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Setchell Carlson...588 dial, 407 cab, 420 chassis(?) Serious need of a schematic.
Hi guys...sure could us some help.
First time poster here at videokarma, but spend a good deal of time at AK. This one is a puzzler. After researching the unit, it appears to be a "blended" radio. I picked up this Setchell Carlson tube radio about a month ago at a Garage sale for $15.00. It was a "sort of working" unit. The broadcast sound was very "mechanical" like an a emergency weather broadcast. Could be the hybrid speaker. By all the photos I've seen the cab is a Model 407, except the for the dial face (attached to chassis) and trim, which looks to be a 588. It appears to be the original dial face trim. But the 407 and 588 schematics do not show the same tube line up as mine. I can't seem find a Setchell Carlson schematic that sports my tube configuration anywhere. A 330 is close. So after much searching with no luck finding a schematic, I decided to just swap components value for value. This unit has been worked on before as there were plenty of component clippings still left at terminals. My efforts yielded only static. So without a schematic, any further effort would be a shot in the dark at best. All tubes test low end of good. The bottom 25L6 has been replaced with a 25Z6. And...I'm guessing the UTAH speaker is a replacement...or were they common to setchell carlson? Looks like a field coil / PM hybrid Any help and info on this radio would be appreciated. Martin P.s. I am being mindful of the "hot chassis". http://i1194.photobucket.com/albums/...rlsonFront.jpg http://i1194.photobucket.com/albums/...on/Chassis.jpg http://i1194.photobucket.com/albums/...arlsonback.jpg http://i1194.photobucket.com/albums/...sisnumber1.jpg http://i1194.photobucket.com/albums/...son/25L6_s.jpg http://i1194.photobucket.com/albums/...sonunderRS.jpg |
First you need to put the grid cap back on the 6A8.
Measure the supply voltages on each plate and screen grid. You should get at least 50 Volts or up to about 100 Volts. See that the cathodes all draw current. Then see if the local oscillator is running. You really need an isolation transformer and an oscilloscope to do this properly. |
bob91343,
Thanks for the help. The photo is only a reference photo of the units original state. Believe me, the cap is on. What I really need is an ID on the chassis and schematic to match. But again, thanks for the reply. martin |
Setchell- Carlson made a lot of 32 volt farm sets. It looks that it was converted to 115 volt ac-dc. First thing I would do is the capacitor heater dropping trick after a complete recap. Not too many of that make around. Good find.
PS. The voice coil in the speaker is probably dragging on the pole piece. It gives that real mechanical sound. |
dieseljeep,
Being a new hobbyist to tube radios and not having much of a electronics background, a lot goes right over my head. I have recapped the unit to the best of my abilities with the limited information that is present. Basically I recapped it and replaced the resistors value for value. I am not familiar with the "capacitor heater dropping trick". Can you expound? Since you mention that it has been converted to 115V from 32V, I'm guessing that a schematic would be usless anyway...correct? Hmmmm? Can I bypass the fieldcoil xfmr and test the speaker on a common speaker lead? I'll keep plugging away at it. Thanks Martin |
If it's not working, a tube isn't drawing the right amount of current or a tuned circuit isn't tuning. If you have limited experience with this stuff you ought to find a mentor to walk you through. What test equipment do you have?
A schematic diagram wouldn't help much; all those circuits are pretty much the same. A superheterodyne with IF of most likely 455 kc (more rarely 262.5 kc). If the local oscillator is running, you have a good start. If not, you need to get it running. That's the 6A8 circuitry. Do you get a hum when you put your finger on the 6Q7 cap? If so, the audio works okay. All that's left are the IF stages, the 6K7s. |
Gentlemen,
Before I go any further with questions, I'm going to go over what I've already done and double check everything. So far I have found a capacitor that I replaced with the wrong value and a broken solder to ground. Also I need to digest what you have asked of me so far. The revelation that the original schematic wouldn't help much has given me a bit to think about. I took a good number of "before" photos, and I'll post those with like "after" photos. Maybe the problem is obvious to "experienced" eyes. Bob91343, The unit was working before I did the component swap, albeit, not very well. At present all I get out of it is hum. The hum is volume sensitive and increases in pitch as I move the tuner up in kilocycles. I also have what sounds to be a brief arching in the on/off switch about 5 sec. after it's switched on. I don't get any difference in hum when I touch the cap on 6Q7. The only test equipment I have is an emissions tube tester. Dieseljeep, Thanks for the nod on the find. It gives me some sorely needed encouragement. The cabinet is in wonderful shape. I've got it stripped and the inlay is really quite nice. I'll need to replace the grill cloth and a new dial cover...crystal or plastic. I haven't decided. I really do enjoy restoring things and don't give up easily. My time with tubes has been short. My only other venture was a restoration of a Fisher 800c. If you feel inclined to have a look you can view it here. It is from start to finish, so it's quite long. But it will give you an idea about what my level of comprehension is. http://www.audiokarma.org/forums/sho...d.php?t=373498 Thank for taking the time to help. I'll post again when I've got something worth going over. martin |
Don't think you need to do the capacitor drop for the heater voltage, seems to be already taken care of. In the underchassis shot, underneath the 6Q7, I see what looks like a 50 ohm wirewound resistor. I did the calculations and that would be about right for dropping the heater voltage if you used 115V as the line voltage. For 120V line voltage you'd need 70 ohms. Although the calcs say smaller, I'd use a 20 watt for either ohms value for it to run cooler. Don't know what watts the present one is. The 50 ohm is probably OK but I'd measure voltage at the tube heater terminals to see how close it is to spec. I think I'd also move that resistor away from directly underneath the tube socket, towards the rear of the chassis, moving any wires in the way. That resistor will get hot.
