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  #16  
Old 11-04-2011, 01:20 AM
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Reese,
I can't thank you enough for effort your putting forth here.
I'll try to be as concise as I can. I won't do any changes until I hear back from you on my findings and the changes that need to be made.
I'll add a side note or two if it seems to pertain.



1. The tech who wired this for 120VAC made it into a hot chassis set. Notice that one side of the line goes to the power switch and the other side of the switch is grounded to chassis. It is dangerous like this in that if the hot side of the line goes to the switch, the chassis with the switch on will be a shock hazard with respect to ground. Even with the switch off it will be a shock hazard if the hot side of the line goes to the rectifier as the chassis would be hot back through the heater string. You can find the other errors and get the radio to work this way, but you should use an isolation transformer between the set and line. Also you should be insulated from a concrete floor and be careful and don't rest your arm or other hand on the chassis when taking readings. The live chassis can be made safer with a floating B- bus but I'd say get the set working first and tackle that as a later project.

2. 25Z6: pins 3 and 5 are tied together and one side of the line cord goes to them.

The neutral side (I traced this side back to the plug and marked it to ensure it is always plugged in as neutral) of the line cord is attached to pin2, jumped to pin3 and jumped again to pin5 of 25Z6.

3. 25Z6: pins 4 and 8 are tied together and go to the + side of the first electrolytic cap. The - side of this cap goes to chassis ground. Be sure you have + and - correct on all electrolytics. It looks like you have two electrolytics with their + sides tied together. Why is that, and where are they in the circuit? There's another underneath them. What are the mfd values and voltage of each?

Yes…pin 4&8 are tied together and go to the (+) of the first electrolytic. I assume your referring to the 2 brown electrolytics, 39uf/400V each. They replace a 40mdf/40mdf/150V cardboard can cap that had two (+) leads and a single (-) lead. The 2 (-) leads (striped side of labels) of the new lytics were tied together to mimic what was removed. There isn’t another lytic below them, unless you mean the blue one, 15uf/400V out to the side…semantics?
Note: The 130.5V tap from the lower transformer is also attached to pin 8 of 25Z6


4. 25Z6: cannot see but there should be an approximately 1000 ohm 1 watt resistor also connected to the + side of the first electrolytic cap and the other end of the resistor should go to the + side of the second electrolytic cap. The - side of this second cap goes to chassis ground. Where does the + side of this second cap go?

I do not have 1000 ohm 1 watt resistor spanning to two (+) leads of the elecrtolytics.
The 2 (-) neg leads of the elecrtolytics (which are tied together) go to pin 8 of the 6Q7 tube. Then pin 8 is jumped to pin1 which is soldered to the chassis ground tab.
The (+) lead of the second electrolytic goes to pin 6 of 6Q7.


5. 25L6's: tube at front of chassis near speaker plug let's call tube A, other one tube B. Tube A has a wire from nearby input transformer going to pin 5 (grid 1.) Should be another transformer wire to pin 5 on tube B? What is resistance between the two pin 5's? Is there a third wire from the transformer on the same side as the other two and if so, where does it go? What is resistance between this wire and each of the two other ones going to the two pin 5's? Transformer can't be open or shorted or there'll be no audio.

The resistance between pin 5 on the two 25L6’s is 276.1 ohms
The third wire goes to pin 1 on 25L6 tube (A)
The resistance on the third wire, pin1 to pin 5 on 25L6 (A) is 141.7 ohms.
The resistance on the third wire, pin1 to pin 5 on 25L6 (B) is 134.3 ohms.
Note: Pin 1 on tube A is also attached to the chassis ground tab.



6. 25L6's: each tube should have separate wire from pin 3 (plate) to speaker plug. Another single wire from speaker plug to both pins 4 (screen) jumped together. Pin 4 on tube B appears to have another wire which should be going to B+ (the + side of the second electrolytic.) Does it?

Yes…pin 4 on 25L6 B has a wire that attaches to the (+) lead of the second at the pin 6 of 6Q7.
Pin 4 of 25L6 B has an additional wire that goes to the opposite side of the afore mentioned (upper) “3 wire input xfmr”.


7. Unplug the speaker. Determine which wire going to the output transformer on the speaker is the one that goes to pin 4 on tubes A and B. Connect one lead of your ohmmeter to this wire at the plug. Check the ohms between this wire and each of the other two wires going to the transformer in turn. They should be about the same.

133.7 ohm & 153.7 ohm


8. 25L6's: there is a resistor (what ohms?) from pin 8 (cathode) to ground on tube A, and an electrolytic cap (what mfd. and volts?) from same pin 8 to ground. Is there a similar arrangement on pin 8 of tube B?

