[Collins] Re: 75S3 Questions - Update

[Dr. Gerald N. Johnson]

On Sun, 2008-02-24 at 17:10 -0500, Bill Stewart wrote:
> 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)

6EA8s are better than 6U8A and some say they have a much longer life.

> And, the s/n is 12xxx, which apparently doesn't help much as far as dating 
> the rcvr.

The serial numbers were issued in random order.

> 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.

Low bias will make that tube draw extra plate current so run hot, also
that will make the power transformer run hot.

> 
> 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.

All the electrolytics bad. Sitting and age has worn them out. They all
need to be replaced. There is an 8/25 marked as C29 on the 75S-3B
schematic. Its wired at the point in the bias divider where the grid
resistor for the audio output tube is connected. Its not in the 75S-1
schematic I have from 1959. The tubes are also numbered differently? Is
your V8 the 6BF6? If so, pin 9 is just a spare pin used for a junction
point.
> 
> 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

New power supply filters may also help cool off the power transformer.

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