Hello,
After cleaning the chassis installed a new dial light and cord, I replaced all paper caps, electrolytic caps, and some crumbling wiring. I powered up the unit via a dim bulb tester and everything seemed to be coming to life ok. Then the 4700 ohm resistor (#29) tied to the 6J8G DET-OSC tube began to smoke! I shut down the unit, replaced the resistor and began to troubleshoot.
I rechecked all other resistors and made sure all caps were correctly wired.
I removed the 6J8G and the resistor still got very hot - shut the unit down - checked the make sure the resistor was still good.
I replaced the 6J8G and removed the 1232 tube and the resistor still got very hot - checked the make sure the resistor was still good.
I replaced the 1232 and removed the 7B7 tube and the resistor still got very hot - checked the make sure the resistor was still good.
I replaced the 7B7 and tore apart the IF can (#30) and replaced the old rubber wires.
Powered the unit up and the resistor still got very hot.
Please let me know what next steps I could take to figure out where the problem is.
Many thanks!
Check the wiring to the 6J8 tube socket. There should only be two connections to pin 6. One will be the 4.7K resistor 29 and the other should be the 250 pf cap 22 going to the push button switch. If you have any additional connections there is a miswiring problem.
With the 6J8 out of its socket, the only other part connected is the 250 pf cap and that should not draw any current to overheat the resistor. You must have a short in the associated wiring or something is miswired.
Hi Mondial.
Thank you for the reply!
I had to replace the 6J8 tube socket as you may be able to see in the picture.
I also happen to have four other 40-180 radio chassis, of which, 2 are complete and work fine, and the other two are in line for renovation. So, I have quadruple checked the 6J8 wiring and cannot see a problem. I did find that I miswired cap 10 to the 1235, but after correcting this, and firing up the unit, resistor 29 still smoked.
Stumped.
One of the ways to find a persistent short is to start immediately after the resistor that is getting warm, with the schematic beside you, then follow and disconnect one wire, one component at a time, followed by a test each time. If nothing changes, reconnect the wire or component and go to another branch of wiring. If the resistor stops overheating, keep following that particular path. Don't skip any wire or component. Just disconnect, test, reconnect until you find the short.
That sounds like a plan - thanks John!
Looking at your photo of the 6J8 socket. I see pin 6 connected to the 4.7K resistor and a red wire. What does the red wire connect to? It should only connect to the 250 pf cap, nothing else.
Its hard to tell from the photo but it looks like the red wire connects to a terminal with more than one connection (brown wire?). The red wire should only connect to the 250 pf cap.
As John said, disconnect anything associated with the 4.7K which gets hot. Start with the red wire connected to pin6 of the 6J8 and follow it to the terminals it connects to.
I disconnected the red wire and the resistor does NOT get hot.
The red wire connects to a terminal that has a 150 ohm resistor connected to it. The other end of the 150 ohm resistor connects to a terminal that has the 250 pf (#22) cap connected to it. This design is seen on all of my other 40-180 units.
Bad #22?
Forgot to mention that the 150 ohm resistor and 250 pf cap are connected to the pushbutton switch. This connection was never touched...
It was difficult, but I disconnected the 250 pf cap (#22) from the unit. It tested okay.
I reconnected #22 to the 150 ohm resistor which has the red wire attached at the other end.
I tested the unit and #29 resistor was operating fine.
I cleaned the junction where I disconnected #22, reconnected #22, rearranged some wires...
IT WORKED!
Attached the antenna, and was able to tune to numerous stations.
Burn-in is next...
Thank you all for the assistance! Much appreciated!
So, I've been following along and it's nice that the problem is resolved but I don't see any 150 ohm resistor in the schematic where he describes it. The only 150 ohm resistor near by is the cathode bias resistor. What's happening?
Seems that a 150 ohm resistor was added in series with the 250 pF cap connected to pin 6 of the 6J8. Appears to be an undocumented production change as it does not appear on the schematic.
Rob - you are correct. There are no other 150 ohm resistors in the schematic besides the cathode bias resistor. I currently have 6 of these radios and they all have this 'extra' 150 ohm resistor where I indicated! I can only speculate that it was a mid-stream manufacturing change that did not get into the schematic, or, the schematic I have is an earlier (or later) version then what my radios have.
Someone smarter than me will have to deduce why this design was implemented.
Excellent find! Mystery solved.