75A-4 Blues

[k4kyv]

Don, is your receiver still using the 6BA7 as V5?

Yes.

I completed the changeover, not permanent but such that component swapping is easy. The gain is way over the top though and as you indicated Don, it has nothing to do with the warble and shift of the signal. The front end is very hot now and I have to use judicious amounts of RF gain reduction to make listenable. I imagine there are ways to combat this but at least I have successfully removed the high voltage from the filter.
There must be a pad ahead of this stage.

I think there is an i.f. gain adjustment pot somewhere IIRC.  I use a 20 dB pad between the dipole and vertical and receiver.  Don't use it with the beverage or indoor loop.  The rx seems to have adequate rf sensitivity, maybe a little too much.

[KM1H]

I have a low Serial, a mid and a very high last run unit.

 

I had a real low serial for awhile, like 330 or something.
The one in regular use is middle as in the 2400's and another is very late. So late that it has a factory noise blanker and a Collins historian presently has it trying to trace its history. The last time I actually used it was in the 80's and its never been recapped and the blanker wasnt very effective unless someone with pure copper ignition wires drove by. I shelved it when I sold the pair of Drake C Lines and got the first TS-930.

[N6YW]

My ER 75A-4 Compendium showed up today!
Sure enough, there is a section devoted to repairing the frequency jump problem.
I guess I had better get organized and prepare the bench for extracting the PTO.
Repairing it looks to be easy and straightforward.

[KM1H]

Just take your time especially with the front panel and wiring.

[N6YW]

I will, trust me. I don't like re-working my own work.
The sticky tubing for the PTO wires will finally get 86'd too.

[N6YW]

Just take your time especially with the front panel and wiring.

BTW...
When loosening the shaft coupler, can I remove the PTO without messing with those pesky
cam disks?

[n2bc]

The cam disks are inside the PTO body.  If you are refering to the Oldham Coupler on the PTO shaft, it is not a big deal to separate it.  It will need to be cleaned anyway.

Be careful you do not over rotate the the shaft of the PTO.  The turns limiter is external to the PTO (on the front panel).  If you start cranking on the shaft when the PTO is out of the A-4, it can be turned too far and the result is not pleasant at all.

[k4kyv]

Be careful you do not over rotate the the shaft of the PTO.  The turns limiter is external to the PTO (on the front panel).  If you start cranking on the shaft when the PTO is out of the A-4, it can be turned too far and the result is not pleasant at all.

Agreed! If you turn the shaft on the PTO far enough beyond its limits to feel physical resistance to further rotation, damage may already be done.

Before  removing the PTO, best to set the frequency at 500 or somewhere else at midrange, carefully note the setting, and keep track of any rotation before the unit is disassembled. Once apart, note the exact position of the slug assembly as it rides oven the cams, even to the point of counting the leaves.  Try to re-assemble it exactly as found.

While it is apart, that would be a good time to re-adjust the leaf-cams for maximum linearity.  Most older PTOs have developed some error over the years.  It might not be a bad idea to fabricate an extension cable to allow the PTO to operate outside the receiver, and a frequency counter would be very useful for checking linearity. But be careful! This requires running the PTO all the way across its tuning range, so be wary of the range limits.  Temporarily attaching some kind of 360° precision calibrated dial to the shaft would make adjustment a lot easier.

Those PTOs are basically all the same.  I have worked on ones from the T-368, R-390 series and T-195, but never have (yet) gone into the ones from my 75A-4s.

That little slug that expands or compresses the tuning range to exactly 1 mc/s per 10 turns of rotation should be checked and adjusted as well.  If the slug is against the  stop and all the adjustment is used up, that can be somewhat corrected by removing turns from the coil (or adding turns if necessary, but you usually need to remove turns).  I believe this is all explained in the compendium.

Unfortunately, the problem with the PTOs going out of calibration appears to be from ageing of the powdered iron slug composition material, somewhat analogous to the resistance shift in carbon composition resistors.  It may not be possible to restore the PTO to perfect calibration, but it can be adjusted to minimum error. In extreme cases, perhaps some kind of outboard digital frequency display would be in order. In years past I used to see ads for those, adaptable to most vintage receivers. Otherwise, something else could be adapted to the purpose, or for the ambitious, homebrewed.

