[ccomer1955]

Morzh has inspired me to revisit an old nemesis while I wait for Gary to reopen at Playthings so I can start getting serious about that 77. I put it aside as beyond my very limited skills a while back, but I've been reading my "Elements" and lurking here and at that other board. I had recapped, checked coils, etc. no luck. Voltages were never right, and rather than fix it until I killed it, I put it away (the cabinet turned out beautifully after some minor veneer fixes). I just ran voltages again, and my voltages are still off, but more informative to me, now (I think). The 80 was filaments=5v, plates=344 (all heater voltages were around 5.3v for all the rest of the tubes)-

42: plate=209, screen=215, grid=-6, cathode=1
6A7:plate=211, screen=75, anode grid=135, control grid=1.8 (positive voltage)
78: plate=210, screen=75, suppressor and cathode (pins 4 & 5 are connected)=1.6, grid=0
75: triode plate=149, diode plate=-80.6, cathode=.1, grid=-.5

http://www.nostalgiaair.org/PagesByModel...013827.pdf

The antenna coil and oscillator coils have good primaries and secondaries, and both IF towers measure with valid primary and secondary numbers. Tubes all test OK on my Triplett 2314. Seems to me all voltages are low to begin with, plates are 50v or so low. Grid voltages are preventing oscillation? So, I have a shorting/grounding issue that is sapping voltage, or some of those new capacitors need to be replaced again. The wave trap was shorted, but I jumpered around it. No open trimmers. I get resistance without shorts across the dial on the tuning cap.
I guess maybe doing some resistance math and point-to-point checks might be in order. Checking all grounded connections (I have some concerns about how C27, R47, R45, C46, R43, & C42 ground)?
I get a loud buzz touching the 75 grid cap--actually a kind of squeal, a bit less loud touching the cap of the 6A7, and a soft buzz touching the volume pot if that helps.
It's still more than my limited knowledge and skill can solve right now, but if anything jumps out at you, I'd appreciate the input.

[morzh]

Diode plates bing -80V !!! No good.

[ccomer1955]

C24? or C54? R25 shows perfect resistance. Ground of the volume pot?

[morzh]

Don't know. But 1) remeasure, looks weird. 2) is your volume pot OK and soldered well? 3) Is your 44 wirewound resistor OK and soldered well?

Diode plate should be negative, but 80V....

[ccomer1955]

Good catch, morzh. My old man eyes failed to notice the tiny little m in front of the V. I ran major resistances, and R44=254.5, R27+R29=140K, both coils gave good numbers for primaries and secondaries, choke is 2400, field coil=1200, output transformer is good, and both IFs have good windings. Used a dim bulb and variac to bring the set up with no tubes in, and the bulb stayed dark. While under load, the bulb has a dim glow going on. All heaters run about 2.3v.
voltages:

80:filaments=4v, plates=300 different and lower.
42: plate=205, screen=209, grid=-6.4, cathode=0
6A7:plate=213, screen=76, anode grid=136, control grid=1.9 (positive voltage) and grid cap= +1.9
78: plate=212, screen=75, suppressor and cathode (pins 4 & 5 are connected)=1.6, grid=1.6 grid cap= -82mv
75: triode plate=148, diode plate=-96mv, cathode=.1, grid=0 grid cap= -.5

Still gets buzzing where it did before. I need to get the heater voltage higher and the 6A7 grid in the negative, right? I still wonder about the grounding for several components, so I will look there (especially the ones that have their grounded sides connected together on the daughterboard (are all 66s done that way?). I also have the service note components i this set (the 20K resistor off the bandswitch, the 70 ohm resistor in the oscillator coil.

That's it for tonight. I'll think about it tomorrow as Scarlet says.

[PAradiogeek]

Have you checked continuity on the volume pot itself? You need to verify that the pot has a resistance somewhere between 0 -350K from the top connector to the wiper (center contact) as you rotate the control.

I recently had a Zenith radio with a bad volume pot. The resistance element was good, ~300K from top to bottom, but there was NO connectivity to the wiper. Radio was completely dead. No stations on either BC or SW.

I tried cleaning it, but was unable to get connectivity. Ultimately had to get a new on/off/volume pot. After that, it worked like a champ.
GL
Jon

[ccomer1955]

Yeah, for a while I was sure that was the issue, but I bought a Mallory replacement from Gary at Playthings, and have the same issues. It (and the old Philco one) both show resistance across the rotation. I've taken the Philco one apart and cleaned it out pretty good. There was some carbon or other dirt on the strip, but it's nice and shiny, now. Thing is, it doesn't show 350K of resistance. It is more like 300K. The Mallory is 500K, but I checked and it is the one Mallory says is replacement for the Philco part. The exact resistance is not crucial, then?
I'm still wondering about the grounding for just about everything except the filter circuit. Especially the capacitors and resistors that are tied together on their grounded sides on the daughterboard. If someone has pictures of an old restoration, I'd appreciate it if they looked to see where those components are physically grounded so I can put my mind at rest on that score.
Also, it bothers me that my filament voltages are so low. I get a good number at the 80, then its 2.3 volts for all the heaters on all the other tubes. Generally, I wonder about the combined resistances of the old wiring and soldering.
Almost related to that, I noticed yesterday when I pulled the tubes, that pin #5 (K,G3) on the output tube (42) is lose and wiggles freely. A shorted socket actually arcs, right?

