Collins 75A-3 IF alignment

[k7mdo]

I spent a good deal of the heavy metal night trying to learn how to align the IF in my newly restored 75A-3.  All went well up to the point that I did not have a VTVM so I first tried my Tek 465 and then my Fluke 77.

The "connection point" on the receiver is at the diode load junction of V8 (plate circuit of the detector AVC 6AL5 to cathode of the V10 noise limiter) and it was easy to find.  Here though was where things went astray.

(remember the radio was playing fine, I just couldn't keep my hands out of it)

The problem was instruction (step 8 in th emanual) that says to keep the VTVM reading below 5 VDC during the tune process.  The scope seemed to make no sense as it showed a low level sinusoidal signal but no DC potential so I hooked the Fluke up and sure enough a NEGATVE zero to -10 VDC depending on the signal generator output level.  So,I followed the steps assuming that the manual MEANT (-) not (+).  All went well and following the steps seemed to work well and the radio now plays as well as it did before (no better).

Do you suppose Collins manual is wrong on the voltage?  Any ideas? Does anyone think the input impedance of the Fluke is too low for this type of measurement? Thanks in advance.

73, Tom

[K5UJ]

Like you I am certainly no receiver expert, so I recommend getting the prescribed test equipment in the manual.

On step 8 in the first part of the alignment, if you follow the DC path from the junction between R39 and R42 you wind up going through an IF coil to pin 2 of V8.  The tube v. chart says the quiescent v. there should be -2.5 v. so it is probably a negative v. you should see.  You also backoff on the sig.gen. output to get below 5 volts.    

You need a sig. gen. with 100 ohm output Z; and a VTVM.  The VTVM won't load the circuit down.  I recommend getting them before you do any other alignment attempts and follow the instructions in the manual exactly.   Genuine vacuum tube v. meters are all over the place at hamfests, almost being given away.  There are some things that are still measured best with them.  The ones from the '50s, Heath V-6s and V-7s, RCA Voltohmnysts, etc. are all basically identical in design so any manual for any of them will work with any others.  All you usually have to do with one is remove the cover, pop out the battery and throw it away (you don't need the VTVM to measure resistance) and replace a few old wax coated and paper wrapped caps.  Most everything else is usually good enough.  

Rob

[WQ9E]

A negative going AGC voltage is correct for this receiver so you aligned it properly.  It is much easier to use an analog meter (or scope) for these adjustments and this is one case where a VTVM should be used so as not to load down the AGC bus.  Your scope would work fine but you probably had it set for AC instead of DC coupling (look just below your input volts/div switches) to set it for DC coupling.

[k7mdo]

Thanks for the notes and encouragement.  I am reading the manual for the umteenth time to see if I somehow misinterpreted it but it looks like they simply left off the minus sign on the instructions.  Everythig else worked just as I might have expected... just minus.

I do have a pretty decent rf generator (AN URM 25 D) and was able to keep the input low enough as to avoid the -5 VDC limits.

Radio plays well and so I am leaving it for now.

As to the VTVM, there is a hamfest coming in Rickreal and I have always seen VTVM's for sale there... will pick up a clean looking one to see if I can get it to work.

I did try the scope but I was sure I had it on DC input....  but I will try again.  I think the thing I saw when I had it hooked up was a sinusoidal pattern of very low amplitutde... not sure what that meant and maybe I needed to just reduce the vertical deflection sensitivity some to see the DC component I was looking for.... but of course I was looking for it to go positive... will try again with the scope.



73, Tom

[WQ9E]

Don't forget to hook up the ground clip on your scope probe.

Also the most common scope probe is a 10X and using a 10X probe means the actual input sensitivity is 1/10th of the marked value.  So on the .5 V range with a 10X probe the actual deflection sensitivity is 5 volts per division.  The advantage of the 10X probe is significantly reduced resistive and capacitive loading.

[W3RSW]

My 75A2 manual also has the Unsigned value as 5 volts in at least two sentences.
It does say, hook up the VTVM DC lead to the diode lead, jct. of r39 & r42.

It also says to "carefully tune signal generator until maximum diode load voltage is obtained. Maintain the input below the AVC threshold point. (approx. 8V.)" . --  Again an unsigned voltage.

AVC operation with voltages derived from power supply, rectified signal voltages overcoming pedestal voltages, etc is described in section 4.5.6   The AVC line is low Impedance.  V9 the Avc amp is described as having 11 volts of bias when not drawing current in no signal conditions.  Collins assumed that one knows that the 11 volts of cutoff bias for a 12AX7 in that circuit is negative.
Nowhere in their illustration, fig. 4-4, or in their detailed description is there any signed voltage values.