Remember, the factory specs for frequency calibration are only plus/minus 300~, when the nearest 100 kc/s calibration point is zeroed to the xtal calibrator.  That is a worst-case scenario, actually rather poor, given the inherent precision of the PTO and dial mechanism, and most receivers can be adjusted to meet or exceed that specification before/without even going inside the PTO.

One more hint regarding calibration and linearity adjustment: manuals for both the 75A-4 and R-390 series say to reference the end-point adjustment settings to the extreme ends of the tuning range: at the 0.000 and 1.000 mc/s points, and repeat several times until they fall in place at both ends.  I have found that this often leaves a substantial error near the middle of the range, where most of the popular ham frequencies lie.  Instead, use 0.100 and 0.900 mc/s for the adjustment points.  That may take it off a little at 0.000 and 1.000, but the intermediate calibration points will more closely line up into place.  Linearity error tends to be substantially greater within the first and last 100 kc/s of the tuning range, than across the intermediate 800 kc/s range between the 100 and 900 kc/s points.

[N6YW]

The cam disks are inside the PTO body.  If you are refering to the Oldham Coupler on the PTO shaft, it is not a big deal to separate it.  It will need to be cleaned anyway.

Be careful you do not over rotate the the shaft of the PTO.  The turns limiter is external to the PTO (on the front panel).  If you start cranking on the shaft when the PTO is out of the A-4, it can be turned too far and the result is not pleasant at all.

Thanks for that clarification. I am referring to the several disks that reside in front of the PTO body
that have the little stop tabs stacked in a row prior to that final coupler arrangement.
I guess the trick here is to carefully map everything out as I disassemble and reverse order assemble.
According to the compendium, it echoes what you all have said about presetting the PTO frequency and marking it to make recalibrating easier when done.
I appreciate all of your sage advice about this operation. I can see how it would be easy to royally screw
things up and render it useless.
Onward!

[sndtubes]

Well, my 75A4 did exactly this.  I replaced the ceramic and mica caps inside the little box at the front and bottom of the PTO.  You do not need to disassemble the PTO.  It's tight in there , but you can replace them.  It totally cured the PTO instability I had.  It would sometimes sound as if someone was rocking the pto back and forth across a signal.  Try that before taking the PTO apart.

[N6YW]

Well, my 75A4 did exactly this.  I replaced the ceramic and mica caps inside the little box at the front and bottom of the PTO.  You do not need to disassemble the PTO.  It's tight in there , but you can replace them.  It totally cured the PTO instability I had.  It would sometimes sound as if someone was rocking the pto back and forth across a signal.  Try that before taking the PTO apart.

THANK YOU!
Here is what it looks like inside the compartment. Those little cubes of treachery are history!
While I am at it, the two tubular coupling caps are gone too. Replaced with Mallory M-150 series caps.
More to follow.

[N6YW]

Problem solved
After replacing C-204, 205, 206 & 208, it is very steady without the "Dances with Wolves" that plagued it before.
Now that it is staying relatively on frequency, I can attend to the AVC and S meter problems.
THANK YOU EVERYBODY!!!!!!!!!!
Onward and upward!

[sndtubes]

Mazel Tov!  I finally helped somebody!  As soon as I read your post, I knew what the problem was.  GL with the rest.  They can really be a pain, but totally worth the trouble when finished.

73,
Mike WB0SND

[N6YW]

Mike
It will be on frequency tonight for the West Collins Collectors 75 Meter Net.
Hopefully I can attend to the AVC problem. Shouldn't be too hard.
Thanks again!

[ke7trp]

Congrats!   I got my DC relay in the mail.  I should have my Gold dust twins on the air next week if all goes as planned.  I cant wait to make a contact with you, Twins to twins.

C

[N6YW]

Congrats!   I got my DC relay in the mail.  I should have my Gold dust twins on the air next week if all goes as planned.  I cant wait to make a contact with you, Twins to twins.

C

Yahoo!!!!
I will have mine on tonight! I just hope the band holds up... it was very sloppy last night.
See you next week!
Here's the Twins online and in the hunt tonight!

