Hello All,
I just restored a Zenith H500. But it has a problem that's probably well known. When listening to a SW band, the station gradually fades away, and so does the entire band, until it's dead. I can try pushing the buttons and going back to that band, and it doesn't help. But what does help is going to the BC band, tuning to a very strong strong station and let it play for a few minutes. Then when I go back to the dead SW band it's alive again for a short time. The longer I leave it on a strong BC station, the longer the SW will stay working.
I replaced all paper and electrolytic capacitors and the rectifier. The BC band never fades out. There is a 1L6 in the socket. I can always bring back a SW band by doing this. How it "re-energizes" the band, I have no idea. But my guess is that it stops oscillating (oscillation collapses) on the SW band, and going to the lowest band, starts back it up again.
If this is a known problem, please help.

Thanks,

Dave

Sounds like a tube is drifting into its cutoff region. This is just a shot in the dark, but you might try disabling the AVC bus and seeing if your problem disappears. If so, you might have a bad/mis-connection somewhere on the bus, or the AVC cap is the incorrect value.

Presumably you replaced the original selenium stack rectifier with a silicon device. This can boost your B+ voltage and cause biasing issues. Not as much of an issue with this transformerless set, but still something to check out.

Hello Again All,
It was my choice of current limiting resistor I was using with the 1N400x diode. The 75 ohm section of the original resistor seems to work ok. Usually I use a 100 ohm resistor in other radios. 75+55 ohms was too high. What does everyone else use as the current limiting resistor in the H500?

Thanks,

Dave
:Hello All,
: I just restored a Zenith H500. But it has a problem that's probably well known. When listening to a SW band, the station gradually fades away, and so does the entire band, until it's dead. I can try pushing the buttons and going back to that band, and it doesn't help. But what does help is going to the BC band, tuning to a very strong strong station and let it play for a few minutes. Then when I go back to the dead SW band it's alive again for a short time. The longer I leave it on a strong BC station, the longer the SW will stay working.
: I replaced all paper and electrolytic capacitors and the rectifier. The BC band never fades out. There is a 1L6 in the socket. I can always bring back a SW band by doing this. How it "re-energizes" the band, I have no idea. But my guess is that it stops oscillating (oscillation collapses) on the SW band, and going to the lowest band, starts back it up again.
: If this is a known problem, please help.
:
:Thanks,
:
:Dave
:

:Hello Again All,
: It was my choice of current limiting resistor I was using with the 1N400x diode. The 75 ohm section of the original resistor seems to work ok. Usually I use a 100 ohm resistor in other radios. 75+55 ohms was too high. What does everyone else use as the current limiting resistor in the H500?
:
:Thanks,
:
:Dave
::Hello All,
:: I just restored a Zenith H500. But it has a problem that's probably well known. When listening to a SW band, the station gradually fades away, and so does the entire band, until it's dead. I can try pushing the buttons and going back to that band, and it doesn't help. But what does help is going to the BC band, tuning to a very strong strong station and let it play for a few minutes. Then when I go back to the dead SW band it's alive again for a short time. The longer I leave it on a strong BC station, the longer the SW will stay working.
:: I replaced all paper and electrolytic capacitors and the rectifier. The BC band never fades out. There is a 1L6 in the socket. I can always bring back a SW band by doing this. How it "re-energizes" the band, I have no idea. But my guess is that it stops oscillating (oscillation collapses) on the SW band, and going to the lowest band, starts back it up again.
:: If this is a known problem, please help.
::
::Thanks,
::
::Dave
::The 1L6 tube was designed by raytheon especially for ZENITH. The 1L6 tube does a MUCH, MUCH better job at the higher frequencies than the 1R5. In the older days ZENITH had a lot of trouble with station drift on the higher frequencies. Wasn't very stable so ZENITH got someone to correct the situation & RAYTHEON came up with an answer. But the cost was high & ZENITH didn't care about the cost; THEY WANTED QUALITY. And the surprising thing is they still sold! But at that time people didn't know about the technacalities, they wanted something that was stable & ZENITH gave it to them. When new the 1L6 was only about 30% higher than a 1R5. Now it's probably about 200% higher because of supply & demand. A 1L6 can be replaced by a 1R5 but there might be some wiring problems, for instance lack of space to work in. Another thing to do is to make a tube socket adapter (pin changer) from an old tube socket. MAKE SURE THE WIRES ARE INSULATED!!!! I made one & it works "pretty much OK". But DOUBLE & TRIPLE check your work!!!! The 1R5 & the 1L6 are a close match. The difference was that the 1L6 was better on higher frequencies than the 1R5 & ZENITH insisted on QUALITY as everyone knows!
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