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#1
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Help Needed on Halli SX 140
Hello to the group,
I have been restoring a Hallicrafters SX 140 and it now receives well and is quite sensitive, but the S meter barely moves. Is this normal for this receiver or should it move quite a bit like most other Halli's? Does anyone know if there were any errors on the schematic or if this radio has any trouble spots? Any hints? 73 Mike KF6KXG |
#2
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Help Needed on Halli SX 140
tchrme wrote:
Hello to the group, I have been restoring a Hallicrafters SX 140 and it now receives well and is quite sensitive, but the S meter barely moves. Is this normal for this receiver or should it move quite a bit like most other Halli's? Does anyone know if there were any errors on the schematic or if this radio has any trouble spots? Any hints? 73 Mike KF6KXG Look at the voltage on the AGC line and see. If the AGC voltage is in the ballpark, there's an issue with the meter or the resistors that feed it. If the AGC voltage is way off, the receiver may be TOO sensitive. --scott -- "C'est un Nagra. C'est suisse, et tres, tres precis." |
#3
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Help Needed on Halli SX 140
A short while back I posted my findings on an SX-140 resurrection.
You should be able to find it by searching here. Silver mica disease is all too prevalent nowadays so if thats not an issue for you then the rest is pretty reliable. As for the meter...I really haven't paid much attention to it because its such a cheap bouncy thing but it does seem to read about what I would expect. Bill W X 4 A |
#4
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Help Needed on Halli SX 140
On Feb 21, 11:30 am, exray wrote:
A short while back I posted my findings on an SX-140 resurrection. You should be able to find it by searching here. Silver mica disease is all too prevalent nowadays so if thats not an issue for you then the rest is pretty reliable. As for the meter...I really haven't paid much attention to it because its such a cheap bouncy thing but it does seem to read about what I would expect. Bill W X 4 A Thank you Scott and Bill for your suggestions. After some more time spent on it, the S meter will read almost full scale when picking up a nearby sig gen or for the built in crystal calibrator, but barely moves on other signals. I replaced a tube, most caps, the out of spec resistors and checked the pots. Could the caps in the IF's be bad if the audio reception works well but the S meter only work on very strong signals? I thought the symtom of bad IF caps is thunder crashes and poor audio. I haven't found anything pulling down the AVC. 73 Mike KF6KXG |
#5
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Help Needed on Halli SX 140
"tchrme" wrote in message ... On Feb 21, 11:30 am, exray wrote: A short while back I posted my findings on an SX-140 resurrection. You should be able to find it by searching here. Silver mica disease is all too prevalent nowadays so if thats not an issue for you then the rest is pretty reliable. As for the meter...I really haven't paid much attention to it because its such a cheap bouncy thing but it does seem to read about what I would expect. Bill W X 4 A Thank you Scott and Bill for your suggestions. After some more time spent on it, the S meter will read almost full scale when picking up a nearby sig gen or for the built in crystal calibrator, but barely moves on other signals. I replaced a tube, most caps, the out of spec resistors and checked the pots. Could the caps in the IF's be bad if the audio reception works well but the S meter only work on very strong signals? I thought the symtom of bad IF caps is thunder crashes and poor audio. I haven't found anything pulling down the AVC. 73 Mike KF6KXG Bad tuning caps in the IF can cause frequency instability. The tuning may change from cold to warm. This is typical of bad mica caps. Bad by-pass or decoupling caps can cause instability in the form of oscillation, low gain and other problems. Paper type caps can go bad with age and should be replaced with more modern ceramic or plastic types. I think the mica cap desease mentioned above may be a rapid instability which shows up as an irregular warbling where the caps are in the local oscillator. This is sometimes called scintellation. I am not sure of the cause but it seems fairly common in silvered-mica types in molded cases. -- --- Richard Knoppow Los Angeles, CA, USA |
#6
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Help Needed on Halli SX 140
There's several different bad modes of failing/failed mica caps in the
IF xfmrs. As related to gain as long as the xfmr shows good peaks during alignment the value should be correct. The thunderstorm noise and intermittent operation usually occur before the thing goes completely open. Are you getting regeneration? I found I could not achieve regeneration until everything was quite up to par. That seems like a good measure of its overall health so if there are still problems with the meter in the AM mode that might be localized to something more specific than general IF gain. -Bill |
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