75A-4 mods: 1s that work well

Discussion in '"Boat Anchor" & Classic Equipment' started by W5INC, Jan 25, 2015.

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  1. W5INC

    W5INC Ham Member QRZ Page

    Replaced all of the known bad tubes, hooked the speaker up, plugged in the rig and threw the juice to it. Lights lit up and there was no smoke :) coming from the rig. No sound was coming out from the speaker, then I switched the mode selector over to the AM position and out came the noise. Was on 40 Ms and I could hear some strong signals 20 over, around 7.030. I was checking to see if the 100 KC calibrator was working but that was as dead as a door nail. The pass band tuning was working and I was able to copy some signals in the pileup around 7.030.

    After 15 minutes the signals dropped out and I tapped the 6DC6 RF amp and they popped right back up again. So I changed out this guy and there is plenty of sound now, but only in the AM position. I will clean the wafer switch with Tarnx (?) today, as I am thinking this is the preferred method of doing this job. I will throw the rig on the R&S signal generator to check alinement once I get the mode switch problem figured out. The SSB detector 12AU7 is new, but will check this out also. There is a noticeable hum in the rig when the noise limiter is up all of the way and you can also notice it when the AF is cranked all of the way down. I will have to check out the hum mod shielding to make sure it is grounded correctly also. Good to be making progress on this guy and I sure appreciate the help given forth by the Forum members here, nice enough to supply answers to my questions, on this BA. :)
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    I am not sure about using Tarn-X unless you can rinse it off.

    Old timers use WD-40. Not the best choice, but works.

    A good quality alcohol works for cleaning.

    Have fun.
  3. K9STH

    K9STH Ham Member QRZ Page


    When TarnX, or Goddard's Silver Dip (same thing), is applied with a cotton swab ("Q-tip"), it doesn't have to be washed off. It evaporates in a hurry.

    WD-40 works fine for things like potentiometers and "OK" for most switches. But, one needs to be VERY careful as to not get too much applied. Also, any cleaner, not just WD-40, can cause swelling of rotary switch wafers, especially those used by certain equipment manufacturers.

    Glen, K9STH
  4. W5INC

    W5INC Ham Member QRZ Page

    Back to the 75A-4 and wanting to iron out all of the problems with this guy. Been having a problem with the RF amp, the V2 guy that has been intermittent and needs to be taken care of 1st, I believe. Found the E2 part number, grid suppressor was shot and needs to be replaced. Has # 24 bus wire wrapped around it as part of the suppressor, which I was just going to try and use again. Resistor is open instead of being 47 ohms so I was just going to replace the resistor and hopefully not FUBAR up the bus wire in the process. Trying to find the plug in 2 prong calibrator can for this guy, but have had no luck as of yet. Hopefully this part isn't unobtainium at the moment. :)

  5. W5INC

    W5INC Ham Member QRZ Page

    I did clip 1 side of the bus wire off of the resistor lead to check on the resistor to see if it was good, but it was open and this is now changed. TNX for the emails. :)

    Been having a problem with the AVC circuit. All voltages check good on the Bias regulator V19 tube. All the voltages check good on the AVC noise clipper tube V16. Resistance measurements are off on pins 5 + 6 of both tubes. Measuring between pin 7 of the bias tube and pin 1 of the noise clipper tube it shows over 1 meg in the slow AVC position. It looks like to me it should be only in the 10K ohm range between these 2 tube pins. I don't have the face off the rig as of yet and can only access 1 side of the AVC switch. Checking the terminal I can access to pin 1 of the noise clipper tube it shows about 11K, which should be right. So it looks like the front will have to come off so I can check the other resistors that are on the switch/circuit. Hopefully not chasing my tail in this AVC problem and getting closer to finding out what the cause of the problem is. Definitely feel like the Tortoise and certainly not like the Hare, in this project. :)
  6. W5INC

    W5INC Ham Member QRZ Page

    I bought the smaller number of pieces for the Bristol spline wrench set, that works on the S Line but doesn't have the correct size spline wrench to take off the A series knobs. :( The knobs need to come off, to get the face of the radio off. I have the spline wrench for the big 4 to 1 VFO knob, but the knob set screws are a tad bigger on the A line then the S Line. Looks like I will now need to buy the big set to gt the right size Bristol wrench. Somtimes the old saying is true: Penny wise and pound foolish. :mad:
  7. K9STH

    K9STH Ham Member QRZ Page

    The X'cellite set that has the handle and several different sizes of Bristol wrenches is well worth the money.

    I have the small, blue plastic holder versions, as well as the X'cellite set. Most of the time I use the X'cellite ones. There are occasions, where space is really cramped, that the small, "L" shaped, wrenches have to be used. But, most of the time, there is plenty of room and the X'cellite versions are definitely easier to use.

    Glen, K9STH
  8. W5INC

    W5INC Ham Member QRZ Page

    TNX Mr. Glenn, I am glad I seen this post before ordering the Bristol wrenches. I have the 508 small Blue plastic holder set for the S Lines and didn't realize I would need different sizes in the future, before the A Line bug bit. I am actually enjoying working on this piece of iron, although a few cuss words come out every now and again in the process. No real progress happens without a struggle. :)
  9. W5INC

    W5INC Ham Member QRZ Page

    The 508 set does indeed have the correct size wrench, it was just that the 5 of set screws in the knobs themselves have been FUBARed up sometime in the past. Time to drill out the set screws that are toast to get the knobs off. Just the normal course of problems that are associated with rigs that are 50+ years old, I reckon. :)
  10. K9STH

    K9STH Ham Member QRZ Page

    I got this set, if I remember correctly, from a dealer on Amazon.com several years ago. There were quite a few different prices for exactly the same thing. Of course, I added the shipping charges to the price of the set and bought the one that had the lowest total price.

    002a.jpg 005b.jpg

    Glen, K9STH

    P.S. The "system" doubled up the 2nd photo!

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