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Old 16th September 2018, 04:56 AM   #371
w5jag is offline w5jag  United States
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It took quite a bit of hot air, but the SMA fittings came off without any damage to the board.

I normally bypass the MW filter, but with this broadband amp, reception is significantly improved with it in place. It might be unneeded with a tuned antenna.

It seems stable over a wide range of voltage. The listing suggested the gain could be varied with applied voltage; I'm not really seeing that with the current test setup. I may try to measure it at some point.

Seems worth the 6 bucks or so it cost - can't buy the parts for that. Likewise that AD831 mixer board - the AD831 chip is almost fifteen bucks at Mouser. Assuming the chip on the board is genuine.

Win W5JAG
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Old 23rd September 2018, 09:17 PM   #372
w5jag is offline w5jag  United States
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This is the ebay Analog Devices AD831 prefab mixer board, behind the narrow band MOSFET RF amp.

There are some things I really like about this - low LO drive, preassembled, cheap price. Some things I don't like are the size of the module, and it has a bone crushing current requirement - 125 ma. It also has a narrow operating voltage range, 9-11 VDC. Right now I am running it at 8 VDC, and it seems to work fine, even though it is out of spec. I ran it at 10 VDC initially, and didn't like how warm it ran.

This setup works pretty well, I expect it would be satisfactory in actual usage.

I don't have enough room on this board to make double conversion with these prefab boards, so I need to get another protoboard, or just stick with single conversion, if I go with the prefab modules. I've been leaning toward single conversion for a while since this will be a single band radio, at least in the first version.

Win W5JAG
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Old 30th September 2018, 12:00 AM   #373
w5jag is offline w5jag  United States
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Quote:
.... This setup works pretty well, I expect it would be satisfactory in actual usage. .....
Been using it for a week or so; my impression is still the same - the cheap AD831 mixer board with the narrowband 40673 RF amp and SG-9 IF board makes a very satisfactory single conversion radio. The sensitivity is good, the AD831 doesn't seem to care that it is not matched on either port, it doesn't show any sign of overload on big signals; no indication of local broadcast stations breaking through, AGC action is good, audio quality is good.

Actually, it is kind of refreshing to listen to a radio that doesn't go through multiple conversions, filters, DSP, etc. like has been the norm for at least the last couple of decades. They do sound different.

Other than a better tunable LO, I'm not sure there is much, or any, practical improvement to be had here, given that the end product is to be a single band mobile rig.

I was getting some AF ringing with those twisted wires for the low level audio leads, so I replaced all of those with shielded cables. The NEC AF chip on the SG-9 has 50 dB gain according to the data sheet for the ECG equivalent, so not too surprising it would ring. The shielded cable cleaned that up.

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Old 30th September 2018, 01:58 AM   #374
PRR is offline PRR  United States
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I am not an RF guy, but a friend brought me a cross-needle SWR meter because it looked like something I might like. Not quite the same as this model (attached). A real elegant way to do computation of a ratio.
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Old 30th September 2018, 02:47 AM   #375
PRR is offline PRR  United States
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Found it: Workman HP202s -- 26-30MHz.
Workman HP202S SWR / Watt Meter Review
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Old 30th September 2018, 03:41 AM   #376
anatech is offline anatech  Canada
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No RF gear here?
Hi PRR,
Those have been out for quite some time now. Yes, they are kinda cool.

So, are you going to get your HAM license or chicken bander license?

A lifetime ago I had a CB home station and a mobile as well. That meter would have been the thing to have back then. I was 15 ~ 17 at the time. I even have the antenna lead still, I couldn't bear to throw it out. RG-6 might have been. I should take a look at it. I terminated it with PL-259 connectors (soldered).

-Chris
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Old 2nd October 2018, 03:52 AM   #377
PRR is offline PRR  United States
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Quote:
Originally Posted by anatech View Post
Those have been out for quite some time now.
Decades for sure.

The brand on mine may have been defunct in 2004, and they sold it for years before that. There's several/many other minorly-different products, like my attach. I had never seen one up close though. Last-chance, because the few who still need to know an SWR can buy a digital cheaper.

Ah, but mine is tagged $3.

What occurs to me: good audio needle-meters are becoming scarce. This is two fairly sensitive meters in one: Stereo! Hack the RF stuff off. It should be possible to read >1W speaker levels with simple diode circuit. Line level wants a 19-cent buffer-rectifier. The meters are linear so only 20dB range, but there's a 3-way switch so 30 or 50dB can be covered.

Lastly: if the several internal diodes are Germanium there are people who will pay the $3 price just for the few Ge diodes. Shotkees are not-the-same in fuzz boxes.
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Old 2nd October 2018, 10:37 AM   #378
jfetter is offline jfetter  United States
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I have an amateur extra license but still buy pretty much any CB gear at garage sales.
Over the years have picked up a Galaxy SSB and Motorola MoCat etc.
Found a couple of linear amps too. One is 250 watts. Of course they are grandfathered for selling and are illegal to use. I've picked up a couple of zenith transocean SWL receivers too.
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Old 8th October 2018, 03:10 PM   #379
w5jag is offline w5jag  United States
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During extended "burn in" testing of the TX, a switching transistor failed, and had to be replaced.

The pic is RF TX output. The second harmonic seems high. After the filter, the TX signal passes through a single tuned MOSFET RF amp, and then an untuned JFET buffer to the outside world. Drive is a 1 KHz sine wave from my audio generator. The SG-9 is not being overdriven; in fact drive level seems to make no difference. The second harmonic stays at - 26 dB at any drive level that I can see within the 8 bits of the FFT.

I've decided the RF performance of the MOSFET preamp / AD831 / SG-9 is completely adequate for the intended application. The AD831 looks to have a somewhat high SSB NF, but this does not seem to be a real world issue in a single conversion configuration, when preceded by the reasonably quiet RF amp.

I've thought some about the TX part of the transverter, but haven't melted any solder yet.

edit: Is NF anything I should be concerned with in a TX mixer?

Win W5JAG
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Old 11th October 2018, 04:31 AM   #380
w5jag is offline w5jag  United States
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I had a generic NPN transistor tacked on the back of the board as a replacement for the bad switch; tonight I found an old pull out part that had the correct ECB pinout, and made a proper repair.

I'm not all that impressed with the frequency response of the transmitter; playing with it some, it seems flat from about 600 Hz to 1700 Hz, and -3 db from about 320 Hz to 2800 Hz. I'm not sure if this is the actual filter response, or the audio channel response to give it a bit more apparent talk power. On receive, it does not sound rolled off on either end of the passband. I haven't tried to measure the tx audio channel; the board is packed tight - I would probably have to solder in a test point on the back of the board to be able to clip up a scope probe.

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