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Author Topic: sticky relay and relay confusion  (Read 2401 times)
N4RSS
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Posts: 258




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« on: March 06, 2010, 07:51:36 AM »

I have a sticking T/R relay I believe in my SB-220. I attempted to clean the contacts with
some paper but problem still there.

The relay I THOUGHT was the right one seems to appear near one of the RF in/out
jacks. But from the schematic, there seems to be one near the tube sockets,
more in the middle. I did see that when I had it open but didn't recognize what
it was, it had a clear plastic shroud over it.

The schematic also doesn't show the relay I did clean near the RF jacks. The
amp has been modded to add all of the Harbach except soft key.

So, I wan't to clean the T/R relay, did I do that ?
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WX7G
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Posts: 5908




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« Reply #1 on: March 06, 2010, 07:54:45 AM »

The best solution to a sticky or dirty relay is to replace it. It is worn out.
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K6AER
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Posts: 3468




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« Reply #2 on: March 06, 2010, 12:56:51 PM »

Teh SB-220 has only a open frame T/R relay. I suspect an additional a low voltage keying relay has been added.
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N4RSS
Member

Posts: 258




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« Reply #3 on: March 07, 2010, 06:58:07 AM »

Yes, I believe I cleaned the wrong relay.  However, the correct relay is unfortunately a replacement one that is sealed in a plastic housing.

So I hacked a hole in it to get access and sprayed with DeOxit.  I'll see what happens.
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W8JI
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« Reply #4 on: March 07, 2010, 09:58:29 AM »

The best solution to a sticky or dirty relay is to replace it. It is worn out.

This is generally the case, except on the receive side of the system. On the receive side the contact current is so low the contacts can't "clean" and so they develope poor connections. This is because the 15-30 ampere contact button is too large and the wrong materials for zero-current use.

If you have intermittent receive problems, then there are things you can do to help the relay. Replacing the relay generally isn't the best idea. Neither is abrasive burnishing.

For other problems, like transmit problems, it is a good idea to just change the relay. Like Dave says, it is probably  bad.

The number one problem with large relays is on receiving, and it is caused by a very thin contact film.

73 Tom
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K5JX
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Posts: 7




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« Reply #5 on: March 15, 2010, 09:11:29 PM »

Tom, I'm currently experiencing intermittent receive problems with the relay in my AL-82. What are the things you can do to help the relay in that case? Thanks & 73.
Rene, K5JX
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G3RZP
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Posts: 4325




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« Reply #6 on: March 16, 2010, 02:19:27 AM »

The FT102 is a classic case of using relays to switch very low level signals with the wrong contact material. Some relays were made with platinum contacts in order to get over it, but the usual method back in the days of Strowger telephone exchanges was to bleed some DC through the contacts. Using  under 1mA at 24 volts has worked on my FT102 for over 20 years. Platinum contacts were not very popular because of cost, but the scrap metal merchants loved them!

Some relays eventually stick in the energised position because of residual magentism. Where there's a copper stud on the armature, it has, over the years got flattened down to the point where the air gap has virtually disappeared. Usually, a layer or two of Scotch tape will effect a cure, although with small relays, it is such a pain to do that it's easier to change the relay.
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WX7G
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« Reply #7 on: March 16, 2010, 03:46:47 AM »

You guys are correct on 'dry switching' of relays with low signal current.

I have tried cleaning the contacts with DC current and it works for awhile but the problem comes back, more and more frequently. For microwave relays in test systems I have seen a huge differernce in life between brands before dry switching comes on. Some brands would go 6 months, others 6 years.  

When I design with small signal relays I try to keep the current above at 5-10 mA. I do not have good with and without data to say if this is effective but I keep doing it.

As a technician I got tired of replacing a soldered in relay in a semi-HV circuit and tried cleaning it with Tarn-X. Not good. It soaked into the fiber insulator making it conductive. That relay exploded. Another lession learned.
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AA4HA
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Posts: 1377




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« Reply #8 on: March 16, 2010, 07:08:36 AM »

You could go with a hermetically sealed relay. They are available from the surplus distributors (FairRadio, Surplus Sales of Nebraska, etc...). Ice-cube relays are notorious for contact contamination and oxidation. In some cases you can open the plastic case and clean the contacts.

Here is an article that will explain the low current issues;

http://www.findernet.com/en/country/united_kindom/press/press_05.php?lang=it

To improve relay contact reliability you could also go with several contacts in parallel and tie them together. This will increase the probability of relay engagement under low current applications but also double current carrying capacity with high loads.

I have the low current switching problems on the Harris RF-350K transceivers. Each radio has 16-18 ice cube relays with SPDT contacts rated up to 10 A. They are used to switch the low-pass filters in and out of the circuit and the switching current is very low. Almost every owner of a RF-350K has times where the radio will not pass a BITE test when switching through the filter combinations. This is an endemic problem in those radios and I am trying to design a daughterboard that can hold a small DIP relay and be a solder-in replacement for the original relays that are now unobtanium.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
W8JI
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Posts: 9304


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« Reply #9 on: March 16, 2010, 08:10:23 AM »

Tom, I'm currently experiencing intermittent receive problems with the relay in my AL-82. What are the things you can do to help the relay in that case? Thanks & 73.
Rene, K5JX

Rene,

Sorry I missed this.

One thing I did early on with Ameritron relays, and a change I put into Heath relays, was to remove the unused normally closed contact on the bias switching line. Since it is a three pole relay, this increased contact pressure on the remaining two normally closed contacts in the signal path by 33%. 

This problem is NOT a problem that is addressed just by using a sealed relay. Sealed relays, unless they are vacuum relays, have the same problem. The problem is low current allows a very thin layer (just a  few molecules thick) of contaminants to build up. Even sealed relays have this issue because things leach out of the plastics, plus the air inside the relay can still oxidize the contacts.

The problem is a contact designed to switch current will not clean itself at low current. This is a problem with everything from switches to relays and potentiometers.

Replacing the relay isn't a good solution either, because the new relay will have the same issue. What you DON'T want to do is burnish, file, or sand the contacts.

What I do is find a good well-bonded paper like a matchbook cover, and wet it with WD 40. I rub it back and forth between the contacts and polish them with the wet cardboard. It has to be something that does not leave wood or paper fibers. I do not polish or rub the transmit side, just the receive contacts.

I find this almost always works. I rarely change relays.

If you want a relay that really lasts, buy a vacuum relay.

73 Tom
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G3RZP
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Posts: 4325




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« Reply #10 on: March 16, 2010, 10:01:34 AM »

Tom's comment about sealed relays is right, although they often last a bit longer! But even nitrogen filled ones have been known to have this problem with stuff leaching out of the plastic.

I think it's not so much the current that you bleed continuosly through the relay, but making sure that there really are enough volts to break that oxide film. I find 24 works: one contact, I use 300 because that's the easy way in that part of the circuit.  I understand one of the newer Icom rigs does this trick, too.
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WX7G
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« Reply #11 on: March 16, 2010, 10:27:34 AM »

It's not the continuous DC current but the peak current that is needed to prevent dry switching.

Say you are using the relay to switch a 5V logic circuit with a 10k pull-up resistor. That is only 500 uA. What might help, and I do this for both signal integrity and for relay life, is to terminate the line into an R and C in series. 50 ohms and 0.1 uF for example. We not only have a peak relay current of 100 mA as well as matching for the T-line. This keeps the logic input from being beat by the mismatched t-line.
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