[n2bc]

The disks you are referring to is the PTO turns limiter.  I don't recommend taking it apart, just flush them out well with solvent and lightly lube.  I tried to re-purpose an old one and the tolerances are pretty tight, have up on it.

The Oldham coupler I mentioned is the gizmo with the spring between the two outer faces and the slotted brass disk in-between. It connects the outer shaft to the PTO and the purpose is to provide non backlash compensation for any minor misalignment of the PTO with the outer shaft.  Oh, the spring will typically fly off into an adjoining room. Not to worry, you will find it a few months after you "make do" with something close.   

[ke7trp]

I was wondeing if it was ok to stack the units like that.  That makes me feel better about getting the rig into my small ham shack. 

I will get flamed by this next question by any Collins snobs that happens to be on the forum, but has anyone thought about Transcieving with these units?  I am sure someone has done this at some point.  EIther external VFO for both or having a buffer board that Drivers the other?  Probably to much work and you would really upset the CCA members at the though of any modifications.  It sure would be cool to have external Solid state ROCK solid VFO.

Do you find you have to chase the reciever and transmitter around?
c

Congrats!   I got my DC relay in the mail.  I should have my Gold dust twins on the air next week if all goes as planned.  I cant wait to make a contact with you, Twins to twins.

C

Yahoo!!!!
I will have mine on tonight! I just hope the band holds up... it was very sloppy last night.
See you next week!
Here's the Twins online and in the hunt tonight!

[N6YW]

I was wondeing if it was ok to stack the units like that.  That makes me feel better about getting the rig into my small ham shack. 

I will get flamed by this next question by any Collins snobs that happens to be on the forum, but has anyone thought about Transcieving with these units?  I am sure someone has done this at some point.  EIther external VFO for both or having a buffer board that Drivers the other?  Probably to much work and you would really upset the CCA members at the though of any modifications.  It sure would be cool to have external Solid state ROCK solid VFO.

Do you find you have to chase the reciever and transmitter around?
c

Congrats!   I got my DC relay in the mail.  I should have my Gold dust twins on the air next week if all goes as planned.  I cant wait to make a contact with you, Twins to twins.

C

Yahoo!!!!
I will have mine on tonight! I just hope the band holds up... it was very sloppy last night.
See you next week!
Here's the Twins online and in the hunt tonight!

They are both fairly stable, with the KWS-1 being best. Stacking these is not an issue because of the venting of the cabinets. I cheat a little bit when I am in QSO. As a courtesy to the other OPS on frequency, I monitor my on air transmissions with headphones and my trusty 756 Pro. Therefore, I make absolutely sure that my TX is on freq. I am quite surprised at how well the rig stays on frequency but it will get better as I go through and eliminate those problematic molded mica's.
After being in QSO for over an hour tonight so far, the 75A-4 is rocking big time and staying relatively on frequency. I am also impressed with the floor noise quality compared to my other radios. The lack of AVC action is very apparent with strong signals and I must fix the problem. The beauty of the radio demands better AVC action.
I love this receiver. It will be a real outstanding unit when I am done with it because I am completely committed to getting it right. It has a lot of promise and is in very good shape considering it's age. It's 6 years older than I am and now stays on frequency better than I do! Hi! I am also observant of not going too far with the modifications, especially those that have been proven not reliable or warranted.
The compendium has been a savior, as well as several of the OPS here with the excellent contributions.
Having a real ball tonight.

[flintstone mop]

Glad that the caps worked out. Better than getting into the PTO....Seems like PTO's get nasty after very very hard use / abuse
If the radio has been in a decent environment and not twisted violently around, the PTO may never need opening.
CAPS are our enemies. But hats off to them for lasting this long..

Good luck with your goodies

[sndtubes]

I recommend the AGC mods by George, W1LSB.  They are in Electric Radio about a year or 2 ago.  This mod is far superior to the more common and popular K7CMS mod.

[ke7trp]

If they are hooked up the way the manual says with teh BNC,  connector.  You can listen you yourself talk on the transmitter through the 75A4 without transmitting over the air. No need for the other radio on with headphones. You can flip the Driver stage on, Talk a little and fine tune the Kws1 vfo, Then flip the switch back and you are ready to go.

