Solid State Relay for PTT?

[MBDiagMan]

I have a small ice cube relay for the push to talk, that does not energize when cold. I am quite certain that the relay is the culprit.

I am moving toward replacing the relay, which is not a trivial task, since it is buried at the top and forward under the panel. It's going to be a big job to replace it, so I don't want to do it twice.

My question is this: under most circumstances, it would seem that a solid state relay would be an obvious improvement. I am thinking, though, that a solid state relay would not give the audible click in the headset as is heard when energizing an electromechanical relay.

Am I correct in assuming that I would not hear that click if a solid state relay were used?

As always, the help and expertise offered on this forum is greatly appreciated.

 

[JeffDG]

Why would it be different?

Are you thinking that you're hearing the mechanical click of the relay? Or is it an electrical effect of the circuit path changing?

If the relay is up under the panel, the odds of you hearing the mechanical click are pretty low.

 

[MBDiagMan]

I'm not talking about the MECHANICAL click. I'm talking about the electrical noise caused by the surge when the relay energizes and deenergizes. Will this electrical noise still be present with a semiconductor completing or opening the circuit rather than mechanical contacts?

 

[pericynthion]

It depends on how the audio input circuitry of the intercom or radio is designed, but typically yes you would still expect to hear some sort of click. When designing such things if you want it to be click-free you have to go to special efforts.
The PTT is a boolean input so it really shouldn't make any difference whether you use a switch, relay, SSR or just a transistor - as long as the voltage is pulled low enough to trigger the comparator on the intercom/radio's input.

 

[Henning]

You'll hear the energy surge into the system when the relay closes regardless what type relay. The question is can you legally use a solid state one?

 

[MBDiagMan]

Good question Henning. It might lead to one more reason to stay with an old fashioned electromechanical relay. Most everything else on my ol' girl is stuck in the fifties, so maybe this is another area where old tech might not be best, but it would be legal.

 

[MBDiagMan]

It depends on how the audio input circuitry of the intercom or radio is designed, but typically yes you would still expect to hear some sort of click. When designing such things if you want it to be click-free you have to go to special efforts.
The PTT is a boolean input so it really shouldn't make any difference whether you use a switch, relay, SSR or just a transistor - as long as the voltage is pulled low enough to trigger the comparator on the intercom/radio's input.

Another great point. If the electronic relay involves a transistor output, the .7V drop across the junction, might be just enough to cause problems with the intercom circuitry.

Great and thoughtful responses. It sounds like the best approach is to stay with an ice cube relay. It will be such a job to get at it, I really don't want to do it twice, even if I haven't put everything back in place before having to change it.

Thanks guys!

 

[Henning]

Good question Henning. It might lead to one more reason to stay with an old fashioned electromechanical relay. Most everything else on my ol' girl is stuck in the fifties, so maybe this is another area where old tech might not be best, but it would be legal.

The answer to the question may lie in if the new relay has a MilSpec number. If you can get one of correct capacities with a MilSpec number, you should be able to use it legally. As for the elecro mechanical design, this is probably the original and has only taken what, 45 years to fail? At that rate I wouldn't be particularly fussed with having to do it again. Besides, you could relocate it to an easier location.

 

[MBDiagMan]

Yes, relocating the relay is a very good idea.

 

[BigBadLou]

FWIW, IF the relay is serviceable and can be opened up, the chances are that it is just stiff lube preventing the moving parts from making contact in colder weather. Inspection, cleaning and applying new (not 4 decades old) lube should not be too hard for an avionics shop.

 

[weirdjim]

A. The click you hear is the audio power amplifier in the transceiver switching from being a headset/speaker amplifier for the receiver to a modulator for the transmitter. We actually design the best we can to minimize that click but can never completely eliminate it. It has to do with a power amplifier going from partial gain to full gain for complete transmitter modulation.

B. I'm having great fun listening to chatter on this subject about the relay not being "legal" if it is (a) solid state and (b) un-mil-spec. Let's see now, it is taking the output of a legal headset that does not require TSO and switching it to a legal radio that does not require TSO. Not only that, but *gasp* those headsets and radios aren't on the original equipment list for the aircraft for the simple reason that the transistor was INVENTED in 1947 and wasn't widely used in avionics until Genave came out with the revolutionary A-200 in 1968.

Can the relay be replaced with a solid state relay and a logbook entry?
Yes.

Can the relay be replaced with a plain old industrial quality mechanical relay and a logbook entry?
Yes.