The first thing you need to do to get the set working is for the audio section to work. If it's not working you'll never know if the IF and RF are since you can't hear them. Always start at the speaker and work backwards. We know the speaker works since you can hear hum. You can use the diagram you found around the 6Q7 and the two 25L6's and check your wiring. With a two foot piece of insulated wire, bare the end of it and touch to the grids of the 25L6's, one after the other. Should hear hum. With the volume halfway up, should hear loud hum when touched to the center terminal of the volume control. Check the grid wire going to the top cap of the 6Q7 and see if it has continuity or shorting anywhere. Check all components in the audio section for wiring mistakes or bad components. |
Martin. There's very poor coverage of Servicing information in Riders for Setchell-Carlson. My 6volt tombstone farm radio isn't in there either. It's a real neat design using engineering I haven't seen before.
Your radio looks like it used three 25L6's instead of four. |
No difference in hum while touching 6Q7 cap indicates a problem in the audio section. If there is a change when tuning, it seems the IF may be oscillating.
Check deeper for errors. This is why I never advocate a total recap - too many chances for error. Once it works, you can change a part at a time to make sure you aren't screwing up. Check that the B+ is adequate. You'll need a voltmeter for that. See if the ripple is excessive; you will need a 'scope for that unless you have a spare electrolytic capacitor to parallel the existing one. If the hum goes away, you need to fix the power supply. One sneaky way to check the B+ is to run the radio, pull the power plug, then quickly short the B+ to ground with a screwdriver. Should get a major spark. |
Gentlemen,
I'm hoping this will help you help me. I haven't got thru all that has been suggested, but I'm working on it. bob...which component is the "local oscillator"? Reese...it is a 50ohm wire wound cap and it does get hot. Watts not specified. Lesson learned on recapping all at once. Voltage at the bulb (not working, filament intact) (-)1.0mV On the 6Q7 datasheet it looks as though the screen goes to ground and didn't know what to make of that. The rear 25L6 looks way out of whack. Yes? I've checked continuity on all open ended wire and found no breaks. I've been looking around the net reading about older radios and found this in an article on the All American Five. I'm sure you guys know this stuff, but I found it interesting and informative...for a noob like me. http://en.wikipedia.org/wiki/All_American_Five Thanks again for the help. Martin http://i1194.photobucket.com/albums/...markedcopy.jpg |
That rear tube marked 25L6 is really a 25Z6. It's used as a rectifier in the conversion. It has two plates and two cathodes that should be paralleled. The original tech should have re-labelled the chassis. In the radio's original state, they had the plates and screens on all the tubes running at 32 volts and the audio output was very weak. That's why they used three 25L6's.
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I'm going to suggest a way to tackle this beast. I can see how some of the wiring goes, but some parts aren't clear from the pix. Can we do it section by section? Some of this will be going over sections that are already working, but may not be quite right. I'll number the items for reference. Please confirm or explain each.
PLEASE anyone chime in here with any suggestions or if I flubbed anything. All tests with set unplugged. 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. 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? 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? 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. 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? 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. 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? That's enough for a while! :yes: |
Reese,
At work right now, but will get after your recomendations tonight. thanks dieseljeep...your's as well thanks also. martin |
There are some more components around the 25L6's but the above should cover the most important ones (I hope!) Get the above straightened out before proceeding.
