w8cbc
07-09-2006, 05:27 AM
This review will mostly be a comparison with the later 751A.
I was looking for some gear to keep at the place in northeastern VE3 so I wouldn't have to haul the 751A there and back once or twice a year, subjecting it to being banged around and temperature-stressed. I picked this one up in eBay.
I figured there couldn't be much difference.
Wrong.
First of all, I had a fair measure of fixing and aligning to do. In addition, the accompanying schematic doesn't quite match the circuitry in some places so there was some additional figuring-out to do. This has nothng to do with the design differences of course but it sets some context.
The major design differences are:
The 751's final IF stage is 350 kc, not 9 MC. There's no filtering there whereas the 751A has a 3 kc filter for its PBT (SSB/CW).
There's no negative feedback in the first mic amp; no RF bypassing there either. Also, the mic amp starts with high-frequency pre-emphasis and the transmit tone control counteracts it. In the 751A, it's designed to be flat with a high-frequency boost/cut after it.
Metering circuitry is also rather different. The functions are the same but there was a redesign in the 751A to make them much more stable and meaningful.
Filter switching is another thing that was updated in the later version.
So.
Receiving:
Sensitivity and selectivity just meet spec.
The stock filtering does not include a 500-cycle CW filter.
There's an optional 6 kc AM filter in the 9 MC IF stage that doesn't make much of a difference over the stock 6 kc filter in the 455 kc stage.
Dynamic range is somewhat less than the 751A. This 751 will overload from very strong signals.
The noise blanker works a little better than the 751A's. It'll get rid of some power-line buzz that the 751A's can't quite handle.
The 751's notch filter is quite sharp and is a bit shallower than the 751A's. This one doesn't do as much for generalised noise.
Filter switching is rather limited in the optional configuration as I got it. Each mode (except FM) has the same two combinations - AM (6 kc) and SSB (2k3), both double-filtered. In SSB/CW, PBT works at 2k3 but not 6k.
I changed this. The 9 MC filter is now selected by mode - 2k3 for CW/SSB, 6k for AM. The filter switch selects the 455 kc filter - 2k4 or 6k. There is a small difference in SSB as the double-filtering makes for steeper skirts. In AM, the SSB filters were offset, resulting in a bandwidth of just over a kc. Now I get either 2k4 or 6k. This modified behaviour mimics the 751A's as far as it goes. I'll need a FL-52A to get proper CW selectivity (the "CW Narrow" mode).
For FM, the bandwidth is 15 kc, same as the 751A. There's no difference in FM reception as far as I can tell.
Frequency stability is somewhat worse than the 751A. The reference oscillator is solid but those BFOs drift some. It's not horrible - maybe +/- 50 cycles - but it is noticeable.
Preamp and attenuator both have less effect than their later versions.
Receive audio is full and clean, just like the 751A.
I could not get the S meter to line up accurately. The necessary adjustments aren't there. It reads 10 dB high at S9, 10 dB low at +40, and rather arbitrarily outside that range.
Transmitting:
Power output is to spec (105 watts maximum) into the IFR in all bands but 15 metres. For some reason it'll only manage about 70 watts there. It's actually capable of 150 watts in most bands and was set to do 120; I backed it off to 105 when I went over the thing.
FM transmit depends upon a relay that's known to get flaky. Mine has. I get a variable amount of output, 50-100 watts. I'll have to get a replacement relay from ICOM as I don't have one that'll do.
AM transmit is full DSB and makes better than 150 watts PEP (the IFR's metering limit) for 50 watts carrier. This is in contrast to the 751A which mostly modulates negative and makes 70 watts PEP for 50 watts carrier. The 751 isn't knobs-cranked-to-the-right-proof in other words. However, I prefer it this way. I'd rather back off the carrier power myself and have a good clean signal than have some screwy negative-modulation scheme going on.
The filtering was initially only useable "narrow" for SSB transmit. In "wide" (with both AM filters switched in), it passed full DSB. My modification to the switching (see under Reception, above) did away with that. Now it passes only the selected sideband.
SSB transmit audio is sensitive to RF on the case. My RF ground situation is dicey due to living in a third-floor apartment. This thing sounds clear and clean in 20 and 40 metres, somewhat distorted in 17 metres, pretty bad in 15 metres, and garbled in 80 metres. I didn't dare try it in 10 metres where my worst problems are. I did some bypassing at the input and it helped a little. By contrast, the 751A has moderate distortion in 10 metres at full power due to the same causes, no other problems.
Aside from the RF bypass, I flattened the mic amp's response by component substitution. I also had to raise the stage gain a bit. The tone control, when adjusted "flat", cut the overall gain to the point that I couldn't get full output with the mic gain cranked against its stop. All of this further reduced the effect of RF getting into it - what I described above is the result, it was initially much worse.
Ic metering is touchy and drifty. It was way off when I got the 751. I aligned it and it's drifted off again.
Final bias (idling current) is also a little drifty. I set it a bit high because of this, to ensure that the finals stay class AB.
General Operation:
Just about identical to the 751A. The only difference is there's no VOX button - it's switched on the gain control. I couldn't care less. I only use the VOX circuit for QSK CW.
Conclusions:
The IC-751A is a major refinement of and update to the IC-751. Aside from the VOX switch, the changes are all internal. ICOM learned from their mistakes and applied what they learned. With that said, the IC-751 is a decent basic rig. Given some maintenance and a good ground system, it ought to do the job.
