66B missing dot redux expert help requested - Printable Version
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66B missing dot redux expert help requested - sk373a@gmail.com - 01-29-2022
Hello to all,
I apologize for bringing this subject up but I need some expert advice on the subject. I have recapped and changed bad resistors etc. Tubes test good. Reading all the threads about the missing dot(s) surrounding C50 here and on other sites I wired in one 22uf cap for C50 and slowly brought the unit up to power with a variance. It worked well pulling in local stations and at night I was impressed with the DX reception. However, I noticed that my voltages on the tubes listed in service bulletin #197 were a lot higher than those listed in the table ie: 340v on the 6A7 vs 260v, the other tubes were higher as well. Also I noticed a spark when moving the tone control switch to bring in the higher value cap (yes I had replaced these caps). So, I decided to install two separate caps for C50 and wire them separately per the earlier thread posted by klondike98 a few years ago. So now the pos side of C50a goes to the filament of tube 80 and the pos side of C50b goes to R28 et al. The set still pulls in local stations and DX, the tube voltages are a lot closer to those listed in the service bulletin and the arcing went away from the tone switch but now it is hyper sensitive to the scratchiness of the volume pot and other transient noise. I plan on replacing the volume pot/switch next. Anyway, so which way is the correct wiring as it works in both configurations except running higher voltages at the tubes? Sorry for the long thread, this has been a complicated repair and stopped being fun weeks ago. Thanks in advance for everyone’s time and help.
RE: 66B missing dot redux expert help requested - Mondial - 01-29-2022
There should be two separate capacitors for C50 and NO connection at the red dot. Connection at the blue dot is the correct one.
It is not unusual that the volume control makes scratchy noises as it is rotated and it has nothing to do with the power supply. There is no DC blocking capacitor between the top end of the volume control and the detector circuit. When a signal is received, a DC voltage is generated by the detector and applied across the volume control. This will cause noise as the control is rotated.
Most radios have a DC blocking capacitor between the detector and the end of the volume control which minimizes the noise, but to save money some manufacturers omit the cap which eliminates the need for extra parts.
RE: 66B missing dot redux expert help requested - klondike98 - 01-29-2022
+1 on what Mondial said. That had me going for awhile but the folks here at the Phorum helped me get it right.
RE: 66B missing dot redux expert help requested - sk373a@gmail.com - 01-29-2022
Thank you for your help and reply. I think the increased sensitivity to the volume pot is because I had the bias to the field coil shorted when I had it configured with only one capacitor at C50. The volume pot is really scratchy and non linear when I rotate the shaft so I will go ahead and replace it. Thank you again for your time and help.
Mark
RE: 66B missing dot redux expert help requested - mikethedruid - 01-29-2022
I don't know if you did a search and found it, but I restored a 66B not long ago. Here is a link to that thread. https://philcoradio.com/phorum/showthread.php?tid=22924 I hope it can be of help to you.
RE: 66B missing dot redux expert help requested - sk373a@gmail.com - 01-29-2022
Great thank you I will check out the link.
RE: 66B missing dot redux expert help requested - sk373a@gmail.com - 01-30-2022
Well Gents the project is finally complete. I changed the on/off volume switch today and it was a HUGE improvement to the audio quality. No more very loud static pops and scracthiness. The old pot was non linear in its movement plus it was ohming out at 900K and would actually increase at first when moving the shaft. I installed 500K switch/pot part # R-VS-500KA from Antique Electronic Supply. I had to shorten the shaft by 1/4 inch and use a file to make the flat spot for the wood knob to slide on. Thanks again for everybody’s help and time answering my questions.
All the best,
Mark
RE: 66B missing dot redux expert help requested - radiorich - 01-31-2022
Welcome aboard Mark,
That is great news and I also have had same issue with a radio .
Sincerely Richard
RE: 66B missing dot redux expert help requested - sk373a@gmail.com - 01-31-2022
