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Title: Collins 75A-2 Limiter Operation Post by: W1KSZ on June 02, 2017, 05:05:46 PM I just picked up a very nice looking Collins 75A-2 from a local. The Cabinet and Front Panel are pristine.
There have been a few mods made to it, but nicely done and I don't think I will reverse them. One thing that bothers me, when I switch on the Limiter in the AM Mode, the receiver goes silent. It works fine in CW, but not in AM. I am hoping that one of the "Mods" isn't responsible for this. Any comments ? Thanks, Dick, W1KSZ Title: Re: Collins 75A-2 Limiter Operation Post by: Pete, WA2CWA on June 02, 2017, 06:05:20 PM I just picked up a very nice looking Collins 75A-2 from a local. The Cabinet and Front Panel are pristine. There have been a few mods made to it, but nicely done and I don't think I will reverse them. One thing that bothers me, when I switch on the Limiter in the AM Mode, the receiver goes silent. It works fine in CW, but not in AM. I am hoping that one of the "Mods" isn't responsible for this. Any comments ? Thanks, Dick, W1KSZ Check with the previous owner if it worked. 75A-2 has an AM/FM noise limiter, V10, and a CW noise limiter, V16. Maybe V10 is bad. Maybe the switch contact is bad. Whatever the mods were might have screwed up this particular function. Maybe the AM/FM noise limiter was disabled so as not to be annoyed by the distortion that generally is present when you use this noise limiter design in the AM mode. Title: Re: Collins 75A-2 Limiter Operation Post by: W1KSZ on June 02, 2017, 06:52:45 PM This radio has been through 3 owners that I know of. I believe the Mods were done previous to that.
I'll take a look into it further. It's really a beautiful Receiver for 60 years old. Thanks for the reply, 73, Dick, W1KSZ Title: Re: Collins 75A-2 Limiter Operation Post by: w1vtp on June 14, 2017, 11:42:30 AM This radio has been through 3 owners that I know of. I believe the Mods were done previous to that. I'll take a look into it further. It's really a beautiful Receiver for 60 years old. Thanks for the reply, 73, Dick, W1KSZ I just tried my 75A2 out and the noise limiter seems to work OK. It does impose a little bit of clipping on the modulation peaks and cause the incoming signal to be a tiny bit distorted but I think that's normal. I usually keep the noise limiter off. On the URL below, scroll down to the first set of pics and you will see my Collins vintage station. That black thing with the meters is my class E station. Check your manual. I think there were two revisions of the 75A2 but look at V10, a 6AL5. That's the noise limiter tube. Is it lit? Anyway, poke around that circuit and see of you can make it work. I must say that the limiter is a disappointment to me the way it works in my A2 GL, 73, Al VTP Al https://www.qrz.com/db/w1vtp (https://www.qrz.com/db/w1vtp) Title: Re: Collins 75A-2 Limiter Operation Post by: W1KSZ on June 14, 2017, 12:12:12 PM I tested all the tubes on my trusty TV-7, all were OK.
There's not much to that circuit, just need to go through the wiring to see if it got disabled somehow. Thanks for the reply, 73, Dick, W1KSZ Title: Re: Collins 75A-2 Limiter Operation Post by: w1vtp on June 16, 2017, 12:29:05 PM I tested all the tubes on my trusty TV-7, all were OK. There's not much to that circuit, just need to go through the wiring to see if it got disabled somehow. Thanks for the reply, 73, Dick, W1KSZ That would be my next step - maybe use a scope / probe and trace the audio using a beat note as a tone to trace. GL, Al |