Resistor from pin 8 to grnd…620 ohms
Electrolytic (blue) 15mfd/400V.
Pin 8 on 25L6 B is jumped to pin 8 on 25L6 A


Whew!
Hope this answers your questions and gets us closer.
Thanks again.
Martin
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  #17  
Old 11-04-2011, 11:46 AM
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I've gone over your response and here's what I've found. Again, anyone looking over our shoulders please chime in if I missed or goofed anything:

1. and 2: OK

3. It almost looks as if there is a component underneath the two brown 'lytics? If so, what is it and where is it connected? Or am I just seeing a terminal of tube B?

3. The 130.5 tap on the choke (looks like a transformer but has only 2 wires) goes to pin 8 of the 25Z6 and should not: change it to pin 6 of the 6Q7.

4. A resistor should be added between the two + sides of the brown electrolytics. It can go between pin 6 of the 6Q7 and pin 8 (or pin 4) of the 25Z6. This is part of the high voltage filter on the set. You can use 1000 or 1200 ohms at 1 or 2 watts. I'd go with the 2 watt for cooler operation.

5. though 8: OK

I think once you go through 9, 10, and 11, we may be able to get a buzz through the speaker from touching the top cap of the 6Q7.
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  #18  
Old 11-05-2011, 02:12 AM
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Reese,
Got in late tonight. I will make changes tomorrow and work on steps 9 - 11.
But here are some photos of the area below the brown lytics and the pot.
On the pot...black wire goes to pin 6 on the rear 6k7.
The grey/green wire goes to the 6Q7 top cap, but has a .022uf/630V cap in the circuit.
On the 6Q7 side of the .022uf/630 cap, that cap lead has a 1.3Mohm resistor attached that goes to pin 8 of 6Q7which is jumped to pin 1 which is soldered to the ground tab
thanks again
martin

P.s. I think I've got some shorting in the pot as I can hear the brief crackle of arching going on at the pot about 5 sec after the unit is switched on. Originally the pot had a gummy feel when turned. So I did a real thorough deoxit washing. Was that some kind of ëlecrto-lube" that I washed out?





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Old 11-05-2011, 07:16 AM
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Nice clear pix. So far the grid wiring to the 6Q7 looks OK. Should not be any high voltage in the pot to arc. The arcing you hear may be in the switch. Probably no way to wash out the switch as it's enclosed, but maybe better not to as there's grease in there. With the set unplugged you could try switching on and off a dozen times to see if that helps clear it. It's possible to drill out the rivets and disassemble the pot and switch and check the switch contacts and clean them etc. but it's a bit watchmakerish to do. For now I'd let it go and you might want to just replace the pot/switch later on if the crackling persists. If the pot seems stiff a drop of light oil on the shaft where it goes into the threaded bushing should help: tip the chassis with the shaft pointing up.
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  #20  
Old 11-05-2011, 07:44 PM
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Reese,

All suggested changes have been made. I had to use a 1k/1watt resistor to bridge the two b+ leads. It’s all I could lay my hands quickly.

9. 6Q7: pin 3 (the triode plate) goes to the interstage or input transformer that feeds the two output tubes. The other wire coming from that side of the transformer should get B+ to feed the 6Q7 from somewhere, probably from the + side of the second electrolytic filter cap? Check ohms between these two transformer wires: transformer should not be open or shorted.

Resistance between is…962ohms.


10. 6Q7: pin 8 (cathode) either goes directly to ground or to ground through another component or two?

Pin 8 is jumped to pin 1, which is soldered to the ground tab.


11. 6Q7: top cap (grid) should go through a cap to the center terminal of the volume control. Can't tell from the pix but this line should not be grounded anywhere or any signal will be shunted to ground and there will be no audio. Test: remove the clip from the grid cap of 6Q7, turn set on and volume up, touch grid cap. If loud buzz, audio section is working and something seems to be shorting out signal back to the volume control.

I did indeed get a loud buzz that quickly dissipated.

What next?
martin
P.s. I am more than willing to take photos of any section or component you need to see.

Last edited by marloubow; 11-05-2011 at 07:50 PM.
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  #21  
Old 11-06-2011, 09:14 AM
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Sounds like the audio section is working! Why it died out, maybe it sucked all the juice out of the electrolytics since it was at top volume. That should just be a quick test as it's not good for the speaker, either. What happens when you reattach the top cap; can you hear any radio stations or static or control scratching as you turn the volume control up and down? Is there any DC voltage between the high side of the volume control (not center and not ground terminal) and chassis?

Q: where do pins 4 & 5 go on the 6Q7?

Q2: where do all the different wires (colors etc.) go from the three aluminum cans up top of the chassis? Two square and one round.

