75A-4 Blues

[N6YW]

As noted in my "Christmas Twins" thread, the A4 is becoming a real had scratcher.
The biggest problem I am having is instability. Example: While on any band, I tune into a decent signal
and it just dances around like a drunk chick at a Grateful Dead show. This happened before and after my re-cap job, so I reasoned I am not the reason it occurs. To give you a better idea of what it sounds like, imagine the musical saw sounds of a bad B-rated horror movie while using the Calibration signal.
I have targeted a possible problem that I mentioned earlier, of the 175 vdc on the mechanical filter, occurring on the side of the shield after V5 second mixer. There is supposed to be a blocking cap (1000) C-144, but it was never there. I tried 1000pf and .001 and all it did was attenuate the hell out of the signal.
At this point I am starting to realize my limitations of receiver understanding. I know this radio has the potential to be a star performer, but right now it sounds awful, and not very sensitive either.
I have not started the Mica replacement quest either, as it will take a few hours to accomplish. I am awaiting the sage advice and direction from those who know better than I.
I will supply some photos to support my descriptions here. All of the original paper caps were replaced, along with the electrolytics.

[N6YW]

Here are some pics of the area of V-5 & V-6.

[N6YW]

If anyone has any pictures they can share, this would help greatly.
Thank you.

[k4kyv]

Sounds like the PTO.  Sometimes the lubricating grease hardens and causes the cam on the slug thingy to ride flakily over the linearity guide.  Requires breaking the seal and working on the inside of the unit. See the articles in ER or in the compendium amongst available sources.

To rule out the crystal oscillator, try 160m, since the rcvr is single conversion on that band and no xtal is used.

[N6YW]

Sounds like the PTO.  Sometimes the lubricating grease hardens and causes the cam on the slug thingy to ride flakily over the linearity guide.  Requires breaking the seal and working on the inside of the unit. See the articles in ER or in the compendium amongst available sources.

To rule out the crystal oscillator, try 160m, since the rcvr is single conversion on that band and no xtal is used.

Don
Thank you. On 160, the calibrator still jumps all over the place. Tuning into distant heterodyne's  confirms that the instability exists there too.

[WA1GFZ]

crystal filter has nothing to do with frequency stability.
Sounds like you have a bad cap in the pto. you might get lucky and it is just a bad tube in the pto.

[N6YW]

Sounds like the PTO.  Sometimes the lubricating grease hardens and causes the cam on the slug thingy to ride flakily over the linearity guide.  Requires breaking the seal and working on the inside of the unit. See the articles in ER or in the compendium amongst available sources.

To rule out the crystal oscillator, try 160m, since the rcvr is single conversion on that band and no xtal is used.

Sorry, I misused the term. I know you meant Oscillator.
I replaced the PTO tubes and swapped out the mixer and IF tubes, same result.
I tune into a loud station on 40M for example, and it sounds like I am jogging the PTO back and forth randomly.
The task of dealing with the PTO disassembly doesn't bother me. Ughhhh.

[Todd, KA1KAQ]

Don't forget the passband tuning. It's mechanically linked to the PTO. Mine flakes out from time to time, needs a good going over.

[N6YW]

Thanks Todd.
I just now replaced the mica bypass caps on the SSB/CW detector (V-11) and the transformer grid feed cap into pin 2 of that tube. It improved the audio somewhat, but no real improvement on the "wiggle".
Before I commence taking the PTO apart, can anyone suggest something else?

[n2bc]

Before you disassemble the PTO,  make sure the ground strap (wiper) where the shaft enters the body is clean and has tension on the shaft.

[Bill, KD0HG]

Before you disassemble the PTO,  make sure the ground strap (wiper) where the shaft enters the body is clean and has tension on the shaft.

Yes, exactly. Before opening up the PTO...The PTO shaft needs to be clean and the wiper needs to press against the shaft at all times. See if that fixes the problem. Also check the DC voltages at the PTO tube socket to make sure they are stable.

[N6YW]

Before you disassemble the PTO,  make sure the ground strap (wiper) where the shaft enters the body is clean and has tension on the shaft.

Check!

Before you disassemble the PTO,  make sure the ground strap (wiper) where the shaft enters the body is clean and has tension on the shaft.

Yes, exactly. Before opening up the PTO...The PTO shaft needs to be clean and the wiper needs to press against the shaft at all times. See if that fixes the problem. Also check the DC voltages at the PTO tube socket to make sure they are stable.

Check!

Are there any molded mics'a within the transformer cans?

[WA1GFZ]

Why not find the problem and fix it rather thqn shot gunning guesses.

[N6YW]

Why not find the problem and fix it rather than shot gunning guesses.

Not sure what the problem is.

