be the case. Yep, c-8.  Resistors don't use mfds.  Also noticed B+ is about 14 volts low at the rectifier cathode, but plate voltages only about 5 volts low, i.e., 140 reading 135-138.  Possible problem?  Unfortunately don't have any 25z6's to replace it with.  Anyway, will replace c-8 and see what happens.  Thanks, Terry.  Ed

Doubt 5v is going to make much of a difference.
If you like excitement you tie a couple of 1N4007 across your 25Z6. Did I say that???  [Image: http://philcoradio.com/phorum/images/smilies/icon_eek.gif]  That will get ur 140 up the abt 160. On second thought probably not a good idea. This set uses a voltage doubler where the DC output should be much higher than 140v. Typical doubler would have an output of abr 250vdc w/120ac input. Not quite sure how they get the power supply to work with such low output.