I was looking for some gear to keep at the place in northeastern VE3 so I wouldn't have to haul the 751A there and back once or twice a year, subjecting it to being banged around and temperature-stressed. I picked this one up in eBay.
I figured there couldn't be much difference.
Wrong.
First of all, I had a fair measure of fixing and aligning to do. In addition, the accompanying schematic doesn't quite match the circuitry in some places so there was some additional figuring-out to do. This has nothng to do with the design differences of course but it sets some context.
The major design differences are:
The 751's final IF stage is 350 kc, not 9 MC. There's no filtering there whereas the 751A has a 3 kc filter for its PBT (SSB/CW).
There's no negative feedback in the first mic amp; no RF bypassing there either. Also, the mic amp starts with high-frequency pre-emphasis and the transmit tone control counteracts it. In the 751A, it's designed to be flat with a high-frequency boost/cut after it.
Metering circuitry is also rather different. The functions are the same but there was a redesign in the 751A to make them much more stable and meaningful.
Filter switching is another thing that was updated in the later version.
So.
Receiving:
Sensitivity and selectivity just meet spec.
The stock filtering does not include a 500-cycle CW filter.
There's an optional 6 kc AM filter in the 9 MC IF stage that doesn't make much of a difference over the stock 6 kc filter in the 455 kc stage.
Dynamic range is somewhat less than the 751A. This 751 will overload from very strong signals.
The noise blanker works a little better than the 751A's. It'll get rid of some power-line buzz that the 751A's can't quite handle.
The 751's notch filter is quite sharp and is a bit shallower than the 751A's. This one doesn't do as much for generalised noise.
Filter switching is rather limited in the optional configuration as I got it. Each mode (except FM) has the same two combinations - AM (6 kc) and SSB (2k3), both double-filtered. In SSB/CW, PBT works at 2k3 but not 6k.
I changed this. The 9 MC filter is now selected by mode - 2k3 for CW/SSB, 6k for AM. The filter switch selects the 455 kc filter - 2k4 or 6k. There is a small difference in SSB as the double-filtering makes for steeper skirts. In AM, the SSB filters were offset, resulting in a bandwidth of just over a kc. Now I get either 2k4 or 6k. This modified behaviour mimics the 751A's as far as it goes. I'll need a FL-52A to get proper CW selectivity (the "CW Narrow" mode).
For FM, the bandwidth is 15 kc, same as the 751A. There's no difference in FM reception as far as I can tell.
Frequency stability is somewhat worse than the 751A. The reference oscillator is solid but those BFOs drift some. It's not horrible - maybe +/- 50 cycles - but it is noticeable.
Preamp and attenuator both have less effect than their later versions.
Receive audio is full and clean, just like the 751A.
I could not get the S meter to line up accurately. The necessary adjustments aren't there. It reads 10 dB high at S9, 10 dB low at +40, and rather arbitrarily outside that range.
Transmitting:
Power output is to spec (105 watts maximum) into the IFR in all bands but 15 metres. For some reason it'll only manage about 70 watts there. It's actually capable of 150 watts in most bands and was set to do 120; I backed it off to 105 when I went over the thing.
FM transmit depends upon a relay that's known to get flaky. Mine has. I get a variable amount of output, 50-100 watts. I'll have to get a replacement relay from ICOM as I don't have one that'll do.
AM transmit is full DSB and makes better than 150 watts PEP (the IFR's metering limit) for 50 watts carrier. This is in contrast to the 751A which mostly modulates negative and makes 70 watts PEP for 50 watts carrier. The 751 isn't knobs-cranked-to-the-right-proof in other words. However, I prefer it this way. I'd rather back off the carrier power myself and have a good clean signal than have some screwy negative-modulation scheme going on.
The filtering was initially only useable "narrow" for SSB transmit. In "wide" (with both AM filters switched in), it passed full DSB. My modification to the switching (see under Reception, above) did away with that. Now it passes only the selected sideband.
SSB transmit audio is sensitive to RF on the case. My RF ground situation is dicey due to living in a third-floor apartment. This thing sounds clear and clean in 20 and 40 metres, somewhat distorted in 17 metres, pretty bad in 15 metres, and garbled in 80 metres. I didn't dare try it in 10 metres where my worst problems are. I did some bypassing at the input and it helped a little. By contrast, the 751A has moderate distortion in 10 metres at full power due to the same causes, no other problems.
Aside from the RF bypass, I flattened the mic amp's response by component substitution. I also had to raise the stage gain a bit. The tone control, when adjusted "flat", cut the overall gain to the point that I couldn't get full output with the mic gain cranked against its stop. All of this further reduced the effect of RF getting into it - what I described above is the result, it was initially much worse.
Ic metering is touchy and drifty. It was way off when I got the 751. I aligned it and it's drifted off again.
Final bias (idling current) is also a little drifty. I set it a bit high because of this, to ensure that the finals stay class AB.
General Operation:
Just about identical to the 751A. The only difference is there's no VOX button - it's switched on the gain control. I couldn't care less. I only use the VOX circuit for QSK CW.
Conclusions:
The IC-751A is a major refinement of and update to the IC-751. Aside from the VOX switch, the changes are all internal. ICOM learned from their mistakes and applied what they learned. With that said, the IC-751 is a decent basic rig. Given some maintenance and a good ground system, it ought to do the job.