[k4kyv]

One way to tell for sure whether or not it is the PTO is to monitor the oscillator on another receiver (one with a BFO or CW/SSB mode).  The pto operates at 455 kc/s higher in frequency than the 75A4 dial readout on 160m, and has high enough output that you should be able to hear it with another receiver on the bench,  with a short wire connected to the antenna terminal and the other end poking inside the A4 cabinet.  Try poking the wire between the oscillator tube and shield if you can't find the signal with the wire just in the vicinity of the oscillator. Once you find the oscillation on the other receiver, turn on the BFO in the second receiver and get a heterodyne. If you notice the same wobble in frequency on the received oscillator signal, then the PTO is changing frequency.  

I assume you notice this shift in the CW/SSB mode, and you are hearing the beat heterodyne shifting about. If the main oscillator seems clean, look at the BFO. That could be the culprit, and the BFO is a lot easier to work on than the PTO. If you are hearing this shift in the AM mode, the PTO jump would have to be extremely severe to affect the signal as received through the pass-band of the i.f. filter.

I'm also assuming you  have eliminated a flaky problem in the  crystal controlled 1st oscillator circuit.

[N6YW]

One way to tell for sure whether or not it is the PTO is to monitor the oscillator on another receiver.  The pto operates at 455 kc/s higher in frequency than the dial reading on 160m. Once you find the oscillation on the other receiver, turn on the BFO and get a heterodyne. If you notice the same wobble in frequency, then the PTO is  changing frequency. 

I assume you notice this shift in the CW/SSB mode, and you are hearing the beat heterodyne shifting about. If the oscillator seems clean, look at the BFO. That could be the culprit, and the BFO is a lot easier to work on than the PTO. If you are hearing the shift in the AM mode, the PT

Don
Thank you. I will try that. When you refer to the "BFO" on the 74A-4, I assume you are referring to the PBT control. I'll hook up my HQ-170 and loosely couple an antenna close by. The thing is, no matter what mode or band, the problem is occurring, and with the crystal calibrator on, I can zero beat and it shifts right along with the RX signal.
Should I be concerned with the 175 vdc on the mechanical filter? If someone had a photo of a stock 75A-4 showing the area of V5 & V6, it would clear up some things for me because C-144 is missing and apparently never was installed, or was removed at some point for a mod. When I tried putting a .001 in place I lost a ton of gain.

[k4kyv]

I don't have the schematic right here in front of me, but one of the first modifications you want to make  to an early run 75A-4 is to take the +DC plate voltage off the mechanical filters.  A simple tube or component failure could place the full plate voltage across the input side of the filter and likely destroy it.  Collins later made the modification on their production runs.  Feed the DC to the plate of the tube through an rf choke, and couple the plate to the input of the filter through a disc ceramic blocking capacitor.  I use a 1000 volt rated capacitor on mine - in fact, two in series, just in case one ever shorts out. The bottom end of the filter is removed from the +HV and as I recall, wired directly to ground.

That mod is in the ER compendium, and no doubt can be found on line. Instead of a regular rf choke, in mine I used the coil from the remains of an old Heathkit Q-multiplier, which as I recall is self resonant in the vicinity of 455 kc/s.  It may have a slug to adjust the inductance, but I don't recall using a fixed cap across it.  I'll have to look next time I open up the receiver; don't think I documented and recorded the details of the mod. A single coil and cap from the primary of an old 455 kc i.f. transformer could be made to work.  The idea is to use a coil that is broadly resonant at the i.f., in order to get more gain than what you would get from a simple rf choke. I believe Collins just used the choke. The choice of choke may make a difference in any loss of gain you might experience.

That DC voltage on the mechanical filter could destroy the filter if something broke down and shorted to ground, but I don't believe it would have anything to do with the receiver jumping about in frequency.

[N6YW]

One way to tell for sure whether or not it is the PTO is to monitor the oscillator on another receiver.  The pto operates at 455 kc/s higher in frequency than the dial reading on 160m. Once you find the oscillation on the other receiver, turn on the BFO and get a heterodyne. If you notice the same wobble in frequency, then the PTO is  changing frequency.  

I assume you notice this shift in the CW/SSB mode, and you are hearing the beat heterodyne shifting about. If the oscillator seems clean, look at the BFO. That could be the culprit, and the BFO is a lot easier to work on than the PTO. If you are hearing the shift in the AM mode, the PT

Don
Thank you. I will try that. When you refer to the "BFO" on the 74A-4, I assume you are referring to the PBT control. I'll hook up my HQ-170 and loosely couple an antenna close by. The thing is, no matter what mode or band, the problem is occurring, and with the crystal calibrator on, I can zero beat and it shifts right along with the RX signal.
Should I be concerned with the 175 vdc on the mechanical filter? If someone had a photo of a stock 75A-4 showing the area of V5 & V6, it would clear up some things for me because C-144 is missing and apparently never was installed, or was removed at some point for a mod. When I tried putting a .001 in place I lost a ton of gain.

