[glyphmon]

Just some additional information on the two issues:

1 - Noise independent of volume: present on all band switch settings; no sound at all (probably obvious) with 6F6 (glass) tube removed; issue returns after insertion and delay; 6F6 tests good and the noise is also present with a different 6F6 (known working from the RCA 6K3).  There doesn't seem to be a terrible amount of components between the volume control and the 6F6.  All the resistors have been replaced in this radio, so everything should be in-spec.  Suggestions on where to poke/prod first?

2 - Band switch works fine in shortwave position and I can receive what stations there are.  I can't say for sure on the middle band due to the dearth of activity there, but AM is silent.  The only exceptions being the noise issue mentioned above and at the very bottom end of the dial when the tuning condenser is fully meshed, or nearly so.  At that point, there appears a higher-pitched, whiny noise.  The rest of the range is dead - no ambient interference, static, nothing, again, with the exception of issue 1 above.

So, for the band switch, since it was working but intermittently before but stopped after Deoxit, I suspect a bad contact?  Or is it more likely that a transformer or connection to it has been weakened?

Thankful for any guidance you could suggest.

Best wishes,
Roy

[glyphmon]

Well, in case more information will be useful for others to chime in, I managed to do some tracing through the band switch today.

With the band switch in the broadcast position I tested for continuity between the following points:
Antenna terminal to ground terminal - good
B12 to B6 - good
6K7G plate to Resistor 33 - good
any pin on 6A8G to Resistor 44 - no continuity
F1 to Capacitor 27 - good

Someone can point out the correct terminology to use with these if I get it wrong, but to me it means the primary and secondary are good on the broadcast antenna transformer, the primary is good on the broadcast RF transformer, and the oscillator transformer is good.  As far as the secondary on the broadcast RF transformer, I think my test points were not ideal because lo and behold, I later discovered that pushing in on the band switch knob in AM position allows me to receive AM and receive it quite well.  And after cutting power with working AM I tried the same test points and still had no continuity.  AM came back when I turned the power back on, anyway.

So I think it IS in fact the band switch, and since everything else tested fine, I suspect the contacts on the D wafer (secondary side of the RF transformers) is where the weak point is.  BUT since I know about this now, I'm not inclined to go much deeper on this rabbit hole IF I can continue to get the radio to work on AM with this trick of pushing on the band switch knob.

So, that is one mystery now a little less mysterious.

Secondly, I still have some noise with the volume control all the way down, but now that I could be sure AM was working, I started out with voltage measurements per the schematic.  The loudness of the noise varied from very quiet to quite loud throughout much of testing.

Now, I didn't remember to try measuring the voltages on the electrolytic that is marked with the other tube voltages - I can easily enough, I hope, if that becomes necessary, but here are the measurements for the tubes with expected values in parentheses, Isotap set to 120V:

6F6G
Plate: 250V (240V)
Screen Grid: fluctuated constantly in the range of 265V to 270V (250V)

RF 6K7G
Plate: 266V (245V)
Screen Grid: 78.5V (70V)
Suppressor Grid: -3.3V (?)

6A8G
Plate: 260V (245V)
Pin 4 (grids 3 and 5): 78.3V (70V)
Osc Control Grid: -10.5V (?)
Osc Plate: 179V (150V)

6Q7G
Plate: 193V (150V) ***

IF 6K7G
Plate: fluctuated constantly in the range of 166V to 180V (250V)
Screen Grid: 77.8V (70V)
Suppressor Grid: 0V (?)

So, there are some significantly high voltages on some of these pins.  Not sure exactly what that suggests or if people think that it's problematic.  The sensitivity seems pretty good.

Of note: the 6Q7G plate, when making contact from the probe to the pin, I got a loud pop of noise (no smoke) and it blew the 1A fuse.  Because there was no way I had inadvertently made a short, I put a 3A fuse in that came with the fuse holders, and tried again.  Still got the loud pop but noticed the noise present with the volume all the way down went away.  In fact, I can just barely hear the station come in with the volume all the way down though maybe that's always just been buried in the noise.  Could tune through much of the AM band without the noise recurring.  Eventually, though, the noise came back, and while making contact with that pin didn't guarantee it went away the first time I touched it, it did by the second contact with the probe.

