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Post by Double D on Jan 4, 2009 16:01:12 GMT -5
My Mark III has started taking spells when the transmitter won't transmit. It still cuts the receiver off, the spot still works, and shows modulation on the built-in meter, but has no RF output. It really comes and goes with no other symptoms. It does seem like I can keep flipping the key, and it will start working sometimes. Other times it will just start working on it's own. Could a bad relay do this? I don't think I have ever replaced the relay in this one, and I bought it new in 1976. It sure has been a good one. It still has almost all it's original parts.
I know it's impossible to tell for sure without a bench test. I guess I'm just making conversation. I'm the service manager at a Chevy dealer, so I know how it is to get these "stupid" questions. I just don't get to ask them very often. ;D
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Post by gator7 on Jan 4, 2009 16:59:48 GMT -5
Hi Dan,
Well am not a tech. But a relay may be the culprit here. Since keying the transmitter brings it back at times.
But I think its time for that 100,000 mile tune up. As you said it has many of the OEM parts.
Good luck with the MK III. I am sure someone will be able to help out here on the forum.
73
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Post by Tombstone (R.I.P.) on Jan 4, 2009 17:12:42 GMT -5
Yeah, I agree. Cleaning the relay contacts is the place to start. You can cut a match pack down, soak it with tuner spray, and run it gently inside the relays. There are other causes of relay failure. As far as I know exact replacements aren't available but they're the same physically but the wiring isn't the same.
Tombstone
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Post by BionicChicken on Jan 4, 2009 17:37:26 GMT -5
My Mark III has started taking spells when the transmitter won't transmit. It still cuts the receiver off, the spot still works, and shows modulation on the built-in meter, but has no RF output. It really comes and goes with no other symptoms. It does seem like I can keep flipping the key, and it will start working sometimes. Other times it will just start working on it's own. Could a bad relay do this? I don't think I have ever replaced the relay in this one, and I bought it new in 1976. It sure has been a good one. It still has almost all it's original parts. I know it's impossible to tell for sure without a bench test. I guess I'm just making conversation. I'm the service manager at a Chevy dealer, so I know how it is to get these "stupid" questions. I just don't get to ask them very often. ;D Sounds like possibly a bad crystal or dirty crystal selector. When it does this change channels and see if it is all channels or just one. BC |
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Post by Tombstone (R.I.P.) on Jan 4, 2009 18:18:23 GMT -5
Yeah, BC. I didn't take tuner kludge and other things into consideration.
Tombstone
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Sandbagger
Administrator/The Boss 
Posts: 6,250 
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Post by Sandbagger on Jan 4, 2009 19:10:14 GMT -5
My Mark III has started taking spells when the transmitter won't transmit. It still cuts the receiver off, the spot still works, and shows modulation on the built-in meter, but has no RF output. It really comes and goes with no other symptoms. It does seem like I can keep flipping the key, and it will start working sometimes. Other times it will just start working on it's own. Could a bad relay do this? I don't think I have ever replaced the relay in this one, and I bought it new in 1976. It sure has been a good one. It still has almost all it's original parts. I know it's impossible to tell for sure without a bench test. I guess I'm just making conversation. I'm the service manager at a Chevy dealer, so I know how it is to get these "stupid" questions. I just don't get to ask them very often. ;D Sounds like possibly a bad crystal or dirty crystal selector. When it does this change channels and see if it is all channels or just one. BC I'm leaning toward the relay, especially if it's original. If a crystal or switch were intermittent, then the spot would be inoperative when the transmit dropped as well. |
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Post by FIXR on Jan 4, 2009 21:52:56 GMT -5
You can tell if you have a relay problem in the antenna output section by placing the meter switch in the FWD position and setting the needle to the middle scale or wherever with the SWR CAL control. When the power drops out, look and see if the needle is where you set it at. If it is,then the output section of the relay is bad and not letting the power out to the ant connector.
The SWR indicator board is between the output tank circuit and the relays.
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Post by Sniper..Unit 305 on Jan 4, 2009 21:53:41 GMT -5
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Post by 2600 on Jan 5, 2009 0:33:04 GMT -5
Be advised that an exact replacement relay is not available. Not even from Barkett's. Hasn't been made for almost 30 years.
The one that's commonly used looks the same size, but the layout of the pins is not compatible. It won't fit the old socket, and the layout of the new relay's base diagram is not the same. The standard solution is to replace the relay and the socket at the same time. Barkett's catalog shows the relay and socket sold as one item.
I'm not aware of a clear, bug-free step-by-step guide to this project on the web.
If the one sold by Barkett's comes with instructions, you're good to go. Just be sure to get all the wires on the right pins of the new socket. Getting them wrong could turn that first time you power up the radio into a real "smoke" test. You don't want to let too much of the smoke out of it. It's expensive to put back in.
73
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Post by Double D on Jan 5, 2009 20:14:30 GMT -5
Well, the dang thing is working perfect now, and I didn't do anything to it. When it does act up it affects all channels. The next time it quits I'll try that SWR meter test. I do have a copy of the relay rewire directions from Barkett's left over from another project so if I have to replace it I'm OK with that.
Thanks for all the responses.
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Post by Tombstone (R.I.P.) on Jan 6, 2009 7:50:20 GMT -5
Gremlins??
Tombstone
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Post by Junior on Jan 6, 2009 8:05:01 GMT -5
Like Gizmo? You didnt get it wet did you?
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Post by mark4 on Jan 6, 2009 17:04:37 GMT -5
No! It's just like the economy.
Lost it's output!
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Deleted
Deleted Member
Posts: 0
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Post by Deleted on Jan 6, 2009 22:51:46 GMT -5
Corndog !
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