[philco4me]

Hello all
Got a 41-608 that I recapped about a year ago. Lately the highs have been distorted on the phono. I replaced the cart and needle but made no difference. Could a tube be an issue or a cap went bad ?

Thank you

[jrblasde]

Good evening! Could you elaborate a bit more on the distortion? Is it perhaps static-y sounding, for instance, or is it higher or lower in tone than it should be? Also, was it a rapid onset or has it been more progressive?

[philco4me]

Good evening
For example s sounds are distorted you cringe when you hear it. It seems to have come on suddenly. Highs are the issue lows are fine

Thanks for the reply

[jrblasde]

I was formerly an audio-video system engineer. The “s” sound is a great test to the frequency reproduction of a system. I think I understand exactly what you’re hearing now. Probably sounds like a radio that’s not quite tuned to a particular station, doesn’t it?

Sounds like a change in capacitance if you ask me. But given that you say it was a sudden onset, I’m almost inclined to say it’s a tube issue. Certain failures within tubes can cause changes in capacitance from pin to pin.

Just as a sanity check, have you tried playing different records? Does the radio mode sound normal?

[philco4me]

I have Tried several different records with same result. I even played the same records on my 46-1209 and they play fine. The radio seems to play fine.

[jrblasde]

To me it definitely sounds like an issue with the 7C7 preamp tube. You’ve pretty well narrowed it down. You might just verify that there aren’t any damaged capacitors in that section of the circuit, though I doubt it if you recapped the system last year.

Let’s see if any of the experts here have an opinion! I’m pretty new to tube circuitry, myself.

[philco4me]

Thank you so much for the fast reply. I will Test that 7c7 and let you know

Thanks again

[jrblasde]

You’re welcome! I’m following with a curiosity. I have a 49-906 which had a tube go out on the FM RF circuit, and it caused a similar issue with audio reproduction. Just the same, I was able to narrow it down to just the one function, and found it to be the tube. That’s really the only reason I’m feeling confident in my assumption.

[philco4me]

This set has 7c6 tubes. I will Test both.
Thank you. I’ll post the result

[jrblasde]

Ah, so you have a code 122 model of the 41-608. I had to do some digging!

[jrblasde]

Good evening! I don’t know if you’ve had time yet to test that 7C6 tube, but I had a thought. You could try swapping the two 7C6’s in your set. If it really is the tube (and not another problem), then the record player should suddenly sound fine whereas your radio will not. Just a thought for a potentially quick test.

[philco4me]

Hello
Tested both 7c6 and they tested slightly on the good side so I am replaceing them both. Also my 7b7’s tested weak so they are getting replaced also. 
Thanks so much for the help and I will post the result when I replace the tubes.

[jrblasde]

Fingers crossed that we’ve got a solution!

[MrFixr55]

Hi Joseph,

Unless the preamp tube is gassy, I don't really think that the weak but good tube will cause distortion.

The original turntable in this unit used a unique "Bem of Light" pickup, described in the Philco documentation for this set. I have never troubleshot (or even seen) the "BOL" (Beam of Light) pickup, but there are 2 factors here. The pickup emits a signal much smaller than a crystal or ceramic cartridge in common usage in the 1940s and 1950s, hence the extra amp stage. In addition, the light source appears to be driven by the radio's local oscillator. This was some serious "tech" for 1940 -1941, and I believe it was unique to Philco.

Verify that the wafer switch is fully clean and not intermittent. Another "divide and conquer" technique would be to substitute another signal for the phono pickup. Either a turntable with a magnetic pickup or a dynamic mic would work. A line level source such as a CD player, MP3 or computer would require a lot of attenuation. If the sound through the preamp is good with the external signal but bad with the BOL, continue to troubleshoot the pickup. If the BOL has been replaced by a crystal or ceramic cartridge, then this is likely your problem. You would likely overdrive the preamp with a crystal or ceramic cartridge. In addition, I assume that if this is the original turntable, it plays only 78RPM records. The stylus must be for 78RPM records, as the groves are wider than 33 1/3 or 45 RPM records. If you can't find a BOL pickup, a GE RPX would be a good substitute. There are also modern magnetic pickups still made, particularly by Shure and Audio Technica. Ensure that you buy a 78RPM stylus for the pickup.

If the turntable has been replaced, and has a crystal or ceramic pickup, then this (overdriving the preamp) is your issue. If you intend to use a crystal or ceramic pickup, then you must bypass the preamp by disconnecting Cap # 44 from the wafer switch at point C9A on the schematic and connecting the pickup directly there. Alternately, either attenuate the signal from the pickup or replace the cartridge with a magnetic one. If the turntable is a 3 speed, then consider a cartridge marketed for "Scratch DJs". (They now prefer to call themselves "turntablists" as they consider themselves as playing a musical instrument.)

Hope this helps!

[jrblasde]

I appreciate you joining in, sir! I'm certainly no expert yet, but I'm happy to bounce ideas around and learn more about the troubleshooting process. 

This is all very interesting. It was actually just this week when I first read about the Beam Of Light audio pickup. Sounds as though there is more to investigate. Myself, I do not own a turntable of this vintage. I'm more familiar with the magnetic pickup systems. When I was on the search for my 46-480, I saw a physically similar model 47-1227. It was an upright console with AM-FM radio (no shortwave like mine) and a turntable. It was very appealing to me, but was in New York and the seller insisted upon local pickup.

Are you able to elaborate a bit more on the mechanics of the BOL pickup? Does it still have a stylus which physically touches the platter, or does it simply emit a light and observe the intensity of the reflected light? Sounds like I've got some reading to do!