I've got a resistor that keeps frying- can anyone help? - Videokarma.org TV - Video

fltyrap · #1 12-29-2008, 10:34 AM

Hi,

I've finished recapping a Zenith C845 (paper and electrolytic caps), but there is one resistor that was burning up before the recapping that is still frying. It's a 680 ohm, 1/2 watt resistor that is on the line running into the grid of a 6BJ6 RF tube. I was hoping the recapping would solve this, but no luck.

I've attached a scan of the schematic here, with the troublesome resistor pointed out by a black arrow. Any ideas what would be causing this? Any obvious suspects based on the schematic? I thought maybe it was the extra juice from the silicon diode, but I put 100 ohms on the line and it still burns up.

Even with the resistor smoking, the radio powers up, the tubes light, and sound comes through the speakers (albeit not so great sound).

jeyurkon · #2 12-29-2008, 10:46 AM

Did you replace C7?

Could also be a shorted 6BJ6.

John

marty59 · #3 12-29-2008, 10:52 AM

Even take C7 and lift one end and see what happens as well as check voltages at that tube socket...and yes, tube ok? May need to swap it out regardless.

toxcrusadr · #4 12-29-2008, 01:34 PM

+1 on all that advice. I've found that smart people almost always tell me to check voltages at the socket, and since they're shown on your schematic, it's easy to see if they're wonky.

ke4jhj · #5 12-29-2008, 10:51 PM

I agree with the above advice. Either C7 or the 6BJ6 is shorted or there is a problem with the wiring at the tube socket. Good luck.

Reece · #6 12-30-2008, 12:56 PM

Agree with all above suggestions to check. Either the cap, the tube, or the tube socket is at fault. Sometimes there may be internal charring between layers of a tube socket that short out a terminal. Pull the tube and check with your ohmmeter to see if pin 6 is shorted to any other pin: should not be. If cap is OK, unsolder the wire(s) from pin 6 and check ohms between that and all other pins and to ground. Should have infinite ohms.
Reece

avionic · #7 12-30-2008, 01:12 PM

Quote:

Originally Posted by Reece

Agree with all above suggestions to check. Either the cap, the tube, or the tube socket is at fault. Sometimes there may be internal charring between layers of a tube socket that short out a terminal. Pull the tube and check with your ohmmeter to see if pin 6 is shorted to any other pin: should not be. If cap is OK, unsolder the wire(s) from pin 6 and check ohms between that and all other pins and to ground. Should have infinite ohms.
Reece

Sage advice...

Reece · #8 12-30-2008, 01:15 PM

Avionic wrote:
Sage advice...

***************
You callin' me sage, boy?

Reece

cooljjay · #9 12-30-2008, 11:14 PM

Make sure no one soldered the 2 pins together on the tube, also check for solder drops, they can sometimes get lodged between the pins or a cap/resistor and chassis. You may already know but if not, make sure to disconnect the old can cap and check the pos/neg of the ecaps to make sure they are correctly installed.

fltyrap · #10 12-31-2008, 08:18 AM

Thanks for the advice, everyone. C7 was hidden by a metal shield of some sort. I removed this and it appears that the body of the capacitor (ceramic tubular) is melting into another socket tab.

Can I replace a tubular ceramic capacitor with an orange dip film capacitor in this application? I don't have any .001 uF on hand, but would a .022 work?

jeyurkon · #11 12-31-2008, 09:01 AM

It will probably work, though ceramic would be better. It shouldn't hurt anything to try it.

John

Chimes · #12 12-31-2008, 12:02 PM

Quote:

Originally Posted by fltyrap

Thanks for the advice, everyone. C7 was hidden by a metal shield of some sort. I removed this and it appears that the body of the capacitor (ceramic tubular) is melting into another socket tab.

Can I replace a tubular ceramic capacitor with an orange dip film capacitor in this application? I don't have any .001 uF on hand, but would a .022 work?

A .022 will work just fine.

fltyrap · #13 01-01-2009, 12:24 PM

Hey, replacing C7 worked like a charm! No smoke, great sound. I can't believe it. I'll post some pics when I get everything back in the cabinet and the whole thing is polished up.

Thanks to everyone for the great advice in this and other threads.

Jeffhs · #14 01-01-2009, 02:44 PM

You have an excellent radio. The C845 was one of Zenith's best table sets of the late 1950s-early sixties. With an 8" main speaker, 5" tweeter, and tone/loudness compensation, not to mention a true tone control (it changes the entire response curve of the audio stage, rather than simply cutting down the highs), this set sounds great. I have one and use it quite a bit here; sounds much better than most of my other sets, especially my solid-state portables, with the exception of my Zenith Royal 1000 T/O and R-70. With an RF stage and two IF stages on both AM and FM, the C845 is very sensitive and selective; using only the built-in antennas on mine, I can get stations from 60-70 miles away much of the time (and all Cleveland stations 24/7). I live very near the south shore of Lake Erie, so I often hear stations in southwestern Ontario, Canada and Detroit on this radio when the FM band opens up.

Your set should work just as well. Being in Washington, DC, you should be able to get stations on your C845 from that city, its suburbs in Virginia and Maryland, and most if not all major Baltimore FMs as well. In fact, as sensitive and selective as the C845 is, you may also find yourself getting stations up and down the Eastern Seaboard. Put a really good antenna on your C845 and you may hear stations up and down the East Coast as well. Zenith did not skimp on anything when they designed this radio; as I said, it was one of Zenith's best table model radios in the late '50s through about 1961. As I always say about radios made by the former Zenith Radio Corporation of Chicago, they don't make them like that anymore.