The AM Forum
March 28, 2024, 01:05:45 PM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: B&W 5100B Modulation Observations.  (Read 6648 times)
0 Members and 1 Guest are viewing this topic.
KD1SH
Member

Offline Offline

Posts: 737



« on: January 18, 2020, 10:14:13 PM »

  Well, my B&W 5100B is all re-capped and functional; might even have it up in the shack and running for the rally.  Looking at the modulation on my REA Mod Monitor, though, I find the symmetry disappointing.  Very asymmetrical in the negative direction - can't even get a consistent 80% positive without banging 100% negative.  Realistically, I average around 65% if I want to avoid hitting 100% negative.
   But, a little experimenting reveals that if I load the PA lighter than the manual says, loading it for around 180 mils rather than the suggested 225 mils, the modulation becomes pretty nearly symmetrical.
   Looking at the schematic, I see that the rig uses self-modulating screens with both series resistors and a choke (20 H).  The choke must do most of the work because the resistors are only 100 ohms - probably more for equalizing than for modulating.
   Now, is a self-modulating screen circuit more sensitive to output loading than one that applies modulated B+ to the screens though a dropping resistor?  My Ranger uses the latter method and doesn't seem to care nearly as much about loading.
Logged

"Gosh, Batman, I never knew there were no punctuation marks in alphabet soup!"
由obin, in the 1960's Batman TV series.
KK4YY
Member

Offline Offline

Posts: 565


Your best isn't as good as you can be.


« Reply #1 on: January 18, 2020, 10:32:39 PM »

Might be a weak modulator tube. Try swapping the modulator plate cap leads.
Logged

All your worries won't add a day to your life, or make the ones you have any happier.
kb3ouk
Member

Offline Offline

Posts: 1640

The Voice of Fulton County


« Reply #2 on: January 19, 2020, 08:29:39 AM »

Definitely sounds like a screen modulation issue, my 814 rig uses a dropping resistor off the B+ same modulates great but if you put just fixed voltage on the screen it will act like that. Increasing the loading would be much like loading up a final for pure screen modulation. Really a plate modulated tetrode is probably mostly screen modulation, and modulating the plate is just what allows the efficiency to come back up to close to what a plate modulated triode does.
Logged

Clarke's Second Law: The only way of discovering the limits of the possible is by venturing a little past them into the impossible
KD1SH
Member

Offline Offline

Posts: 737



« Reply #3 on: January 19, 2020, 08:51:29 AM »

  That was the first thing I tried, being the easiest. Swapped the plate leads, then the tubes.  Today I'll try a pair of new (NOS) 6146's.
   The 5100B's modular construction is very nice, but in this situation it's a bit of a pain, since I can't probe into the modulator without removing it from the chassis.
   I've already gone through the modulator in my recap process: replaced the electrolytics, and both of the screen resistors in the RF deck, one of which had drifted up to 174 ohms from the original 100.  Everything else looked nice in there, but I'm going to go back into it today.
   I'm also going to get my LCR meter on that 20H screen choke; shorted windings in there would certainly banjack the modulation.  I'll check the screen bypasses on the final PA 6146's, too.
   Many other hands have been into this rig before I got it - I've already set right a couple of wiring errors, - one of which had shorted out the filter choke in the pi-section bias supply filter - so maybe there's something like that going on that I missed.

Might be a weak modulator tube. Try swapping the modulator plate cap leads.
Logged

"Gosh, Batman, I never knew there were no punctuation marks in alphabet soup!"
由obin, in the 1960's Batman TV series.
KD1SH
Member

Offline Offline

Posts: 737



« Reply #4 on: January 19, 2020, 09:06:54 AM »

  BTW: I also tried swapping the leads of the D104 crystal head - no significant difference.  Tried injecting a clean sine into the mic jack, and it's still asymmetrical, so it's not my voice (which probably is somewhat asymmetrical) or the microphone.  Also tried using my IHY box, which gives me positive asymmetry with the Ranger, and that made it just a tad better, but still strongly favoring the negative.
Logged

"Gosh, Batman, I never knew there were no punctuation marks in alphabet soup!"
由obin, in the 1960's Batman TV series.
WD5JKO
Member

Offline Offline

Posts: 1996


WD5JKO


« Reply #5 on: January 19, 2020, 12:32:39 PM »



I have no direct experience with the BW5100.

A good friend had two of them, and he discovered a few things.

On 40m with the internal VFO, these things FM...badly. There was evidence from prior owners to better shield the VFO cover. Using a direct crystal (7160) eliminates the issue.

The Audio modules between his two rigs were wired differently at the plug. Caused lotsa smoke!

The audio modulation was never right on either rig. Had to drive it hard to get the average up, and when doing so, was pretty wide. Most likely he was over modulating on the negative side like yours.

I just looked at the schematic from BAMA, and boy that schematic is busy, and the connectors sure clutter things up. Noticed R23 on the 6146 PA grid bias...22K. That means with 5ma grid drive, this resistor adds -110v grid leak bias on top of the fixed bias from the big tapped power resistor. What if this resistor has drifted up in value?? Does varying the grid drive have an effect on the modulated positive peaks?

