(12-31-2014, 08:03 AM)Diverted Wrote: Hi again. This is an edit to a previous response I had about the filament not having continuity. Well it does; I pulled the cap off the rectifier and jiggled the socket and got ohms! So a little remedial soldering is in order.
MOVING ON to other things, my next question has to do with candohms and one particular resistor that appears to have been swapped out.
On the schematic, resistor 82 is listed as a 5K wirewound. On the chassis, it's a candohm-style resistor bolted to the chassis. it runs between the field coil and the 3mfd electroytic to ground. Looking last night, I noticed that the field coil side of the resistor was disconnected. ?? Ohmed it out and it read many megs. OPEN. So I looked around and the only other likely replacement resistor I could find was an aftermarket Ohmite 15K, 10W tied in on one end to the middle lug on bakelite capacitor number 58. The other end goes to ground. That middle is used as a tie in post to the field coil, so I'm thinking this resistor is a replacement and whoever put it in just found that to be a convenient spot to tie it in.
But it's a 15K! Three times what's called for on the schematic. Obviously I'm going to replace it with a five. But how many watts should I use?
Similar question for the five section candohm (part 82). Several sections are quite high so I'm going to replace it. But I don't know what wattages to use, and space is at a premium. Anyone have any idea what wattages I should use for each section? Thank you!
Ted, Believe the 4 section candohm is 196+18+22+22 = 258 ohms total used for the (-) negative control grid biasing of the audio tubes, etc. Use a 200 ohm to replace the 196 ohn section. Will make very little difference. The schematic and associated info. from Nastalgia Air shows about -33VDC developed on the C.T. of the high voltage transfomer feeding the candohm at 115VAC in. So say at 120VAC may be worst case -35VDC. Since this is a series string resistor the current to ground is common thru the four sections. Once you know the I. you can calculate the power rating of the individual resistors...P=IxIxR. I=E/R. I = 35/(200+18+22+22)= ~.134 amps. or 134 mamps. For the 200 ohm section...P= (.134)(.134)(200)= ~3.6 watts. I would use a 200 ohm, 10 watt resistor. You can calculate the power rating needed for the other three sections. The result would indicate that 1 watt resistors should be sufficient. R82 is a 5600 ohm bleeder resistor in parallel with the 3 ufd filter cap. on the B+ buss to the driver tubes, 6J5's. The schematic data shows about 235vdc at 115VAC into the set. I would think that the value of this resistor is not critical as it looks to me to mainly be used for discharging the B+, and to minimize pops, at turn on and turn off of the set. The B+ is primarily determined by the filter choke, capacitors, field coil in the power supply circuit. Say you get 245vdc on this B+ buss at 120VAC. The power dissipated by R82 would be P=ExE/R = (245x245)/5600= ~10.7 watts. Holy Toledo no wonder in burnt out . A 5600 ohm resistor here would require at least a 20 watt rating. I can see why someone put in a 15K resistor which I feel would have essentially no effect on the circuit operation. A 15K would limit the current rating needed to 10 watts. If you wish to stay with original values then a 20-25 watt resistor is needed. I seems that R82 accounts for slightly less than 10% of the power dissipated by the entire set if my calculations are correct. Good luck, John
[/quote]
Thanks for all the help so far.
I have done very little with this set, apart from replacing the candohm and burned out 5.6K resistor today. I am looking around the set for issues and today found a cut ground wire attached to the RF section. See pic. From the parts diagram/layout drawing, this appears to be part 20, a .05 cap that connects to Position 4 of the RF transformer, one side of the shadow meter, and some other parts.
I have looked around this part of the circuit and cannot find a replacement cap; so I am unsure why someone in the past cut this out of the circuit and didn't replace it.
Two questions:
What does this cap do?
Also, would I be able to tie a replacement .05 cap anywhere along this node? Or should I try to keep it as close to the transformer as possible? Getting it in there will be tight, as that transformer is buried in the RF section. I would be easier for me to tie it into the shadowmeter, etc., but don't know if that's kosher.
Thoughts appreciated, thank you!
PS: Turns out the on/off switch is shot. Someone intentionally shorted it out with a soldered in wire, so I'm looking for a replacement if anyone has one. See the WTB section.
Ted
(This post was last modified: 01-15-2015, 04:40 PM by Diverted.)
Here's another. Anyone?
I am going through the radio and found these two .003, 1000V caps (parts 73 and 74) running from either side of the audio output transformer and output tubes to ground. Didn't have any .003 caps in that voltage rating, so for each I ran two .0068, 630V caps in series. Should cover the voltage requirement. But my real question: Is such a high voltage rating required?
Thanks.
(This post was last modified: 01-26-2015, 02:50 PM by Diverted.)
The .05 uF cap, part (20), is tied into the B+ line feeding the plate of the RF tube (through the appropriate coils, of course), and it looks to me that it acts as a bypass to keep stray RF out of the B+ line.
The second question is more tricky.
1000 volts were specified due to the possibility of voltage spikes at the plates of the audio output tubes. Now, there has been disagreement among experienced collectors/restorers as to whether they really need to be 1000 volts or not. All I can tell you is that I've been using 600/630 volt caps as these plate bypasses for years, and have yet to have one blow up on me. But then again, I do not run my radios at high volume levels. If you run a radio full blast, then it *might* need the 1000 volt rating. YMMV.
