[Collins] Re: 75S3 Questions - Update

[Bill Stewart]

I've been working on the 75S3 for a while, and now for some updates.

As mentioned previously, I replaced all diodes, B+ and bias with 
1N4007's...which worked just fine.
Also, mentioned earlier, this rcvr uses 6U8A's instead of 6EA8's (which are 
used on the schem. I have...no problem)
And, the s/n is 12xxx, which apparently doesn't help much as far as dating 
the rcvr.
The rcvr worked well, from the beginning, but had a awful hum and squeal 
when the volume got up around 2 o'clock. I later noticed this hum was 
non-existent when using the AM mode.

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.
ANSWER: The 1N4007's work just fine and I will not put in  the older type 
diodes. This rcvr had the S/T cube rectifier
                 for bias and it was replaced.

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.
ANSWER: A stingy S-meter is normal.

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.
ANSWER: I got numerous suggestions from the list on helping this situation. 
I will probably make an insert of white card stock or photo paper and see 
how that works. Mine has a #47 lamp which I will use.

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.
ANSWER: No problem using the RS spkr.
                  Normal for 6BF5 to be hot.
                   I found the bias to be low.
                  With 115VAC line voltage the 6BF5 bias, originally, was 
around -8vdc (pin 1 to gnd, in opr position).
                   Plate voltage was +133vdc.
                   After replacing the bias diode w/1N4007, bias is -11vdc 
and plate was +137vdc. Bias now right
                   on per my schematic....B+ a tad high.

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.?
ANSWER: The trans. seems to have cooled off a bit...can now hold hand on it. 
I measured the temp after
                   operating for several hours and the laminations measure 
around 115 deg. This seems to still be a tad
                   on the warm side so will do some more checking on the 
trans. (cking current draw, etc)

I previously mentioned the hum. It had a loud hum as the volume was advanced 
up near 12 o'clock plus would break into a squeal at 2 o'clock. This only 
occurred in the sideband and cw emission positions. On AM, there was only a 
slight background hum, at high volume, and sounded normal. I pulled V8, the 
prod. det. and there was no hum in the sb/cw positions and AM still sounded 
normal. I found a small 8uf/25vdc 'lytic (with the minus lead connected to 
pin 9 of V8 & plus lead to gnd) to be the culprit. As is normal, it was the 
last part I changed. That 'lytic is not shown on the schem. I have but looks 
to have been factory installed.

The rcvr seems to be working well now with the loud hum gone. The trans. 
still seems to be a bit warm, but will do some more checking on that. Thanks 
to all for your input and special thanks to J, W7KSG, for the 'lytic and 
power cord.....73 de Bill K4JYS