Philco model 70 rebuild - by Tim P. - Printable Version
+- The PHILCO Phorum (https://philcoradio.com/phorum)
+-- Forum: Philco Radio Discussions (https://philcoradio.com/phorum/forumdisplay.php?fid=5)
+--- Forum: Philco Electronic Restoration (https://philcoradio.com/phorum/forumdisplay.php?fid=8)
+--- Thread: Philco model 70 rebuild - by Tim P. (/showthread.php?tid=8626)
+- The PHILCO Phorum (https://philcoradio.com/phorum)
+-- Forum: Philco Radio Discussions (https://philcoradio.com/phorum/forumdisplay.php?fid=5)
+--- Forum: Philco Electronic Restoration (https://philcoradio.com/phorum/forumdisplay.php?fid=8)
+--- Thread: Philco model 70 rebuild - by Tim P. (/showthread.php?tid=8626)
RE: Philco model 70 rebuild - by Tim P. - TV MAN - 02-21-2014
Ok, alignment done (at least attempted). The good: It has REALLY brought it to life! It is MUCH louder. The IF was way off, and so was the LF osc. The bad: I still have issues in the higher freq's. Here is what I have so far. It seems much more selective and sensative below 110 on the dial. Above that, all I get is noise, motorboating and oscillations, depending on the location of the tuning cap.
It also goes microphonic at certain points. (That is, in the problem area - does NOT do this below 110 where things are working properly) I am guessing the HF oscillator is going out of whack at that point (I can about play the piano tapping the tuning plates). I have also checked for shorts on the cap. None found so far. I also could NOT get the #4 adjustment to go tune right. I have a lot of noise when adjusting, and it had a very weak signal (much more than low freq'). It just didnt respond like the others. Have to crank up the RF generator pretty high to hear. The others, I set the gen about as low as it will go. I have improved it a lot, but it still isnt right. I pulled the screws on the side with the adjustments and cleaned the contacts, put the screws back in. Seemed to help a little. When I get below 110, it kicks back in and I get stations and 'normal' static.
Mike, does your HF osc adjustment cap #4 have a small metal washer between the nut and tha cap? Mine does, the other 2 (#5 & #6) do not. Also, the tuning cap has bent out plates on the end. See my pic. You can also see the washer. This seems to be where the problem area is. I feel everything from the IF on out is working ok.
Here is a shot of the tuner - see what you think. Any help appreciated. Bout ready to call "Ghostbusters"
RE: Philco model 70 rebuild - by Tim P. - TV MAN - 02-21-2014
Here is the volume control. I drilled a couple of small holes, just below the center contact, shot Deoxit in, but still have some issues. I believe is the RF side giving the most trouble. The cleaner helped, but didnt fix. I have tried several times. Has anyone opened and repaired this type of control? These final details seem the most frustrating, and I suspect these issues may have been what put the radio "in the attic" to start with. Thanks for any advice!
RE: Philco model 70 rebuild - by Tim P. - morzh - 02-21-2014
Tim
I look at it tonight. Will let you know.
RE: Philco model 70 rebuild - by Tim P. - Mondial - 02-21-2014
That metal washer on the osc trimmer does not look right. The adjusting nut should be insulated from the brass metal contact underneath. Perhaps there is an insulating washer under the metal one?
RE: Philco model 70 rebuild - by Tim P. - morzh - 02-21-2014
Tim
There are two problems.
1. The washer is supposed to be a fiber one. I do not know if it is a problem or not in your case as you need to find out if it is insulated somehow. Basically it is in parallel to the main group of the tuning cap so if it shorts you will see the whole group shorted.
2. The other obvious problem I see is the upper plate of that trimmer cap is cut to a small size which may or may not affect the operation but if it does not tune right when you try to align it it might be the case. You could try to add a small capacitance in parallel, or simply solder a regular 20-40pF trimmer cap across and see if this helps.
RE: Philco model 70 rebuild - by Tim P. - TV MAN - 02-22-2014
I spent a couple of hours doing some careful checks. The cap in question on the tuner has a fiber washer under the other washer. I did some voltage checks on the 1st det and the osc tubes. I can PM with everything if needed, but the most obvious 'oddity' is the voltage on the cathode of the 24 1st det tube. When the radio is in the good part of the range, where it picks up, the voltage sits about 8v. I can adj the dial from 55 up to where it cuts out - same, that is, until I hit the place where it goes bad - then the voltage jumps up to 12v, and climbs to 24v as I tune all the way up, drop back, it falls until it hits the working part, then stabilizes at 8v again.
