How to set the filiment controls on the Radiola 17. Part 2
paul @removeppinyot.removecom Paul_P
Well I am making slow progress with the Radiola 17. I subbed in temporary
and new resistors for the blown power supply tapped bias resistor. And
subbed in some resistors in the receiver portion. I want to test the audio
coupling transformers to see if they work or not.
I did inject and pass a tone at the 2nd AF (audio output) tube grid to
speaker. And passed a tone at the plate of the 1ST AF tube. It seems the
1st AF tube is dead. Can't pass tone at it's grid (plate volts is good).
But I will tackle that tomorrow by swapping it with one of the RF #26s.
This 26 tested intermittent when the globe was twisted. (I did resolder the
pins but still no good).
My goal is to get a solid confirmation of the transformer just after the
Detector. I want to know if I need any serious iron before I begin.
BTW where would I find a replacement transformer if it proves to be open?
There seems to be two transformers under one metal case. One for the
Detector output and one for the 1st AF output.
Thanks all,
Paul P.
Kenneth Scharf
>
> BTW where would I find a replacement transformer if it proves to be
> open? There seems to be two transformers under one metal case. One for
> the Detector output and one for the 1st AF output.
>
> Thanks all,
>
> Paul P.
DumpsterDiver
I'm away from my home base & don't have the schematic in front of me,
but I imagine you are probably referring to the pots that sit across
the filament windings and adjust the "center" ground points for
minimum hum.
The easiest way I know is to tune between stations and adjust for
minimum audible hum. Of course, you can do a more precise adjustment
with a scope, setting for minimum 120 Hz amplitude.
With sets of this type (early AC TRF sets with #26 tubes), you will
find that swapping the 26's around requires re-balancing the filament
pots for best results. And some tubes balance out better then others,
presumably due to slight differences in their internal elements
spacing.
Generally, the 1st audio stage is the most critical for hum, and it
pays to put your "best" (least hummy) #26 in that socket. Temporarily
pull the #26's in the RF stages and the #27 detector, and try each 26
till you find the one that can be adjusted for least hum. The RF
stages are much less critical in terms of filament balance, so it
makes sense to use the hummier tubes there. Sort of a pain, but it can
make a noticeable difference.
If the audio output tube (UX-171?) also has a filament pot, then it
should also be adjusted for minimum hum with no signal coming in from
earlier stages.
Adjust the RF filaments pot last.
All of these adjustments are "fine tuning" and should only be done
after the set is in basic, working condition, with good tubes in
place. Transformers are generally best checked for continuity and
shorts with a meter, rather than by signal tracing. If you need to
replace the 1st audio transformer, I believe a "modern" Stancor 3:1
(Model??) will do. I don't know what you would need for the second
audio transformer, maybe a 2:1, again a low-power interstage type is
all you would need ... no significant power involved, just signal
voltage amplification.
DD
Jeff
as they interact some.
Perhaps best if I could fax you the pages from the RCA Redbook for
that set, that way you'll have them.
I have not come across a '17 with open audio transformers (yet)..
If you need one, I can look thru the bone pile and see if I have a '17
chassis with good iron on it...
Jeff the Antique Radio Store www.oldradiosrus.com
On Dec 9, 9:04 pm, "Paul_P" <REMOVE paul @ REMOVE ppinyot . REMOVEcom>
wrote: