The pilot just refuses or doesn't believe the meter so he taps the glass on the altimeter gauge a few times instead of pulling up... ... and then..CRASH.
Well here I was tonight ....trouble-shooting a low volume problem:
.. After restoring this nice little 4-tube TRF .. a little 'ole Sentinel 30-A ("Arrow").
Here's the schematic:
http://www.nostalgiaair.org/Re ... 018154.htm
A few months ago when I first got it I had replaced EVERY capacitor including any micas and measured and replaced all out-of-tolerance resistors. It was playing just fine.
I put it away for a few months to get some construction work done here but yesterday I put it back on the bench to finish cleaning the chassis etc.
After a thorough kitchen sink washing and rinsing of all areas except the transformer ... I let it dry in warm blowing heat for 24hrs and reassembled it all and let it play for a day or so.
Again last night I had it just sitting on the bench playing nicely for 8 hours till I went to bed.
Tonight when I turned it on however the volume was way down.
My first thought was maybe there was something loose or maybe a bad tube-pin contact.
I checked all that and also measured the B+ which was fine at the rectifier and on the output-tube plate... so I stopped checking B+ and instead just poked around looking for solder globs and anything that might be shorting. All was fine.
Being too lazy to bother thinking very hard.. I decided to swap some tubes starting with the 6D6 RF- amp and the 6C6 detector... then the 42 output tube and finally the 80 rectifier... all were NEW tubes and none made any difference.
Now I knew it was time to do what I should have done first... START THINKING and measuring and troubleshooting.
The first suspicious measurement came on the Plate of the 6C6 detector... it was only +6vdc !!! DUH.
WHY?
Something was pulling that B+ down hard. .. So I thought maybe something was turning the tube on very hard pulling it down.. but NOPE ....even with the tube out of the socket.... the plate pin was still at only +6volts!!
Now here's the problem with 'stoopid' thinking. I looked at the only 3 components connected there.. a 200k resistor, a 500pf mica cap and a .01 coupling cap to the output.
I 'stoopidly' believed that since I had replaced the two caps already...the problem MUST be that 200k resistor.
I measured a good +250v on one side of the resistor.. but the plate side was down at only +6vdc!
Maybe the resistor value is off?
But nope... when I removed it to measure it was fine.
So what else??
Can't be that brand-new 500pf mica I thought.
..... so the only other thing there was the brand-new .01 @ 630v cap.
I removed that .01 and the voltage was STILL down at +6volts!!!
ARRRG.
This is amazing I thought.
.... the only thing left is that brand-new 500pf shiny-reddish-brown dipped silver mica cap.
CAN'T be that... I thought.
.....after all ... it's MICA and it's BRAND new... lol
Well of course that was it...
I removed and measured the leakage... 9k !!!
WOW.
... Replaced it and now all is well again in radio-heaven.
But I just didn't seem to want to believe it because.. of some silly reason ..like it was new..
But the leakage didn't care if it was new or not .. it was STILL leaking... lol.
So who made that 500pf cap?
Johnnysan-
And guess what...last night I soldered a new 470 PF ceramic disc to a plate pin on a Zenith I'm working on. When I fire that set up I'll be thinking about this story should the plate voltage read much too low. Thanks for sharing!
::Now I understand why pilots sometimes crash into mountains in a fog even though their instruments clearly indicated they were loosing altitude.
::
::The pilot just refuses or doesn't believe the meter so he taps the glass on the altimeter gauge a few times instead of pulling up... ... and then..CRASH.
::
::Well here I was tonight ....trouble-shooting a low volume problem:
::
::.. After restoring this nice little 4-tube TRF .. a little 'ole Sentinel 30-A ("Arrow").
::Here's the schematic:
::http://www.nostalgiaair.org/Re ... 018154.htm
::
::A few months ago when I first got it I had replaced EVERY capacitor including any micas and measured and replaced all out-of-tolerance resistors. It was playing just fine.
::
::I put it away for a few months to get some construction work done here but yesterday I put it back on the bench to finish cleaning the chassis etc.
::
::After a thorough kitchen sink washing and rinsing of all areas except the transformer ... I let it dry in warm blowing heat for 24hrs and reassembled it all and let it play for a day or so.
::
::Again last night I had it just sitting on the bench playing nicely for 8 hours till I went to bed.
::
::Tonight when I turned it on however the volume was way down.
::
::My first thought was maybe there was something loose or maybe a bad tube-pin contact.
::
::I checked all that and also measured the B+ which was fine at the rectifier and on the output-tube plate... so I stopped checking B+ and instead just poked around looking for solder globs and anything that might be shorting. All was fine.
::
::Being too lazy to bother thinking very hard.. I decided to swap some tubes starting with the 6D6 RF- amp and the 6C6 detector... then the 42 output tube and finally the 80 rectifier... all were NEW tubes and none made any difference.
::
::Now I knew it was time to do what I should have done first... START THINKING and measuring and troubleshooting.
::
::The first suspicious measurement came on the Plate of the 6C6 detector... it was only +6vdc !!! DUH.
::
::WHY?
::
::Something was pulling that B+ down hard. .. So I thought maybe something was turning the tube on very hard pulling it down.. but NOPE ....even with the tube out of the socket.... the plate pin was still at only +6volts!!
::
::Now here's the problem with 'stoopid' thinking. I looked at the only 3 components connected there.. a 200k resistor, a 500pf mica cap and a .01 coupling cap to the output.
::
::I 'stoopidly' believed that since I had replaced the two caps already...the problem MUST be that 200k resistor.
::
::I measured a good +250v on one side of the resistor.. but the plate side was down at only +6vdc!
::Maybe the resistor value is off?
::
::But nope... when I removed it to measure it was fine.
::
::So what else??
::
::Can't be that brand-new 500pf mica I thought.
::..... so the only other thing there was the brand-new .01 @ 630v cap.
::
::I removed that .01 and the voltage was STILL down at +6volts!!!
::ARRRG.
::
::This is amazing I thought.
::.... the only thing left is that brand-new 500pf shiny-reddish-brown dipped silver mica cap.
::
::CAN'T be that... I thought.
::.....after all ... it's MICA and it's BRAND new... lol
::
::Well of course that was it...
::I removed and measured the leakage... 9k !!!
::WOW.
::
::... Replaced it and now all is well again in radio-heaven.
::
::But I just didn't seem to want to believe it because.. of some silly reason ..like it was new..
::
::But the leakage didn't care if it was new or not .. it was STILL leaking... lol.
::
:
:
:So who made that 500pf cap?
:Johnnysan-
:
:
:
:
:
I'm thinking it wasn't sealed properly causing some moisture to get in there during the bath. However it took a day of good play before showing up.
It went upwards today from 9k to 48K so maybe it's drying out?