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Author Topic: 75A1 is aligned but does not hear as well as Kenwood TS-820s  (Read 25811 times)
K1YTG
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Posts: 486




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« Reply #15 on: January 21, 2014, 01:47:54 PM »

I have been listening on the 80 and 40 meter bands a lot with the 75a1 and read all the comments here.
I am thinking that it is working fairly well but the noise floor is higher than it needs to be.
On the Kenwood TS 820s when I dis connect the antenna the noise drops to near zero.  On the 75A1 when I dis connect the antenna the noise drops only slightly.  And it is the noise that makes the weak signals difficult to understand.
I have ordered replacement tubes for the If section .  That is the only tubes I did not have others for comparison. 
Once I know all the tubes are good performers then I guess grounding the rf input to each stage working from the AF backwards would let me know which stages have the noise in them.
Does that sound like a good plan.  Or what can you suggest to check the stages and see if there is a problem one.
I do have a good scope.
Norm
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WB2WIK
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« Reply #16 on: January 21, 2014, 06:33:31 PM »

I sure agree that with any sort of normal antenna connected, the 75A-1 should have plenty of sensitivity on 80m and 40m, compared with anything.  You don't need a low noise figure on those bands, a NF of 15 dB should be as good as an NF of 1 dB.

However, if the receiver has a lot of noise even with an antenna disconnected, that's weird.

I'd look more at the front end and first mixer.
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G3RZP
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« Reply #17 on: January 21, 2014, 11:23:44 PM »

There's very few times that a 15dB NF isn't good enough on 10m, let alone on 40!

Check the volts around the 6AK5 RF amp. I suspect you need a signal generator to start looking at the actual step up in the tuned circuits around the front end and check their bandwidth to see if the coil Q has degraded. You also need to check carefully the IF bandwidths - again, coil Q may have degraded. Especially with mouse pee...

Back in the 1960's, a fellow apprentice got an AR88 that proved to have the IF transformer Q degraded, and he had to  unwind each transformer to find the number of turns and the spacing between the windings and the width of each pie and the number of strands in the wire..... Then a bottle of gin to a lady in coil winding section and he had the set really up and running...
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K1YTG
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« Reply #18 on: January 22, 2014, 09:33:19 AM »

I have a good signal generator , a good scope 100Mhz.  I guess everything I need. 
But where can I look up the procedure to check out the gain of each stage and the Q of the coils.
This is more advanced than I can remember reading in radio trouble shooting guides.
Or in the signal generator manual.  My generator has the frequency shift function in order to see the response curve on the scope.
Norm
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G3RZP
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« Reply #19 on: January 22, 2014, 11:32:27 AM »

Hang the scope on the 6AK5 plate. Damp the plate coil with a 1 k resistor in series with a .01 disc ceramic from plate to ground. Feed a signal in the antenna terminals. Find the frequency where you have most volts, and then tune the generator  each side until you have 0.707 of those volts. The centre frequency divided by twice the difference   between the 0.707 amplitude frequencies gives you the Q. It ought to be about 50 to 100. Now put the scope from grid to ground, tune the generator for maximum signal on the 'scope. Measure that, and transfer the scope to the antenna terminals. This tells you the step up which should be over 3. Do a similar test at the plate of the mixer.

Measure the IF bandwidth by feeding the sig gen into the 2nd mixer grid. Connect the scope DC coupled to the detector, switch off the AGC, back off the RF gain and sweep the sig gen through the IF (455, I think). Find the maximum signal and again, the 0.707 points - these are the 3dB points, and you can plot the IF selectivity and compare with the handbook spec.

Measuring stage gain is a bit more difficult since even a 10x probe has enough capacitance to throw off the tuning. If you have two identical probes and a two channel scope, hang one probe on the grid and the other on the plate, tune the generator for maximum - it should be the same frequency for both, at least on the LF bands, and the ratio is the stage gain.
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KH2G
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« Reply #20 on: January 22, 2014, 11:34:13 AM »

The ambient noise floor on tube rigs will be higher due to thermionic emission than what you will see from a FET front end. Lower noise floor equates to more signals heard.
Dick KH2G
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N2EY
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« Reply #21 on: January 22, 2014, 02:39:38 PM »

The ambient noise floor on tube rigs will be higher due to thermionic emission than what you will see from a FET front end. Lower noise floor equates to more signals heard.

Maybe in theory.

But in practical terms a 75A-1, or almost any tube receiver of decent design, has a low-enough noise floor for use below 10 MHz.

If the 75A-1 is functioning normally, and a typical antenna for 80 or 40 is used, connecting/disconnecting the antenna should cause a MAJOR change in background noise. If that doesn't happen, something is definitely wrong in the front end.

The first things (after the usual tube/switch/alignment) to check are voltages at the RF amplifier and mixer tubes. (Changing the resistors and capacitors was a good step, but it's still possible that something odd is going on that causes a major change in operating voltages).

The second thing is to check the tuned circuits in the front end. It is possible that one of the coils is open, or as others have suggested, has seriously degraded Q. (A shorted turn in a winding will make a real mess of things).


