Relay Chattering On Amplifier. Help Please...

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thundir
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Relay Chattering On Amplifier. Help Please...

Post by thundir » Thursday 23rd Oct 2008, 14:51

A friend has an amplifier that is a 2 pill driving an 8 pill. The problem is that when he goes up on the hill Perhaps next to the towers his Relay start to chatter or buzz. This also happens if he drives by the police stations. I believe the transmissions from the beacons are causing this chatter. other wise it works great if not next to the towers. The funny thing is that this never happened on his texas star DX667 turbo amp. Everything is properly grounded. Any Ideas as opposed to staying away from such towers. Any solid info would be great. Could his relay be going funny? Perhaps can some type of filtering be installed...................Thanks Guys............. :roll:

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Post by timrim » Thursday 23rd Oct 2008, 15:38

It sounds like the RF keying circuit is either too sensitive or poorly designed. Can you supply me a schematic of the amp, or at least the keying circuit?

One other possibility is that he may be using low grade coax between the radio and amp, the RF signal from the towers may be getting in through the poor shielding of the coax.

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Post by thundir » Thursday 23rd Oct 2008, 15:58

Hi,
Thanks for replying really quick. We dont have a schematic but if you want i can take a picture of what it looks like where the keying relay is with surrounding components . Will this help. The cable that is being used is The gray RG8mini cable. From radio to amp. And from amp to antenna. Swr is 1 to 1.2. Barely moves on all chanels. This Box is a pirate 1.5 KD. :roll:

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Post by timrim » Thursday 23rd Oct 2008, 16:06

A pic may possibly help, it would be great if you could shine a light from behind the board so that I can see the traces too. Try to get a very close up of the keying circuit. The relay may or may not be near the RF keying circuit, so just to be safe, get at least 1 shot of the whole board.

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Post by thundir » Thursday 23rd Oct 2008, 17:46

I WILL POST PICS SOON. THANK YOU. PLEASE STAY WITH ME ON THIS.

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Post by thundir » Friday 24th Oct 2008, 13:10

Here are the pics. Couldnt get to the bottom of the board to show you the trace. Its a pain to get to.......So what do you think :roll:
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Pirate 1 Relay.jpg
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Post by thundir » Friday 24th Oct 2008, 13:12

here is another
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Post by thundir » Friday 24th Oct 2008, 13:21

I had a 3rd pic but it limited me to only 5 megabytes........Hope these shine some Idea. I can also be your eyes if needed.................Thanks again.............................John...........

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Post by timrim » Friday 24th Oct 2008, 16:03

that TO220 transistor with the diodes attached to it is the RF keyer, can you get me a pic of it with a camera angle facing it, so that I can better see how its connected and also see the part number?

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Post by thundir » Friday 24th Oct 2008, 16:17

Ok ill try and get another pic if The forum allows me. Usually only 5 mb. Let me try. If not i will post it again tomorrow. But will try tonight. Thanks man..........

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Post by thundir » Friday 24th Oct 2008, 17:00

My Digital camera is Junk. So Traced it out on pencil and drew it for you . Please let me know if you can understand writing. Thanks................... :roll Hey they wont let me upload. Can I email it to you please..........I dont understand why Im being limited to 5 mb.........My Email is Thanks..........John.......
Last edited by thundir on Saturday 25th Oct 2008, 17:20, edited 1 time in total.

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Post by thundir » Friday 24th Oct 2008, 17:02

by the way the transistor is ST WFS748 China TIP 31A

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Post by timrim » Friday 24th Oct 2008, 18:42

email sent

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Post by timrim » Friday 24th Oct 2008, 21:44

The circuit you sent me is very crude at best and has no bias. When I get to work, I will draw a suggestion to try with the transistor you are using, but I will also send you a schematic of a more reliable RF keyig circuit that you probably should use to replace that one.

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Post by thundir » Saturday 25th Oct 2008, 13:10

Thanks TimRim for letting me pick your brain there. I will try the combination that you will send me. Patiently awaiting your message................................John. And will let you know of my Results........

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Post by timrim » Saturday 25th Oct 2008, 16:14

Ok, before we consider rebuilding the RF keyer, there are 3 thing that I would like you to do.
First replace the capacitor going to the keying circuit, its currenty a 47pf which is larger than most RF keyers, put a 15pf in its place.

Then give the transistor some bias by adding a 470-1000 ohm resistor from the emitter to base. This value is a starting point, most RF keyers use a 2n2907 small signal type, and a 470 works well in those circuits.

Take a .047uf cap and place it between the emitter and collector of the transistor.

The following is optional, but may help reduce false keying in areas with high RF, I.E...(Towers).

