[Joe Rossi]

Did I mention the voltage measurements?

7A8 pin 2, 102v, schematic 107v
7A8 pin 5, 56v, schematic 40v (9/2/26, actual 46v)
7A8 pin 3, 102v, schematic 105v
7A8 pin 4, -5.5v, schematic -4.5 to -7.5v
12BA6 pin 5, 102v, schematic 107v
12BA6 pin 6, 57v, schematic 40v (9/2/26, actual 46v)
14B6 pin 2, 97v, schematic 60v
50L6 pin 3, 108v, schematic 108v
50L6 pin 4, 102v, schematic 105v

1st filter cap, 132v, schematic 130v
2nd filter cap, 119v, schematic 118v
3rd filter cap, 101.5v, schematic 105v

Joe

[RodB]

Silver migration disease didn't occur until the late 40s and only in one brand of transformer, the miniature cans. That's not to say it won't be anywhere else, but it's not worth chasing a problem that isn't likely there. Motor boating is a feedback problem, SMD is high voltage leaking through the mica caps causing continuous crackling noise.

Quite often motorboating is caused by something in the AVC circuit.

[MrFixr55]

Interesting set in the fact that it contains Octals, Loctals and Miniatures.

A lot of sets from small manufacturers made running changes from Octals to miniatures (7 Pin) using wafer type miniature sockets that were large enough to use the holes punched in the chassis for octal sockets, and implement the "new" miniature tubes as their supply of octals dried out. Even RCA did this. There were some 1948 or so models with metal octal tubes for the converter, IF and detector / 1st AF only to have a 35W4 rectifier and 50B5 (soon replaced by 50C5) output tubes. It is interesting that this set was punched to use the miniature socket for the 12BA6 as the IF tube.

[MrFixr55]

Joe, you replaced the 12AV6 Jury Rig with the 14B6? That small cap was likely used to filter RF out of the audio and probably would not cause motorboating.

Most of your voltages are well within what most people would accept as limits. the exception here is the 1st Audio plate and the IF and Converter screens.

Since the 50L6 Screen value, 12BA6 and 7A8 plate values are all good and that section of the power supply is the source of the 14B6 triode plate and converter and IF screens, check the resistors R202 for the 14B6 Plate and R300 for the converter and IF screens. The conundrum is that when you have several problems, look for the common. The common between the 3 tubes with out of spec voltages is the B+ from the second filter cap which is well within range. So you have either 2 separate problems, one of which is causing the motorboating or 3 separate problems, one of which (the motorboating) is not caused by the out of spec voltages discussed above.

Is the motorboating there at low volume? Does the frequency change with the volume? If so, look at the audio portion of the set. If not, look at the RF section of the set. There should be a rather large (0.05uF) cap between common and the AVC line this is C304 on the Canada Philco 83 schematic. There is a parallel cap C403, a 0.02 uF cap between the AVC line and common. I don't know why there are 2 caps across the same line unless the schematic is wrong (Schematic missing a resistor between the 2 caps in the AC line?) By common, I mean the junction of the low side of the heater string, the negative of the twin electrolytic caps, the return from the volume control, the cathode of the 1st AF tube, the cathode resistor of the 50L6, etc. There should be also a large value cap between this "Mecca" and the physical chassis. This is C303, a 0.1uF cap.

Re the power switch. Hear is a very useful link to the "Switch in the low side" conundrum from Philip Nelson's "Phil's Old Radios":
https://antiqueradio.org/safety.htm

If you did this mod (Using a polarized power cord, connecting the neutral directly to the "Common" (Mecca), placing the power switch between the "Hot" side of the power cord and the junction between the 35Z5 filament and plate, make sure that you removed any "jumper" wire from the switch to the "low" end of the volume control.

Hope this helps.

[Joe Rossi]

Good Evening MrFixr!  Thank you for responding.

"Check the resistors R202 for the 14B6 Plate and R300 for the converter and IF screens ".  R202 measures 467K, as it should and is attached where it is supposed to be..  R300 measured intermitant; I replaced it with a new 39K. (As an aside, R300 wasn't even in the radio when I began working on it.)

"Is the motorboating there at low volume? Does the frequency change with the volume?" Motorboating is there no matter what the volume control is set at.  The frequency does not change with volume.

"There should be a rather large (0.05uF) cap between common and the AVC line this is C304 on the Canada Philco 83 schematic. There is a parallel cap C403, a 0.02 uF cap between the AVC line and common. I don't know why there are 2 caps across the same line unless the schematic is wrong"  C304 is there as is C403.  Of note, the US equivalent model of the 81 is the 50-520. The 50-520 schematic doesn't have a C403, or an R304.

