slightly distorted audio model 52 after rebuild - Printable Version
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slightly distorted audio model 52 after rebuild - gribbled - 12-18-2018
I've finished the electrical rebuild and alignment of my model 52 and the receiver is working well, possibly too well. With my 20' wire in the ceiling of my basement, I can listen to clear channel stations in Philadelphia and Quebec from Iowa!
Local stations are another thing. I use KMRY (local oldies station) a lot and it is so strong that it is hard to listen to. You almost can't turn the volume control down far enough. "Volume" on this receiver is really an RF control at the antenna.
I've noticed that the audio is slightly distorted - not horrible at all but it is worse on strong stations. Is this just typical for a 1934 design? The output tube (47) tests ok but has a fair amount of blue glow. It also runs REALLY hot, by far hotter than the rectifier or any of the other tubes.
I haven't checked the bias voltages on the output tube, I can do that next.
Thanks for any help,
dave
RE: slightly distorted audio model 52 after rebuild - Phlogiston - 12-18-2018
OK, I'm too lazy to look at a schematic, but I know that this is a fact with models like the 70 and possibly the 90 too. So it is normal operation. All you can really do is to use a shorter antenna wire when listening to locals.
You might want to check all of the voltages on the 47 toob.
RE: slightly distorted audio model 52 after rebuild - Radioroslyn - 12-18-2018
> "Volume" on this receiver is really an RF control at the antenna.
Correct. And this set doesn't have an avc circuit. Which can make it more susceptible to overload.
>Is this just typical for a 1934 design?
Well it's more like 1929 technology although Philco was the designer of avc (1st used in the model 95) they decided to not incorporate it in this set.
>I haven't checked the bias voltages on the output tube
Good idea. Should see abt -16vdc on the 47's control grid.
If you are not using a ground connection on the chassis do so this will help make the volume control more effective.
RE: slightly distorted audio model 52 after rebuild - gribbled - 12-18-2018
(12-18-2018, 04:01 PM)Radioroslyn Wrote: If you are not using a ground connection on the chassis do so this will help make the volume control more effective.
That's interesting... I'm not currently using a ground. I added caps on the AC line to ground which seems to provide an RF path to ground through the powerlines, a bit.
I'll try adding a real ground.
RE: slightly distorted audio model 52 after rebuild - gribbled - 12-19-2018
I checked the bias on the 47 output tube, all voltages measured DC with a digital meter.
Plate: 270V
Grid2: 284V
Grid1: -15V
About 2.6 VAC Across the filament.
This seems ok to me.
RE: slightly distorted audio model 52 after rebuild - morzh - 12-20-2018
52 is a TRF with a plate detector. In which respect it is same as 20, and the superhets like 70 and 90 also use the plate (also called Power) detectors. They all sound a bit distorted, especially at the low signal level, considering the volume in them is changed by actually attenuating the antenna sgnal.
Of course you can have other problems but these radios never sound very well, they sound acceptable.
RE: slightly distorted audio model 52 after rebuild - Radioroslyn - 12-20-2018
>52 is a TRF with a plate detector.
Half right does have a plate detector but is a superhet.
RE: slightly distorted audio model 52 after rebuild - morzh - 12-20-2018
Terry
How is this
http://www.nostalgiaair.org/Resources/638/M0029638.htm
A superhet?
RE: slightly distorted audio model 52 after rebuild - gribbled - 12-20-2018
I can confirm the 52B is indeed a superhet. It has a very low IF (175 kHz) but it certainly mixes the incoming RF down to the IF frequency.
RE: slightly distorted audio model 52 after rebuild - Ron Ramirez - 12-20-2018
(12-20-2018, 04:09 PM)morzh Wrote: Terry
How is this
http://www.nostalgiaair.org/Resources/638/M0029638.htm
A superhet?
I'm not Terry, but...as gribbled said, the 52 is indeed a superhet. Look at the second schematic at the bottom of the page at that link.
The 51 and 52 were, basically, stripped down, bare-bones versions of the late version of the 70.
RE: slightly distorted audio model 52 after rebuild - morzh - 12-20-2018
Oh...they showed 50 first.....me seeing just one page I thought the first one was the 52.
They fit both on one page
Anywho, the plate detector stays and should explain at least part of the distortion.
RE: slightly distorted audio model 52 after rebuild - rfeenstra - 12-21-2018
Interesting! I was just about ready to signal trace the 70 I recently restored cuz of the very slight distortion I noticed. It bugs me. I want the audio to be as clean as other radios I have. I guess I'll live with it!
RE: slightly distorted audio model 52 after rebuild - Ron Ramirez - 12-21-2018
Those older Philcos with the "power detector" (plate detector) were not capable of producing high fidelity sound.
RE: slightly distorted audio model 52 after rebuild - Radioroslyn - 12-21-2018
<It bugs me.
Well if are you up for a little debauchery remove the 24A detector tube and socket and replace w/a 2A6 tube/six pin socket (2.5v version of the 75,6Q7, 6SQ7 ect). Use the triode for a resistance coupled 1st audio and the diodes for detection/avc. You can use an audio gain control rather than the dual rf gain control.
RE: slightly distorted audio model 52 after rebuild - morzh - 12-21-2018
But that's butchery
It's the way it's supposed to be. In fact, as many of us have more than one radio, restore another with a diode detector and then you have a nice demo of one vs another detectors' sound.
There are more than one way to butcher the detector if one wants to go there.
One is replacing C23 parallel to R18 in the detector cathode feedback with small value cap, like a few hundred pF. This makes sound noticeably better but lowers sensitivity.
Another is replacing the detector with a triode in the diode configuration (as it is in 95, 111, or 37-116 abd many others). Again, will be a gain loss. But little wiring.
I would just leave it be.
Just make sure everything is good with voltages, tubes, values and then the little distortion that will remain is native to these radios.