Author Topic: The old GE radio  (Read 2647 times)

Motorola Minion

  • Hero Member
  • *****
  • Posts: 951
  • Southern Pennsylvania
    • View Profile
Re: The old GE radio
« Reply #45 on: February 08, 2019, 02:00:13 PM »
Check all your coils/transformers for continuity. Most of the tough dog problems I encounter in 1930s radios are due to an "open" somewhere that does not always cause a hard failure. Fortunately, the coil forms from this era are large with slack in the windings and easier to perform surgery on.
Tubes - Magical - Tubes

Dave

ed from Baltimore

  • Sr. Member
  • ****
  • Posts: 345
    • View Profile
Re: The old GE radio
« Reply #46 on: February 08, 2019, 09:05:38 PM »
   The 340 volts DC on the 6L6 screen grid is probably why there is the red glow on the plate. The plate current is higher than it should be  because of the overly high screen voltage  (should be only 250 ? )
       The too high plate voltage (350 instead of 290) multiplied by the high plate current is 2 reasons why the plate power dissipation is too high, hence the red glow. (Hopefully it's only visible in a darkened room ?)
       The 370 volts DC on the rectifier tube looks like the total supply output measured between rectifier cathode (+ lead) to transformer center tap (- lead ),  not to ground. Subtract 18 volts negative bias from that 370  and you get 350 or so between rectifier tube cathode and ground. Guessing 100 mA or so total current from the supply to all the tubes gives about a 45 volt voltage drop across the speaker field coil. By the way does that field coil speaker look like it's original, and is its resistance 460 ohms or whatever the schematic says ?  My Rider manual print is hard to read numbers on.  Then subtracting another 5 or 10 volts lost through the resistance of the audio transformer primary resistance and that gets you pretty close to 290 as specified.
          The current through the 1000 ohm power supply voltage dropping resistor feeds all the rest of the tubes plus the 6L6 screen grid. (but not the 6L6 plate)and also feeds the 12000 and 13000 voltage divider and that should be another 40 odd volts because each RF tube is probably 8 to 10 mA and another 10 mA to the 25000 total resistance (12000 and 13000)
          There should be 250 volts on the RF tube plates and about 120 volts on the RF tubes screens (from the 12000 and 13000 voltage divider. Its a hard schematic to read, though.


ed from Baltimore

  • Sr. Member
  • ****
  • Posts: 345
    • View Profile
Re: The old GE radio
« Reply #47 on: February 08, 2019, 09:43:05 PM »
      That GE circuit is rather unusual because every tube (except 5Z4 rectifier) has its cathode grounded with no separate cathode bias resistors. So you have no easy way to see if an individual tube is conducting current or not. If all the RF tubes were non-conducting, say, because of an open solder connection on the wire feeding the 3 screen grids,  the 6L6 try to compensate by conducting the missing current in addition to its own to give you the 18 volts bias as before. So the defect in the bad section results in incorrect readings in the good section.
        Are all the tubes metal type on the schematic ? I didn't see the letter G after the tube type, at least not on the 6L6. Some of those tubes came only in the metal version for a year or two until the glass version came out. At least the RCA 6L6 and 6V6, they were the only beam tubes at first, 6F6 was a power pentode ? I believe.   Some, like 6AC7 or 6AG7 may never have been available in glass version. They were the high-transconductance "hot rod" tubes for pulse and TV circuits.
         If all those voltage divider resistors and maybe the field coil speaker, have been replaced, that might explain why the voltages are all way too high. Someone may not have had access to the correct schematic years ago. Or miswired something. If the 6L6 screen is higher than the RF tube plates, by more than a few volts, something is connected wrong. They should all be 250 or so Volts DC.



ed from Baltimore

  • Sr. Member
  • ****
  • Posts: 345
    • View Profile
Re: The old GE radio
« Reply #48 on: February 09, 2019, 05:41:02 AM »
   Boy did I goof up. Now that I have a magnified print of the schematic and socket voltages in front of me instead of relying on memory, all my numbers were off. Negative bias looks like 15 or 16  volts instead of 18. So that makes 355 or 354 volts minus 300 or 54 volts across the 460 ohm speaker field coil.     About 120 mA total current for all the tubes .   Ten volts dropped across the audio transformer primary 133 ohm resistance is about 75 mA plate current which is about 21 watts of plate power dissipation. I think that's about the maximum allowed for a 6L6. 
         Meanwhile I thought all the tubes had their cathodes grounded but it was only the 6F5 and 6L6.
         That 340 volts on the 6L6 screen is particularly scary because it means its also 340 volts on the plates of the RF tubes, instead of 250 and then probably 150 volts on the RF tube screens instead of 100.

