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Author Topic: 75A-3 S-meter calibration  (Read 5518 times)
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k7mdo
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« on: October 29, 2012, 01:42:49 PM »

I am working on a recently acquired 75-A3 that has a couple of problems.
My question is about the pot that adjusts the S-meter.  For some reason when I follow the instrucitons to set the S-meter to its zero point I am at the end of travel on the potentiometer.  The meter will not quite get up to "0".  It isn't pegged below zero but is a couple of "S" units below "0".

If I reduce the RF gain some then of course the meter rises up to some point on the meter scale and the potentiometer works smoothly through its range.

Another, and possibly related, issue is that when I check plate and screen voltages on the RF and IF tubes they are all very high by about 20-30 VDC.  At the 5Y3 the output measures correctly at about 250 VDC and the dropping resistors all have what appear to be correct resistance markings.

This set has been modified per the 1955 change to 6BZ6 first RF tube and the work looks to be per that mod. diagram. 

Thoughts?

Tom
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K5UJ
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« Reply #1 on: October 30, 2012, 06:38:44 AM »

hi Tom I don't have my manual with me right now but I can say that my A-3 S meter drops below zero with the RF gain pot fully cw and no signal input, and it seems pretty accurate on signal strengths up through S9.  at around 10 dB over 9 it starts piling on and gets to be about 25 dB generous on strong signals (so when it says 50 over S9 the signal is really around 20 over 9). 

I don't have an answer for you about the plate and screen v.  If the 5Y3 checks okay and all the other tubes are high, maybe there is a v. regulator problem?  There's an OA2 in there I think.  Just throwing out ideas.

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« Reply #2 on: October 30, 2012, 08:03:19 AM »

Start by checking the values of R47, R50, and R67 in the S meter circuit to see if one or more of these have drifted up in value.

According to my 75A-3 manual the voltage at the cathode of the rectifier is 210 instead of the 250 you measured, I expect that the increased voltage you are measuring is largely due to your actual line voltage being higher than that used for the Collins measurements.
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K5UJ
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« Reply #3 on: October 30, 2012, 08:57:13 AM »


According to my 75A-3 manual the voltage at the cathode of the rectifier is 210 instead of the 250 you measured,

Ah ha.  I wondered about line v. but since he said the 5Y3 was normal I wrote that off.  I run mine on a small variac at 110 v.  so if the seasonal fluctuation goes up, it won't get over 115.
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k7mdo
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« Reply #4 on: October 30, 2012, 01:37:12 PM »

It is odd as when I got the set a couple of weeks ago it was kind of weak on receive but the s-meter worked "in range".  Only after I checked and replaced a few of the weak tubes did it brighten up and that is when the s-meter took a turn for the worse.

The manual has a simple instruction for setting it's zero by shorting the antenna leads and then zeroing the meter with the R-49 pot. 

I will start with the associated resistors first as they aren't too hard to get at on the chassis.

Will report findings, particularly if I solve it.

73, Tom



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K5UJ
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« Reply #5 on: October 30, 2012, 10:19:11 PM »

Tom, when you get the 75A-3 fixed up I think you will like it.  It seems many hams go after the earlier A line receivers or the A-4, making the A-3 a bit of a sleeper.  To me (admittedly I don't have a great deal of vintage rx experience) it has that black wrinkle bakelite rack cabinet aesthetic combined with PTO stability.  I like the ham band only coverage and the mechanical filter selectivity option.  That 3.1 kc filter that they usually have is great under QRM.  The downsides are pretty easily worked around.  You can (I must give W3JN credit for this) tap the AF gain pot wiper with a .1 uF cap and bring that signal out via a shielded audio cable to an external p.p. tube amp to a speaker and get around the stock single ended audio section (pull the audio tube if there is no load on the speaker transformer). 

