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Electronics
Tinkering success!
I cobbled together a good power supply for the iron that Mark Balliet gave me! I was having a hard time finding a barrel jack on a thrift store transformer that matched it, but eventually I bought a cheap little 7.5 volt, 3 watt wall wart with the right barrel, a Sharp power supply from something-or-other that provides almost 6 amps at 12 volts, and soldered the cord from the little one into the big one. I had to resort to a little saw on my multi-tool to get the little one open, but the sharp is in a much bigger package with press-fit snaps and screws and opened much more straight-forwardly. And actually went back together, unlike the other one!
How did I use the soldering iron without a supply for it? I found another wall wart with a jack that didn't actually insert much at all into the iron's receptacle, and doesn't provide near as much power as the new one does, but it made just enough of a connection to make it work until the job was done.
More about the Heathkit I'm trying to fix . behind the cut.
I am totally stuck on the Heathkit volt-ohm meter. Someone on a Reddit forum suggested a Heathkit mailing list, so I may try asking folks there.
Replacing Q1 and Q2 did not change anything. I tried replacing C3 and that didn't help (though I'm not sure how critical the value is). Voltage on Q1 source / Q2 Base is still goes from full positive rail (about 9 volts) with the bias adjust set to one extreme and down to 8.57 at the other bias adjust extreme.
I removed Q2 entirely in case voltage was leaking in through it somehow, then did two rounds of testing: one with Q1 gate disconnected and one with Q1 source disconnected. Behavior was consistent.
Bias adjust can move Q1 gate voltage from 2.37 volts (corresponding to full positive rail at Q1 source) to about 0.32 volts (corresponding to Q1 source 8.57 volts). If I tie Q1 gate to circuit ground, Q1 source drops to 7.56 volts, which is still way higher than the apx. 5 volts the circuit needs to balance the bridge.
I'd think that maybe the JFET I used as a replacement for Q1 has the wrong values, but this is the same behavior I was getting with the original Q1. Is this what this component is supposed to do?
I found a detailed review and explanation of the IM-17 here: https://www.w6ze.org/Heathkit/Heathkit_078_IM17.pdf
But it doesn't help me figure out my issue. I also tried watching a couple of Youtube videos of people working with or (supposedly) disassembling and restoring the IM-17, but they were not enlightening.
I cobbled together a good power supply for the iron that Mark Balliet gave me! I was having a hard time finding a barrel jack on a thrift store transformer that matched it, but eventually I bought a cheap little 7.5 volt, 3 watt wall wart with the right barrel, a Sharp power supply from something-or-other that provides almost 6 amps at 12 volts, and soldered the cord from the little one into the big one. I had to resort to a little saw on my multi-tool to get the little one open, but the sharp is in a much bigger package with press-fit snaps and screws and opened much more straight-forwardly. And actually went back together, unlike the other one!
How did I use the soldering iron without a supply for it? I found another wall wart with a jack that didn't actually insert much at all into the iron's receptacle, and doesn't provide near as much power as the new one does, but it made just enough of a connection to make it work until the job was done.
More about the Heathkit I'm trying to fix . behind the cut.
I am totally stuck on the Heathkit volt-ohm meter. Someone on a Reddit forum suggested a Heathkit mailing list, so I may try asking folks there.
Replacing Q1 and Q2 did not change anything. I tried replacing C3 and that didn't help (though I'm not sure how critical the value is). Voltage on Q1 source / Q2 Base is still goes from full positive rail (about 9 volts) with the bias adjust set to one extreme and down to 8.57 at the other bias adjust extreme.
I removed Q2 entirely in case voltage was leaking in through it somehow, then did two rounds of testing: one with Q1 gate disconnected and one with Q1 source disconnected. Behavior was consistent.
Bias adjust can move Q1 gate voltage from 2.37 volts (corresponding to full positive rail at Q1 source) to about 0.32 volts (corresponding to Q1 source 8.57 volts). If I tie Q1 gate to circuit ground, Q1 source drops to 7.56 volts, which is still way higher than the apx. 5 volts the circuit needs to balance the bridge.
I'd think that maybe the JFET I used as a replacement for Q1 has the wrong values, but this is the same behavior I was getting with the original Q1. Is this what this component is supposed to do?
I found a detailed review and explanation of the IM-17 here: https://www.w6ze.org/Heathkit/Heathkit_078_IM17.pdf
But it doesn't help me figure out my issue. I also tried watching a couple of Youtube videos of people working with or (supposedly) disassembling and restoring the IM-17, but they were not enlightening.