TC Chris

  • Hero Member
  • *****
  • Posts: 1840
    • View Profile
Re: The old GE radio
« Reply #49 on: February 09, 2019, 11:04:16 AM »
Every time I look at that schematic it makes my head spin so I do it in small doses.  I'll check out the RF tube voltages next.  They're all metal tubes.  If they're scary-high too then that might explain why the first AF tube shorted out. 

The speaker field looks original.  GE cheaped out by using three wires instead of four for the speaker field and the output transformer, so I have to process that with my poor brain when looking at the schematic.  It makes electronic sense but is harder to visualize.  I'll check resistance too.

Chris Campbell

TC Chris

  • Hero Member
  • *****
  • Posts: 1840
    • View Profile
Re: The old GE radio
« Reply #50 on: February 09, 2019, 10:28:25 PM »
Tonight after a movie I went out to explore voltages and resistances.  It's a lot easier to find a failed/out of spec part than somebody's mis-wiring. 

First, the field coil and audio transformer primary, both right on spec or very close at 440 ohms/130 ohms.

Second, checked 6K7 screen and plate voltages (IF amps).  Way scary--180/370 on one, 150/350 on the other. 

Third, checked the voltage divider resistances (resistors replacing the Candohm unit)--all pretty close except R-23, a bit high
Here are spec/measured:
  R-21   1000/996
  R-22   12,000/11,700
  R-23   15,000/16,800
  R-24    210/224
  R-25     30/36
  R-26    110/113

I've been measuring with a cheapie digital meter from Harbor Freight that my brother gave me.  It sometimes takes a while to settle down but the digits are easier on my eyes in the shop than the needle on my VOMs or the solid-state VTVM or the real tube VTVM (which isn't calibrated since it was given to me).  And since a lot so the numbers make sense, I'm guessing that it's accurate enough for tube radio work.

My next job is to study that old Rider schematic and under-chassis parts diagram, and try to see where I might be getting the wrong voltage all over.  I wonder if there's an accidental connection point somewhere putting the high power supply voltage where it's supposed to be reduced by the voltage divider.  I've replaced almost all the old rubber insulated wires (all that seemed to be in danger of contacting the chassis or metal parts).

Chris Campbell



TC Chris

  • Hero Member
  • *****
  • Posts: 1840
    • View Profile
Re: The old GE radio
« Reply #51 on: February 09, 2019, 10:31:34 PM »
One more thought--I may be better off running this device on my dim-bulb tester, the only voltage-reducing option I have, until the problem is found.  It;s kind of a miracle that I have killed more tubes or burned up an IF transformer.

Chris Campbell

ed from Baltimore

  • Sr. Member
  • ****
  • Posts: 345
    • View Profile
Re: The old GE radio
« Reply #52 on: February 10, 2019, 06:29:05 AM »
Yes I agree 100 % that the dim bulb tester should be in the circuit, at least to get the rectifier output down to 355 volts with respect to ground. (370 minus 15 volts bias) The way the arrows and dotted lines are drawn for the 370 volt reading, it seems to mean the total power supply output as measured across the first filter capacitor, not measured with respect to ground. Then you would have 55 volts voltage across the 460 ohm speaker field coil  which works out to 120 mA total power supply current. Subtracting 50 mA from 120 mA (the 50 mA is the 50 volt drop across the 1000 ohm dropping resistor, 250 minus 200) leaves 70 mA 6L6 plate current. When calculating plate current from a 10 volt drop across the audio transformer 133 ohm primary you get 75 mA so the current must be around 70 to 75 mA
         You might want to measure the speaker field coil resistance to see if it is really 460 ohms. If there are some shorted turns the resistance may be way down from that value, and the very fine wire used to get the resistance that high makes shorted turns a real possibility.
          Also, if that first filter capacitor really 16 microFarads or did someone get ambitious in the past repair history and put in a 20 or 30 microFarad cap. The larger the first filter capacitor, the closer the DC voltage rises to 1.4 times 330 or 460 volts DC. The capacitors beyond the filter choke field coil don't affect it nearly as much, just tge

ed from Baltimore

  • Sr. Member
  • ****
  • Posts: 345
    • View Profile
Re: The old GE radio
« Reply #53 on: February 10, 2019, 08:00:09 AM »
          Anyway...………...the 80 volts difference between a 370 volts DC supply and a 290 volts plate voltage o he 6L6 comes from the 55 volt drop across the speaker field coil, the 10 volt drop across the 6L6 transformer primary, and the 15 volt difference between measuring the 290 volts with respect to ground and measuring the 370 volts with respect to the negative lead of the power supply where it is different from ground by  the amount of bias on the 6L6.