Would be nice to have a scope, and a 100X compensated probe (~$20 on Ebay). Need to determine if the peak modulation is limited to audio power, or the RF side where the 6146's are limited for some reason.

Jim
Wd5JKO
Logged
ka8gef
Contributing
Member
*
Offline Offline

Posts: 34


« Reply #6 on: January 19, 2020, 02:19:55 PM »

I have a 5100B and have not experienced these issues. However, if you haven't talked to Marc Bell, K3MSB, you may want to give it a try. He is very experienced with this rig.

One more note- there is a filament dropping resistor on the 6u8 audio amp, I believe it is R415 and 4.7 ohms. This resistor takes a beating and it's value should be checked if you have not already done so.

ka8gef
Logged
KD1SH
Member

Offline Offline

Posts: 737



« Reply #7 on: January 19, 2020, 04:19:14 PM »




The Audio modules between his two rigs were wired differently at the plug. Caused lotsa smoke!


  I'm quickly discovering that this is the curse of many boat anchors: they were made in many iterations, with the differences being poorly documented, if at all.


I just looked at the schematic from BAMA, and boy that schematic is busy, and the connectors sure clutter things up. Noticed R23 on the 6146 PA grid bias...22K. That means with 5ma grid drive, this resistor adds -110v grid leak bias on top of the fixed bias from the big tapped power resistor. What if this resistor has drifted up in value?? Does varying the grid drive have an effect on the modulated positive peaks?


  I think you're looking at a schematic for the 5100, non-"B".  In the "B", that resistor is R318 and it's 3.3K.  The "B" doesn't have that big tapped power resistor.
  Grid drive changes don't seem to have any effect on the modulation.


Would be nice to have a scope, and a 100X compensated probe (~$20 on Ebay). Need to determine if the peak modulation is limited to audio power, or the RF side where the 6146's are limited for some reason.


  Yes - having a 100X probe would be fantastic!  Was out running errands earlier today and thinking that I wish I could put the scope on the modulated B+.  Thought of kluging up some sort of resistive divider, but the additional load might have a Heisenberg effect.

Logged

"Gosh, Batman, I never knew there were no punctuation marks in alphabet soup!"
由obin, in the 1960's Batman TV series.
KD1SH
Member

Offline Offline

Posts: 737



« Reply #8 on: January 19, 2020, 04:21:45 PM »

Good idea - I've seen K3MSB's site, and he sure seems to know the rig.  Pretty sure I checked that resistor, but I'll take another look.


I have a 5100B and have not experienced these issues. However, if you haven't talked to Marc Bell, K3MSB, you may want to give it a try. He is very experienced with this rig.

One more note- there is a filament dropping resistor on the 6u8 audio amp, I believe it is R415 and 4.7 ohms. This resistor takes a beating and it's value should be checked if you have not already done so.

ka8gef
Logged

"Gosh, Batman, I never knew there were no punctuation marks in alphabet soup!"
由obin, in the 1960's Batman TV series.
WD5JKO
Member

Offline Offline

Posts: 1996


WD5JKO


« Reply #9 on: January 20, 2020, 07:16:53 AM »


P4100 High-Voltage Oscilloscope Probe Kit 100:1 100 MHz fits Tektronix, HP

$18.95, Free Shipping. Ebay URL from US based seller:

https://tinyurl.com/wgo2x22

I use one of these with commercial 3KW Class E amplifiers at 13.56 Mhz. Takes about 5 years of daily use before they crap out. Not bad for under $20!

Jim
Wd5JKO
Logged
KD1SH
Member

Offline Offline

Posts: 737



« Reply #10 on: February 29, 2020, 12:16:26 PM »

  Okay, the asymmetrical modulation problem has been pretty much resolved - a balky 6146 in the modulator was mostly the culprit there.  But now, according to both on-air reports (sounds like a telephone) and sine sweeping (huge roll-off starting at 1 KC and almost entirely gone by 500 HZ), there's a massive audio bandwidth issue.
   My scope shows a nice flat response from 375 HZ all the way up to 5 KC (probably wider, but that's the span I used) when measured at the grid of the 6AQ5, so the 6U8 speech amp is working FB, but at the plate of the 6AQ5 the bottom catastrophically drops out of the low end.
   The secondary of the driver transformer isn't delivering any appreciable current to the grids of the 6146's, so I can't imagine any loading of the secondary playing a part here.  I'm suspecting the primary of the driver transformer.  Maybe shorted windings?  It's a simple modulator - no rocket science - especially in the drive circuit (see the attached schematic).  You can't see it on the little section of the schematic, but the cathode bypass cap on the 6AQ5 is partially bypassed by the 470 ohm resistor to ground in phone mode.  (The path from the 6AQ5 plate to the 6U8 cathode through the 220K isn't present in this version.)