Jim

 

[weirdjim]

Another great point. If the electronic relay involves a transistor output, the .7V drop across the junction, might be just enough to cause problems with the intercom circuitry.

From memory ... not always perfect ...

The RTCA specification for switching (12 volt system) between transmit and receive is >7 volts on the key line is receive. <3 volts on the key line is transmit. In between 3 and 7 volts is no-man's land and undefined.

That 0.7 volt drop across the junction doesn't exist in a solid state relay because they use the transistor in saturation and Vce(sat) is less than 0.1 volts for any reasonably decent transistor at these current levels.

Thanks,

Jim

 

[MBDiagMan]

Some great information.

I have thought about the possibility of cleaning and lubing the relay. If it were easier to get at, I would have already given that a chance. What would be the best lubricant after flushing it with contact cleaner?

That said, after Jim's explanation, it sounds like a solid state relay might be the best solution, and relocated in the bargain.

 

[Henning]

A. The click you hear is the audio power amplifier in the transceiver switching from being a headset/speaker amplifier for the receiver to a modulator for the transmitter. We actually design the best we can to minimize that click but can never completely eliminate it. It has to do with a power amplifier going from partial gain to full gain for complete transmitter modulation.

B. I'm having great fun listening to chatter on this subject about the relay not being "legal" if it is (a) solid state and (b) un-mil-spec. Let's see now, it is taking the output of a legal headset that does not require TSO and switching it to a legal radio that does not require TSO. Not only that, but *gasp* those headsets and radios aren't on the original equipment list for the aircraft for the simple reason that the transistor was INVENTED in 1947 and wasn't widely used in avionics until Genave came out with the revolutionary A-200 in 1968.

Can the relay be replaced with a solid state relay and a logbook entry?
Yes.

Can the relay be replaced with a plain old industrial quality mechanical relay and a logbook entry?
Yes.

Jim

That could well be true which is why I posed it as a question. There is a distinction from your examples though, the headsets are not mounted to the plane which is where there is a distinction drawn, and the radios (at least the ones I got) were installed on an STC. I'm not sure where all the lines are drawn.

 

[weirdjim]

Some great information.

I have thought about the possibility of cleaning and lubing the relay. If it were easier to get at, I would have already given that a chance. What would be the best lubricant after flushing it with contact cleaner?

It all depends on what the relay is switching. If all it is doing is switching the + supply to the transceiver to key the transceiver then cleaning the whole relay with either contact cleaner or denatured alky and then a very light lubricant like lubriplate on the moving part junctions would be fine.

However, if the relay is also switching the audio from the microphone, then the contacts need some sort of tlc. The problem is that unplated relay contacts that don't switch a fair slug of current don't burn off little tiny bits of crap (that's a technical term, you'll get used to it) when they switch. Those little bits of corrosion accumulate over time and gradually keep the contacts from closing. Some folks say that they can use a LITTLE TINY BIT of the silver bearing goose grease used between a computer CPU and its heat sink to lube the contacts. I don't know; I've never had to use this technique.

IF you replace the relay, and it DOES switch audio, get a "dry circuit" relay that has the contacts flashed with gold, which never corrodes. They are nickels and dimes more than a regular base metal contact relay. Some swear by coin silver contacts, but silver forms the sulfide (tarnishes) and eventually winds up with the same problem of crud on the contacts.

That said, after Jim's explanation, it sounds like a solid state relay might be the best solution, and relocated in the bargain.

No contacts to corrrode. Ever.

.....

 

[weirdjim]

That could well be true which is why I posed it as a question. There is a distinction from your examples though, the headsets are not mounted to the plane which is where there is a distinction drawn, and the radios (at least the ones I got) were installed on an STC. I'm not sure where all the lines are drawn.

Yet there is a TSO procedure for headsets and microphones. Why go to the bother if not necessary?

99% of part 91 aircraft radios are installed with a logbook entry if not TSO'd.

Jim

 

[Henning]

Yet there is a TSO procedure for headsets and microphones. Why go to the bother if not necessary?

99% of part 91 aircraft radios are installed with a logbook entry if not TSO'd.

Jim

Yep, but they are all MilSpec or AN... parts. I don't see where a relay is different from a switch, and I can't go to Radio Shack and use any switch.

 

[weirdjim]

Yep, but they are all MilSpec or AN... parts. I don't see where a relay is different from a switch, and I can't go to Radio Shack and use any switch.

We can agree to disagree.

Jim

 

[MBDiagMan]

.....