I'm hoping someone has some thoughts about this revelation with the noise at minimum volume?  

Also, would it be expected that the screen grid voltages on the 6F6 and plate voltage on the IF 6K7 fluctuated so constantly?

Thoughts on the low plate voltage on the IF 6K7 in general?

Best wishes,
Roy

[MrFixr55]

Hi Glyphmon,

Re the screen of the 6F6, it depends on which 6F6 you are measuring the screen on. On one 6F6, the screen connects to the same B+ that the output transformer center tap connects to. However, the other 6F6 screen connects to B+ through a 3.5K resistor. This is done so that the varying voltage from the screen of that 5F6 represents a signal 180 degrees out of phase with its input. This is the inverted signal sent to the other 6F6. This is in lieu of using a phase inverter stage; a Philco trick in a lot of the 1940-1942 sets that generally works. (Later Philcos used a genuine Cathodyne phase inverter.

The varying plate voltage on the IF 6K7 may be rather worrisome. Measure the B+ to the transformer. If this is stable and the plate voltage varies by that much, then the transformer may be at issue, maybe a bad solder joint or bad socket connection. the transformer may ohm good under no current but with the tube plate current as a load to the transformer, the issue may manifest itself. there could be issues with the tube itself. Swap the tube with the RF 6K7.

Suppressors of both the RF and IF 6K7s should be the same. -3V does not seem outrageous. note the 25 Ohm resistance between chassis ground and the next resistor, the 168 Ohm portion of resistor #68. the other end is tied to the HV center tap. This is how negative grid bias is taken.

What happens if you pull the 6Q7? does the noise quit? If so, check the following parts of the 6Q7 circuit:
Plate resistor (50), Coupling cap (44), resistor (51). Check the following parts of the 6Q7 and 6F6 grid bias circuit:
Resistors 54, 56, 57, 68 (the wirewound, tapped resistor).

Hope this all helps.

[glyphmon]

MrFixr55,

I will check what you suggested later today.  Just a point of clarification - there's only one 6F6 in this radio.  How does this change your perspective on that front?

Best wishes,
Roy

[MrFixr55]

OOpsie!!! I must be looking at the wrong schematic! Yes, this changes things!

OK, for the 37-620. If the 6F6 screen voltage is unstable, this is a major issue. The screen and output transformer are both fed by the same voltage, generated at the junction of the speaker field and 8 uF cap #60. If the voltage at the 6F6 plate (pin 3) is stable but the voltage at the screen (pin 4) fluctuates, check your wiring from the screen terminal on the socket (pin 4) back to the above stated junction. You may find a cold solder joint.

The plate voltage for the IF Amp is from the primary of the 2nd IF transformer. The B+ for that transformer also comes from the same source as that for the output tube stage. Measure the B+ to the transformer. If this is stable and the plate voltage varies by that much, then the transformer may be at issue, maybe a bad solder joint or bad socket connection. The transformer may ohm good under no current but with the tube plate current as a load to the transformer, the issue may manifest itself. However, I think that this may be a cold solder joint condition. Depending on the wiring, what is causing the screen fluctuation in the 6F6 can also cause the plate fluctuation in the 6K7.

The suppressor grid issues are likely not a big issue. I would expect a slightly negative voltage on both, with the RF being slightly more negative than the IF. The suppressors are tied not to chassis ground but to taps on the bias resistor that is between chassis ground and the center tap of the HV winding of the power supply. Note the 25 Ohm resistance between chassis ground and the next resistor, the 8 Ohm section, then the 202 Ohm portion of resistor #61. The other end is tied to the HV center tap. This is how negative grid bias is generated in many sets of this vintage.

The bottom line here is to trace through the B+ supply. Note that where the junctions are made on the schematic may not be where they actually are in the physical wiring, but the result is the same.

