Adding AVC Mod to Philco 70 - Printable Version
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RE: Adding AVC Mod to Philco 70 - Ron Ramirez - 06-23-2019
Good idea to always replace the mica cap in parallel with the low frequency padder.
It's the only remaining original part in this radio...that is, other than the "B-C" resistor, the power transformer, the filter choke, the trimmer/padder caps, the tuning condenser, the coils, and the IF transformers.
RE: Adding AVC Mod to Philco 70 - Ron Ramirez - 06-23-2019
"You Big Dummy"
Doing some research, I noticed that the Model 19 and 89 sets, which also use a 260 kc IF, use a padder capacitor (04000-S) which has more "leaves" and therefore requires no mica cap in parallel with it.
So, I decided to just replace the original padder with an 04000-S since I have a (very) small stash of trimmer caps salvaged from junk Philco sets.
And when I went to replace the padder, guess what I found...
I had forgotten to install resistor (11), 51K, which is also mounted in parallel with padder cap (10).
Reattaching a speaker and a test antenna, the set came to life - with the tube shield in place - and is now picking up two or three stations weakly. Best of all - no more popping or buzzing.
It needs a complete alignment, but it is now working.
Thanks Ed and Rob for your help with this one.
RE: Adding AVC Mod to Philco 70 - Ron Ramirez - 06-23-2019
(Almost) Final Touches
I can't get to my VTVM right now (long story so I'll spare you), but a quick alignment by ear brought up the high end of the 70 nicely. While it picks up the 600 kc signal from the signal generator very well, on antenna the low end seems...well...very low.
Oh, and now it requires the tube shield to be in place to prevent the unit from squealing (oscillating). Which, I think, is a good thing.
Rob, I would be most interested in reading more details on how you went about bending tuning cap plates to achieve more uniform response across the AM band.
RE: Adding AVC Mod to Philco 70 - Ed Locker - 06-23-2019
And the dreaded distortion?
RE: Adding AVC Mod to Philco 70 - Ron Ramirez - 06-23-2019
None that I could tell. But then again the few stations I could pick up were all talk, no music. I really need a longwire...
But in general it sounds like any other radio with AVC, no "blasting" between stations.
This is an excellent mod you came up with, Ed.
RE: Adding AVC Mod to Philco 70 - rfeenstra - 06-24-2019
Woo hoo! Success at last. Plate bending... lots of trial and error! What I found was that when I got the compensator caps tuned in for maximum sensitivity at the high end of the band, the sensitivity was quite low on the low end of the band. By tuning the compensator caps, I could greatly increase the sensitivity on the low end. In my case that meant decreasing the capacitance of the compensator caps on the RF and 1st IF compensators that are on the tuning capacitor. So, to bring that into line, I had to decrease the capacitance of the plates on the tuning capacitor. I tuned the compensators for 1.4 mhz. then tuned to mid band. Bending the slotted plate out reduces the capacitance. I checked to see if i was close by seeing if I could increase signal using the compensators. If I could, I increased the bend on the plates. I then retuned at 1.4 mhz and checked again. I repeated this procedure until I was reasonably close. Then I moved on to 600 khz with the compensators tuned for 1400 khz. In my case, I could again increase sensitivity by reducing the capacitance of the compensators. so, that meant bending the slotted plates out of the now almost fully "engaged" tuning capacitor. I would then repeat the process by re-tuning to 1400 Khz and then re-checking at 600. this is a back and forth process until there is reasonably good tracking on all 3 gangs of the tuning capacitor.
If the oscillator is not tracking, you can do the same thing by tuning at 1400 with the compensator (most of the tuning capacitor is not "engaged") and then checking at lower frequencies, noting whether you need more or less capacitance in the "engaged" portion of the tuning capacitor for the indicated frequency. I would say that if you want to improve tracking of the oscillator, do that before the RF and 1st IF tuning. I am known to confuse myself in the process so think it through to determine whether you need more or less capacitance. e.g. If a 1000 khz signal reads on the dial at 1100, there is too much capacitance when the dial is at 1000. So bending the "engaged' slotted tab out will reduce the capacitance. Remember, only the engaged portion of the capacitor is contributing capacitance. Like I said, it is easy to get confused in the process and bend the wrong way! Been there, done that! it's kind of satisfying to get it tuned in better than it probably was ever tuned by the people on the assembly line.
