The AM Forum

A few weeks ago I acquired a 75A-3 and have gone through and replaced caps and tested and replaced resistors. I want to do the Service Bulletin 2 that changes the RF Amp's 6CB6 to a 6DC6 (or 6GM6 as some have recommended). However, I need to access the innards of the rig's Mechanical Filter assembly for some of these changes.  Any tips, suggestions, or warnings on doing this?

Thanks...Harry

Harry,

I am puzzled as to why you need to get into the mechanical filter for an RF amplifier tube change?   Seems to be quite a bit downstream from the 1st RF stage! 

Joe-GMS

Joe,

Me too. However, the last step in the bulletin states "remove AVC from pin 3 of the mechanical filter box assembly. Connect pin 3 filter box to junction of R57, R58 (RF gain control and minimum bias resistor).

Harry

I have been following this thread with interest, because I currently have my dad's 75A3 SN 1075 on the bench, working on an elusive AGC problem.  I am considering the application of Service Bulletin 3B when I get everything else resolved.  (Note:  Service Bulletin 3B supersedes SB 2 and SB 3A.  I would ignore the obsolete documents and apply the changes in the document linked below.)

Here is a link to Service Bulletin 3B:  
http://www.collinsradio.org/cca-collins-technical-archives/collins-radio-service-bulletins-and-information-letters/

In step 5 of the service bulletin, the procedure calls for verification that pins 2 (suppressor grid) and 7 (cathode) of the second IF amplifier  (6BA6 V-18) are indeed grounded.  Rather than disassemble the unit to inspect it, perhaps it would be easier to just remove the tube and test for continuity at the tube socket.  If they are not grounded, then it would be necessary to disassemble the filter chassis to resolve.

Step 6 requires changing the AGC connection to pin 3 of the filter chassis, and this should be possible from under the main chassis without disassembly of the filter chassis.  Pin 3 connects to 10K R77 inside the filter chassis, which should not need to be altered.

Rick,

Thanks....The serial number of my 75A-3 is 1367 ... hence I was looking at Bulletin 2.

I too am having an AVC issue I'm tracking down.

Harry

It seems Service Bulletin 3B applies to earlier models with serial numbers 1299 and lower, using the high-loss mechanical filters, while Service Bulletin 2 applies to serial numbers 1300 and higher, using the lower-loss mechanical filters.

We can assume which bulletin applies to our receiver based upon model number, if it has filters originally installed in the factory.  However, if the filter was installed later, it may or may not be compatible with the high or low loss specification.

I believe the filter in my receiver serial 1075 is original, high-loss, but I cannot be 100% sure.  My filter type no is F455B-31, and on the side it is identified as Collins part number 5926-2924-002.  There is no other uniquely identifying information on the filter case.  The only relevant information I found regarding this filter ID can be seen at http://jlkolb.cts.com/site/MFid.htm

By the way, in regard to 75A3 Service Bulletin 1 addressing internal receiver noise, I discovered a discrepancy in the value of the resistor to be added.  In step 3, it specifies a 100,000 ohm resistor to be connected between pins 1 and 2 of the mechanical filter assembly, however in the explanation below it indicates a 10,000 ohm resistor is available from Collins.  Further, it is not clear how this resistor would improve the noise figure, considering the schematic shows a 4,700 ohm (R81) resistor at this point.  

The schematic was printed from the 75A3 instruction book dated 15 April 1954.  Following the cover page, the 75A3 addenda indicates that resistor (R81) should be changed to 10,000 ohms.  So it appears that 10,000 ohms is probably the correct value for Service Bulletin 1.  These changes should be verified before proceeding with Service Bulletin 2 or 3B.

Rick,

I noticed the same. I also suspect there may be variations in the parts lists and schematics based upon when the manual being used was published.

As for the Service Bulletins....regardless of serial number one could start with Bulletin 2...see how things work/improve then proceed with 3B by just adding the recommended resistors for the subsequent steps as the first half of 3B is basically the same as Bulletin 2.

Harry

Ok...I'm embarrassed (to the point of being humiliated) to ask this....but the last step of Bulletin 2 is to connect pin 3 of the filter box to the junction of R57 and R58. I found these on the schematic but for the life of me not the actual physical location. Guidance please.

thanks....harry

R58, 560 ohms (green-blue-brown) is located under the chassis on the left side, just to the right of T3.  I attached three photos, one very close up.  Follow the curved green wire, the resistor is just to the right of the cable clamp.  R57 is the RF gain control. 

The point to connect to is where the green wire attaches to the resistor, toward the back of the chassis.  The end of the resistor toward the front of the chassis is grounded.

Rick,

Thanks...it's where I suspected, but for some odd reason I had doubts.

I take it that after removing the AVC line from pin 3 of the mechanical filter box I leave the line unterminated.

Harry

Update on my AGC issues:

My 75A-3 worked fine in CW, but was DEAF in AM.  Worked fine before the last AM Rally.  I had to use the 51J-4 for that event.  After much head-scratching, we located the problem.  C82, at the grid of AGC amplifier V9A was leaky, causing the bias to shift to the point of shutting down the IF stages.  I also discovered R35, at the plate V9A was 330 Kohms; the schematic indicates it should be 100 Kohms.  After these components were replaced, all is well, ready for complete alignment.

Thanks to W4BYT who assisted in the troubleshooting endeavour.

It is interesting to note that the failed capacitor was replaced previously with a black beauty.  No problem found with any of the original brown dog-bone capacitors, which are reportedly more reliable than anything available today.  I have found other resistors in this receiver that have gone high in value and needed to be replaced, but other than electrolytic capacitors, no others needed attention.  In addition, I discovered that V1 was indeed a 6DC6, and the Service Bulletin 3B updates were already applied!

