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

    I stepped back for a few days from the rig to reevaluate the proceedings. The AVC does work but the negative bias is out of kilter on most tubes. Spent some time tracing out the bias circuit and all roads lead back to the SSB detector tube. I changed C95, the CW detector audio output coupler already and found out it is now toast. Checking around I found that the noise limiter output coupler cap C96 is out also. So it is good I guess that the C95 cap went out and made it easier to find what might have been a problem with the rig for a while now. Getting closer said the tortoise. I don't feel too bad as there is a well known AR OP who has some videos documenting his repairs on YouTube. He spent over 10 hours looking for the problem with his particular 75A-4 and has the videos to prove it. I did learn some good pointers from watching a few of the videos and will use his very good video when aligning up the 75A-4, once all repairs are completed. :)
  2. W5INC

    W5INC Ham Member QRZ Page

    This rig is beating me like a rented mule to say the least. trainwreck.gif I am going to have to look for an early schematic for the 75A-4 which mine is. The schematic I am using belongs to the late 75A-4s and has me scratching my head. Looking at the SSB detector tube V11 according to the schematic there should be L28 @ 2 MH coil coming off pin 6 along with 2 @ 220pf caps going to ground, C90 + C91. Looking thru the Collins 75A-4 manual I have, it shows L28 should be located right around V11 and has no listing at all for C91's location. :confused: I sure can't find either piece. C90 becomes toast when replaced. This cap does indeed come off pin 6 and goes right to ground on a terminal strip right where R59 is grounded also. This latest schematic doesn't show pins 4 and 5 either on V11 as my box has these 2 pins tied together with pin 4 going to ground and a mica cap coming off pin 5 to the terminal strip where R 60 resides. I see there is a diode load test point in this circuit but the manual gives no information on how or why this test point is there. Any ideas on the use of this test point would be appreciated as stumbling around here in the dark is certainly no fun. :(

  3. K9STH

    K9STH Ham Member QRZ Page

    Those points are used in the alignment of the 455 kHz i.f. and are referenced in Section V.5 of the older manual.

    If you need a PDF of the older manual, contact me, by E-Mail, at:

    k9sth (at) sbcglobal (dot) net

    and I can E-Mail you the files.

    Glen, K9STH
  4. W5INC

    W5INC Ham Member QRZ Page

    Thank You Mr. Glenn, might make things a tad easier knowing what is factory specs and what is actually inside of the 50+ year old box. I have hacked out some previous modifications done by past owners and getting the rig back to stock should help to get it back on the bands. TNX agn! :)
  5. W5INC

    W5INC Ham Member QRZ Page

    Are the 2 capacitors on top considered to be polarized capacitors? Taking some time off working on a 65 Chebby C20 PU truck and looking at this project some more. Starting to feel like Ahab and the White whale, as this rig is starting to "task" me. :D

  6. K9STH

    K9STH Ham Member QRZ Page

    No! Those are silver mica capacitors and are among the "deadly" capacitors that should be replaced.

    Those are coupling capacitors and I have found that disc ceramic capacitors work to replace them. Of course, there are other mica capacitors that can replace them as well.

    Glen, K9STH
  7. W5INC

    W5INC Ham Member QRZ Page

    TNX Mr. Glenn, 100% correct in your description of coupling caps. Still have the original problem with the rig as it keeps destroying new caps I have put in. I have cut out some more added disc ceramics that weren't on schematics, either the early (TNX Mr. Glenn) or later versions, as these had to have been added by an end user at 1 time in the past. It looks like it is very close to stock condition once more and hopefully make it easier to find the problem. The 65 Chevy truck electrical system pales in comparison to the 75A-4s. :D
  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    What do you mean by "destroying new caps" ? What voltage caps are you using ?

    Good Luck on the Chevy.

    By the way the arrows help to tell you the direction on reading the cap value.


    Last edited: Apr 12, 2015
  9. K9STH

    K9STH Ham Member QRZ Page

    The highest voltage in the 75A-4 is listed on pin-5 of V-22 the plate of the 6AQ5 audio output tube. That tube is fed, through the audio output transformer, directly from the junction of the 2-filter chokes. The manual lists 220-volts at 117 VAC input to the receiver. Even with the higher line voltages of today, that voltage should not exceed around 250-volts.

    I use only 400-volt capacitors, or higher voltage, in the 75A-4. If you are having capacitor failures, then I strongly suspect that they do not have a true 400-volt, or higher, rating.

    Has someone replaced the 5Y3 rectifier tube with solid-state? Even with solid-state, the maximum voltage shouldn't be more than around 270-volts.

    Has someone put an electrolytic capacitor between pin-8 of the 5Y3 and the input to the first filter choke L-29? If there is a capacitor there, that changes everything because that would make the power supply a capacitor input instead of a choke input. Doing so would increase the voltage, in the receiver, considerably. If there is such a capacitor, remove IMMEDIATELY!

    The truth be known, most of the black beauty / bumblebee capacitors, since they are used as bypass capacitors, can be replaced with disc ceramic capacitors instead of the "poly" types that most of us use. There are exceptions, especially where coupling capacitors are in audio stages including A.V.C., or in certain filter applications. However, the majority of the paper type capacitors were used as bypass capacitors. In the S-Line, bypass capacitors are disc ceramic.

    Glen, K9STH
  10. KM1H

    KM1H Ham Member QRZ Page

    Those caps are well known for SMD (Silver Mica Disease)
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