K3MSB

B&W 5100 CW/AM Transmitter




Service Notes & Addendum/Errata


I used a B&W 5100B as a Johnny Novice in the early 1970's and currently use one as my main AM transmitter. The B model is an excellent radio for AM -- it has good audio quality and will produce around 90 Watts of carrier. I've many a fond memory of nice rag chews on 10M AM in the early 2000's as we came off the peak of the last solar cycle...

My fondness for the "B" made me want to pick up the 5100, the "B"s predecessor. The Barker and Williamson 5100 is a CW and AM transmitter produced between 1954 and 1956.

One could purchase the separate 51SB SSB generator if desired. The 51SB required the 5100 to be modified accordingly, and B&W provided a kit for this purpose. Factory wired 5100 transmitters were designated "5100S". I have an old QST advertisement for the "S" variant, but have never seen one. Check out the "service notes" link above for details. One can determine if one has an "S" or "straight" variant by the presence (or lack thereof) of the "NORM/SSB" switch located to the right rear of the RF compartment.

I picked up this 5100 in Pittsburgh around 2005.

Click on any image for larger version... Latest entries are last on the page!!

1 Oct 2007

From the shelf to the bench.....

4 May 2008

During the winter, I tried preliminary testing of the radio. To make a long story short, the RF Multiplier unit was so filthy that erratic operation resulted. The SSB/AM switch needed to be completely disassembled, cleaned, and restored. I figured if I was going to do that, I might as well disassemble the entire RF Multiplier stage for cleaning and restoration. Since I wanted to get the radio on the air as soon as possible, I used an RF Multiplier stage from a second 5100 I had on the shelf.

I remembered that C36 (grid current adjustment by the front panel "Exciter" control) had been damaged when the second 5100 was shipped to me. The control was pushed back causing the rotors and stators to touch. So, I used a small screwdriver to bend them apart, and tried initial tune up. This didn't work too well. Ip and Ig were very high, and I didn't catch it fast enough and managed to zorch screen resistor R25 (or R26).

I removed the RF Multiplier stage and proceeded to remove C36 in hopes of further repair. While "fixing" the cap, I cracked the ceramic casing. I figured some JB Weld would fix the cap, but I wasn't confident I could do anything to help the rotor and stator issue. My buddy Dean K5DH provided me with a replacement for C36. The replacement was 36 pF instead of the required 28 pF, but I figured that aside from being more touchy on tune up, it should work just fine. Besides, it was free and fit the mounting holes.....

Here are some pix of the repaired RF Mutlipler unit. I've annotated some of the caps for reference, and also Bandswitch Wafer "E".

8 May 2008

Replaced the RF Amp bias pot and associated resistors. The 450V cap is overkill, but when it's the only part in the junkbox of the correct value...

Meter Shunt Resistors.

Here's a picture of the meter shunt resistors on the Meter Selector switch.

The schematics do not mention the values for these resistors, only that the grid resistor provides full scale meter deflection at 40 mA, and the other two provide full scale deflection at 400 mA.

I measured the grid current shunt resistor to be 5.2 ohms (both on my DVM and VTVM).

I tried to measure the other 2 shunt resistors, which should be 1/10 of the grid shunt resistor (0.52 ohms) as they both want full meter deflection at 400 mA. My DVM gave me a reading of 0.7 ohms, and my Heathkit VTVM was around 0.3 ohms, so not a good correlation there.

However, the VTVM and DVM did give me the same value for the grid position, that being 5.2 ohms. So, one can infer that if 5.2 ohms is supposed to give full deflection at 40 mA, then the correct value for the other 2 shunt resistors should be 0.52 ohms to give full scale deflection at 400 mA.

Jim W5JO sent me the following: "I just dug out the old switch with the shunts attached that I replaced in my 5100. My Triplett and digital meter said 7.5 and .7 ohms. Neither can read below on ohm to more than one place"

So, what are the correct values for these shunts? Good question; we have some data points. When I start indepth work on the 5100, I'll gin up a test circuit and see what provides the correct meter deflection.

Well, here's a shot of the old girl on the bench; from the pots on the right, it must have been while I was replacing the RF Amp Bias Adjust pot. Yeah, yeah, I know, fix the shelves.......

14 July 2008

Made several contacts on 10M AM as well as one on 15M AM over the past several weeks. Here's the 5100 paired up with my Hallicrafters SX-101A:

Power output is a bit low on 10M, but I didn't align it before initial testing. I wanted to basically make sure that the radio played before I spent the time on a more in depth restoration. Audio reports have been good... but.... there's always a but.... two folks said my audio sounded like it was going into "oscillation"; this was accompanied by a very slight crackling from the modulator stage. The first time this happened it went away and the audio returned to normal. I made several contacts afterwards with no one reporting any problems. The last contact I again heard the crackling, and the audio remained degraded. I experimented by turning down the audio gain (barely enough to modulate) and was told the audio still sounded bad.

So; time for detailed work to begin. This will start with a thorough check of the modulator stage and upgrading the audio as per the mods provide me by Moe W5KD some years ago.

30 July 2008

Removed the modulator subchassis.

The modulator subchassis is secured by 6 screws under the main chassis. You do NOT have to remove the 2 nuts securing C71 as indicated in the following photo. The screws that secure those 2 nuts are accessible from the top of the main chassis (that is covered by the modulator subchassis.....).

Unlike the 5100B which connects the mic and audio pot control to the subchassis via plugs, you have to unsolder all the connections on the 5100 and feed them through a hold in the main chassis.

Some corrosion is evident on the inside of the subchassis. There are also components at the base of the 6U8 socket which is on the subchassis "tower" that may need replaced. Right now, I'm thinking of removing everything from the subchassis, not just replacing components that need replaced. If there's visible corrosion around one tube sockets, who knows what lurks under other sockets or ground lugs!!

12 October 2008

The 5100 paired up with a BC-342-N for the Fall Classic Exchange on 80 and 20 CW. The modulator subchassis is still removed.

Updates to follow......

Copyright Mark S. Bell 2007-2008