Here is the schematic of the area that is troubling me as it pertains to the filter.
Mine isn't wired in this manner, and the LC network feeding the plate of V5 and the wiper of the switch is missing entirely. Mine must be an early version before the schematic that I have. Sorry for the low res picture.

[N6YW]

I ordered the entire compendium today. This ought to help clear up some mysteries contained in my receiver, which is serial numbered 1621. Here is a photo I found of one that has been modified to match the schematic drawing that I have. I just may have the molded inductor too.

[k4kyv]

Here is the schematic of the area that is troubling me as it pertains to the filter.
Mine isn't wired in this manner, and the LC network feeding the plate of V5 and the wiper of the switch is missing entirely. Mine must be an early version before the schematic that I have.

Yep, that's what you have. C144 (1000pf) blocks the DC off the filter, and the bottom end of the filters go directly to ground. C145 and L32 form a resonant circuit at 455 kc/s, to eliminate the loss in gain that a simple rf choke would cause. In yours the plate goes directly to the filter and the bottom end goes to the B+ line. This is a disaster waiting to happen.  If the tube plate shorts to ground or somehow the wiring to the switch happens to get grounded out, the full B+ is directly applied right across each filter as it is switched in.  You could  wipe out all 3 filters in one shot!  Even if the tube doesn't short out, who knows how well the insulation inside the 50 y.o. filters will continue to hold up to the B+ voltage.

You need to immediately re-wire the circuit according to your schematic.  Use the best quality capacitor you can find for C144, and use something rated for several times the actual B+ voltage, since if that capacitor shorts out, the full B+ will appear across the input side of the filters.  Making this mod and using a poor quality capacitor is even worse than leaving the original circuit intact.  In mine, I use a couple of 2000 pf capacitors in series, each rated at 1 kv, for C144.  That way, if one of the capacitors were to short out, the other would serve as a back-up and save the filters from destruction.

C145 and  L2 form the tuned circuit.  Due to variations in components, just any 2 mh and 62 pf capacitor may not hit exact resonance. This would not only cause signal loss; it would detune the input coil in the filter, resulting in excessive pass-band ripple. If you can find a physically small mica trimmer, about 75 to 100 pf maximum, use that for C145.  That will allow you to adjust the input circuit for exact resonance. Tune for the best  compromise that results in the least overall ripple in all three filters.  Getting one filter perfect may result in more ripple in others, so a compromise may be necessary.  Collins decided to save on costs by using a fixed 100pf mica (C61, C130, C132 and C64, C131, C133), instead of a variable trimmer, across each end of each filter for resonance.  This makes it impossible to adjust the input and output of each filter for optimum performance. The filters still perform pretty well anyway, but this might be upset when you add C145 and L2 if they don't happen to resonate precisely at the i.f. frequency.

[N6YW]

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

[N6YW]

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.
Onward.

[KM1H]

The A4 PTO is notorious for defective ceramic caps and this has been documented several places on the web. Changing will affect calibration and you may have to remove the PTO a few times to get it on. It can be a PITA job and one I dont relish doing.

The ground wiper as mentioned is an important first step as is being sure its not a tube problem.

Have you tried a new PTO tube, V-15? You have 3 available right in the radio.

Carl

[N6YW]

The A4 PTO is notorious for defective ceramic caps and this has been documented several places on the web. Changing will affect calibration and you may have to remove the PTO a few times to get it on. It can be a PITA job and one I dont relish doing.

The ground wiper as mentioned is an important first step as is being sure its not a tube problem.

Have you tried a new PTO tube, V-15? You have 3 available right in the radio.

Carl

Carl
Thanks for the reply. Unfortunately, I am up for the task but don't look forward to it if I have to.
All tubes have been swapped with no change in the problem. I haven't ruled out a bad cap or resistor, but as soon as the radio comes on from a cold start, this behavior starts as the radio settles in. This doesn't sound
like a bad resistor problem, at least from a thermal standpoint.
After I get back from my NYE gig in PHX, I'll have another go at it. What I plan on doing then, is to go stage by stage making careful measurements of the components and voltages. While I have a very nice Sencore LC-75, I do not have a real honest in circuit able ohm meter. My Fluke 87 III isn't the ticket for that sort of thing. Thanks for everyone's replies and guidance. I'll see you guys on the other side.
Happy New Year.

[ke7trp]

I have three 75A4's here in PHX and you are welcome to come by and take pictures or inspect them if that helps you.  I have a low Serial, a mid and a very high last run unit.