[morzh]

Power off. Pull 80 tube. Power on. Keep pulling all other tubes one by one to see if at any point the filament volage becomes normal. Also see if your pilot light has a short or the bulb is wrong. If a bad tube, the voltage will jump to normal once pulled.

If does not help, power off, pull all tubes and the pilotblight, and desolder one of the transormer filament leads from where it goes on one of the tube panels. Measure across the filament pins on any socket ( except 80's). See if there is any load present (should not be).

[TV MAN]

If all the voltages are low, including filament voltages, make sure you are getting full line voltage to your power transformer (no bad power switch, line cord, connection, etc). If you are, follow Mikes advice to pull tubes, pilot lamp and recheck (removing the load). If still low, you have a bad power transformer.

Power supply problem = nothing will work right

[morzh]

Which would be strange, I mean bad transformer.

Theoretically at least, if you do not have smoke, heat etc occurring in your transformer, and then you have one winding outputting correct voltage, the rest of the windings, PROVIDED the transformer is original and not had been rewound in the past, the rest of the windings, if unloaded, MUST output correct voltages. And as I understand, both 80 filament voltage and the High voltage are correct. Which means the 6.3V filament voltage if unloaded, has to be correct.
Again, should you have a turn short internally in ANY winding, NONE of your voltages will be correct AND you will have heat/smoke/blown fuses (if installed) etc.

[ccomer1955]

5v at the filament without the tube. Depending on the 80 I use, 3.4-4.5 with the tube. Lower past the 6A7. I unsoldered and cleaned the 6A7 lugs, then made good physical contact and resoldered. Voltage came up to 4.5v. Still, have other issues preventing oscillation, but progress, I suppose. I'm going to look at my filter circuit again. If I did damage to the pair of caps in C50, it would be a "works or doesn't work" situation, right?
What I have in the set right now, is as follows (the filter circuit was largely disconnected when I got the set, so I had to reconstruct it from the schematic, so I restuffed the filter can and color-coded one of the 8uf caps green and the other yellow): the yellow cap is connected to the filament of the 80, to the choke, and to the green/white lead to the field coil. The green cap is connected to the lug of R28 which it shares with a lead that I ran from C22 (because that lug on C22 has a lead that traces directly back to the output tube's grid lug which it shares with the white wire coming from the output transformer). Both filter caps share a common ground lead to C51 where the CT is connected from its lug on R44. C49 is also destuffed and has a white lead connected to the lug for R47 which it shares with R15.
I used 10uf 500v caps for both C50 caps and for C49 because they are what I had on hand, and I have C49 connected to the black lead on the choke and the yellow C50 to the white lead (but the choke connection isn't crucial, right?). If that is correct, then one question I have is do I have the yellow C50 connected to lead #6 or lead#7 from the transformer?

[morzh]

When you say yellow/green etc, you probably refer to some sort of picture.
Where is it?

[ccomer1955]

No, I'm just using the colors to differentiate between the two sections of C50. I put different colors on the leads for the two caps in C50 so I could make notes on my schematic. I can post a picture if you like.

[morzh]

So far what you said is OK. As long as that Green cap and all it connects to also includes the other (not the green/white) lead of the field coil. If you do get the High voltage there, it does, just confirm it.

Then again, this directly does not affect the filaments' voltage.



What is your input voltage across the primary of the power transformer? I mean, have you measured it?


Last thing: have you done what I asked you to with tubes' pulling? We need to know the FULLY UNLOADED (no tubes in sockets AT ALL) voltage in 80 socket and other (any of the rest) socket with EXACTLY 110VAC measured across the power input.

Last last thing. Is your meter OK? Do you have another one? Does it read a known voltage as it should??

[ccomer1955]

I have not measured the line voltage for that socket. Come to think of it, I need to see if the socket at my bench (relatively new set up) is even hooked up correctly (the house was built in 96). I did pull each tube in turn, and when I had resoldered the leads from the transformer to to 6A7, it was a consistent 4.5 at each heater in turn.

By high voltage at the other (yellow) cap, you mean measure it where the line from the field coil and the 42 connect? That would be before the two 70K resistors on the daughterboard, right?
About the DMM, that is a good question. It is a Kein MM200. I have one other (cheap Radio Shack model). I can use both to measure line voltage for the home and get back to you.
Fully unloaded voltage for the rectifier filament sockets (no tubes in the radio at all)=5v but that is with the unknown house voltage.
I apply voltage through a dim bulb tester and variac, so I will need to measure voltage for them as well. I will do so and get back to you.
By the way, with no tubes in the set, the dim bulb doesn't show light at full power with a 40 watt bulb.
Thanks for taking time with this. It seems the 66 can be a troublesome set for many folks.