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 Post subject: Ramsey AM-25 Transmitter.
PostPosted: Jul Wed 11, 2018 1:11 am 
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Joined: Jan Mon 17, 2011 2:10 am
Posts: 98
Location: Revere,Massachusetts U.S.A.
Hi Folks, I need your help I bought this unit off eBay, it’s drawing about 1.8 amps . I’ve checked all the transistors and IC’s for dead shorts. Nothing yet also I replaced all the electrolytic’s . Still same problem.
The power transistor and Fet are running super hot. Any idea’s would be appreciated.
Thanks Tony


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 Post subject: Re: Ramsey AM-25 Transmitter.
PostPosted: Jul Wed 11, 2018 2:53 am 
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Are you sure that it's drawing 1.8 amps? Seems that level of current would fry things. It sounds like a dead short somewhere. I'd begin by checking the trace side of the board for solder shorting between traces or landings.

Here's a link to the instruction manual;
https://www.manualslib.com/manual/80163 ... tml#manual

Unfortunately they deleted the schematic diagram.

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 Post subject: Re: Ramsey AM-25 Transmitter.
PostPosted: Jul Wed 11, 2018 4:28 am 
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Do you have polarity correct?? Power supply producing DC & not AC?

I know with the Ramsey supplied 18v power pack, there is one transistor that's prone to short out. One in my AM-25 popped completely apart. Don't remember any issue with other components overheating.

I'll see if I can dig up info, think still have the manual out in garage.

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 Post subject: Re: Ramsey AM-25 Transmitter.
PostPosted: Jul Wed 11, 2018 9:36 am 
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Location: The Old Dominion VA 23518
The schematic for the AM25C is available at mods.dk

It may differ slightly from the OP's. Most of the AM25s I've seen suffer from bad modulator transistors and/or mis-wiring by an amateur failing to follow the manual when assembling the unit.
The only high current path is through the modulator, but the modulator series resistors would burn up before you got that high in current. Look for solder bridges - there are no electrolytics except C34 (470uf, near the power switch) directly connected ot the 12V line, just 0.01uf and 0.1uf disc caps.

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 Post subject: Re: Ramsey AM-25 Transmitter.
PostPosted: Jul Wed 11, 2018 4:04 pm 
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Joined: Jan Mon 17, 2011 2:10 am
Posts: 98
Location: Revere,Massachusetts U.S.A.
Thank You to all for the quick reply, will keep trying to find the problem.


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 Post subject: Re: Ramsey AM-25 Transmitter
PostPosted: Jul Thu 12, 2018 9:00 am 
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Joined: Jan Mon 17, 2011 2:10 am
Posts: 98
Location: Revere,Massachusetts U.S.A.
Hi Again,I tried to include the schematic but the file is too large. it was drawing 1.8 amps and both power NPN & power FET were hot enough to fry eggs. What the builder did was jumped R34 & R33 in order to boost the output to 1 watt.

Once I removed the jumpers and replaced the power transistors it was drawing the proper current around 300MW. Also, I was able to set the proper bias levels and it's overall performance greatly improved. I also swapped out all the electrolylics just to make sure if any were damaged by the excessive current.

Now R34 & R33 are overheating current draw is still at 300MW and its performance is still good, there must be damage from running everything was to hot.
I guessing that the coils L4& L6 may have shorts across there windings ??? They also carried the heavy current.

If you folks would look over the schematic and maybe give me a couple of hints, I would greatly appreciate any help.

Thank You Tony


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 Post subject: Re: Ramsey AM-25 Transmitter.
PostPosted: Jul Thu 12, 2018 12:12 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Arlington, TX, USA
The two 10 ohm resistors are discolored in my unit as well. I believe they are just undersized for the application as far as power rating goes. I doubt the two matching transformers (L4 and L6) are damaged.

This transmitter wants to see a normal RF load of about 50 ohms for the output filter network to do a good job, otherwise the output waveform looks very messy indeed. A normal short wire antenna is not enough, either use a resistor or loading coil to get the load impedance down.


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 Post subject: Re: Ramsey AM-25 Transmitter.
PostPosted: Jul Thu 12, 2018 1:05 pm 
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Location: Utica, NY 13502 (USA)
transoceanic8523 wrote:
Now R34 & R33 are overheating current draw is still at 300MW
Do you mean the current draw is 300 milliamps (ma) or do you mean the power dissipated in the resistors is 300 MW?

Dave


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 Post subject: Re: Ramsey AM-25 Transmitter.
PostPosted: Jul Thu 12, 2018 3:40 pm 
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Joined: Jan Mon 17, 2011 2:10 am
Posts: 98
Location: Revere,Massachusetts U.S.A.
Hi Dave, Thank You for your help, the 300ma is what the unit itself is drawing. Seems odd that it appears to be working correctly even with the 2 (10 ohm) overheating???

Looking forward to hearing from you.....Tony


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 Post subject: Re: Ramsey AM-25 Transmitter
PostPosted: Jul Thu 12, 2018 7:24 pm 
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transoceanic 8523 wrote:
I guessing that the coils L4& L6 may have shorts across there windings ??? They also carried the heavy current.


Easy enough to find out. Just use your meter across each winding to see if there's any resistance.

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 Post subject: Re: Ramsey AM-25 Transmitter.
PostPosted: Jul Fri 13, 2018 2:16 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 19763
Location: Utica, NY 13502 (USA)
What is the voltage drop across the resistors?

Dave


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