[Collins] 75S3 Questions

[Dr. Gerald N. Johnson]

On Mon, 2008-02-11 at 09:56 -0500, Bill Stewart wrote:
> I recently received a 75S3 (s/n 12xxx) which was blowing fuses. I found one of the B+ diodes shorted and replaced both with 1N4007 diodes. This rcvr used 1N1084 S/T B+ diodes which fit into a open type fuse holder. The schematic which I have, apparently not for this rcvr, calls for 1N1490 diodes. Also noted is that this rcvr uses 6U8A's instead of the 6EA8's as noted on the schematic I have. Now it's working fine, but I have a few questions.
> 
The Sarkes-Tarzian fuse block was a popular replacement in TVs at one
time. Could have been used in the S-line or have been a replacement as
far as I know. The original silicon diodes were a top hat wire leaded
type. Other than originality, the 1N4007 is a fine substitute and will
live longer than the originals.

> Question 1: Is there anything special about the B+ diodes. The 1N4007's are working ok and the current rating is well
>                   above rcvr requirements. Can the older 1N1084 diodes still be found should I want to return it to orig. config.

My 75S-3B/C book says 1N1096 or 1N1492.

> Question 2: I am hearing plenty of signals, but the S-meter is very stingy...with only a few sigs going over S-9. Is this 
>                   normal S-meter action or could the RF need some tweaking. 

Collins S-meters have always been rated as stingy, but with weak IF
tubes and leaky AVC time constant black beauty capacitors the S-meters
become more stingy. There could be a need for alignment, but some of the
many ceramic trimmers in the front end may be stuck and not allow
adjustment. And some may have gone open detuning the front end but that
will only affect a single band unless its a 10 meter trimmer.

To say nothing about cleaning the many bandswitch contacts. DeoxIT
applied with the dropper bottle half a drop per contact followed by
vigorous switch operating cleans them up. A spray is not good because it
can saturate the switch insulator leading to undesired leakage. 

There are several black beauty oiled paper capacitors that are known to
be excessively leaky. They need to be replaced with orange drops for a
permanent fix. The black beauties are molded plastic, domed ends, and
with color code strips, kind of like resistors but much larger. There
are so few good black beauties that its not worth the bother of removing
them with full length leads and then testing properly for leakage. Its
far more productive to replace them all. Then low gain, poor AGC, and
hot audio output tube problems are nearly all cured.

> Question 3: Another S-meter question- Should there be enough light from the meter lamp to read the meter in the dark. 
>                  Mine has very little light coming around the meter face and is about useless as far as reading it in the dark.
>                  Anything special about the meter lamp make up. 

May have a 12 or 24 volt lamp in place of the original 6 volt lamp that
didn't last so long. A type 44 will give better light than a 47 or 51,
if the 44 will fit which my book says it should.

> Question 4: The 6BF5 audio tube runs real hot...makes spit sizzle. Is this normal? I'm using a Radio Shack 'scanner'  
>                   speaker and am guessing its 8 ohms.

The audio coupling capacitor to the grid is one of those leaky black
beauties and it takes very little leakage to drastically increase the
6BF6 plate current. Also there was a negative bias supply that supplies
bias to the 6BF6 and cut off bias for the receiver as well as negative
bias for the RF gain control. In early receivers it used a selenium
diode block about 3/8" by 1/2 by 3/4" also S/T. That is almost open
removing bias.

The electrolytics are not likely in good shape because of the B+ diode
shorting and old age. They will give more trouble soon if not already. 

> Question 5: Last one for now...the power transformer runs pretty warm...it gets uncomfortable after about 15-20
>                   seconds...is this a normal temp.? 

That could be coming from the 6BF6 bias problems, leaky filter
capacitors, or it could have a shorted turn and be failing. Pull the
6BF6 and see if the transformer heats as rapidly. Then unhook the three
rectifiers and make the same test.
> 
> I haven't received a manual yet so I'm operating in the dark a bit. Would this rcvr be toward the end of the plain S3 series being it has a high ser. no. and diode and tube changes. Thanks for any help...73 de Bill K4JYS 

Can't tell anything by the serial numbers, they were issued in random
order. By decree from Art.

6U8A and 6EA8 are barely distinguishable from the performance side. 6EA8
went into later production, but I don't know if the change was mid run
or with a model change because the 6EA8 pentode had slightly better gain
and so lower second mixer noise, but the main thing was the 6EA8 has a
longer working lifetime.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer