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Problem with AM-19

Posted: Wed Oct 05, 2016 7:44 am
by RodeoJack
I have a 2-tower AM-19 at a client station. Recently, the unit stopped showing loop current on the reference tower. Without changing the reference adjustments however, I'm getting normal phase and ratio readings from tower 2. Loop current coming in from both towers is about normal, though I don't have those numbers here.

I've seen one of these things with the cover off but, other than changing a power switch, haven't touched anything inside. Does this problem sound like anything that can be easily fixed in the field, or am I looking at sending it in?

Re: Problem with AM-19

Posted: Wed Oct 05, 2016 8:18 am
by Deep Thought
It is almost certainly the unobtainium tower select relay on the reference tower input module and is a common failure. The loop current ratio for the non-reference tower is a function of the attenuator setting, which the reference tower gives you, well, a reference for the 100% setting. The phase reference select is a different relay on the same input module and is operated by the day/night buttons on the monitor (or is hardwired on a single-mode monitor). If you didn't change the attenuator you would get the same non-reference readings as you did before even with the select relay not working.

The good news is it is a plug-in fix. The bad news is the relays require a contortionist with tiny fingers to plug back in.

Don't even bother trying to get PI to fix it...they'll say it is no longer supported, will cost a minimum of $1,500 and/or try to sell you an AM-1901 for $15K. They do have retrofit replacement relays but will charge you an arm and a leg for them. Mike Patton also has them for much more reasonable prices. I also have a bunch of used relays if you can't get one from Mike.

Re: Problem with AM-19

Posted: Wed Oct 05, 2016 12:04 pm
by kcbooboo
Another much lower probability is the SCRs that drive the relays on the input card. If one opens, it won't pull its relay in. They are all mercury-wetted relays that must be installed in a particular position so the mercury doesn't short out the contacts when it's not supposed to. You can pull them out and measure the coil resistance, or if possible, measure the coil voltages when that tower is selected to verify that the SCRs are working.

New relays are terribly expensive, so if it's a $3 SCR instead, better to check before you spend the money.

While you've got the unit open, check that none of the electrolytic caps are leaking. If you have to replace one, replace them all. I repaired one with bad caps (meters drifting and indicating upscale with no input signals) several years ago that required a complete adjustment afterward, which I thought was quite easy to do.

Bob M.

Re: Problem with AM-19

Posted: Wed Oct 05, 2016 6:57 pm
by RodeoJack
Thanks, Both. Will check as advised.

Re: Problem with AM-19

Posted: Sun Oct 09, 2016 6:32 pm
by rrlane
I have some of those relays somewhere if you need one.
Stui

Re: Problem with AM-19

Posted: Sun Oct 09, 2016 8:17 pm
by RodeoJack
rrlane wrote:I have some of those relays somewhere if you need one.
Stui
Thanks. Will know after I get back out there...