41-610 High plate and grid voltages - Printable Version
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41-610 High plate and grid voltages - atvman101 - 12-30-2019
Greetings everyone,
Working on a Philco 41-610P code 121. It's my desire to spend as little as possible (time or money) to figure out if it's worth saving. To start i did some basic repair to the unit,replaced the filter caps,repaired broken/chewed up wiring. Even managed to rewind a coil on the oscillator. I also replaced one cap that the mice completely chewed through. At this point i decided to power it up on a variac using the dim bulb method. At about 50v the indicator bulbs began to glow, drawing about 20 watts at this point. Slowly increased voltage. At about 80v ,30 watts I started to hear faint noise as I let the radio warm up. I was able to tune in a local station.It sounded weak of course but the audio was distorted too. I checked plate and grid voltages which were kinda high, tube 41 plate was at 190v, schematic called for 165v, for example. Checking the other tubes revealed higher voltages as well. I then decide to re-cap the radio as most of the original caps were still in place and checked the resistors i could while i had the tubes out. I then slowly powered the radio back up. At about the same voltage on the variac i began to hear noise,tuned in the local station. Audio sounded much better. I then went up to 90v on the variac, drawing about 43 watts and the radio really came back to life. Audio was crisp and clear and was able to tune in more stations.Short wave and the phono amplifer also worked. I then checked plate and grid voltages and to my disappointment all were reading even higher relative to their rated voltages on the schematic. The 41 plate was now reading about 210 volts,should be 165.Voltage just after the rectifier but before the field coil was reading 250v,should be 225v. I can only guess what the voltages would be if i went any higher on the variac (roman candles came to mind).
I was expecting lower voltages with the radios age. To me it seems like the radio isn't drawing enough power. My question is this, As tubes age do they tend to draw less power,causing voltages to rise? I don't have a tube checker yet,haven't really needed one up until now i guess. I'm having trouble zeroing in on any one cause of the higher voltages. In the forums experience,am I looking at a total rebuild? My experience is limited with antique radios,it is a hobby. I've repaired a half dozen or so pre 1960's radios with success. I'm kind of at a loss on this one. I attached the schematic i'm using.
Thanks in advance for any help or pointers.
RE: 41-610 High plate and grid voltages - morzh - 12-30-2019
If you use a modern meter vs then models that had very low impedance and would load the circuitry, this should then account at least for part of the difference.
RE: 41-610 High plate and grid voltages - Radioroslyn - 12-30-2019
Hi and welcome to the Phorum,
Would check the plate voltage on both 41's it's not uncommon to find one side of the output transformer to be open.
RE: 41-610 High plate and grid voltages - Ron Ramirez - 12-30-2019
+1 what Mike (morzh) said, plus:
Today's line (AC) voltages are generally higher now than they were in 1941. Back then, 110 to 115 volts was the norm. Now, 125 or even as high as 127-128 (depending upon location) is considered "normal".
Higher AC line voltages, combined with voltmeters with higher input impedances, both contribute to higher voltage readings compared to the measurements taken when the set was new.
Oh, and also check what Terry (Radioroslyn) suggested.
RE: 41-610 High plate and grid voltages - atvman101 - 01-01-2020
Guys,Many Thanks!
Indeed i used a brand new DMM to work on this particular radio. Meter impedance was in back of my mind but unfortunately that's where it stayed. I just had it stuck in my head there wasn't that big of a difference. I rechecked the radio again with my old analog VOM and the voltage readings made much more sense. I ran the radio at 110v and let it play for about 6 hrs yesterday and it sounded great. Lesson re-learned for me.
Thanks Again!
Kent