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Old 13th August 2018, 07:55 PM   #331
Wavebourn is offline Wavebourn  United States
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Originally Posted by DF96 View Post
As a general rule it is best not to apply AGC to mixers. The ideal mixing device has a square-law characteristic. Reducing gain automatically reduces the maximum signal it can handle, because you are nearer the cutoff.
I prefer to apply AGC before the mixer, in RF amp/preselector. For better selectivity and less of unwanted results of intermodulation. Also, application of AGC to the mixer can be reflected on the frequency of heterodyne.
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Old 15th August 2018, 03:45 PM   #332
w5jag is offline w5jag  United States
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The 1 uF smoothing / filter capacitor on the AGC line was bad. It tested OK out of circuit, but was not.

I replaced it and this restored normal operation. Resting voltage is now - 0.06 volts, and, using the FY6600 as an over the air test at 9 MHz, increasing signal strength would drive it to -0.6 volts, so seems to be working properly. It could be trimmed to exactly zero by adjustment of that 100 pF sampling capacitor. Normal background noise begins AGC operation, so it is sensitive enough as is.

This is the second time that I have observed a bad electrolytic capacitor to cause an excessive dc voltage on the circuit it is serving. I don't understand this failure mechanism. As a lay person, it is counter intuitive.

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Old 16th August 2018, 11:20 AM   #333
DF96 is offline DF96  England
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There are vague similarities between an electrolytic and a cell: both have metals separated by an electrolyte.
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Old 16th August 2018, 12:44 PM   #334
Tubelab_com is offline Tubelab_com  United States
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Sketchy electrolytics can cause all sorts of weird problems. Sometimes they act like caps at one frequency, but are open or inductive at another. Sometimes they can even act like crummy diodes.

This can let high frequency crud through confusing the circuit or your voltmeter. It can also create DC from RF energy. This is particularly problematic when the suspect cap is in a feedback loop like an AGC circuit.


I got a batch of 470 uF 35 V electrolytics long before the counterfeit parts issue (early 80's), but they were junk. I was trying to use them for filter caps in the power supply for a TV microwave receiver. The DC voltage was OK but there were weird noises in the TV picture or sound caused by the caps in about 10% of the units. Others had frequency drift. Replacement caps fixed the units.

This was not an oscillating regulator since there was none, but bad electros are notorious for making 78XX or LM317 regulators oscillate.

They wouldn't work for cathode bypass caps in small guitar amps either (bad sound). The only use I found for those caps were electric firecrackers!
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Old 17th August 2018, 11:15 PM   #335
Wavebourn is offline Wavebourn  United States
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Quote:
Originally Posted by DF96 View Post
There are vague similarities between an electrolytic and a cell: both have metals separated by an electrolyte.
Aluminium - oxide capacitors (AKA electrolytics) have conductors separated by an oxide film. They act actually as diodes between layers of an inductor, in series with capacitors.
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Old 18th August 2018, 01:49 AM   #336
gerhard is offline gerhard  Germany
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yes. P - Al2O3 film - N diodes.

you can fool all the people all the time, can you?

Gerhard
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Old Yesterday, 02:52 PM   #337
w5jag is offline w5jag  United States
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This is a double conversion setup with the HP mixers for testing. Still needs quite a bit of work.

No first IF selectivity yet.

Could need an RF preamplifier. Hard to say, yet - when driving the FT-817 as a back end, the overall noise combination of the two is misleading.

An LM375 based oscillator had no trouble driving the low impedance of the HP chip, but the simple Pierce oscillator shown here, and used on the other board, needed a buffer to be able to drive the chip. That part still needs some refinement, or I might use an LM375 here.

Not sure about optimal LO drive levels.

The TX part of the Mizhuo board seems to work, in that grounding the keying line energizes the transmitter, and putting a 1 KHz tone into the mic input yields a 9 MHz SSB supressed carrier signal, modulated at 1 KHz, at the output.

I can't start on the TX part of the transverter, because I am still undecided as to whether or not the RX will stay a double conversion up converter, or a simple single conversion down converter. The latter seems more appropriate for a simple one or two band mobile radio, but no decision has been made yet.

It occurred to me while mowing the other afternoon, that one of the 54 MHz crystals discussed above, mixed with the traditional 5 MHz LC VFO or even a VCXO, could make a clean 59 MHz tunable LO for a twenty meter up converter to 45 MHz, and get rid of that DDS whatever. So there is also that.

So no real point in working on the TX until some decisions are made.

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Old Yesterday, 06:03 PM   #338
jackinnj is offline jackinnj  United States
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anyone need a 1N263 mixer diode?
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