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Author Topic: 75A1 is aligned but does not hear as well as Kenwood TS-820s  (Read 25805 times)
K1YTG
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« on: January 17, 2014, 12:37:26 PM »

I have spent much time on my 75A1.  It had so much rodent pee in the if section that it all had to be taken apart and all the trimmers replaced.  Many of the screw coil trimmers had to be tapped and new threaded rod put in.
It is back together now and aligned.
Side by side comparisons with a Kenwood TS 820s has the 75a1 not able to make out weak stations as well as the Kenwood.
Does this mean there is a problem with the 75A1 .  Or is that just the way it is comparing a 1949 receiver with one from the 70's.
Shall I keep working on it with the assumption that it can do better?  Or is it time to put it back in the case?
Thanks for your comments.  Norm
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WB2WIK
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« Reply #1 on: January 17, 2014, 01:26:52 PM »

Are you connecting the antenna transmission line (presumably coax) directly to the ANT and GND terminals on the rear apron of the receiver?

I haven't used a 75A-1 in about 35 years but seem to recall it was really designed for balanced line and I used an outboard balun to match it for coax, and by doing so the sensitivity improved.  Hazy recollection, though.
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K1YTG
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« Reply #2 on: January 17, 2014, 01:31:59 PM »

That is a good point.  I have the coax braid to ground.  One antenna terminal to ground and the other to center of coax.
I will connect up a transformer 75 ohm to 300 ohm and try that.
Norm
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WB2WIK
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« Reply #3 on: January 17, 2014, 03:04:29 PM »

That is a good point.  I have the coax braid to ground.  One antenna terminal to ground and the other to center of coax.
I will connect up a transformer 75 ohm to 300 ohm and try that.
Norm

As I recall (haven't seen one in a long time) there are three screw terminals for the antenna: Two are for balanced line, plus a third one that is "ground."

If you use a balun, you want to use the first two screw terminals, and DO NOT connect the middle one to the GROUND screw terminal at all, just leave it floating.

I think. Wink

Also, I've found that "some" television type 300 Ohm to 75 Ohm transformers may work down on HF, some don't and many work poorly since they're really not designed to operate below 56 MHz.  I found some that will work pretty well on 10m but were terrible on 80m. 

Might have to find a really good one, or one specifically made for lower frequencies, or homebrew one.

A transmitting antenna balun designed for HF ham work (4:1 voltage balun), although very "overkill" for power rating, would probably work fine for this; it should have enough ferrite in it to get down to 3.5 MHz.
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K1YTG
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« Reply #4 on: January 17, 2014, 03:14:11 PM »

I have a Denton antenna tuner rated at 100w.  I will try that and see how it compares to the tv coil.
Norm
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K1YTG
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« Reply #5 on: January 17, 2014, 03:43:36 PM »

I put the antenna tuner between the antenna coax and the receiver.  I was able to get a clear peak in signal when adjusting the tuner.
I think it did help bring up the signals slightly. 
But the Kenwood TS-820s still beat the Collins in making out weak ssb signals .
The weak signals are easier to understand on the Kenwood.  It could be partly due to the Kenwood having a product detector.
norm
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K0IZ
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« Reply #6 on: January 18, 2014, 05:25:05 PM »

Is the receiver really quiet (background hiss)?  If so the IF amps may not be functioning correctly.  Older receivers had fairly noisy front ends.  You mentioned problem with IF transformers.  If you adjust the tuning on each transformer, can you hear the hiss (or signal) going up and down?  It's possible one or more transformers are not functioning correctly.

That old of a receiver could have other problems, bad bypass capacitor somewhere, resistor changed value such that an amp stage is not working well, etc.  When working well a 75A1 should hear quite well, especially on 20M and lower freq.  Product detector makes for cleaner audio, but not your problem.  You may have to start at last IF and use a signal generator or tracer, checking for reasonable gain per stage, then go one more stage towards antenna, etc.  Good luck.
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AC5UP
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« Reply #7 on: January 18, 2014, 07:03:07 PM »

It's assumed that since Collins gear is "the best" it will always out-perform a lesser brand name in every situation.

Not true.

"The Best" can be a subjective rating and judged on several levels like overall build quality, noise floor, IMD specs, audio quality, etc. Some venerable combinations like an R-390 with CV-591A SSB convertor http://www.isquare.com/personal_pages/CV-591.htm had to be "The Best" because either piece was so impressive to look at individually the combo * must * be beyond peer.

Time marches on.

It's possible the IF transformers in the 75A1 have been permanently compromised thanks to time and mouse tinkle as previously suggested. It's also possible the TS-820S has the weak signal advantage of zero thermionic (filament) noise inherent with solid state receivers. Today it's common to believe that tube designs have advantages.  They do.  Unca' Sam ran our fleet of R-390A spook receivers well beyond the hollow-state transition elsewhere, but back in the day it was known that newer was almost always better and the JA's had evolved well beyond their cheap transistor radio days of the 60's...

The Collins is good stuff. The Kenwood is too. The better preserved piece is more than capable of outperforming the other because at this stage of the game it's more about condition than reputation or technology.  Tongue
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G3RZP
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« Reply #8 on: January 19, 2014, 01:51:40 AM »

If the noise comes up 6dB or more with the antenna connected, you have all the sensitivity you can use.  The 6AK5 RF amp tube is very quiet, with an enr of 1880 ohms. Assume a passband of 5kHz, that's about 400nV of noise while the step up from the antenna is likely to be about 5. So the tube noise referred to the input is around 80nV, and tuned circuit noise will likely exceed that. The IF passband will be more than the TS820, which will add noise, and as '5UP says, the IF transformers (and maybe RF ones too) may well have lost Q over the years - especially when treated with rodent urine! - so the bandwidth is much wider and the gain less. The AF passband of the 75A1 is likely to be more than the TS820 -it was designed for the days of AM, not SSB - which can make weak signals sound noisy because of demodulated noise from the IF and maybe the BFO not being rolled off. Some pre WW2 receivers even offered variable BFO injection level to allow minimising the BFO noise, but sacrificed signal handling under those conditions. But receivers of that era didn't have AGC on CW anyway, requiring the operator to 'ride' the RF gain control.

The real test is a good signal generator and distortion meter to check s/n and real selectivity.
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K8AC
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« Reply #9 on: January 19, 2014, 07:27:06 AM »

You didn't mention which bands you were doing the comparison on and it's unclear just what you meant by "not able to make out weak stations".  Back in the 1960s and 70s, it was pretty common knowledge that these older receivers, regardless of manufacturer,  were lacking in sensitivity on the higher bands.  The use of an external preamp/preselector, such as those offered by RME, was widespread.  Even in the 1960s, more modern receiver designs such as the Drake 2B ran rings around the older receivers when it came to weak signal work on 15 and 10 meters.  When the 75A1 was brand new, it wouldn't compare to later receivers when it came to copying signals on the higher bands.  Collins cult followers of today won't buy that and will try to convince you otherwise. 

73, K8AC
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G3RZP
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« Reply #10 on: January 19, 2014, 09:18:30 AM »

true, Floyd, but we don't know if the 75A1 is working to full specification or not. If it isn't, that could well explain it.

As far as 'Collins must be best' is concerned, I was very unimpressed when I started looking inside the 30L1 I bought.....
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KH2G
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« Reply #11 on: January 19, 2014, 01:32:19 PM »

Compare thee sense spec on each then there will be no question. I believe the old collins, good old radio that it was in its day, won't be quite as hot.

Regards, Dick KH2G
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K1YTG
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« Reply #12 on: January 19, 2014, 01:57:21 PM »

I have been comparing the copy on 80 and 40 meters.  There is more noise on the 75A1 and that is what makes copy on weak signals more difficult.  Perhaps the wider bandwidth makes that normal for this comparison between receivers.
Each paper and oil capacitor has been checked and most replaced.  Most all resistors have been checked and many replaced. 
I have been substituting tubes to check for weak ones.  Several I have not located a spare so I am held back on this.  The set did have several dead tubes when I got it. 

I was hoping this receiver would perform well enough that I would want to use it regularly.  But from what I have seen so far it will not be one I use often.  So all the time I have put into it was not put to good use.  It would have looked nice sitting on the shelf as it was.
I do get some satisfaction out of bringing it back to life, though. 
Norm
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W1BR
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« Reply #13 on: January 19, 2014, 02:51:44 PM »

I have several boat anchor receivers. Most can hold their own below 10 MHz. A 75A1 should be able to hear weak signals on 80 and 40 meters. My SX-101A holds its own compared to the JRC and Ten Tec gear in the shack, except for selectivity, stability and strong signal handling ability. Otherwise it has a good set of ears. I'd suggest comparing the 75A! to a known good boat anchor RX.
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G3RZP
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« Reply #14 on: January 19, 2014, 03:42:41 PM »

You have to do a selectivity comparison. The 75A1 won't be as good on SSB without a filter about 2.7kHz wide at 6 dB down and a decent shape factor.

On CW, with the narrower bandwidth, it should be just as good. Check out the CW end of the bands.

Certainly on the LF bands, if the sensitivity isn't up there with a semi-modern transceiver on an antenna, there's something wrong. Even a 1935 HRO will have adequate sensitivity on 40 and 80 with a semi-reasonable antenna...
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