* 5100B Modulator.PDF - Adobe Acrobat Reader DC.jpg (73.84 KB, 978x520 - viewed 432 times.)
Logged

"Gosh, Batman, I never knew there were no punctuation marks in alphabet soup!"
由obin, in the 1960's Batman TV series.
KK4YY
Member

Offline Offline

Posts: 565


Your best isn't as good as you can be.


« Reply #11 on: February 29, 2020, 02:00:25 PM »

Make sure the cathode bypass capacitor on the 6AQ5 (C405) is good. Probably best to just replace it.
Logged

All your worries won't add a day to your life, or make the ones you have any happier.
KD1SH
Member

Offline Offline

Posts: 737



« Reply #12 on: February 29, 2020, 02:26:34 PM »

Make sure the cathode bypass capacitor on the 6AQ5 (C405) is good. Probably best to just replace it.

  Every electrolytic has been replaced.  Original spec was 20 uf, new one is 22 uf.  confirmed also that it is indeed being shunted with the 470 ohm by the mode switch.  Tested the 6AQ5, too.
Logged

"Gosh, Batman, I never knew there were no punctuation marks in alphabet soup!"
由obin, in the 1960's Batman TV series.
KD1SH
Member

Offline Offline

Posts: 737



« Reply #13 on: February 29, 2020, 02:33:35 PM »

  More info:  K3MSB has a nice web page about his 5100B restoration, including his DC analysis of the driver and mod transformers.
  He says primary DC resistance is 379 ohms, while secondary measured 625 ohms.  I just pulled mine and checked it.  Primary measures 144 ohms, and secondary 250 ohms.
  Also checked the impedance: I measured 1K at 1 KHZ, and 242 ohms at 100 HZ.  Sounds awfully low to me, but I don't have access to any impedance specs for that transformer.
  Inductance of the primary measured only 1.3 H, which again seems awfully low.
Logged

"Gosh, Batman, I never knew there were no punctuation marks in alphabet soup!"
由obin, in the 1960's Batman TV series.
W4EWH
Member

Offline Offline

Posts: 833



« Reply #14 on: February 29, 2020, 03:12:06 PM »

Tried injecting a clean sine into the mic jack, and it's still asymmetrical, so it's not my voice (which probably is somewhat asymmetrical) or the microphone. 

Just as an experiment, please reverse the leads on your microphone. Sometimes broadcast stations find announcers with "negative" voices, and they have to provide them with a specially-wired microphone that only they can use.

HTH.

73,

Bill, W4EWH
Logged

Life's too short for plastic radios.  Wallow in the hollow! - KD1SH
KK4YY
Member

Offline Offline

Posts: 565


Your best isn't as good as you can be.


« Reply #15 on: February 29, 2020, 03:14:17 PM »

A lot of those driver transformers are pathetically small things. You can replace them with a low power modulation transformer and probably get better performance from it. I've used a 15 watt Stancor poly-pedance mod tranny as a driver and it worked FB. If you have something like that on hand you could clip lead it into the circuit as a test.
Logged

All your worries won't add a day to your life, or make the ones you have any happier.
KD1SH
Member

Offline Offline

Posts: 737



« Reply #16 on: February 29, 2020, 04:08:33 PM »

  It is indeed a pathetic little thing; mod transformer is no beast either.  I suspect that B&W was quite conservative in their design, giving the rig no more than they felt necessary in the modulation department.  The transmitter is rated at 140 watts input on phone; the manual claims that the modulator can deliver 75 watts of audio, but I think that's optimistic.  
  I'm going to look through my parts stash and see what I can find.  There are plenty of options - the tube audio amp places have lots of inexpensive transformers.
  One problem is the modular construction - it offers precious little room inside the modulator enclosure for anything bigger than stock.


A lot of those driver transformers are pathetically small things. You can replace them with a low power modulation transformer and probably get better performance from it. I've used a 15 watt Stancor poly-pedance mod tranny as a driver and it worked FB. If you have something like that on hand you could clip lead it into the circuit as a test.
Logged

"Gosh, Batman, I never knew there were no punctuation marks in alphabet soup!"
由obin, in the 1960's Batman TV series.
KK4YY
Member

Offline Offline

Posts: 565


Your best isn't as good as you can be.


« Reply #17 on: February 29, 2020, 04:26:31 PM »

Be sure to use a transformer with a gapped core as it will need to handle DC on its primary.
Logged

All your worries won't add a day to your life, or make the ones you have any happier.
KD1SH
Member

Offline Offline

Posts: 737



« Reply #18 on: February 29, 2020, 05:04:33 PM »

  Thanks - I'd forgotten that and was almost ready to order.  Thought of reconfiguring the circuit to feed the 6AQ5 plate with a resistor and then capacitor coupling to the transformer, but I'm not sure if I can get away with that without changing more things.


Be sure to use a transformer with a gapped core as it will need to handle DC on its primary.
Logged

"Gosh, Batman, I never knew there were no punctuation marks in alphabet soup!"
由obin, in the 1960's Batman TV series.
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone ゥ 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.098 seconds with 19 queries.