BTW, note that the polarization indicated on the schematic is incorrect. The upside down U is usually the can. This is definitely NOT connected to positive. However, I'm sure you wired the caps correctly. If not, you woud have heard a banf and found cap confetti all inside the radio. (Been there, done that, got the scars.)

[glyphmon]

MrFixr55,

Thank you again for your assistance.  I've checked at a couple junctions and I don't think I've run into any bad solder joints, though admittedly, some of them are not pretty, and not all mine.

At the junction with R33, I get fluctuating between 264V and 272V, this of course then proceeds through the 2nd IF transformer to the plate of the IF 6K7.
At the junction of the field coil, C60, and connections to the junction above and the screen grid of the 6F6, I similarly get fluctuating between 266V and 271V.  This junction is essentially the lead of C60 and the field coil output tying together directly at the tab for the pin on the tube socket.

Would the next step in tracing this be to go to the field coil?  I have replaced the three wires that run from the chassis to the speaker since the old ones were in very poor condition with exposed conductor.

I will say, the fluctuation seems to get more pronounced on the meter when the noise from the speaker (again, with volume still all the way down) is more actively changing.

UPDATE:

I pulled the cover off of the speaker connections and tested each side there.  The input (green trace) side coming from the junction with C62 was a little bit variable, starting out at 342V but gradually increased and settled at 349V.  The output (white) side connecting to the junction with the 6F6 and C60 was again variable between 269V and 278V over the period that I observed.

Flaky field coil, perhaps?  Possible that something in there is leading to the noise that would be independent of the volume control?  If the output from the field coil is variable, it would make sense to me that everything downstream in that circuit would likely experience variability as well.

Best wishes,
Roy

[MrFixr55]

Hi Roy,

So the plate of the 6F6 is fluctuating now? If there are fluctuations in voltage with the volume control down, then there is an issue.

You may want to heat up all the joints, especially those not made by you. Verify that all caps, electrolytic and paper are replaced. Electrolytics definitely go bad over the years and especially the older pre-war wet electrolytics. For this set, for Caps #62 and 60 should be 450V. Make sure that you replaced caps, especially those in the Bakelite blocks. Don't bridge caps with new ones.

After that, do the tap test. Tap individual components, tubes and junctions (of course, use something non-metallic but with some weight). Whatever you tap that causes a disturbance, investigate there.

Output transformers and field coils can arc. This will definitely wreak havoc. If you have another set with a single 6F6, 6K6, 41, 42 or 2A5 and all the radio's B+ load goes through the field coil, consider subbing the speaker if you can't find the cause of the issue.

Hope this helps.


BTW, set your Isotap to 115V if you can. (The schematic specs the AC power supply to be 115V.) 120V is about 4% or so high, as is some of the DC voltages. For example, 250 V on the 6F6 plate as opposed to 240V is about the same amount of error. Check the AC at the radio's power transformer input lead connections to ensure that it is not fluctuating also. Verify that the power switch is not erratic.

[glyphmon]

MrFixr55,

No, the plate of the 6F6 is not fluctuating that I've observed.  Places that I've observed fluctuation are:
Output side of field coil (white wire) at the field coil terminal, 269V - 278V
Junction of white wire above with C60, 6F6 screen grid, 265V - 270V
Junction of R33 to 2nd IF Transformer, 264V - 272V
Plate of IF 6K7, 166V - 180V

Of note, I'm not seeing fluctuation on the input side of the field coil, so I feel like the power coming from the rectifier is stable, at least (settled at 349V).

All of this testing has been with the volume control all the way down and in that state there is still some intermittent/random static-like noise (think wind blowing across a microphone) from the speaker.

All caps non-mica have been replaced, though I didn't replace the electrolytics myself.  I'm wondering if I should replace them again.  They have a few years on them now, and admittedly, that's not a few years with any kind of regular use.  Bakelite blocks were excavated and reused with new caps.  Electrolytics were not restuffed, but originals are disconnected from the circuitry.  Everything else was cleanly removed before replacing.  Any out-of-tolerance resistors, in fact, I think all resistors, barring the voltage divider have been replaced.