I rarely have seen the tuning plates bent on a "production" radio. But I have seen pics of the tuning capacitor of a McMurdo Silver and noticed obvious bending of the outside plate. Same with my Scott 16A. There was obvious tweaking to make it track.
Feel free to call me, Ron, if this doesn't make sense. You have my number!
RE: Adding AVC Mod to Philco 70 - Ed Locker - 06-30-2019
Ron,
Any updates on your modified 70? Get it aligned? Did the low end perk up?
How about a video?
Ed
RE: Adding AVC Mod to Philco 70 - rfeenstra - 10-07-2019
Ron, I too am looking for an update. How did this thing turn out?
RE: Adding AVC Mod to Philco 70 - Ron Ramirez - 10-08-2019
Okay, okay, folks...
I did experiment with bending plates which seemed to improve low end reception somewhat. At the time I did not have my outdoor longwire up (which my son-in-law has since built for me) so I couldn't really put it to the test.
Since then I've moved on to other things and yes, I do need to drag it back out and try it again with my longwire to get a really good idea of whether it is truly better now, or not. If I feel up to it I'll try to do that this weekend. No promises.
RE: Adding AVC Mod to Philco 70 - rfeenstra - 10-08-2019
RE: Adding AVC Mod to Philco 70 - Ron Ramirez - 10-13-2019
All right. You folks insisted...so I took some time a little after 7pm EDT this evening, moved my RCA T8-14 chassis out of the way, and tried out the Philco 70 chassis on my longwire antenna.
Here are the results as I slowly tune from the high end of the band to the low end:
[Video: https://youtu.be/UJJbJ-ObRx0]
Notice how it begins to get quieter as I go below 1000 kc, which indicates that bending plates did no good. You will also notice that it picks up a station broadcasting at 760 kc at 70 on the dial. So the alignment is definitely off and needs to be redone.
Right now my RCA T8-14 has priority, so nothing more will be done to this 70 until the T8-14 is finished. I have a used B+K 4040 signal generator on its way here and if it turns out to be OK, I'll use it to realign the 70.
I think that if nothing else, this video clearly shows that it is past time to retire that old Clough-Brengle signal generator I have been using.
RE: Adding AVC Mod to Philco 70 - rfeenstra - 10-13-2019
Thank you, Ron. Looks like you have a bit of work to do yet! When I had an improvised antenna coil on my 16B, the low end of the dial was all messed up. When I got the proper coil from Ed, it straightened it all out. Persistence will pay off.
RE: Adding AVC Mod to Philco 70 - Ron Ramirez - 12-26-2019
So, finally, after months of procrastination...I revisited this Philco 70 tonight and realigned the set, now using my B+K function generator.
Results: MUCH, MUCH better. The IF alignment was really close as was the high end RF. But the low frequency padder was WAY off.
Now, the radio picks up stations up and down the dial with ease and AVC seems to be working quite well indeed. No, it isn't an RCA 10K, or even a T8-14, but it is doing much better than it was before.
I might post a video later if I get a chance to do so.
RE: Adding AVC Mod to Philco 70 - Ed Locker - 12-26-2019
Glad to know that all your work paid off.
Didn't you have a problem with the low frequency padder earlier?
RE: Adding AVC Mod to Philco 70 - Ron Ramirez - 12-26-2019
Yes - I had removed the 51K resistor that was connected across it, and then forgot to install a new one.
Here is a quick video of the modified Philco 70 in action.
[Video: https://youtu.be/bQA3NXhEObc]
I was picking up WBZ, Boston, again on this set (as I did on the RCA 10K chassis yesterday).
So, until I come up with a suitable speaker (Ed, I was using the test speaker I got from you several years ago), that is it for this Philco 70. One final thought on this subject - the AVC mod is well worth doing to the non-AVC Philco 70 sets - it improves their performance considerably.