Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Oct Sat 16, 2021 9:06 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 11 posts ] 
Author Message
 Post subject: 75A-4 signal/voltage flutter
PostPosted: Sep Tue 21, 2021 3:51 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 449
Location: Missoula, MT
Freshly recapped, NOS tested tubes, voltage-checks, etc. Receives well enough but have not gone thru RF alignment yet.

Issue is that there is some mild 'flutter' going on at about a 5 Hz rate. The OA2 regulator glow dims slightly at this rate also. I've tried several OA2's with no change. Is there an oscillation going on? But where? The flutter is not steady - it will come and go, but always at the same rate when it occurs and it is more apparent in CW-
SSB mode than in AM mode.


Top
 Profile  
 
 Post subject: Re: 75A-4 signal/voltage flutter
PostPosted: Sep Wed 22, 2021 2:40 am 
Member
User avatar

Joined: Nov Mon 02, 2009 7:01 am
Posts: 5376
Location: Lincoln City, OR 97367
Greetings to stevef and the Forum:

Sounds like a de-coupling problem to me. If the regulator tube glow is fluctuating, then the oscillation is on the B+ buss. Get out your scope and scope the unregulated supply to the 0A2 as well as the regulated voltage. One or the other will be oscillating in level. Usually, this is due to what I referred to above as a de-coupling problem. De-coupling is what filter capacitors do... also bypass capacitors. Shunting unwanted signals to ground or "de-coupling" those signals from the rest of the circuitry. I would guess that a capacitor from B+ to ground is not installed correctly or was missed altogether in your re-cap. Be sure that all filter capacitors and bypass capacitors are correct as to value and connected to the correct points around the receiver circuitry. Pay particular attention to capacitors around the regulator itself or the audio output stage... these are the first suspects because they are in relatively high current (low impedance) circuits and thus their absence can have a large effect on the receiver's B+.

Good Luck,

_________________
Jim T.
KB6GM


Top
 Profile  
 
 Post subject: Re: 75A-4 signal/voltage flutter
PostPosted: Sep Wed 22, 2021 3:55 am 
Member
User avatar

Joined: Oct Sun 15, 2006 12:57 pm
Posts: 7885
Location: Liberty, Missouri
You might also check the value of the dropping resistor feeding the regulator and the load of the circuits on the regulated buss.

This sounds like a classic relaxation oscillator issue. You might want to read up on them starting on page 14 of THIS book. A gas regulator is just a big glow lamp, and under similar conditions will oscillate.

_________________
Pics, Bold & underlined text, are usually links.

https://mikeharrison.smugmug.com/

Mike


Top
 Profile  
 
 Post subject: Re: 75A-4 signal/voltage flutter
PostPosted: Sep Sat 25, 2021 3:51 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 449
Location: Missoula, MT
I scoped the B+ line and there was a 20v p-p ripple at 6.14 Hz, with a small amount of 120Hz ripple (about 1v) riding on that. The ripple was a sine wave and not a sawtooth as shown in the relaxation oscillator reference you sent Mike.

I poked around quite a bit checking resistances against the Table 5-1 in the manual, and isolating various stages.
In the end, I found that when I disconnected the B+ line to the PTO from pin 5 of the OA2 regulator, the voltage ripple stopped. When reconnected, it starts fluttering again.

So now I am going to check the components in the PTO.

UPDATE: On second thought, disconnecting the lead from the OA2 only takes the regulator out of the circuit and keep B+ (higher and unregulated) on the PTO.

Additionally, while checking resistances (calibrated Triplett 850 VTVM), I found pin 2 of the rectifier measures about 13.7k and the table indicates it should be 22k. This was true whether the PTO B+ line was connected or not. Manual says 85 watts power draw, but I am getting 0.9A at 115v line (103w). Further, Page 5-0 of the manual says 170v for the B+, but Table 5-1 shows plate voltages for several tubes of 185 and 190v.


Top
 Profile  
 
 Post subject: Re: 75A-4 signal/voltage flutter
PostPosted: Sep Sun 26, 2021 7:41 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2379
Location: Mt. Airy, Maryland
I'm betting someone hung a electrolytic cap somewhere in the regulated B+ line. Find the cap and remove it - that will create a relaxation oscillator. T= RC, with the 2500 ohm dropping resistor in series with the 0A2 I'd look for a 40-60 uF cap that someone tied on the regulated line.


