I originally posted these questions in the Home Radios forum, but guess this is the more appropriate forum.
I have re-capped my 37-89 except for the two bakelite caps #44 and #54.
1. Can I just substitute a regular tubeular cap of similar value for #44 in the output stage tone circuit?
2. Can I safely substitute regular caps for the dual line-to-gnd noise filter cap #54 or do I need to go with the more expensive safety caps? Either way, can I leave the bakelite cap in place and just bypass it?
Thanks for any suggestions. Dick
#1 Yes voltage rating of 630v is ok.
#2You can leave it out of the circuit or replace it with whatever caps you like/can get. Sets have had paper caps there for decades don't know what all the hubbub about safety caps is about. All the darn things do is shock you when your grounded and the radio isn't
Terry
Safety caps weren't being manufactured in mid-late 1936 when the 37-89 was first manufactured, so radio set manufacturers had to use regular caps instead.
The problem is, when a regular paper capacitor fails, it may fail "open" (it opens up internally, as if there is no longer a cap there), or it may fail "short" (it shorts out and acts as a conductor).
If a cap is connected across the AC line, or if two of these are connected between each side of the line and ground (as in the case of the 37-89 and many hundreds of other Philco models), if a regular cap fails and shorts out, you have a potentially dangerous situation.
Safety caps are designed to fail open only.
I have to admit that I, too, resisted adopting the use of these for a long time, until I realized the benefit of them as opposed to the risk.
Thanks for the info Terry and Ron. It is about what I expected. It is really a safety issue.
Thanks Dick
I have completed the recapping of my 37-89 and gone thru a re-alignment, but have a problem.
The radio plays pretty well, but "motorboats" at about 600kc, which happens to be the frequency for the low-end rf alignment. It does this during alignment with the signal generator connected if I vary compensator 12a slightly off peak output either way, as well as with the antenna connected if I tune down to about 700kc.
With the antenna connected, if I tune past the "motorboating" down to about 550kc it stops and I can pick up a station below that frequency.
Can anyone help me out on the likely problem or how to correct it?
Thanks Dick
Hello Richard, motorboating is usually a bypass cap to ground issue, however there are exceptions. I took a quick look at the Riders schematic for the 37-89 over at nostalgiaair. I will make these suggestions. Go carefully back over your schematic and make sure a cap isn't missing somewhere. Check for good solder joints everywhere, especially to ground. I have run across several previously-hacked radios in the past where parts were missing and not replaced. Make sure grid #2 on both 6K7 tubes have appropriate condensers to chassis ground according to schematic. Also check the cap(s) to ground in the avc line. Make sure the choke ( twisted pair wire) part #11 on Riders schematic hasn't been accidentally moved during the recap. Also, make sure both 6K7 and the 6Q7 tubes have their shields on them. I have also had luck eliminating motorboating in the past by adding a capacitor or two out to ground in the appropriate suspect areas. To do this, take a .05/ 600v cap, use a jumper with alligator clips on both ends and connect one end of cap to ground. Use the wire off the other end of cap and touch it to suspect areas that need bypass with set powered on. Just remember to discharge the cap to ground between test points and work with only one hand near the chassis. Im sure others here can offer other suggestions as well.Best of luck with your restoration!
Texasrider: Thanks for the input.
I think my problem is with the RF choke part #11.
It was open and I had to put in a new one. Rather than take the old one out, I just connected the replacement choke in parallel with the original. Sloppy work I know!
I can affect the motorboating by placing my hand on the choke. So, I am going to see if I can get the original repaired and in it's original position.
Will post another item after I am finished with that.
Thanks again. Dick
Also check the part # 9,( Riders schematic)the two wires (loose coupled capacitance) twisted wires. Make sure they are still in the circuit and move them around slightly with a wood pencil to see if that helps.