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. 10. 6Q7: pin 8 (cathode) either goes directly to ground or to ground through another component or two? 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. |
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 |
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. |
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? http://i1194.photobucket.com/albums/...n/SCrepair.jpg http://i1194.photobucket.com/albums/.../SCrepair1.jpg http://i1194.photobucket.com/albums/.../screpair4.jpg |
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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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. |
Sounds like the audio section is working! :yes: 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 |
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 |
Reese,
Didn't see the other 2 questions till just now. Will post later with answers. thanks martin |
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" :thmbsp: or OK as in "over kill". :thumbsdn: martin http://i1194.photobucket.com/albums/...anstopcopy.jpg http://i1194.photobucket.com/albums/...ttomcopy-1.jpg |
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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Yes Reese, you're right, it is pin 6. Don't know why i put down 8. Corrected.
martin |
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. :thmbsp: |
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 |
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. |
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?? |
Reese,
Thanks for hanging in a bit further. Will go over your suggestions and post. Thanks Martin |
Reese,
It was as you said and I checked it against closer-up, original photos. I removed the 1000ohm resistor and put the right hand choke wire back on pin 8 of 25Z6. Voltages are up.... 25L6 A) 106.4 B) 105.2 6Q7 pin3, 105.8 pin6, 107.2 6K7 Front, 107.0 Back, 106.9 6A8 105.7 There is a loud hum @ 6Q7 when touched with the cap wire removed. Martin |
Good! Power supply and audio section are working.
Hoping against hope, put an operating radio, can be a transistor radio or any kind, tuned to a weak station around 1400-1500 right next to your S-C, turn on the S-C and tune it all up and down the dial. If your oscillator is working should hear a squeal or rushing at some point in the other radio. Does your 6A8 test strong? |
Reece,
The 6A8 test "good", how strong that is I do not know. The only tube tester I have is an emissions tester. I tried the 2nd radio trick and couldn't get the functioning radio to whistle, squeal or break the signal no matter how hard I tried. I tried with a small transistor radio and with a receiver at various points on the dial...no wanna' squeal! What do you think? thanks for the help martin |
Shows the oscillator isn't working. The tube should be OK per your tester. Carefully check the wiring against the schematic of a typical 6A8 oscillator/mixer in the article referenced. Also check continuity of each winding on each of the three exposed coils, just to see that no wires are broken. I have a hunch they aren't, since the radio worked weakly in the beginning. I'm thinking there's a wiring error.
Here's where signal injection would come in handy, working back from the first audio tube. |
Reece,
I have continuity thru all my coil connections. The only place I didn't get a continuity signal was thru the donut ring around the coil, but I did get resistance. I couldn't find any misplaced wires. Thanks for the suggestions. I'm going to ponder my next step. The tech I use is a wiz, but he doesn't work for free. He's good and it's right that he gets paid. As much time as you and I have put into this, I really don't want to shelve it. thanks again martin |
There must be "one of us" nearby with test equipment who could help you out.
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Hi all,
Time to man up... I found the problem and I'm not exactly excited to share the news. Wrong value resistor, 47 ohm instead of 47K spanning pins 1 & 5 on the 6A8...rookie mistake. Should have looked for wrong values first. My apologizes to all who helped, especially reece. At least now I can proceed to the rest of the restore. Stain & poly or Tung oil on the cabinet, replace the grill cloth and new dial glass. Will post photos of finished piece. Thanks again. martin |
Congratulations! :banana: :banana: :beerchug: It's happened to all of us. Glad you found it!
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Reece PM me this...
Quote:
I haven't run 20 ft of antenna wire yet. I'll tackle that tomorrow. I have the unit in my basement at the moment with 10ft wire attached to a copper wire wrapped >< (crisscrossed) antenna. I was able to adjust AM 1390 sports to it's proper place on the dial, as well as 960 news talk station. There are quite a few stations that are weak that I should be able to pick with higher antenna placement. Volume is adequate. All the sockets have been "deoxit-ized". The speaker seems to be fine. It has that "classic" AM sound. :D The mechanical sound that it had before is gone. :thmbsp: The dial light is not functioning. The bulb filament looks intact. Before I try a bulb swap, what kind of voltages should I be seeing at its base? At the moment it's...-.013mV DC. The supply wire terminates at the (utah) 50 ohm wire wound resistor. http://i1194.photobucket.com/albums/...epair/Bulb.jpg http://i1194.photobucket.com/albums/...ermination.jpg Thanks Martin |
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