Now ... the odd part. I found if I turn the volume down until I can barely hear, I can get solid tuning up all the way to the top! It's when I start to turn up the volume, when I hit a point, it begins to oscillate and cuts off. The voltage on the cathode of the 24 also jumps at the point it begins to oscillate (and becomes microphonic). If I turn back down, it clears up. Also, the higher I turn the volume, the lower the freq that it starts to mess up.
I swapped the 27 with very little change (I am still waiting for my new one to show up). When I swap the 24 with a new one, the point I can turn up the volume lowers. I tried a weak 24, and couldnt get anything but static. I double checked the coils in the circuit. Also resistors, which are a little off, but not a lot. Also have voltages from the bias resistor. So, somehow, the volume control (checked also) and tuner and this problem are connected. I also want to comment that this is the area where a cap (#12 see my 2nd pic in the thread- I believe it may be mislabeled as #5) was replaced - years ago in looking at the style, so I believe this problem has been with this radio for quite some time - maybe even since new. This one has me scratching my head... got to be some dumb, simple thing I am overlooking. Any thoughts?
Sorry for the "book" ... just hoping someone can help me solve this mystery.
RE: Philco model 70 rebuild - by Tim P. - morzh - 02-22-2014
Tim
This is the top photo.
RE: Philco model 70 rebuild - by Tim P. - morzh - 02-22-2014
By any chance have you tested the interstage coils in the involved stages for leakage between the primary and teh secondary?
All caps changed?
Cap #12
Last: Caps 5 and 23 have GND between them. Is it good?
RE: Philco model 70 rebuild - by Tim P. - Steve Davis - 02-22-2014
Tim, is your tube shield in place?
Steve
RE: Philco model 70 rebuild - by Tim P. - TV MAN - 02-22-2014
Steve, you and Mike may kick me... shield.
I would have NEVER thought... All this for that stinkin shield. Live and learn. I'll retouch the alignment and post the results. Didnt I mention spending hours fighting a dead set only to find the power cord laying disconnected...?
RE: Philco model 70 rebuild - by Tim P. - morzh - 02-22-2014
I asked about the caps and coils because it did look like inter-cascade mutual influence. And this is what shield does also. That is its absence.
Well, one down.
RE: Philco model 70 rebuild - by Tim P. - TV MAN - 02-24-2014
I finally got the courage to to work on this again, and write in. I bet I am not the 1st that has made this mistake and cost a lot of time, to the point of frustration. Anyhows, I retouched the alignment and brought things in pretty good. It picks up across the dial with only a short clip lead. The #4 condensor adj is still pretty touchy. The other two on the tuner were way out, so I brought them back in. It does seem to work pretty good now.
Got to neat up stuff and finish details. I'll be glad to wrap this one up.
RE: Philco model 70 rebuild - by Tim P. - morzh - 02-24-2014
Actually I aligned mine with no shield first.
Then I decided to realign (for some other reason) and this time did it with shield on.
Did not see any difference, but that I guess can vary from radio to radio.
Actually in the 90 the procedure calls for the shield removed when aligning IF frequency.
RE: Philco model 70 rebuild - by Tim P. - TV MAN - 03-05-2014
Here are some updates. I have been playing this for about a week and have been working out some bugs. On the alignment, it seemed to affect the caps on the tuner when setting the alignment with the shield off the tubes. When I reinstalled the shield, I had to adjust quite a bit the front and middle trimmers, the back #4 wasnt affected much. It didnt change the IF alignment. As I finish this project, here are some notes.
Here is a pic of my RF coil rewind:
Before:
After :
RE: Philco model 70 rebuild - by Tim P. - TV MAN - 03-05-2014
Here is a pic of the metal box capacitor rebuild. I bent the tabs back, removed the cardboard. The guts didnt slide right out, so I had to warm with a heat gun, then stuff came right out.
I put the cardboard wrapper back inside, soldered a new cap inside with a new wire, reassembled, and - done.