455kc IF alignment is the very first of the procedures, so you can't be far off with the results you've obtained.  

[k7mdo]

Ahhh. Now I feel better.... it is the first time assuming the manual was wrong turned out to be "right"!! Now I will sleep better. Thanks again

[N8ETQ]

       Was there not a "swamping" thing to De-Que
the IF cans? Cant recall the details but I have a
seris RC on aligator clips clearly marked 75A3.

/Dan

[k7mdo]

Dan not that I have seen.... T

[N8ETQ]

Hey Tom,

    What serial Number is your rig? Collins changed them
quite a bit around SN 1300 with the "Low Loss" filters.
I have not been able to find the Early 75A3 manual
on the web. I had an early model here a while ago
with the original manual, I should have scanned it.

    Take a look here and see if you have the SB's installed
in your rig.  Any way The "Swamping" tool is just a 1k
R in series with a .001 C.  You would "Swamp" the
primary of an IF can with it and tune the secondary
side then move the tool to the secondary and tune
the primary.

http://collinsradio.org/archives/manuals/75Asur.pdf

GL and 73

/Dan

Dan not that I have seen.... T

[k7mdo]

Dan, the 75A-3 I have is a bit odd in my opinion.  Its chassis is numbered as serial number 1184.  the manual that came with it is a reproduction from 15 April 1954 which states on the addenda that "all changes listed below have been incorporated in serial numbers above 1573".  At first this seems clear enough until I look at the addenda which states in line one that the first RF tube changes to a 6DC6.

MY earlier set has a clear factory marking at V-1 as a 6DC6.  Not the original designed 6CB6 that I would have expected from its early serial number. 

To make it even more interesting, my set clearly has all of the addenda instructions incorporated in the wiring.  BUT, its first RF tube (V-1) is a 6BZ6.

The set plays well with the 6BZ6 and trying a CB6 or DC6 provides significantly poorer performance.

So, what do you make of that?

Tom

[N8ETQ]

Hey Tom,

   Sounds like you have a "Hammy Hambone"
Hacked up Rig. (HHHUR).  The 6BZ6 is the
king of 1954 tubes of that series. I would
see if the cathode and screen "R"'s are
correct. I think think BZ6 had around 8000
uMho's. Have not looked at what the "R"'s should
be, That would be the only reason that other
tubes don't cut it.

   What, in "Radio Terms" does this mean?

"The set plays well with the 6BZ6 and trying
a CB6 or DC6 provides significantly poorer
performance."

    Is poorer performance more noise, less gain
worse overload.. I would correct if needed the "R"'s
and do a MDS check with your URM25.. Good test,
and see what's up. Look at the NC-300 manual
for 6BZ6 circuits, paying attention to the
values of the K and SCR "R"'s.

GL and keep us in the loop Please.

73 Tom

/Dan

[k7mdo]

Great information.  I will keep you in the loop but I have two Johnson Viking II's on the workbench and no room for a Collins for a while.  It is my main receiver for the AM station so it is in daily use but I will look into the resistors to see if they are different.  I didn't think of that when working it over this winter.

I have been through a multitude of receivers in the past few years but this has been my first of the 75A series and I am pretty darn amazed by its performance and stability.  It is a keeper. 

Tom

[W3RSW]

Well, if your using it as your prime am rig, if you haven't already, make an audio pickoff right after the AF detector.  Run mini-coax to an RCA jack on back panel and from there to your Hi Fi amplifier and speakers of your choice. 

Just remember SSB will not be demodulated in your outboard audio setup.  Interesting when you forget.

[KM1H]

The 6CB6 is a poor choice for AGC, something that Collins didnt understand until late in the A3 production. They used a 6AK5 in earlier models which is even worse.

The 6DC6 is a great AGC tube but a RF dog above about 20-22 MHz but the AGC circuit is still poor as it was later in the 75A4. Collins couldnt get a good ham requirement AGC designed until the 75S3C.

The 6BZ6 is an excellent TV IF tube that works well but I wouldnt use the NC-300 as an example since it is a 150V B+ radio. Pick a set such as the HQ-170/180, NC-400 (200V), NCX-5. Ive used the 6BZ6 and 6GM6 in many upgrades for lower performing 7 pin pentodes.

Carl

[W3RSW]

Go nine pin and broadband...  12BY7a's forever !    
Why go remote when you can be sharp?  Ride that gain, baby!
AGC is for wusses.

Thanks for all the regurgitation of "ER's" 6xx6vs. x,y, z article.

Now where did I set that spark advance last week when it was 16 deg. F, like this morning...