You can homebrew little filter cards that seat in one or both of the A/B slots using perf board and different gauge wire for pins.  There are two or three articles in ER on the  A-3 and homebrew filters; you can get cheap Murata ceramic filters from Mouser and use them with a blocking cap and load resistors on the IF plate and IF grid sides the filters.  A 6 kc filter has shallow skirts so it sounds pretty good.  A 4 kc filter still sounds pretty good since the skirts on the ceramic filters are not steep but you won't be bothered by a QSO 5 kc away unless the signal is strong.  With these mods you have a pretty good AM receiver, just no flywheel weighted tuning hi hi.
If you are interested I will try to find my information on the homebrew filters.

Rob
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k7mdo
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« Reply #6 on: October 31, 2012, 11:48:57 AM »

Well, I have gone through all of the resistors I can manage in the s-meter circuitry and found none very far off (<5% error).  I am still puzzled by the seeming change in the circuits operation with only my having replaced a few tubes.  I only operated the set for an hour or so before I "got my hands" into it but I remember pretty decent response of the meter with varying signal strengths and don't remember it going below zero.  Now it does not seem to vary much with varying signal strengths and remains below zero with low level signals.

On the good side, the set plays well, seems sensitive even on 10 meters and is stable... what more could you ask?

Yes, I think it is a "keeper" even though the audio is not up to 183D standards, my ears aren't good either.

I am interested in building a 6 kc filter as my main use of the receiver will be 3885 AM. 

However, no hurry on that as the "wife" is dragging me off on a vacation for about a month.  She thinks I need a break from the ham shack!?!  We are off to Nepal (Katmandu) and then Rome.... somewhat of an odd combination. 

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ke7trp
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« Reply #7 on: October 31, 2012, 11:58:36 AM »

Swap the tubes back.  I have found that depending on the circiut, the tubes have a big impact on the S meter.  For example the RF amplfier tubes in my SX42 with really effect the meter zero and peak with the same Sig level injected.  I went 12 rounds with my SX42 and never did get it to zero and read correctly. Its one or the other... 

C
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k7mdo
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« Reply #8 on: October 31, 2012, 01:08:41 PM »

That is a good idea... one that I have had.  One thing I did was to mix in some of the "W" ruggedized military tubes along with the regular 6BA6 tubes.  Those tubes simply may not be suitable.  If that is it, I will report.  Tom
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k7mdo
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« Reply #9 on: October 31, 2012, 06:11:17 PM »

OK, S-meter is fixed, range is well within the adjustment range of the pot.  I started by swapping tubes and that made no difference so I went back to the meter bridge resistors and noticed one that didn't look to be soldered in very well.  It turned out to be just a bad joint!   What must have happened is that in moving the unit home in the back of my truck the vibration was more than expected and it 'loosened' up the joint.  That more than explains the fact that when I first tried it, it worked, then when I started "cleaning" it up it all of a sudden started acting wrong.

The S-meter circuit is a little odd to me as at low RF gain the meter deflects upward... so it is only calibrated at full RF gain.  Somewhere I read an article once that explained how to "reverse" this but for now I am more than happy.  I have it on an inverted "L" 80 meter 1/4 wave and the sensitivity performance is better than my 183D.

Next is the large filter cap that looks to have been replaced once but is still quite old from appearence and labeled on 30 mfd instead of the circuit diagrams listing of 40 mfd.  It is an odd callout in the parts list as it states -5%, +100% for the value. 

Now I will go on vacation without that problem haunting me...  73 and thanks for the advice folks, Tom
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K5UJ
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« Reply #10 on: October 31, 2012, 06:32:38 PM »

The S-meter circuit is a little odd to me as at low RF gain the meter deflects upward...

What's odd about that.  What gear doesn't do that?
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« Reply #11 on: October 31, 2012, 06:54:40 PM »

Tom,

Glad you found the meter problem. 

Vintage electrolytic capacitors were anything but precision devices thus the spec you found. 
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Rodger WQ9E
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