I don't know exactly what the contacts are keying. I do know that I can remove the tail number plate at the top of the panel which allows me to see the relay. When it is warm enough to work I can see the contacts make and break. When it is not working, I can see that the contacts are not moving when the PTT switch is pressed.

When the contacts make, the radio always works well, so I don't think there is a contact issue, but a mechanical relay energizing resistance that gets worse with cold temperature.

This is a 12V system airplane. Is there a particular solid state relay that you would recommend?

 

[weirdjim]

I don't know exactly what the contacts are keying. I do know that I can remove the tail number plate at the top of the panel which allows me to see the relay. When it is warm enough to work I can see the contacts make and break. When it is not working, I can see that the contacts are not moving when the PTT switch is pressed.

When the contacts make, the radio always works well, so I don't think there is a contact issue, but a mechanical relay energizing resistance that gets worse with cold temperature.

This is a 12V system airplane. Is there a particular solid state relay that you would recommend?

Since this is not something that we can well define (i.e. what are we switching around?) I'd get a long soda straw and blow enough WD-40 (or other cleaner/lube stuff you like) into that sucker to loosen up the congealed grease it came with 60 years ago. No sense reinventing the wheel unless we have to. Even if we take it out, we've gotta trace the wires out to figure out what is being switched.

Jim

 

[MBDiagMan]

Sounds good to me Jim. As far as changing it, I was figuring that there are two wires for the coil and two wires for the contact, but since it is so difficult to get at, I am not sure what I will find.

If I can get enough access to get the cover off and reach it with a length of tubing, I think blowing in WD40 is worth a try. I have yet to be able to touch it, so getting the cover off will be a challenge. I assume that trying to blow something in there without getting the cover off will be near impossible. Is that a correct assumption? Is there a possibility that some of the WD40 could make it in without removing the cover?

Thanks again.

 

[weirdjim]

Sounds good to me Jim. As far as changing it, I was figuring that there are two wires for the coil and two wires for the contact, but since it is so difficult to get at, I am not sure what I will find.

If I can get enough access to get the cover off and reach it with a length of tubing, I think blowing in WD40 is worth a try. I have yet to be able to touch it, so getting the cover off will be a challenge. I assume that trying to blow something in there without getting the cover off will be near impossible. Is that a correct assumption? Is there a possibility that some of the WD40 could make it in without removing the cover?

Thanks again.

How many COM radios do you have? An audio panel?

Jim

 

[MBDiagMan]

Jim,

I will try to remember the Narco model numbers and list it all here. If it would be helpful, there is a picture of the panel in post number 19 of my "Old Coax" thread below. I don't have the picture on this computer or I would add it here to save you the trouble.

Audio Panel AP 136
Com 1 & 2. COM 120's
NAV 1. NAV 121
NAV 2. NAV 122


Hopefully my poor memory has not fouled up these model numbers too bad. There is no intercom panel of any kind. Only the headphone jacks on the pilot side can transmit. The PTT switch on either yoke, will key the mic. When headsets are plugged in on both sides, they operate like an intercom. I considered adding an intercom panel of some sort, but have gotten by well without it.

I hope this answers your questions. Thanks again for the help. Also thanks to all who have contributed in the thread.

 

[weirdjim]

Jim,

I will try to remember the Narco model numbers and list it all here. If it would be helpful, there is a picture of the panel in post number 19 of my "Old Coax" thread below. I don't have the picture on this computer or I would add it here to save you the trouble.

Audio Panel AP 136
Com 1 & 2. COM 120's
NAV 1. NAV 121
NAV 2. NAV 122


Hopefully my poor memory has not fouled up these model numbers too bad. There is no intercom panel of any kind. Only the headphone jacks on the pilot side can transmit. The PTT switch on either yoke, will key the mic. When headsets are plugged in on both sides, they operate like an intercom. I considered adding an intercom panel of some sort, but have gotten by well without it.

I hope this answers your questions. Thanks again for the help. Also thanks to all who have contributed in the thread.

The reason I asked is that now I have ABSOLUTELY NO IDEA what the function of that nonfunctional relay is. With what you've got you need that relay not a bit.

Probably.

Jim

 

[MBDiagMan]

The reason I asked is that now I have ABSOLUTELY NO IDEA what the function of that nonfunctional relay is. With what you've got you need that relay not a bit.

Probably.

Jim

I expect that what you're saying is that the switch(es) could be used for the circuit that the relay contacts are completing.

