I would like to know if anyone would know the value of the grid leak resistor of this radio. It is connected to the grid of the # 3 tube. It is painted a solid red color with no markings on it. It currently measures 9.5 Meg. ohms out of the circuit, must be maybe 1, 2 or 3 Meg? Don't know really.
Your thoughts on this are welcome.
Thank you,
Mitch
Value of grid leak varies with the type of tube used. For a '99 tube the value should be 4 megohms bypassed by an 0.0025 uF capacitor. For best reception it may be necessary to noodle around with different resistor values from the 4 meg nominal due to individual variations between tubes of the same type. Most gridleak sets that I am familiar with use a cartridge-type resistor (sort of like an oversized Buss fuse, but with pointed metal ends) in a snap-in holder to facilitate quick resistor-value swapping for "tube peaking" purposes.
(I hate these sets. To get decent performance (minimum howling, best sensitivity) you have to "align" the chassis to the tubes used. So, if a tube has to be replaced, you have to perform at least a stage re-alignment. Some may find that characteristic part of their charm; I just find it a nuisance. These sets were the electronic equivalent of the Stanley Steamer.)
http://radiolaville.com/graphics/docs/Radiola%2020%20Service%20Notes.pdf
Interesting stuff specific to the Radiola 20 but has some general info relevant to other brands of sets made before the screen grid tube was introduced.
Hello again,
There is a capacitor connected between terminals #1 and #2, I think it is C8. Does anyone have any idea what the value of this capacitor might be, it is shorted out of the circuit, also might it be an electrolytic or just a plain capacitor?
Thank you,
Mitch
This would not be an electrolytic cap and is probably an RF bypass cap on the order of 0.01 uF or so. Don't understand your "shorted out of the circuit comment".
Is the cap shorted or did someone bypass it with a wire?
Thanks Tom,
"out of circuit" The capacitor is disconnected from the terminal strip, isolated from the circuit. It measures 4.2 MFD at range 200 MFD on my BK capacitance meter as well as 2.4 MFD at range 20 MFD and 1.7 MFD on range 2 MFD.
Open, shorted, measuring the metal can?
I will try a 0.01 capacitor.
The capacitor is as large as a king size pack of cigarettes in a metal case.
It is not stable to me.
The radio does receive stations at this point but has no sensitivity and alot of drift.
Thank you,
Mitch
Sounds like your set needs to be "aligned" per the procedure in the link I provided. Some RCA TRF sets were superb performers and others were dogs. Can't say where the R-20 fell as I haven't owned one.
I have this radio playing. I was able to tune in NY and Mo. last night.
FYI for some parts: One audio transformer was open, I used a new one from Antique Radios that had a primary winding of 1K ohms and a secondary of 550 ohms, this was the one that drives the #5 tube (120).
The resistor value that is installed across the primary of the 1st audio transformer is 40K ohms.
The best value for the grid leak resistor turned out to be 3 Meg. with a 0.002 Mfd bypass capacitor.
C8 ?, input capacitor works well with a 0.05 Mfd capacitor.
I would like to align this radio using a Heath signal generator and AC voltmeter. I would appreciate some input from you on this.
If you open the service notes site that Tom posted the alignment is in it but not using standard equipment.
Let me know if there are some ideas on the alignment out there.
Thank you,
Mitch
= = = = = =
The only nonstandard equipment item in the RCA procedure is the "Special Radiotron Tube". This is basically a Type '99 tube with the filament open and is also called a "dummy tube". The tube is needed to disable downstream amp stages while retaining (approximately) the disabled tube's inter-element capacitances. You can make one if you have a burned-out '99 tube by cutting off one of the filament pins.
An alternate approach is to unsolder a filament wire from each (working) '99 tube in turn, in order to emulate the "dummy tube". This is, of course, a lot more work but may be the only approach if you don't own a burned-out '99 tube that you can sacrifice to the Neutralization God. It also (arguably) gives better results since you are adjusting the circuit for the actual tube used in the radio, not a "stunt double".
You don't need a dummy '20 tube since it isn't in the RF amp path.
Neutrodynes: now you know why I hate 'em.
Flash forward a few years from the date of manufacture of the Radiola 20 and screen grid tubes were invented, which eliminated all of the neutralization problems inherent in previous TRF sets.
Clarification: I should have specified unsoldering the filament wire that connects to a given tube at its socket "wired side" (i.e., under the chassis). The way I worded it, one might think that I was proposing unsoldering a filament wire between the glass bulb and its socket pin. This would be very difficult, although probably not impossible.
Unsoldering a heater wire under the chassis is still a hassle, but "do-able". Not sure how radio servicemen did it "back in the day" but I suspect that most of them used the dummy tube approach since it was much less time consuming and probably gave acceptable results.