Top
 Profile  
 
 Post subject: Re: 75A-4 signal/voltage flutter
PostPosted: Sep Sun 26, 2021 8:03 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 449
Location: Missoula, MT
The only place left such a cap could be hiding is inside the PTO, which I have yet to open up. I'm trying to narrow down if that i place to look or not. I've replaced all other electrolytics including a new Nationwide Radio 3-section filter can. Here's another clue I found yesterday: the ripple/flutter is worse when the receiver is upside down vs right-side up (?!)


Top
 Profile  
 
 Post subject: Re: 75A-4 signal/voltage flutter
PostPosted: Sep Sun 26, 2021 8:15 pm 
Member

Joined: Apr Wed 01, 2020 7:22 pm
Posts: 327
w3jn wrote:
I'm betting someone hung a electrolytic cap somewhere in the regulated B+ line. Find the cap and remove it - that will create a relaxation oscillator. T= RC, with the 2500 ohm dropping resistor in series with the 0A2 I'd look for a 40-60 uF cap that someone tied on the regulated line.



That's what I'd be looking for, especially when the OA2 is reacting in time with the flutter....!

73...Jordan VE6ZT


Top
 Profile  
 
 Post subject: Re: 75A-4 signal/voltage flutter
PostPosted: Sep Mon 27, 2021 2:28 am 
Member
User avatar

Joined: Nov Mon 02, 2009 7:01 am
Posts: 5376
Location: Lincoln City, OR 97367
Greetings to stevef and the Forum:

For Mike: I could not open your link.... the bar graph goes fully to the right as if finished, but the process stalls at that point. Fortunately, I already had the glow lamp manual saved so I read it there.

One thing stands out: The relaxation oscillator depends on the difference between the firing potential and extinguishing potential of the glow lamp. In order for oscillation to occur, the lamp must go back and forth between a conducting and a non-conducting state. It is possible to get a sine wave output from a neon relaxation oscillator, but only at frequencies approaching 50 KHz. The oscillation reported by Steve is much slower than that.

Steve also writes:
Quote:
The OA2 regulator glow dims slightly at this rate
(The rate being 5 Hz). Note that he does not report that the 0A2 extinguishes; only dimming. Therefore, the relaxation oscillator theory, at least that pertaining to the 0A2 as the active element, does not seem likely to me at this time.

However, the relaxation oscillator idea may be sound if we switch the active element away from the 0A2 to somewhere else. I would replace both tubes in the PTO and see if the problem remains. If it is cured thereby, one of the PTO tubes is gassy.

Regards,

_________________
Jim T.
KB6GM


Top
 Profile  
 
 Post subject: Re: 75A-4 signal/voltage flutter
PostPosted: Oct Thu 07, 2021 8:10 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 449
Location: Missoula, MT
After narrowing down the oscillation in the audio circuit (as it turns out it was only happening at low AF gain), I looked around checking more capacitors/voltages/tubes with no luck. I finally just removed the famed/recommended K7CMS audio mod (returned back to stock) and the oscillation went away.


Top
 Profile  
 
 Post subject: Re: 75A-4 signal/voltage flutter
PostPosted: Oct Fri 08, 2021 1:44 am 
Member
User avatar

Joined: Nov Mon 02, 2009 7:01 am
Posts: 5376
Location: Lincoln City, OR 97367
Greetings to Steve and the Forum:

Glad you found it. Also, thanks for posting the cause. Some equipment is better engineered than other equipment.... the probability of success and / or usefulness of a mod depends greatly on the quality of the original engineering. Collins engineers were pretty good..... non-factory mods to Collins gear should be approached with caution.

Regards,

_________________
Jim T.
KB6GM


Top
 Profile  
 
 Post subject: Re: 75A-4 signal/voltage flutter
PostPosted: Oct Sat 09, 2021 1:20 am 
Member

Joined: Mar Fri 29, 2013 4:46 am
Posts: 233
Location: Livermore, CA
stevef wrote:
After narrowing down the oscillation in the audio circuit (as it turns out it was only happening at low AF gain), I looked around checking more capacitors/voltages/tubes with no luck. I finally just removed the famed/recommended K7CMS audio mod (returned back to stock) and the oscillation went away.


I've been following this thread in the back ground with out any ideas for solving your problem, but once you mentioned the K7CMS audio mod "DING".
I had what I think was the same oscillation using the K7CMS audio mod in my A4. Fortunately the funky audio was noticed immediately I tried a couple of things nothing worked so I reversed the mod and bingo problem solved.

Dave


Top
 Profile  
 
Post New Topic Post Reply  [ 11 posts ]  Moderator: Sandy

All times are UTC [ DST ]


Who is online

Users browsing this forum: Google [Bot] and 6 guests



Search for:
Jump to:  


































Privacy Policy :: Powered by phpBB