If that relay weren't buried in never never land I would have already investigated. I suspect that either PTT switch can energize the relay. Since only the pilot can transmit, there's no need for a co-pilot PTT switch anyway.

 

[weirdjim]

I expect that what you're saying is that the switch(es) could be used for the circuit that the relay contacts are completing.

If that relay weren't buried in never never land I would have already investigated. I suspect that either PTT switch can energize the relay. Since only the pilot can transmit, there's no need for a co-pilot PTT switch anyway.

The PTT switches should be connected to the audio panel. THe audio panel ought to be the one to resolve whether the pilot or gib pushed their respective switch and put that microphone onto the radio. The audio panel ought to also resolve the "race" condition where both PTT switches are depressed and decide which one should predominate.

Jim

 

[MBDiagMan]

Jim,

This is a little frustrating trying to guess what is there and why while all the while having no schematics to see what's up.

There's something that adds to the mystery. There is an identical relay mounted next to the one in question for which I have been unable to determine its purpose. I sure wish I could access all that to trace the wiring.

 

[weirdjim]

Jim,

This is a little frustrating trying to guess what is there and why while all the while having no schematics to see what's up.

There's something that adds to the mystery. There is an identical relay mounted next to the one in question for which I have been unable to determine its purpose. I sure wish I could access all that to trace the wiring.

Well ... when I bought the 182A out of the farmer's field with the chickens roosting where the hat rack should have been and the wings in the attic of the barn ...

I gutted it. I mean GUTTED it. I put just enough of the piping and wiring in with temporary connections to make it legal (sort of) to ferry it from Fresno to Grass Valley ... about 300 miles.

When it came to the wiring, I put two large aluminum plates on braces underneath the instrument panel. I went to some surplus store down in LA for the stuff that HAD to have milspec wire on it (starter cables, landing light wiring, all the heavy stuff) but all the "radio" and instrument wires went to terminal strips mounted on those aluminum plates.

NOTHING went into that airplane without a sketch (later turned into a formal engineering drawing) as to what wire went where and all that stuff. All those wires went to those terminal strips on those plates.

Now those terminal strips and the crimp terminals that are attached to them are standard hardware from a "respected" source, but I submit for your information that if I needed another strip on Sunday afternoon, the stuff down at the "Rat Shack" had exactly the same manufacturer markings on it, and somehow they may have found their way into a certificated airplane. Let your 1948 conscience be your guide.

So, will you ever be happy until you gut your airplane wiring and make it right? Probably not. Do you want to spend a year hanging upside down under your instrument panel? Probably not. Your call.

Just FYI I took out 75 POUNDS of 1958 cotton coated heavy wire and put back in 16 POUNDS of nylon coated cotten where it was required and PVC coated (which has a mil-spec) for all the audio and sub-ampere wires that were so over-engineered as to be ridiculous.

You will find the same in your ancient machine.

Jim

 

[MBDiagMan]

Jim,

I enjoyed very much reading the previous post. Great and impressive description of your project/airplane.

More background on my little plane:

She was completely disassembled, restored and modified in the late seventies, early eighties and a TON of money and time poured in. If there is a single strand of original wire in the entire aircraft, I would be shocked.

Although it was an impressive restoration job, with lots of modifications for which I have all the field approvals and such, there are two things in play, I have no schematics/wiring diagrams and the restoration itself is probably as old as half the airworthy GA aircraft in the country.

By and large everything in the aircraft works well. The two issues I'm working, the weak antenna signals and the intermittent PTT are the only things at this time that are less than totally satisfactory.

You are correct in sensing that I would like to completely redo the panel, but there is no need to do so if I can sort these two issues.

Your help and encouragement( whether you realize it or not, you are a source of encouragement) is a big part of helping me push through this.

 

[Getonit]

What makes you think a solid state relay is going to last any longer than the old one? The old one lasted decades in a vibrating hot environment, the solid state one might be made in some Chinese factory. I am not impressed with durability of anything modern, look at the issues people have with garmins, aspens, car electronics, big box electronics, etc and anything modern, all built to a price point not a durability standard. Just my thought. Also doesn't mean the ice cube relay wasn't made in China either, or any other third world "low cost provider"

 

[MBDiagMan]

Given that the existing relay was probably built ca. 1980 give or take, I expect that it was not made in China. My biggest problem is access. I am pretty encouraged that if I could get the relay apart, flush it with contact cleaner and then lube it with WD40, i could get another 100,000 miles.

I agree that unless I knew I was getting a high quality part, changing the relay might not be the best plan.