Just curious...why are you using a Canadian 37-3670 schematic? Although it should be the same as the USA 37-670 schematic, I think...
(01-26-2015, 04:28 PM)Ron Ramirez Wrote: Let's examine your first question first.
The .05 uF cap, part (20), is tied into the B+ line feeding the plate of the RF tube (through the appropriate coils, of course), and it looks to me that it acts as a bypass to keep stray RF out of the B+ line.
The second question is more tricky.
1000 volts were specified due to the possibility of voltage spikes at the plates of the audio output tubes. Now, there has been disagreement among experienced collectors/restorers as to whether they really need to be 1000 volts or not. All I can tell you is that I've been using 600/630 volt caps as these plate bypasses for years, and have yet to have one blow up on me. But then again, I do not run my radios at high volume levels. If you run a radio full blast, then it *might* need the 1000 volt rating. YMMV.
Just curious...why are you using a Canadian 37-3670 schematic? Although it should be the same as the USA 37-670 schematic, I think...
Ron,
This is the only schematic I could find, apart from the NostalgiaAir one which isn't so great (honestly I did not know I had Canadian version). I have ordered paperwork from Chuck Schwark at the Repair Bench (comes highly recommended as I'm sure you know) but haven't received it yet; just sent payment a few days ago.
Anyway, the schematic appears mostly correct; a few differences, but I have stuck to the original wiring in those cases. Thanks for the info on those .03 caps, 1000V. I'll never crank the radio, so shouldn't be an issue, esp. with running those .0068s in series.
Thanks! Fired up the radio for the first time last night after replacing the electrolytics, AC to ground and some other caps, and got reception. What a sweet sound!
I wonder if the set ever played after the previous restoration. Among other problems, previous person wired electrolytic 86A backwards he ran the negative to ground; this one has positive to ground.
This is my first Philco and I still have a long way to go on it, but as a collector who has mostly Zeniths, I have to say that this set is pretty impressive. Just finished stuffing the bakelite blocks and replacing all resistors more than 10 percent off spec, plus all paper caps. Haven't got to the caps/resistors in the RF section yet (I will when I remove it) but I am more than impressed with the radio's performance. Without an alignment, after just a partial recap, and in the middle of a blizzard, I picked up many, many SW stations that I am not used to hearing on any of my Zeniths, including my 12S265 and 12S267. And the AM band is very impressive, too. Selective, with great sound and volume. I am really glad I got this radio. Can't wait to get back to it.
(This post was last modified: 01-27-2015, 07:50 PM by Diverted.)
Well, after taking out the RF section, replacing some caps and putting the set back together, all H**l has broken loose. Prior to taking out the floating RF, I was getting great reception and volume across all bands. Now, I am getting very weak AM volume and somewhat decent volume (though there's a lot of squealing going on) on the other bands. In AM, I can barely hear it with volume turned all the way up.
When I squeeze the 6A8 Dector/oscillator grid cap wire, the volume increases a good bit. In SW bands, it's very loud, just about what it was before, I think. In AM, its loud though not as loud as it was prior to pulling out the RF section. When I let go of the grid cap wire, the volume dies again. Reception is poor on AM; on the shortwave bands, there is some reception; however there's also a ton of squealing. The 6A8 voltages are pretty much where they should be. Tubes all test good.
Can someone guide me through some troubleshooting? I would love to solve these issues. It's a beautiful set and I'm very discouraged. Could use some expert help.
Thank you.
Here's a clean copy of the schematic. Click on the photo once or twice to zoom all the way in.
(02-02-2015, 11:11 PM)Diverted Wrote: When I squeeze the 6A8 Dector/oscillator grid cap wire, the volume increases a good bit.
Sounds to me like you have an issue somewhere before the 6A8G control grid. How are the voltages on the 6K7G RF amp?
This sounds very much like the classic bad antenna or RF coil on AM. SW...??? Did you replace caps (8 ), (20) & (21)? Also check resistor (9); if it fails, you lose AVC to the RF tube and the radio will exhibit the same symptoms as open antenna coils.
(02-03-2015, 06:56 AM)Ron Ramirez Wrote: Sounds to me like you have an issue somewhere before the 6A8G control grid. How are the voltages on the 6K7G RF amp?
This sounds very much like the classic bad antenna or RF coil on AM. SW...??? Did you replace caps (8 ), (20) & (21)? Also check resistor (9); if it fails, you lose AVC to the RF tube and the radio will exhibit the same symptoms as open antenna coils.
Found the issue. A stupid oversight on my part.
When I re-installed the tuning condenser, one of the unshielded wire leads leading to the fins ended up getting pushed up agains the side ofthe round access hole on the top of the RF chassis. So the wire was hitting the grounded RF chassis, grounding the fins on the back third of the tuning gang sections. It was just barely touching, but enough. Got home from work, noticed it, desoldered the lead and readjusted ... Bingo. Radio is playing strong and perfect across all bands. Thank god! It was a simple error.
. A 5600 ohm resistor here would require at least a 20 watt rating. I can see why someone put in a 15K resistor which I feel would have essentially no effect on the circuit operation. A 15K would limit the current rating needed to 10 watts. If you wish to stay with original values then a 20-25 watt resistor is needed. I seems that R82 accounts for slightly less than 10% of the power dissipated by the entire set if my calculations are correct. 
Good luck, John
he ran the negative to ground; this one has positive to ground.