73 de Jim, N2EY



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G3RZP
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« Reply #22 on: January 23, 2014, 10:26:55 AM »

In an area where radio noise is equivalent to ITU-R Rec P.372 'rural' environment, a 7 MHz dipole in the clear will produce about 1 microvolt of noise in a 3kHz bandwidth in a 75 ohm input impedance receiver.
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N4UE
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« Reply #23 on: January 27, 2014, 04:07:01 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

One quick question:

How did you check the caps? If you checked them at rated voltage, fine. If you tested them for 'value' with a 9V multimeter, that won't tell you how 'leaky' they are.

I have a 75A-4 and every single tubular cap was no good, as well as one mica.... They checked perfect for value with a NIST traceable cap bridge (that uses a 9 V battery), but leaked horribly at rated voltage.

ron
N4UE
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If you're not the lead sled dog, the view never changes......
K1YTG
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Posts: 486




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« Reply #24 on: January 27, 2014, 06:57:30 PM »

I replaces all the electrolytics and paper caps.  I tested the bathtub oil caps with a heathkit cap tester.  Checked for both value and leakage up at rated voltage.  About half  of the oil ones were good.

Checking the noise stage by stage I found that the really noisy stage is the bfo.  When it is on it really brings up the background noise.
I tried using a signal generator to give me the carrier insertion on ssb and it is much quieter than listening to the ssb with the vfo.
I looked at the waveform of the vfo output and it looks somewhat saw tooth.  Certainly not a sine wave.  There is not much in that circuit.  What could I change to give me a better waveform?  I also noticed that the plate voltage on that vfo tube is low. About the same as the screen.   But I tried increasing it and it did not change the waveform.  Are there caps in the vfo can to replace?  On the schematic the caps in the vfo do not have numbers or values. 

Norm

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KC2QYM
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« Reply #25 on: September 09, 2014, 06:16:49 AM »

Here's my two cents...If you're just disappointed with the Collins receiver performance  why ruminate about it...it's just not healthly.  Cash in on the Collins Cult madness and sell it off. That way you'll have the Kenwood in all it's glory performing to your liking.  I understand there is a propensity for some HAMs to covet this ancient gear and the romantic images they invoke. But let's be realistic, if your tin lizzy cant purr anymore why keep it around.  Get the money from one of the cultists and take your significant other on a vacation but dont take a radio.
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KC4MOP
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Posts: 960




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« Reply #26 on: September 14, 2014, 05:20:29 AM »

My R390A still hears my sig gen less than .8 micro volt. Plus or minus a couple .5 mhz to 30.
Do you have the manual? Did you align according to the procedure in the manual?
The rodents can really destroy electronics.
And KC2QYM, if his Collins is not in pristine condition, as in, looking like it came from the factory, and is not up to specs; then it will not pay for any vacation.
Howard, W3HM, can restore that unit to look and perform better than new.........it will hurt your wallet and be about 2 years before it is complete.
Fred
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G3RZP
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Posts: 1225




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« Reply #27 on: September 14, 2014, 01:14:42 PM »

I got me an Eddystone (a  UK firm) 888A (ham bands only rx). It was about 45 years old and 21 MHz had major oscillator instability. Careful examination showed that solder joint on a capacitor in the 21MHz  osc coil assembly had never been soldered........

A good physical examination with flashlight and magnifying glass can reveal a lot....incidentally, remember that  UK 'torch' is US 'flashlight', and can cause much grief with the TSA!


"Why don't you speak English?"


"I do. You guys only speak American! Do you need English lessons?"



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AC5UP
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Posts: 4546




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« Reply #28 on: September 14, 2014, 02:07:33 PM »


The United States and Great Britain are two countries separated by a common language.
   ---  George Bernard Shaw  (widely attributed beginning in the 1940s)

We have really everything in common with America nowadays, except, of course, language.
   --- Oscar Wilde, The Canterville Ghost (1887)

PS:  Shocked to see a G3 type a phrase like  " I got me an Eddystone ".     Tongue

(we are not amused) 
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KH6AQ
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Posts: 7786




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« Reply #29 on: September 14, 2014, 05:02:25 PM »

I replaces all the electrolytics and paper caps.  I tested the bathtub oil caps with a heathkit cap tester.  Checked for both value and leakage up at rated voltage.  About half  of the oil ones were good.

Checking the noise stage by stage I found that the really noisy stage is the bfo.  When it is on it really brings up the background noise.
I tried using a signal generator to give me the carrier insertion on ssb and it is much quieter than listening to the ssb with the vfo.
I looked at the waveform of the vfo output and it looks somewhat saw tooth.  Certainly not a sine wave.  There is not much in that circuit.  What could I change to give me a better waveform?  I also noticed that the plate voltage on that vfo tube is low. About the same as the screen.   But I tried increasing it and it did not change the waveform.  Are there caps in the vfo can to replace?  On the schematic the caps in the vfo do not have numbers or values.  

Norm


Well you've troubleshot the problem to the BFO. I would not expect a sawtooth from the BFO oscillator. Perhaps a leaky plate coupling capacitor? You could replace the tube then if the problem is still there shotgun the BFO.

Dave
« Last Edit: September 14, 2014, 05:14:38 PM by WX7G » Logged
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