[Crist Rigotti]

Well, it has been a year now since I finished the radio.

Now I have a problem with it. It blows the mains 2 1/2 amp fuse. I pulled the chassis and the speaker and put them on my bench. I hooked it up to my Variac which has a voltmeter and ammeter. When I get to about 50vac the ammeter starts to climb slowly. At 60 volts it goes past 1 1/2 amps and then I shut it down. Then I pulled the 5Z3 rectifier tube and then I can go to full voltage with out any problems. I checked the 5Z3 and it checked good. They way the ammeter operates, it almost seems that a capacitor is breaking down and slowly conducting to ground. I pulled all the other tubes just in case it was a tube, but it still does the same thing.

The radio played well for about 9 months. Then one day it blew a fuse on turn-on. OK, I replaced the 1 1/2a fuse and a week later it blew it again after playing for a while. OK, in goes a 2 1/2a fuse. It played for about 30 seconds and blew. This is about how long it takes for the ammeter on the Variac to get up to 2 1/2 if I left things go.

I pulled the cans off the main filter caps, 103 & 104 and they look OK. I then tried it again with the 5Z3 installed and sure enough after about 30 seconds and 50 to 60 volts, the ammeter slowly climbs and I turn it off at 1 1/2 a. I checked the power transformer, the choke, the electrolytics to see if anything felt warm. Nothing.

So I'm asking what would be a good method of trouble shooting this thing. I do think it is a capacitor that is going bad, but that is just a hunch. Would disconnecting an electrolytic, one at a time cause any harm? Maybe I can find a bad electrolytic that way. Any pointers would be appreciated.

Thanks.
Crist

PS. The thread on restoring this console ended up in the Restoration Notebook Section. If this post is incorrect then please delete it moderator.

[Radioroslyn]

I had a similar problem with the 660L. It ended up being a bad tube socket (one of the 42 output tubes). Ungrounding the filter cans may give you a clue. Also removing the mounting screw from the bakelite blocks to unground them my help too. Seems like the issue is in the HV. If you pull the 5Z3 and power it up with 120v ac all the other tube s light and dials lamps??
> Would disconnecting an electrolytic, one at a time cause any harm?
No the set will just hum but doesn't hurt any thing.

[Mondial]

It sounds like either a filter cap is failing, or the insulation between the 5 v filament winding of the power transformer and ground is breaking down. With the 5Z3 removed, there will be no B+ on the filament winding, so the leakage to ground will not show up.

You can disconnect each filter cap one at a time, but it would be better if you temporarily replaced it with another cap. Otherwise you will subject the speaker and other components to a lot of AC ripple voltage.

Since the problem still shows up without the rest of the tubes installed it would be better to test with just the 5Z3 plugged in.

[Crist Rigotti]

Guys,
Thanks for your input. On a hunch, I disconnected C104 a 8uf filter cap. Yeah! The over current is gone. So I clipped in a replacement and the radio is playing just fine. I then did a permanent repair and as I type, the radio is playing well. I'll play it for an hour or so.

I gotta go out and buy a lottery ticket!

Here is a schematic of the radio.

[Image: http://i1060.photobucket.com/albums/t458...3kfmgb.jpg]

Here is a close up of the schematic. I replaced the single 8uf e-cap C104.

[Image: http://i1060.photobucket.com/albums/t458...kmegle.jpg]

[Mondial]

It would make a lot of sense to use a polypropylene film 8 uF cap for C104, the first filter after the rectifier. This radio requires a lot of DC B+ current and a modern 8 uF electrolytic won't last long in this location because of the high ripple rating needed.

This is what I would use, it will basically last forever:

https://www.tubesandmore.com/products/ca...ylene-fast

[Crist Rigotti]

Wow, what you say makes sense. Would going to a larger uf be bad because I have a faint 60 cycle hum in the speaker. How much larger?

Thanks.

[Mondial]

You really don't want to go more than 10 uF for the first filter cap C104, because its value affects the voltage output. If you have residual hum, you can increase the value of the second filter cap C103 within reason, which will reduce hum without affecting the voltage. I usually go to a 22uF to 33 uf electrolytic for the second cap.

I typically use the 10 uF polypropylene for the first filter, as it is still small enough to fit within the original cap can. That's the cap subject to the most stress from the high ripple current. The remaining filters see very little ripple, so electrolytics are fine there.

Have you adjusted the hum pot 106? This sets the balance between the the two 6A3 tubes and should be set for minimum hum.

[Crist Rigotti]

OK, I'll stick with a 8.2uf and go higher for the 2nd cap.

Yes, I adjusted the hum control. The hum is noticable when warming up before a station is received.

Thanks for the info.

[Phlogiston]

It would make a lot of sense to use a polypropylene film 8 uF cap for C104, the first filter after the rectifier. This radio requires a lot of DC B+ current and a modern 8 uF electrolytic won't last long in this location because of the high ripple rating needed.

This is what I would use, it will basically last forever:

https://www.tubesandmore.com/products/ca...ylene-fast

If there is one thing I could do for other restorers is to impress this point upon them.

I will add: You can usually get a 10uf in those cans. This is the spec for a 80 tube and may reduce hum somewhat.

If you can use these caps in all filters - great! If you just have to save some money (6$ - 8$) use them in the first filter.

After 1930s radios the uf values of caps increase greatly but the B+ also tends to be less. We are stuck with electrolytics here.

[Mondial]

Russ, I have to thank you for introducing me to these PP filter caps. I learned about them in one of your postings a few years ago and have been using them ever since.

They are especially useful when restuffing an electrolytic can, where you never really want to open it up to replace the cap again. Ripple current and ESR are no longer a problem.

I measured less than 0.1 ohms ESR with a 10 uF polypropylene, compared to 4 to 6 ohms with a 10 uF 500V electrolytic. Big difference