electra225

  • Hero Member
  • *****
  • Posts: 2949
    • View Profile
Re: The old GE radio
« Reply #54 on: February 10, 2019, 09:25:10 AM »
The schematic for this radio is one of the more difficult I've seen to follow.  Some of what I see doesn't make sense, such as how they have the cathode on the output tube drawn in the pilot lamp circuit.  Would that not indicate the cathodes are grounded to chassis and not necessarily to power supply negative?  You certainly are not suffering from low B+!  It appears to me (?) that the voltage you have on the plates are directly from the rectifier (?).  I'd suspect a miswire, particularly if it looks like someone has been there ahead of you.  I may not be making any sense, just thinking out loud again.   :-[ ;)
I don't need Google.  My wife says she knows everything.

TC Chris

  • Hero Member
  • *****
  • Posts: 1840
    • View Profile
Re: The old GE radio
« Reply #55 on: February 10, 2019, 11:48:50 AM »
I'm as puzzled as anybody.  I did check the field coil resistance and it right on.  My plan is to print out another copy of the parts location/wiring diagram and trace the various HV circuits in different colored pens.  The diagram is hard to follow and remember.  Then I can study as-built because the physical connections are often not as-drawn on the schematic, but rather to another electrically similar point. 

I do appreciate all the input and comments. 

Chris Campbell

electra225

  • Hero Member
  • *****
  • Posts: 2949
    • View Profile
Re: The old GE radio
« Reply #56 on: February 10, 2019, 12:21:40 PM »
The copy of this schematic grew legs.  I have no idea what happened to it.  Now I can't find it again on ARF.  Chris, would it be too much trouble for you to post it on here?  I hate senior moments.   :-[ :-[


Never mind.  I hate senior moments.   :-[ :-[
I don't need Google.  My wife says she knows everything.

TC Chris

  • Hero Member
  • *****
  • Posts: 1840
    • View Profile
Re: The old GE radio
« Reply #57 on: February 10, 2019, 12:34:44 PM »
Let's try links.  The last time I tried to post a schematic I was stymied by error notices.
7-17 http://americanradiohistory.com/hd2/IDX ... e-0138.pdf
7-18 http://americanradiohistory.com/hd2/IDX ... e-0139.pdf
7-19 http://americanradiohistory.com/hd2/IDX ... e-0140.pdf
7-20 http://americanradiohistory.com/hd2/IDX ... e-0141.pdf

Remove the 7-xx parts before cutting and pasting.

I forgot to add in my last post that all the electrolytics are new so unless a new one was way out of spec, the caps in place are exactly as specified.  The ones I replaced were not originals; they had been replaced long ago, probably when the voltage divider Candohm was replaced, and looked like a disaster waiting to happen.  The old ones were as specified, however.

Chris Campbell

electra225

  • Hero Member
  • *****
  • Posts: 2949
    • View Profile
Re: The old GE radio
« Reply #58 on: February 10, 2019, 12:47:46 PM »
You've probably done this already but.....


What do you find when you measure the voltage at the cathode of the rectifier, 370 VDC, then at the first filter cap, also 370 VDC, then at the output filter cap, after the choke/field coil, 300 VDC.  There is a dual-voltage, 125-250 volt power transformer listed.  Do you have the proper voltage at the rectifier?  It seems to me that field coil is the topic of much discussion.  Would not measuring the voltage at the first place after the field coil, the second filter cap, put that question to bed?  If the voltage is high at the second filter cap, that might give you an indication as to the health of the field coil.  The field coil and the output transformer are the only resistance I see between the rectifier and the output tube.  B+ for the 2nd If amp, a 6K7 comes off the field coil, thru R21 and R13.  I'd sure look for a miswire somewhere.


The 1st If amp and the Converter tube get their B+ from the same circuit as the screen of the 6L6, pin 4.  If the screen voltage is high on the 6L6, then R21 or the field coil become suspects.  Since you say you have essentially rectifier voltage on the plates of almost all the tubes, this places suspicion on R21 or the field coil.  And also opens up the probability of a miswire.  Your idea of tracing each circuit and marking it to eliminate suspects is spot on in this case.
I don't need Google.  My wife says she knows everything.

ed from Baltimore

  • Sr. Member
  • ****
  • Posts: 345
    • View Profile
Re: The old GE radio
« Reply #59 on: February 10, 2019, 11:12:27 PM »
BTW, tubesandmore.com has good schematics