It sure would be nice to have them rock solid.  But I digress, I love old radios and this is one small anoyance.

The 75a4 I will be using is a very late serial number.  I have not tested it much more then about 5 minutes.  Its all working though.  The older 75a4 I have that came with the kws1 has a very fast AGC. TO fast for my tastes. About like my Kwm2a.  To smooth it out, I back down the Gain a bit and its very smooth.

C

I was wondeing if it was ok to stack the units like that.  That makes me feel better about getting the rig into my small ham shack. 

I will get flamed by this next question by any Collins snobs that happens to be on the forum, but has anyone thought about Transcieving with these units?  I am sure someone has done this at some point.  EIther external VFO for both or having a buffer board that Drivers the other?  Probably to much work and you would really upset the CCA members at the though of any modifications.  It sure would be cool to have external Solid state ROCK solid VFO.

Do you find you have to chase the reciever and transmitter around?
c

Congrats!   I got my DC relay in the mail.  I should have my Gold dust twins on the air next week if all goes as planned.  I cant wait to make a contact with you, Twins to twins.

C

Yahoo!!!!
I will have mine on tonight! I just hope the band holds up... it was very sloppy last night.
See you next week!
Here's the Twins online and in the hunt tonight!

They are both fairly stable, with the KWS-1 being best. Stacking these is not an issue because of the venting of the cabinets. I cheat a little bit when I am in QSO. As a courtesy to the other OPS on frequency, I monitor my on air transmissions with headphones and my trusty 756 Pro. Therefore, I make absolutely sure that my TX is on freq. I am quite surprised at how well the rig stays on frequency but it will get better as I go through and eliminate those problematic molded mica's.
After being in QSO for over an hour tonight so far, the 75A-4 is rocking big time and staying relatively on frequency. I am also impressed with the floor noise quality compared to my other radios. The lack of AVC action is very apparent with strong signals and I must fix the problem. The beauty of the radio demands better AVC action.
I love this receiver. It will be a real outstanding unit when I am done with it because I am completely committed to getting it right. It has a lot of promise and is in very good shape considering it's age. It's 6 years older than I am and now stays on frequency better than I do! Hi! I am also observant of not going too far with the modifications, especially those that have been proven not reliable or warranted.
The compendium has been a savior, as well as several of the OPS here with the excellent contributions.
Having a real ball tonight.

[KM1H]

P&H made a transceive adapter that only did 20/80 and Ive only seen one ages ago. They (or somone else) announced a multiband version but it didnt hit the market that Im aware of.

In the late 60's I tried designing a 160-10M one to go with my 100V, even using a lot of SS, at National but never licked the birdy problem. These days it might be a lot easier, and I even have the same rigs, but its not something I'll likely get around to.

Carl

[N6YW]

I'm sure there is a way to do the transceiver operation like the S-Line does, but it sounds like a lot of work.
It would likely involve some internal modifications that aren't worth the hassle. They're a lot fun the way they are.
I had a ball on the 75 meter CCA net last night. There was a lot of the gang on board, including FOUR "Gold Dust Twins" stations! The band co-operated too and was very fun for a change.
This was the first time I had ever used a 75A-4 in QSO and was impressed with the lack of floor noise and the ability to navigate from some QRM. I think it can be better and certainly the AVC problem needs to be addressed. When KB6OQJ transmitted his first signal, the volume nearly destroyed my speaker! He was easily in
excess of 40 db over S9. The quality of the receiver audio is very good despite the crappy output circuit.
I performed the feedback mod in the audio amp and it made a big difference and has a smoother response than it was as stock. I will end up using a direct output into an external power amp once this has been properly installed in the station. Right now, it sits atop the KWS-1 while I go through the trial and test period.
Once both units are completely finished, they will be the center piece of the SSB station
Can't wait.

[ke7trp]

I tuned into the 3895 last night.  I heard a bunch of guys arguing and going on yelling at each other.  Strong signals.  BIG argument.  I got tired of listening and turned it off.

C