Collins 251 problem

[Clark1961]

Used com 2 to call ground today. Had to call twice before getting a response. Ground said they answered 4 times before I heard them. On departure I had used com 2 to pick up the ATIS and to contact ground - no problems.

I'll try the old pull the radio and reinstall/tighten the freaking allen screw fix. Anything else to think about on a Collins radio?

 

[ScottM]

I'll try the old pull the radio and reinstall/tighten the freaking allen screw fix. Anything else to think about on a Collins radio?

Well you may also have antenna or coax problems. So don't be so quick to blame the radio. you could try swapping the comm1 radio with the comm2 radio to see if the problem follows the radio. If not more likely to be the antenna system

 

[docmirror]

They may share a transmit antenna, most dual comms do. The 251 is getting long in the tooth. I have two of them and one is iffy. When you key the mic, look at the front panel on the right side. Begin speaking and look for a faint word vertically that says "xmit". This is an indication that you are getting out. The 'xmit' lamp is directly on the output of the amp, so if you see it, you're transmitting.

As for receive it's a whole different circuit involved. If you're having trouble with both, take it to a shop, and have it looked at. If it'll receive but not xmit, it could be the output stage, which is possible to repair. If it's in the IF stages(likely) it's toast, and you'll have to get another radio. Check ebay.

<edit, yes I know the IF stages can be tuned, but it's problematic in the 251 as it's a cascaded crystal circuit, and the tuning takes for-ev-ar.>

 

[gismo]

They may share a transmit antenna, most dual comms do. The 251 is getting long in the tooth. I have two of them and one is iffy. When you key the mic, look at the front panel on the right side. Begin speaking and look for a faint word vertically that says "xmit". This is an indication that you are getting out. The 'xmit' lamp is directly on the output of the amp, so if you see it, you're transmitting.

As for receive it's a whole different circuit involved. If you're having trouble with both, take it to a shop, and have it looked at. If it'll receive but not xmit, it could be the output stage, which is possible to repair. If it's in the IF stages(likely) it's toast, and you'll have to get another radio. Check ebay.

<edit, yes I know the IF stages can be tuned, but it's problematic in the 251 as it's a cascaded crystal circuit, and the tuning takes for-ev-ar.>

By "share a transmit antenna" do you mean one radio using one antenna for both transmitting and receiving, or do you mean two radios using the same comm antenna. If the former, that's pretty much always the case, if the latter, I've never seen a setup in light plane that did this and I don't think it's possible with two 251s and off the shelf components.

In any case, I second Scott's suggestion of swapping the two 251s to see of the problem moves with the radio, especially since the offender has already been reseated without effect.

 

[Clark1961]

Cleaned contacts and re-seated all coms and navs yesterday. Will see what comes of it.

Both coms worked fine in flight on Friday so I'm thinkin' the ground controller had some sort of line-up problem and didn't 'fess up.

Thanks for the thoughts on more troubleshooting. There may be some hidden damage to the cables from extensive MX last summer. There was damage to the DME antenna cable that was found last fall so there could be more problems lurking about.