http://www.nostalgiaair.org/PagesByModel/089/M0025089.pdf
I originally re-capped and replaced bad resistors etc ... a few months ago and it has been just sitting around waiting for the cabinet painting to be done.
( BTW: ...wait till you see how nice this one looks in two shades of deep metallic violet and purple)
Anyway.. I had it on the bench to check alignment and also for "burn-in."
Good thing, because I noticed that after a few minutes of play the audio began to sound a tiny bit garbled. Now I hate that kind of problem to troubleshoot because it is always so hard and so subjective.
I keep asking myself if it really sounds bad or am I imagining it.. y'know?
.. and being the perfectionist that I am...I would NEVER, knowingly, allow a set to leave my bench that wasn't "perfect" .. so I have to fiddle and fiddle to locate those subtle problems.
Each time I turned it off for a while it sounded great when first back on. .. arrgh.
I checked the speaker.. even swapped it... nope!
Started swapping tubes.. saw that the first RF 12J7gt (sharp cut-off) had been replaced with a 12k7gt.. (remote cut off).. but no that wasn't it.
So then I decided to dig deeper.
I noticed that there were still several dog-bone resistors in there yet.. and I was hoping maybe one had changed value grossly to cause this.
( just poking and hoping)
Then I decided to measure ALL resistors old or new... and that's when I discovered the problem
I saw a nice new 470k resistor on the output tube 35L6.
It looked new but measured very weird [in circuit]... ( 2 megs)
This is supposed to be the grid bias resistor.
The schematic shows this resistor going from pin-5 the grid to B-.
So I un-soldered one end and then re-measured it... OK correct value.
BUT ... the problem is ....one end was properly soldered to B- alright.. but the other end was NOT soldered to the grid(pin-5).
Nope..
Seems that in the midst of re-cap battle ...I guess I inadvertently soldered it to pin 4 (b+) instead of to the grid (pin-5).
That means I had no grid bias at all... the grid was just floating...
And so instead ....I just had a 470k resistor going from B+ to B-...lol
A floating grid will eventually cause the garbled audio I was experiencing.
Wow!
Success.. what luck.
The first resistor I measured was the culprit!
That'll teach me!
Soldered it properly to the grid where it belongs.. ahh.. now what sweet music it makes!
That type of problem can be tricky especially when the radio starts out working right.
This will also happen if the grid bias resistor has increased in value. At some point it won't be able to overcome internal tube leakage.
RCA data books mention value of the grid resistor shouldn't exceed 470K on most output tubes. It would be a good idea to use a lower value to overcome any leakage as the output tube ages.
A 330K would be reasonable. If value is reduced a lot volume and low frequency response starts dropping off.
Norm
:I've been getting the finishing touches done on this terrific little Zenith 6D-510 six-tuber.
:
:http://www.nostalgiaair.org/PagesByModel/089/M0025089.pdf
:
:I originally re-capped and replaced bad resistors etc ... a few months ago and it has been just sitting around waiting for the cabinet painting to be done.
:
:( BTW: ...wait till you see how nice this one looks in two shades of deep metallic violet and purple)
:
:Anyway.. I had it on the bench to check alignment and also for "burn-in."
:
:Good thing, because I noticed that after a few minutes of play the audio began to sound a tiny bit garbled. Now I hate that kind of problem to troubleshoot because it is always so hard and so subjective.
:
:I keep asking myself if it really sounds bad or am I imagining it.. y'know?
:
:.. and being the perfectionist that I am...I would NEVER, knowingly, allow a set to leave my bench that wasn't "perfect" .. so I have to fiddle and fiddle to locate those subtle problems.
:
:Each time I turned it off for a while it sounded great when first back on. .. arrgh.
:
:I checked the speaker.. even swapped it... nope!
:Started swapping tubes.. saw that the first RF 12J7gt (sharp cut-off) had been replaced with a 12k7gt.. (remote cut off).. but no that wasn't it.
:
:So then I decided to dig deeper.
:
:I noticed that there were still several dog-bone resistors in there yet.. and I was hoping maybe one had changed value grossly to cause this.
:( just poking and hoping)
:
:Then I decided to measure ALL resistors old or new... and that's when I discovered the problem
:
:I saw a nice new 470k resistor on the output tube 35L6.
:
: It looked new but measured very weird [in circuit]... ( 2 megs)
:
:This is supposed to be the grid bias resistor.
:
:The schematic shows this resistor going from pin-5 the grid to B-.
:
:So I un-soldered one end and then re-measured it... OK correct value.
:
: BUT ... the problem is ....one end was properly soldered to B- alright.. but the other end was NOT soldered to the grid(pin-5).
: Nope..
:
:Seems that in the midst of re-cap battle ...I guess I inadvertently soldered it to pin 4 (b+) instead of to the grid (pin-5).
:
:That means I had no grid bias at all... the grid was just floating...
:And so instead ....I just had a 470k resistor going from B+ to B-...lol
:
:A floating grid will eventually cause the garbled audio I was experiencing.
:Wow!
:Success.. what luck.
:The first resistor I measured was the culprit!
:
:That'll teach me!
:
:Soldered it properly to the grid where it belongs.. ahh.. now what sweet music it makes!
Peter:
I have been there and done that! On a PA amplifier. The grid resistor - again 470K - opened. I remember 6L6s in the output and the amp was about 35 Watts. I figgered that the grids of the output were picking up snagging enough electrons to get close to cutoff. Turn the thing off, and back on and it would sound great for a few minutes, then get distorted. It had an interstage transformer, and the center tap was grounded thru that 470K resistor. Memory is kinda foggy now, but I seem to remember it was in our 10th grade Gym/Sock hop/whatever auditorium and I had my moment of fame fixing the amplifier for a sock hop that night.
Lewis
This type of problem can really be trying. Many years ago I was working on a trash picked Majestic console AM-FM from the late 40s. It used a 6S8 for the first audio stage. After it was on a bit, it had an annoying slight distortion that wasn't pronounced but was tiring to listen to. In those days the set wasn't old or valuable enough to recap every part. It turned out to be a 22 megohm grid resistor for the 6S8. I didn't have a meter that could measure it. Replacing it cured the distortion, which like yours, happened after several minutes of playing.
FR
:This type of problem can really be trying. Many years ago I was working on a trash picked Majestic console AM-FM from the late 40s. It used a 6S8 for the first audio stage. After it was on a bit, it had an annoying slight distortion that wasn't pronounced but was tiring to listen to. In those days the set wasn't old or valuable enough to recap every part. It turned out to be a 22 megohm grid resistor for the 6S8. I didn't have a meter that could measure it. Replacing it cured the distortion, which like yours, happened after several minutes of playing.
:
:FR
Yes, Fred:
.. and doesn't it make you really feel good when you finally snag the problem... and you know it?
Really helps motivate you and helps to take you on to the next project.. every time you "win one"!.. lol
Doesn't it?
Follow up:
I was just thinking..lol
...Feeling good about fxing problems that you cause by yourself...
.... Also reminds me of the the moron who was hitting himself in the head with a hammer...
.... because it felt so good when he stopped!