Try placing several small ferrite beads on the base of the transistor (between the 15pf cap and transistor/diode junction, but, if you don't have ferrite beads, a small rf choke around 10-30 uh should work also.

Let me know your results and if not satisfactory, we will redesign the RF keyer, with time tested designs that work well.

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Post by thundir » Saturday 25th Oct 2008, 16:57

At what watt rating should be satisfactory for the Resistor, Also for the 15 pf Thanks . Im going to the shack and pick these items. Afterwards will try it in and up the towers to see what happens. Hey Thanks Tim. Appreciate you being with me on this adventure. Hahaha, Im keeping my fingers crossed. By the way if i cant get the rf beads will the rf chokes apply in the same placement as the ferrite beads? Also what is the watt rating. John......... :wink:.

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Post by timrim » Saturday 25th Oct 2008, 18:04

A half watt resistor will be ok and the 15pf cap should be at least 50 volts. Most disc caps don't have the voltage rating on them, but you should not run into any problems if you find one in your junk radios. Radio shack sells a bunch of disc caps (Catalog #: 272-801), perhaps you will find a 15pf in there. If you have a junk CB, you should be able to find everything you need.

If you end up going to Radio Shack, perhaps you should consider picking up a pack of small signal transistors (Catalog #: 276-1604), these will be needed if for some reason we do not get satisfactory performance after the changes to the RF keyer. However, if you live close to the store, then you could hold off on those until we see how the circuit works.

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Post by thundir » Saturday 25th Oct 2008, 19:11

Got it there Tim. I live not even 10 minutes away From The Shack. I'll pick up the parts tomorrow. Will keep you posted on the results.You're Very Astute. Thanks a bunch again. Hey Tim Have a great weekend. I will keep you up to date on the progress..............................John........ :wink:
Can you tell me why its good to bias the transistor with the 470 ohm resistor? Will this change my box from a class c to maybe a class b box?. Also will changing the capicator to the 15pf yield me what?and why placing the .o47 uf cap yeild. Im just starving for this knowledge. Its been 25 years since ive done electronics ...............Thanks

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Post by timrim » Sunday 26th Oct 2008, 6:33

Well, the 470 ohm resistor is a starting value and may need to be increased, 470 is a common value used with the small signal transistors (yours is a power transistor). Giving it some bias will help keep the transistor operating more stable and reliably.
The 15pf cap will not let quite as much RF get to the keyer, that is not going to change the Amplifier class. The .047 is going to help stabilize the circuit as well.

The optional ferrite beads should block some of the higher frequencies, like those picked up by nearby towers from entering the keying circuit, provided that the RF is getting in through the input line.

Many years ago, I used to homebrew amplifiers, and I have messed with all sorts of designs of RF keyers. I have experienced problems and I found out that what works is circuits that many manufacturers are still using today. Almost every amp on the market uses a similar RF keyer nowadays.

What we are trying to do is to keep stray RF from causing your keying transistor to turn on and off repeatedly.

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Post by thundir » Sunday 26th Oct 2008, 9:13

Thanks Tim,
Just got back. Picked up some parts from the shack and the rest from Frys electronics. Thank you for sticking it out through with me. Im getting ready to assemble the project. Then will take a ride to the towers. Keep you posted like usually. Thank you Tim..........................John. :)

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Post by thundir » Sunday 26th Oct 2008, 11:01

Hi Tim,
Well The results are in.......................It worked. I suspected you were an amp builder. No more Chattering. Man that was music to my ears, Hahaha. Yoyu are the best. Went to other forums and no one could explain how to cure the symptom. Listen just wondering, I dont have my meters for checking the watts, But since there is less RF getting to the keyer because of the 15pf cap will this have a tendancy to drop your output. Or perhaps decrease your input. Or is that just to drop the RF interference. Thanks a million there tim. Ill have the meters next weekend and will be able to see if output was affected. Their across the Bay here in San Francisco. One more thing was there a reason why they placed a power transistor instead of the small signal type............................John...........

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Post by timrim » Sunday 26th Oct 2008, 11:51

Glad that worked for you. No, the slight drop is only to the RF keyer circuit, not the finals, so your output will remain the same.

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Post by thundir » Sunday 26th Oct 2008, 12:02

Thanks again tim. I owe you one. I wont forget you. I know you spent time on this. Man what an adventure. Best to you again.................John........... :D

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Post by timrim » Sunday 26th Oct 2008, 15:39

Sorry, I missed part of your question. They may have used a power transistor because it was all that was handy, or, perhaps they wanted to make sure it would withstand a very strong input. The latter don't make much sense though because its 2 driving 8, and it won't take much input to get BIG output power. :D

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