75-A2 common trouble spots?

[N3DRB The Derb]

Since I cant lay down a maul until the electrical guy gets here, I've decided to take care of some unfinished business. JN lent me a 75-A2 for use which it was until it went belly up. I'm starting by putting new filters in it, which I do as a matter of policy in any radio I work on regardless.

I just want to get it playing again. I'm working from the Sam's Photofacts. the S/N is 445.

Anyone have any pointers as to common failure problems? The failure mode seems to be something in the avc, as it is quite deaf and the s-meter hangs around +20 db no matter the rf gain control setting. The set has audio.

It does have a mechanical filter in it.



 

[WA5VGO]

Start looking for leaky mica capacitors. I usually start removing avc from the stages one at a time until I can isolate the stage. It's pretty easy after you narrow it down.

BTW, there's a 100 pF in the filter adapter housing I had to replace once. It's a killer to get at.


73,
Darrell, WA5VGO

[w3jn]

I did some messing with the AVC in that thing, Drb.  Could be something came loose.

I hated the stock AVC and as I remember I added a diode and a resistor for a slow decay or something.  I might have clipped something and soldered it back - as I said it could have come loose again, or shorted against something.

[N3DRB The Derb]

well. the new filters are in. cant do anymore tonight. left hand say no more.

I saw teh diode. I'll test it in the morning and see if it became a friode for some reason. It worked ok fine for months and one day BINK it just stopped. Didnt see anything amiss otherwise, except for a cap that looked like it had waxy buildup on it. one of those long tubular things with the looped wires at each end and colored markings, but it's no resistor. Shouldn't have wax on it. NG.

brings back happy memories from my JN days when I got a 9++ condition 75A2-32V2 station for 100 bucks from a guy in Hampden maybe 2 blocks from Ernie's. 

[KM1H]

I assume then that JN replaced those high value paper clunkers?

Other than that and a few resistors mine works OK. Its nothing special for performance but I havent had the modification urge yet   I must be slipping as I enjoy aggravating the die hard Collins fanatics Whenever I mention my 75A4 to one they climb the wall

Carl
KM1H

[N3DRB The Derb]

nope, there's quite a few paper clunkers still in there.   They are gonna get the heave ho today as long as I have replacements in stock. That's also derb policy: all old electrolytics gotta go, even if they "test" ok.

What about the big red suckers on the back held on with rivet clamps, are those paper too? I'm assuming they are. Ones  .5, havent looked at the other yet.

[KC4VWU]

Derb gotta good policy!

[w3jn]

BTW that thing has a product detector mod plugged into the FM dicktecktor socket.  IF and BFO in, audio out

[N3DRB The Derb]

yeah I love where they stuck the so-239 for that - right through teh back nameplate! Then they wrote "SSB ADAPTER" in big scratchy marks on the back....

Art would NOT have approved....

gotta tie the b minus together and solder,  fire it up and start shooting trouble...I got a xmas present coming via UPS will be here any minute now, so I gotta maintain. Fun getting back into a receiver again.

After this I need to get my $20.00 RAO-7 going. I have a good feeling about that radio, voice is telling me it's gonna be easy, xcep for the 35+ bloated swisher sweet lookin caps I'll have to change....

[KM1H]

What about the big red suckers on the back held on with rivet clamps, are those paper too? I'm assuming they are. Ones  .5, havent looked at the other yet.

Yep both are .5 clunkers, C-83 and 103. Anything that remotely looks like paper goes into the trash but only do 2-3 at a time and then turn on to be sure you didnt screw up.

The RAO will be easy. Just be sure to get the ones under the catacomb and keep the new ones close to the chassis so they dont get hit.

Carl
KM1H