I'll dial the Isotap down to 115V for the next round of testing, and see what I can do following your suggestions further.  Not sure what compatible speakers I may have around.  You say another radio that puts all its B+ through the field coil, but is that the case in this radio?  What about the branch that goes up to R64?  That doesn't seem to go through, or I'm misunderstanding.

Thank you for your patience.

Best wishes,
Roy

[glyphmon]

MrFixr55,

Here are some updated voltages from dialing the Isotap down to 115V, which translated to 116.6V AC at the power switch

6F6G
Plate: 228V (240V)
Screen Grid: 250V (250V) (not fluctuating at time of initial test)

RF 6K7G
Plate: 246V (245V)
Screen Grid: 76V (70V)
Suppressor Grid: -3.2V (?)

6A8G
Plate: 243V (245V)
Pin 4 (grids 3 and 5): 76V (70V)
Osc Control Grid: -9.8V (?)
Osc Plate: 159V (150V)

6Q7G
Plate: 181V (150V)

IF 6K7G
Plate: fluctuated constantly in the range of 120V to 123V (250V)
Screen Grid: 75V (70V)
Suppressor Grid: 0V (?)

Now, the notable differences I saw were the lack of fluctuation on the 6F6 screen grid, and the significant drop in still fluctuating IF 6K7 plate.

Aside from that, the voltages look better at this setting, so I'm not too worried about much other than the fluctuation and the noise.

To the point of the noise, I tried removing the 6Q7 tube as you suggested and of course no signal came through.  There was also no extraneous speaker noise.  Whether smart or not, I inserted the tube with the power to the radio on and observed there was still none of the random noise, though there was a hum and very weak signal until I connected the wire to the cap.  When I connect the wire to the cap, the hum goes away, but the random noise returns.  Removing the cap connector, the noise goes away and hum returns.

For the heck of it, I checked the 6F6 screen grid and the IF 6K7 plate voltages in these two configurations (with and without the cap to the 6Q7 connected).  With the cap on, the 6F6 screen grid tended to fluctuate around 265 +/- 1V, with it disconnected, it was a steady 266V.  Cap connector state didn't seem to impact IF 6K7 plate fluctuation at all, and during this test, it was pretty constant in fluctuating between 188V and 210V.

I see that the Stewart-Warner 91-621 that I have downstairs also has a 6Q7.  I'll try to pull that tomorrow and see if swapping it out makes a difference.  Google suggests that the solder point for the cap inside the tube can crystallize and crack, creating small arcs, and that gently heating the cap with a soldering iron can help.  Of course, I'm sure overheating can do way more damage, but if swapping the tube makes a difference, maybe it's worth a shot?

Not sure if it's just coincidence that the 6F6 grid is fluctuating only when the cap is connected to the 6Q7.  Would that make sense?  I don't see a connection between the two on the schematic; of course logically, when the noise is present there's more power drawn to produce the sound.

If feel like the fluctuation of the IF 6K7 plate (and at other points between it and the field coil) is a separate problem from the noise.  Would a failing electrolytic (C60, in this case) cause this kind of fluctuation?  Like I said, these were replaced, but barely used in the last couple of decades.  Should I just replace the electrolytics again at this point?

I have not gone through and methodically heated all of the joints yet, but I still will try after I swap the 6K7s to see if that makes a difference.  Wanted to give an update and pick your brain based on the behavior of the 6Q7 cap.

Best wishes,
Roy

[MrFixr55]

Hi Roy,

I don't think you will do much harm to the 6Q7 heating the grid cap other than the glue coming loose. If you have solder wick, a solder sucker, etc., you can try to remove all solder, clean the cap and wire as best you can and resolder it. Yeah, I would not worry about anything other than the OF tube plate fluctuation. If you do not get noise with the grid wire off the 6Q7 but DO get noise with it connected, I think that the issue is before the grid cap. I AM worried about the amount of plate fluctuation on that 6K7. If you don't see the same fluctuation at the B+ point closes to the 2nd IF transformer primary (cut off some insulation and connect to it), then the problem is in the 2nd IF transformer primary. Disassemble the transformer and check the connections. If you do see fluctuations at the B+ connection to the 2nd IF transformer primary.

