[heats00]

Hello,
This is my first post after lurking for a bit. I am relatively new to all of this radio business, having recapped 1 Admiral 5X12N successfully so far, and having limited success on a Motorola 57CC1, a Philco F-809, and a Crosley 58TA.

I have now picked up a 46-1209 code 121. After getting a copy of the Sams Photofact and ordering caps and resistors I took time to get somewhat familiar with what I was looking at underneath the chassis.

The AM section worked although it had some loud hum, and was very weak volume-wise, no audible signal of the shortwave, and the phono works as expected. I started recapping and checking resistors as I worked through the chassis and quickly realized even though it was a large chassis, that things were still very tightly packed. (I have tried up until this point to replace the entire component, but found it necessary to clip leads on a few and wrap new leads to them hoping to prevent damage from the soldering iron.)

So far, I have replaced all three electrolytics, all paper caps but 1 which is buried underneath the band switch wafers, and a large majority of the resistors. My first power up after replacing these was a success in that nothing burned up, but I had no sound from the AM band at all and saw that the plates in the 7X7 tube were glowing so I shut everything down.

Back to the bench. I started tracing all of the components I had replaced until this point (lesson learned: check work periodically) and found that I had swapped two resistors accidentally on the 6V6, the 2nd IF Plate Decoupling resistor (1000 ohm) and one of the output grid resistors (330K ohm), and used the wrong value on the other output grid resistor. I corrected these and closely inspected my other work and did not find anything standing out.

Second time powering up was much better. I am finally able to detect signals, but still without much volume and a tiny bit of distortion. I have the volume turned all the way up and most stations are still barely at a conversation level and it seems to only pick up very local stations but at the right location on the dial. The plate glow is still present on the 7X7 but it much less intense than before. With what little knowledge I have at this point, I suspect that the micamold caps on this chassis might not all be mica caps but the mica-looking paper caps. On the 7X7 tube there is only one component I have not replaced and that is the 100 pF Audio Plate bypass, and that looks suspiciously like a paper/mica cap. Would this cap, if it was leaky, cause these symptoms of little volume except on very close by stations, some distortion and cause the plate to glow? The phono plays great with no volume problems. I have no capacitor checker at this point, and not sure how to secure the chassis given it's size to safely check voltages.

Also, how do I get to the capacitor underneath the band switch to replace it? The switch looks to be riveted to the chassis.

Chase

[heats00]

[Image: https://lh3.googleusercontent.com/AFYav8...97-h955-no]

[klondike98]

Welcome to the Phorum!!

[codefox1]

More information here: http://www.nostalgiaair.org/Resources/586/M0013586.htm (free)

If the plate of the 7X7 is really glowing, very unusual, it is most likely defective if there are no bad components or wiring shorts/errors. "Micamold" parts can be wax or mica capacitors (usually paper if >.001), could also be resistors too, so trace 'em on schematic. Many doing complete restorations replace 'em anyway along with carbon resistors which have probably drifted way up.

Leave the "hidden" capacitor for last. Of course, modern replacement will be physically smaller and you may be able to snip the old part in pieces, remove it, and so on. Maybe someone has worked on this model and can help more. Provid a closeup of this area to refresh memories.

I use blocks of wood, just about very set will work OK upside down, sometimes there is a problem if chassis is operated sideways. There are such things as chassis jigs to do this, can't think of who sells 'em, but someone else will chime in.

Welcome!

[Radioroslyn]

As CF said it is a bit unusual. I'd measure the the plate and grid volt and check against the volt chart. What will cause the plate to cherry is way high plate voltage and or lack of grid bias.

Terry

[heats00]

I will check voltages and resistance this weekend and hopefully find an answer, but I have also questioned my choice of capacitor for the output plate bypass. It calls for a .003 @ 1000 v and I used a .005 @ 1000 v. as it was the closest I could find @ JustRadios. Would this cause an issue volume-wise?

Also, the paper capacitor I have not replaced yet is the .01 mfd Conv. Plate Decoupler. If this was not functioning properly, what would it's symptoms be?

[heats00]

Geez, life can sure get in the way of hobbies. I still have not had a chance to check voltages as my workbench is currently full. Hopefully this weekend I can get caught up with the 'fun' stuff again.

[heats00]

So I got to it tonight instead and got some voltage readings. I can see that I am out of range on pin 3 of the 7X7 but not sure what causes this.    I am assuming that getting whatever is causing it will also bring the readings back into line on the 7AF7 as well, or is this incorrect?