"There should be also a large value cap between this "Mecca" and the physical chassis. This is C303, a 0.1uF cap."  Yup, that's there and goes to the physical chassis.

"If you did this mod (Using a polarized power cord, connecting the neutral directly to the "Common" (Mecca), placing the power switch between the "Hot" side of the power cord and the junction between the 35Z5 filament and plate, make sure that you removed any "jumper" wire from the switch to the "low" end of the volume control."  Yes I did the mod and have removed the jumper wire.  Thanks for that link, I'm bookmarking it

MrFixr, I'm bushed, and am going to go to bed and sleep on this pick it back up after the gym in the morning.  God bless you for all you do here.  Joe

[MrFixr55]

God bless you too, Joe, and Thanks! I'm still up because I took a long nap this afternoon.

When you get at it, does the volume level of the motorboating change with the volume? If not, then the problem is in the audio stage, or it's coming through the power supply. If you have a o-scope, you may want to check the supply.

[Joe Rossi]

MrFixr,  I have an OWON PC o-scope, but to my embarassment and shame, I have never learned to use it. I'm going cross-eyed   going over the components and comparing (tracing) to the schematic.  I've noticed that the US 50-520 has a 150K resistor (R100) between the floating ground and chassis ground, where the 81 doesn't have one.  Also, the 81 schematic (and the 50-520) has a dotted line conection in Section 4 labeled C101C.  Anyway, I'm going to hold back for a couple of days and come back to it with a hopefully fresh mind  .  Joe

[MrFixr55]

When you get back to it (and I definitely understand why one must walk away), the first thing to discern is if the motorboating is just as loud with the volume control at minimum as it is at maximum? That is a "Divide and Conquer" piece of information.

The "actual" C101C does appear connected the same way circuit-wise that the "dotted line" C101C appears. I don't know if adding the 150KOhm resistor between Common and Chassis will help, but it couldn't hurt. Adding a 0.05uF cap between B+ and common may be helpful.

C302, the bypass cap for the screens of the converter and IF seems to be too low a value. In earlier model Transitones, and most other AA5 radios, this cap is 0.05uF.

Did you try the radio with the 12AV6 jerry-rig in place? If not, try it. If it works, maybe the 14B6 is bad or there may be a bad connection between the cathode of the 14B6 and Common. Verify the continuity between the pin and Common. If the plate resistor is actually close to 470KOhm but the voltage is that high, then the tube is not conducting correctly.

[Joe Rossi]

MrFixr,

I did not try to fire it up with the 12AV6 installed. The underside of the chassis was really a mess.  I've tried subbing a couple of NOS 14B6's, 7A8's, and a 50L6 (all tested good) to see if maybe the tubes had failed, but no change.  I did find a 22K resistor that was not on the schematic attached on the 7A8.  That resulted in the voltage measurement on pin 5 of the 7A8 and pin 6 of the 12BE6 dropping from 56v to 46v which is closer to the 40v from the schematic.  Clipping the 150K resistor made no difference.  C302 had been .005, but I will try a .05 tomorrow, and recheck that R202.  The volume neither increases or decreases and no change in the motorboating.  I'd say that the put-put is about 120 cycles per minute, steady and rythmic.  More tomorrow.  Thanks friend,  Joe

[Joe Rossi]

I put a .047uf in parallel with C302 giving us .05uf.  No change. Also checked and re-checked R202, that's fine.  Subbed a couple of 12BA6, no change.  I'm going to put this bad boy (bad radio, BAD radio!) back in its cabinet and start looking at the next radio, a 50-520 which is the US equivalent to the 81.  Give my eyes a rest.  Joe

[Joe Rossi]

It's ALIVE!!! My bad, I soldered C203 to pin 3 of the 14B6 instead of the floating ground on pin 4. This is a sweet radio, very sensitive with nice tone. Thank you MrFixr for all your help with this. I have pdf of the 1947 NRI lesson #23 on motorboating and I retraced the bypass caps in the IF and Audio circuits. It's ironic, my Dad graduated from NRI and he would have studied that exact 1947 lesson.

Thanks again, Joe

[RodB]

Doh! (Again) Why do we do that? We keep doing the same mistake expecting different results.

I'll be doing the same later today.

[MrFixr55]

It wasn't a mistake, it was an experiment in positive feedback. An example of the Scientific Method of investigation. You were merely attempting to apply the Armstrong regenerative feedback principle to audio amplifiers. You proved that it doesn't work as well in audio circuits as it did in the Armstrong Regenerative Detector.

[RodB]

Mistakes are humbling, experiments are enlightening.

[Joe Rossi]

Yeah, thats the ticket, I was EXPERimenting. Now I’m on the path to…least resistance. Painting and detailing.  More pix to follow. Thanx gents!  Joe