Good luck with this problem. I'm sure that you will nail it.

[glyphmon]

MrFixr55,

Re: fluctuation, there is fluctuation all the way back to the junction with C60 and the field coil.  No fluctuation at the junction of C62 and the field coil.  So I guess I'm down to swapping 6K7s as a long shot, and warming solder joints from the C60 side of the field coil then out.

I'll update out once I get the 6Q7 and swap the 6K7s.

Thank you again.

Best wishes,
Roy

[radiorich]

Hello Roy,
Sounds good !
Sincerely Richard

[MrFixr55]

Hmmmm,

Roy, I still think that there is a big problem in the area of that IF transformer. The plate voltage of that 6F7 is much lower than it should be. The voltage drop across the primary of the 2nd IF transformer should be minimal, as the resistance of the primary coil is only 11 ohms.

Consider that the specified voltage drop across the output transformer is only 10V for a tube (6F6) that likely draws about 30 mA over a 250 Ohm resistance of the output transformer. Consider that the specified voltage drop for the IF 6K7 is nil (250V vs the B+ output of 250V), the average plate current of a 6K7 is only about 5 mA and the resistance of the 2nd IF primary is 11 Ohms. Your measured voltage is only half the specified voltage. Either something is horribly wrong with the transformer or there is a bad solder connection to it or within it. I believe that you previously stated that the resistance of the transformer was good. However, remember that the current across the circuit when measuring resistance is in the order of microamps when using a DVM. In real life, the voltage drop between the B+ supplying the transformer and the plate of the tube is much more that should be occurring at 5 mA at 11 Ohms.

As to the general fluctuation in B+ after the field coil as opposed to before the field coil may indicate a problem with Cap #60. However, also consider the resistance between the negative lead of the Cap #60 (chassis ground) and the negative lead of Cap #62 (the power transformer HV secondary). As current changes, voltage changes.

Bottom line is that Job #1 is to determine why for a 5 mA load the drop over 11 Ohms is 125 V. This is a violation of Ohm's law. There has to be a bad connection or something else going extremely wrong there, and likely causing the noise you hear.

I think that you are close to solving this issue.

[glyphmon]

MrFixr55,

In saying you were right I am in no way saying I thought you might be wrong.

I just rechecked, based on your guidance, the installation of R33 to ensure I had the right wires in the right places - I did, thankfully.  Testing at the near/B+ side (if this were internet, I'd say upstream) of R33, the junction with the 2nd IF Transformer and B+, was, today stable, but the plate of the 6K7 was not.  It started out at 255, pretty stable which seemed good and then it went to fluctuate around 180 or so.  I didn't catch if the noise started the same time or not, but it may have - I didn't have the cap connected to the 6Q7 at the time.

So it seems to come in after warming up a few seconds.  Heat-sensitive partial open in the primary winding of the 2nd IF transformer, then?  I guess it's time to look at the innards of the 2nd IF transformer?  I see the can is just held down by some spring clips, but looks riveted to what's inside.  I'm guessing de-solder connections and then drill out the rivets on this model, yes?  I should probably wait to see what's in there, but is the primary or the secondary winding usually the bugger to fix to once exposed?  And if it's the primary and a non-obvious, repairable fault, am I better off looking for a replacement somewhere?

Lots of "ifs" in that statement, I know.  I don't have to have answers right now (except on how to get in) but mostly just preparing myself for the fact that this radio isn't quite as finished as I'd like.

Best wishes,
Roy

[radiorich]

Hello Roy,
I have RCA radio that was acting up after warming up and ended up being Mica capacitor in the audio section of the radio .
Replaced that part and the radio has been working ever since .

Sincerely Richard