Also: check DC voltage at all tube plates (pin 3 each tube) with respect to chassis.
25L6 A&B
6Q7
6K7 front
6K7 rear
6A8
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Last edited by Reece; 11-06-2011 at 10:30 AM.
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  #22  
Old 11-06-2011, 11:55 AM
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Reese,
I let the unit warm a little before I tested this morning.
With the top cap off again, I found the loud hum to be constant and not die off like before. So with the top cap off, the hum is loud and constant.
With the top cap clip on there is hum and the pitch changes if I touch the top cap with my finger.
As I turn the volume knob, the hum becomes apparent at about half way and increases as you move to full volume. The increase in volume seems pretty smooth, without alot of pops or crackle.
No radio broadcasts can be found, only hum at a constant level...no scratchyness.
The pitch goes from low to high as I move from one end of the dial to the other.

25L6 - A. 89.5V / B. 88.3V
6Q7 - 89.5V
6K7 - Front 90.7V / Rear 90.7V
6A8 - 89.9V

Thanks
martin

Last edited by marloubow; 11-06-2011 at 12:34 PM.
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  #23  
Old 11-06-2011, 01:56 PM
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marloubow marloubow is offline
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Reese,
Didn't see the other 2 questions till just now. Will post later with answers.
thanks
martin
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  #24  
Old 11-06-2011, 05:27 PM
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Reese,

Pins 4 & 5 of 6Q7...Pin4 to ground / Pin 5 to round can (green) on top.

Next answer is illustrated below. I hope it's clear enough. Let me know if it's OK as in "okay" or OK as in "over kill".

martin




Last edited by marloubow; 11-06-2011 at 07:21 PM.
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  #25  
Old 11-06-2011, 06:37 PM
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Good, I'm scratching down a schematic of sorts, but doesn't the wire from the "green" can go to pin 6 on the rear 6K7 and not to pin 8?
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Old 11-06-2011, 07:01 PM
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Yes Reese, you're right, it is pin 6. Don't know why i put down 8. Corrected.
martin

Last edited by marloubow; 11-06-2011 at 07:28 PM.
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  #27  
Old 11-06-2011, 09:06 PM
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OK. Some of it from the 6K7 rear, on to the front end is hard to follow, so I'll have more questions. If you could take pix at different vertical angles of the underchassis including the 6K7's and the 6A8, I could see where some of the wires go better.

What's the voltage at pin 6 of the 6Q7, if I didn't ask that before?

By the way, your red/blue/green transformer I.D. method worked great.
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  #28  
Old 11-07-2011, 01:17 AM
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Reese,

Thanks for the nod on the illustration.

No problem on the photos...you're putting forth a good deal of effort here, it's the least I can do. I'll try to make this as easy as possible for you.

Pin 6 @ 6Q7...91.7V

I'm going to link you to a cornucopia of photos from different angles.
Too many to post.
Password is...setchell
http://s1194.photobucket.com/albums/...lson%20repair/

Thanks
martin
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  #29  
Old 11-07-2011, 06:46 PM
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Those pictures really show all sides of the wiring. I'm afraid I'm getting to the point of diminishing returns this way though, without having my hands on the chassis to be able to make tests and check continuities, etc. I think now the best thing would be to inject signal into the 6K7 IF stages to see if they carry over through the detector and audio stages, and to see if the 6A8 oscillator/mixer (also called converter) is working. Any wiring mistakes could throw this off greatly but you did say it worked somewhat in the beginning. Is there anyone local with a signal generator and signal tracer that could help? Going that way could localize any trouble a lot more quickly.

I found a good source on line that discusses troubleshooting superhets and they use as guinea pigs a 6A8 converter and a 6K7 I.F. stage (of which you have two) with diagrams and items to check.

http://www.radioremembered.org/superhet.htm

They show a tube in an R.F. amplifier stage before the 6A8 but you don't have an R.F. stage so that could be ignored. The detector and first audio is a 6SQ7 I believe but that's the same as a 6Q7 without the top cap and all electrodes brought out to the base pins. The audio output tubes seem to be OK in your case so just the 6A8, 6K7's, and 6Q7 need inspection for now.

I'd start checking the 6K7 diagram against your radio; they may not be exactly the same in components but close. Then do some of the checks. The voltages you'd get are going to be less than in the article because they're using a transformer set that ups the voltage and you're using line voltage as a source.
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Last edited by Reece; 11-08-2011 at 05:46 AM.
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  #30  
Old 11-08-2011, 06:08 AM
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Thinking back over the power supply on this set, I looked at the original wiring and it "looks" like the choke originally WAS between the two + terminals of the brown electrolytics. That's where it should have been. The way the wiring is stretched out across the chassis it's hard to see.

In the picture, the left hand choke wire goes to a terminal on the rear 6K7 and that same terminal has a wire that goes to 6 on the 6Q7, which is the + terminal of one of the brown 'lytics. The right hand choke wire used to go to 8 on the 25Z6 which also goes to the + terminal of the other brown 'lytic. That would put the choke from + to + which is how it should be. If that's what you see, remove the 1000 ohm resistor and put the right hand choke wire back on 8. That puts us back to the beginning! And the voltages should come up on the tubes. But do you still get buzz from the top cap of the 6Q7??
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