Harry, happy to help, and glad you are making progress on your receiver.  

The AVC line is just a low-voltage DC bus, no need to terminate the disconnected wire.  Just make sure it does not inadvertently make contact with anything.

I usually leave original wires in place if I make modifications, as long as they do not create problems.   This leaves a bread-crumb trail when visiting the receiver at a later date, as the circuit no longer matches the original schematic.

I also make a copy of the schematic, and pencil in all the changes made, including dates and reason for changes and the reference service note identification.

73, Rick

Rick,

Thanks again.

I replaced all the electrolytics and paper caps about 2 weeks ago and then checked all resistors with only 4 or 5 having to be replaced. Audio on AM and CW was very good. SSB not bad either.  Not leaving well enough alone I installed a Treetop Circuits SB-75 product detector (did this with a R388 and it works beautifully). Installation was relatively trivial. However, I lost the BFO signal and AM seemed not as good as before. Add to that I was getting some odd products while listening to AM signals. After hours and hours of screwing around with this and over thinking the problem I went back to the basics.  First, I found a broken resistor lead on V12. By looking at it one could not tell the lead was broken, but prodding it with a soldering tool revealed the problem. Repairing this solved the BFO issue and the product detector began to work as expected.  The odd signal products  issue while listening to AM QSOs was strange as before they weren't present. Turns out the 1st and 2nd mixer tubes were problematic....1st mixer was virtually dead and 2nd wasn't far behind.  Also discovered a resistor on the hairy edge of tolerance. What's odd is both tubes were tested a few weeks ago and appeared to be fine.

Just completed Bulletin 2 and all seems to be good.

I do have a slight "motorboating" in the background when no AM signal is present. Not bad, but since I know it's there it's a "problem".

Harry

Hi Harry,

You are welcome.  I am glad that I was online when you requested assistance, and had the receiver on the bench so photos were easy.

I wonder if you have resolved your AGC problem?  It took a while to fix mine, mainly because so many components are jammed above the V9 tube socket, just hard to see the components clearly.  I hope your motorboating problem is not too difficult.  In audio circuits, problems with power supply decoupling capacitors often cause motorboating. 

I looked at your Treetop adaptor, very interesting approach.  Mine has an SSB adaptor which plugs into the NBFM accessory socket, and requires no mods to the receiver.  The upper/lower sideband selection is variable using an internal oscillator, and tuning is done through a shaft extension and a hidden hole in the panel.  To restore to original, just remove the shaft and knob, and re-install the Collins meatball.  I will attach a few pics.  Mine was manufactured by Universal Service Products, Columbus, Ohio, model A2-3.

73, Rick

Rick,

AGC problem...not quite. Not as bad as before, however, when poking around a couple of weeks ago I noticed someone clipped the micas on V-9 and V-11. These are grid filter caps and I'm not convinced the one on V-9 would affect the AGC. Although micas are said seldom leak I've read that Collins had issues with some micas produced by Micaform. Anyway, I'm awaiting some replacements to arrive today and will see if new ones improve things.

Harry

Well when you get it fixed, it will be a nice present for your bro-in- law, just saying

I've got one that came with a 75A-2.
Works gangbusters!
Maybe the best sounding SSB I have ever heard!

I have a partial schematic...

Two issues:
- the original did not use a shaft bushing on the front panel, it needs one.
- the angle from the front panel to the unit is not quite right.
- the unit itself kind of floats on the octal socket, would be better if there was a solid bracket
to hold it.

(best to use a much better shaft coupler, imho... i think a bellows type would be ideal. )

Ok, Three issues. :D

Hello Bear,

I attached the instruction document and a jpg of the schematic. 

I agree the shaft alignment is not perfect.  But I have not had any problem with using the control.  I don't believe a panel bearing is necessary.  Besides, the flange of a panel bearing would prevent the flush mounting of the legend for tuning.

Regarding the stability of the unit, the instructions provide for mounting the adaptor to the back of the cabinet.  Of course, this is practical only if using the standard Collins cabinet, and would not be an option in a standard rack cabinet with other devices.

I agree it provides excellent sideband detection, but I rarely use it, as the mode selector is normally in the center position, "AM"....

73, Rick

I'd make the hole in the legend plate larger to accommodate the shaft bushing.

according to some conservancy group letter Collins issued an equivalent tube sheet which is available .... the 6dc6 is equivalent to a 6cb6 .... I have used them interchangably in S line .... very small differences noted ... John

Well regarding the AGC issue with AM.....it appears this may be mechanical. With the A-2 upside down to work on it the distortion is there. Right side up and in the cabinet the distortion has disappeared.  I removed it from the cabinet to check again. I have not found anything obvious that could be causing it. Tubes have been replaced and re-seated. Component connections have been poked and prodded.  Heading the wisdom of WA1KPD (my brother-in-law) I'm going to leave it alone until I replace two 2-watt resistors when they arrive.

Thanks to everybody for their help....

Harry WE1X

BTW, for anyone working on a 75A-3 be aware that (a) the various editions of the manuals' parts lists and schematics are inconsistent regarding some component values and (b) in all likelihood the manuals were to coincide with production changes per serial number ranges.  However, when working on my A-3 I referenced three different manuals (one of which was to represent the serial number of my unit) and found several variations.  I suspect that the timing of these manuals did not directly coincide with the production of their represented unit serial numbers. Not a big deal, but just something to be aware of...especially if you're new to restoration as I am.

Harry WE1X