42-PT-93 Please Help - Printable Version
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42-PT-93 Please Help - dseverson - 05-04-2015
Hello, all.
Well, I couldn't resist picking up another radio. It's a 42-PT-93. The schematics are located here:
http://www.nostalgiaair.org/PagesByModel/176/M0014176.pdf
I did the usual recap and replaced the resistors but I am not getting any sound from it but the tubes and dial lamp light up. The tubes are also good. I tried touching the center tab of the volume control but don't get any hum. I replaced the volume control but still don't get any hum but the speaker makes a pop when I connect it to a 9v battery. When I tried taking ohm readings on the 50L6GT, I accidentally touched both pin 2 and 3 at the same time and got a loud hum from the speaker.
I tried taking readings on the loctals by connecting the negative of my digital multimeter to ground, turning the volume control to full, and touching the positive to each pin but I must be doing something wrong because I don't get readings on any of them.
I measured the oscillator coil from ground with the volume turned all the way up and got these readings (not sure if I have the pins correct, though):
Pin 1 - .7
Pin 2 - no reading (DMM doesn't change if that makes sense)
Pin 3 - no reading
Pin 4 - 7.1
Pin 5 - 6.9
I read somewhere on this forum how to rewind this type of oscillator but I can't find it now. If someone could point me in the right direction, I'd appreciate it. A picture of it is attached.
Finally, the case has a built in loop antenna. The two ends were unattached to anything when I got it and I can't figure out where one end goes. I know one goes to the tuning condenser but where does the other end attach?
Any help is greatly appreciated.
Dave
RE: 42-PT-93 Please Help - Nick3092 - 05-04-2015
According to the schematic, the antenna goes to the tuning cap and pin 3 on the antenna coil.
You osc. coil measurements are wrong. Don't measure them to ground. Look at the schematic. Between pins 4 and 5 you should see about 3.3ohms. Between 2 and 3 (which in this case 3 does go to ground) you should get 7ohms. I don't get what #1 on the coil is doing though.
But I think you are jumping the gun on the coil. You should clear the audio and output section first. What equipment do you have (if any). A signal generator with an audio output would be handy. And since this is an AA5, an isolation transformer is highly recommended.
RE: 42-PT-93 Please Help - morzh - 05-04-2015
The other end of loop antenna goes to pin3 of the coil 2.
Were you trying to measure Ohms on powered device? Don't do that anymore if you value your radio and your multimeter.
Touch the grid of 50L6 (be careful not to get grounded when touching stuff) while powered, see if it buzzes first.
RE: 42-PT-93 Please Help - Radioroslyn - 05-04-2015
Hi Dave
You said the magic word so I have to answer!
<When I tried taking ohm readings on the 50L6GT, I accidentally touched both pin 2 and 3 at the same time and got a loud hum from the speaker.>
I'll bet you did!!!!! That's like putting 110vac across the output transformer!!! I'd buzz too!!!
So connect the - probe of your volt meter to pin 6 of the 7C6. This is B-. Connect the + probe
to pin 7 of the 35Z3. should see about 100vdc. Then have a look at pin 4 of the 50L6. Should
see about 100vdc. Next pin 3 of the 50L6. Should see about 90vdc.
Report back with results.
Terry
RE: 42-PT-93 Please Help - dseverson - 05-05-2015
OK, guys, here are the results.
1. Oscillator coil: pins 4 and 5 = 3.5 ohms, pins 2 and 3 = 6.8 ohms
2. No hum when touching grid of 50L6 with a screw driver, powered on
3. With the power on, connected negative probe of my DMM to pin 6 of 7C6 and measured the following:
A. Positive to pin 7 of 35Z3 = 162 volts
B. Positive to pin 4 of 50L6 = infinity (? DMM kept flashing negative)
C. Positive to pin 3 of 50L6 = infinity (? DMM flashes negative)
I can't find the instruction manual for the DMM so I assume flashing negative means infinity.
RE: 42-PT-93 Please Help - Radioroslyn - 05-05-2015
Ok seem like your field coil is open or 26B is in backwards or shorted. Measure resistance across 25 by connecting your ohm meter probes to the two + connections of 26. Should show 500 -1000 ohms. If open you'll need to find a replacement speaker. Can use a PM speaker with some minor modification. If 26B was in backwards chuck it and replace with new one. Old one is shorted now.
Terry
RE: 42-PT-93 Please Help - dseverson - 05-05-2015
I measured the two positive connections of 26 and, yes, it appears to be open. 26B is oriented correctly. It also looks like someone put in a newer speaker but screwed an old coil onto it. I can post a picture of it tomorrow. I would like to find an original speaker assembly as a replacement but if I can't and end up using a PM speaker, what are the modifications that need to be done?
RE: 42-PT-93 Please Help - Radioroslyn - 05-05-2015
<what are the modifications that need to be done?
Add a 500 to 1000ohm resistor @2w between the two + sides of the filter caps(same as across the bad FC). Replace 26A and B with 47mfd@160vdc. Connect PM speaker to the original output transformer.
Terry
ps Just for the heck of it if you have a spare audio output transformer you can connect the primary across the FC without add the resistor and changing the filters. The opt will act as a choke in the power supply which filters much better than the resistor.
pss RU near this guy???
http://www.ebay.com/itm/Antique-Halson-Radio-/291454861233?&_trksid=p2056016.m2518.l4276
Maybe you can help a friend out. (OldRestorer)
RE: 42-PT-93 Please Help - dseverson - 05-05-2015
Sounds like a plan. I'll give it a try.
That ebayer in Oakland is about 15 minutes from my home.
RE: 42-PT-93 Please Help - dseverson - 05-05-2015
What can I help with?
RE: 42-PT-93 Please Help - dseverson - 05-05-2015
By the way, are the filter caps the two electrolytics (26A and 26B)?
RE: 42-PT-93 Please Help - Radioroslyn - 05-05-2015
(05-05-2015, 09:12 PM)dseverson Wrote: What can I help with?
http://www.philcoradio.com/phorum/showthread.php?tid=12204
He beat me to the draw!
<By the way, are the filter caps the two electrolytics (26A and 26B)?
Yup those would be them and #25 is the field coil.
Terry
RE: 42-PT-93 Please Help - dseverson - 05-05-2015
Too bad. That's a really nice looking radio.
Just thinking out loud here but couldn't I just replace the field coil that's screwed onto the speaker? It looks like that's what the last owner did or could that have been the problem in the first place?
RE: 42-PT-93 Please Help - dseverson - 05-06-2015
Pictures of the speaker.
RE: 42-PT-93 Please Help - Nick3092 - 05-06-2015
That is not a field coil screwed on to the speaker. That is the output transformer.
The field coil would be attached to the back. It would act as the magnet you see on the back of modern speakers. It is always i. Line with the center of the speaker.
So either that is some crazy speaker design I have never seen before, or the field coil is straight up missing. I've never seen an EM speaker with the FC removed though.