Hi all, I have a Dentron 2500b that I can key up just fine with a pedel but it's rf sniffer circuit or whatever is not functional and this beast is a little different than what I am use to working on. I guess my question is what do you guys think could be the issue since I don't see a keying transistor, there is a closed relay and a tube style relay . Thanks in advance
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Dentron MLA-2500B key up issues
- Thread starter Danzik
- Start date
There is no RF sniffer. The tube relay is a delay relay. I would highly suggest you read the manual.
SL
10-4 SuperLid, I will read the manual on it however I have friends that have the same amp and they state that theirs will key up without a pedel or using the relay jack and the fan speeds up faster when using it this way but mine does not. I will check out the manual
That Amplifier now going on 50+ years old...is one of the most modified HF/Amateur amps of the resent era...The original tubes over the course of about 10 years, went from about $50 USD each to Unobtainable list...they were Hot Rods but limited production usage soon made them fall off the list of manufacture's replacement productions profit margins.
The amp did NOT have a RF sensing circuit...manual switching only...again highly modified amp.
You can look on the web...gobs of pages on tube swaps/circuit mods etc. I would say there is very few "original" stock versions still running...GL
All the Best
Gary
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Ok, thank you I think I will just leave this one alone then because I believe it has been untouched except the fact some previous owner installed the 2500 band switch to gain 10 meter I believe. It still has the 8875 tubes in it and they seem to put out plenty
Amen to that. Yup......those amps were NEVER built with the RF sensing cct. They are a true amateur amp and had no such need whatsoever. The only RF sensed amps I have seen are the typical CB amp or some VHF/UHF amps. My 2m amp had the option of using RF sensing or straight radio control. When I was using my 2m amp on 6m after I modified it, I was using it with a transverter with a 20m IF. I used the relay control from my HF rig to trigger it. MUCH more reliable.
You could add RF sensing to it.
https://www.ebay.com/itm/1160941166...pid=5336136228&customid=&toolid=10001&mkevt=1
Or you can add a relay module inside the radio you use, and key it with a patch cord from the radio like it was built to do.
https://www.ebay.com/itm/1161584911...pid=5336136228&customid=&toolid=10001&mkevt=1
Always more than one way to skin a cat. Just be nice to those tubes. They are made from unobtanium.
73
https://www.ebay.com/itm/1160941166...pid=5336136228&customid=&toolid=10001&mkevt=1
Or you can add a relay module inside the radio you use, and key it with a patch cord from the radio like it was built to do.
https://www.ebay.com/itm/1161584911...pid=5336136228&customid=&toolid=10001&mkevt=1
Always more than one way to skin a cat. Just be nice to those tubes. They are made from unobtanium.
73
Yeah, I have one of your relay boards already Nomadradio but I didn't know if that transistor could withstand the kind of drive that amp is getting, if so then that would be a easy way of doing it for sure
That board comes with two resistors for the RF input. A 2.2k for barefoot radios and a 4.7k for up to 150 Watt peaks more or less. That amplifier should never have more drive power than that if you want get any lifetime from the tubes. Should hold up just fine.
73
73
Got er installed Nomadradio, I guess I will have to purchase a couple more of those in the near future, pretty handy device works great
The variety of wretched and half-baked keying circuits we found in CB amplifiers made it necessary to design this board. Fixes that problem quickly, so you can move on to the scary and expensive details like filter capacitors and tubes.
Thanks for the purchase and 73
Thanks for the purchase and 73
How about the ones that don't bypass the diode, rectify RF and inject tons of harmonics at the input of the amplifier?
Gotta figure in the bigger picture, the harmonic energy percentage from the keying circuit is a tiny fraction of the harmonics produced by an overdriven amplifier circuit with no low-pass filtering on its output.
The keying circuit certainly could contribute to an amplifier's harmonic output, but if you were to measure it I'd guess the percentage to be a single digit max.
73
I made the same assumption for years and am embarrassed to admit how wrong I was, by copying an RF sensing circuit that did not use a bypass cap directly across the RF diode. When I purchased one of those small, $150 spectrum analyzers, I was shocked.
In my case, the largest harmonic content by far, was coming from my unbypassed RF rectifier diode. I recall it producing a second harmonic no less than 20db down from the fundamental. With the amp off, these harmonics extended well into VHF high band. The amplifier actually blocked the highest harmonics from passing.
I've found adding the bypass cap across the diode, makes the circuit require more drive or, more gain however, it eliminates what could be the addition of much more harmonic content, than I ever expected.
The circuit that produced these results, was probably worst case scenario, using an unbypassed germanium diode. It maximized sensitivity and harmonic content.
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...runna pile-up on 6m SSB