7X7
pin 1 = 3.4 AC
pin 2 = 156
pin 3 = -1.4 (-.5 expected)
pin 4 = 0
pin 5 = -.5
pin 6 = n/a
pin 7 = n/a
pin 8 = 3.4 AC

7AF7
pin 1 = 3.4 AC
pin 2 = 74 (65 expected)
pin 3 = 185
pin 4 = 45
pin 5 = 0
pin 6 = 16 (30 expected)
pin 7 = 27 mV (2 expected)
pin 8 = 3.4 AC

[heats00]

Well, checking things again tonight, I at least figured out why pin 6 and 7 on the 7AF7 were out of range. I misread the schematic and put a 68 ohm resistor in in place of a 6800 ohm on pin 7. SO, fixed that and now the voltage readings look better there.

I did notice a spark once at the terminal point where the .1 uF 7AF7 Plate Decoupler cap and the 470K ohm Phono Amp Plate Decoupling Resistor are joined. It looked to me like an unacceptable solder joint so I reflowed it and I have not noticed any spark since.

The voltage on pin 3 of the 7X7 is still slightly over -1 v DC and there is still a visible glow in the tube. I checked voltages on the 2 6V6's as well and they all read normal other than -13 v dc on each pin 5 (calls for -1.8 and -1.7).

I can still pick up a local station @ 1600 but no others.

Finding 2 mistakes now, I am less confident of my work, and have learned not to shotgun replace everything at once. Hopefully I can learn more troubleshooting from this as well.

[heats00]

Here are all of my problem voltages of the tubes I can get to take readings. I can't reach the 7F8 converter since it is underneath the band switch assembly. What am I overlooking here?

7H7 1st IF Amp
pin 3 = 173v (100v expected)

7H7 2nd IF Amp
pin 3 = 148v (120v expected)

7X7 Det-AVC-AF
pin 3 = -1.02v (-0.5v expected)

7AF7 Phase Inverter-Phono Amp
pin 2 = 69v (65v expected)

6v6 Power Output 1
pin 5 = -13.3v (-1.8v expected)

6V6 Power Output 2
pin 5 = -13.3v (-1.7v expected)

5Y3 Rectifier
pin 2 = 315v (335v expected)
pin 8 = 316v (335v expected)

[heats00]

Anyone?

[heats00]

Well, my best guess is that the 135 ohm 10W candohm needs replacing.  It measures at 139 ohm but I have been reading lots of past posts about candohms measuring fine but still not functioning correctly.  It is also the only component left that I have not replaced in this circuit.  Am I correct in assuming that if it calls for a 135 ohm 10 W rated resistor that I can safely replace it with a 120 ohm and 15 ohm 10W power resistor in series?  

Also, I still need to replace the .01 caps in the bakelite block between the power cord and transformer.  This should be a Y-type cap?

[klondike98]

Yes the .01 caps should be Y2 safety caps. Im not the best troubleshooter but if the candohm is reading OK I'd be surprised its your problem. Did you align the set?

[OZ4]

For what it's worth

From the high minus voltages on the grids of the 6V6s and the low voltage output of the 5Y3 rectifier. It looks like there is something that is dragging down the power supply. With that much negative bias on the 6V6s they should be going into cutoff and drawing very little current and you should be seeing high voltage on the rectifier output not low.

You should check your I*R voltages around the circuit for something drawing too much current. Something with that 7x7 maybe.

Because of the high minus voltages on the 6V6 grids it tells that there is too much current through the ground off set resistor. It sets up the minus voltages for grid bias. Check it's value. I can't read the part number. It's the one between the center tap of the transformer and the ground point.
 
Something else to keep in mind, todays high impedance voltmeters don't load the circuit as much the old ones did and so you will be getting higher readings than those charts. On the grid readings with the old meters ( 20k ohms per volt ) on the 50 volt scale there would be a load of 1meg on the circuit. ( that is if i am not nuts.) With a grid circuit this will really load things down.


 Good luck

[heats00]

Bob,

I have not aligned the set at this point as I did not think that was necessary until the rest of the 'problem' areas had been taken care of. Please correct me if this is wrong. It would also be my first alignment, and I am not to confident of the signal generator that I have.

Chris,

As I understand what you are saying is that there are 2 problems going on. One with the 7X7 tube and one with the 6V6 grid. I think I understand the schematic to show the pin 3 connections on the 7X7 tube to lead to the volume control and to a connection leading to pin 5 of both 6V6's. It also leads back to the (-) side of the 20 mfd electrolytic which has the 135 ohm 10 watt candohm resistor leading to the chassis ground. The center tap also leads to the (-) side of this electrolytic as well. Am I incorrect in assuming that this is the ground off set resistor that you are talking about? I have tried swapping the 7X7 tube with a XXFM tube that was known good and I had the same result of the plates cherrying.

Thanks for the help so far, it is very much appreciated.