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Afterburner 482 help
- Thread starter casios
- Start date
The largest clue you've provided is: I also get a steady 15% modulation. I think its got a humm when keyed like that. Help?
The electrolytic filter capacitors are gone. Well, functionally "gone". Odds are that there is a visible "zit", an outward puncture on the rubber end-cap of one (or more) of them.They're only supposed to be good for 10 to 20 years in the first place. If the filters in your unit are original, that makes them closer to 30 years old.
Never have found a diagram for that amplifier, so I don't know what they used. usually two 450-Volt parts in series. Some manufacturers back in those days (D&A,Varmint) used a series string of three 350-Volt parts.
Don't even know whether it uses a low-voltage 12-Volt relay and a transistor to activate it, or the older 110-Volt DC relay with a tube to sense the RF and key the relay.
Either way, it sounds like you have gotten your "money's worth" out of the electrolytic caps in this unit, ESPECIALLY if they are original. As a rule the only rational way to return a unit like this to regular service is to replace them ALL. Scorched earth, no exceptions, for any electrolytic cap that looks over 20 years old.
If there is a separate bleeder resistor in parallel with each of the H.V. electrolytics, they are probably old, cheap carbon-composition type resistors. They don't age gracefully either, and should be replaced along with the filter caps. If the resistance value of an old bleeder has changed too much, it could cause excess DC voltage to be put across one of the filters. The bleeders are cheap insurance, compared to the cost of blowing out a new filter cap.
And if it has just one bleeder, with NO separate resistor in parallel with each series-connected filter, they need to be added. We use 240k 2 Watt, but there is latitude to this value. Smaller resistance calls for a larger wattage rating. Don't recommend using a resistance any higher than about 500k. Just so that each of the bleeders placed across filters in series are the SAME value. That's part of their function, to equally divide the high voltage two ways, three ways, however it's arranged. You don't want one electrolytic to have more than its 'share' of the HV placed across it, or its breakdown rating could be exceeded, causing it to fail prematurely.
73
The electrolytic filter capacitors are gone. Well, functionally "gone". Odds are that there is a visible "zit", an outward puncture on the rubber end-cap of one (or more) of them.They're only supposed to be good for 10 to 20 years in the first place. If the filters in your unit are original, that makes them closer to 30 years old.
Never have found a diagram for that amplifier, so I don't know what they used. usually two 450-Volt parts in series. Some manufacturers back in those days (D&A,Varmint) used a series string of three 350-Volt parts.
Don't even know whether it uses a low-voltage 12-Volt relay and a transistor to activate it, or the older 110-Volt DC relay with a tube to sense the RF and key the relay.
Either way, it sounds like you have gotten your "money's worth" out of the electrolytic caps in this unit, ESPECIALLY if they are original. As a rule the only rational way to return a unit like this to regular service is to replace them ALL. Scorched earth, no exceptions, for any electrolytic cap that looks over 20 years old.
If there is a separate bleeder resistor in parallel with each of the H.V. electrolytics, they are probably old, cheap carbon-composition type resistors. They don't age gracefully either, and should be replaced along with the filter caps. If the resistance value of an old bleeder has changed too much, it could cause excess DC voltage to be put across one of the filters. The bleeders are cheap insurance, compared to the cost of blowing out a new filter cap.
And if it has just one bleeder, with NO separate resistor in parallel with each series-connected filter, they need to be added. We use 240k 2 Watt, but there is latitude to this value. Smaller resistance calls for a larger wattage rating. Don't recommend using a resistance any higher than about 500k. Just so that each of the bleeders placed across filters in series are the SAME value. That's part of their function, to equally divide the high voltage two ways, three ways, however it's arranged. You don't want one electrolytic to have more than its 'share' of the HV placed across it, or its breakdown rating could be exceeded, causing it to fail prematurely.
73
Okay... I see exactly what you are talking about. One of the filters loooks really old and the other looks new.Someone has replaced it because the goo that comes out of one when bad is still there.(someone did a pisspoor job in trying to scrape it away
). I don't see any physical "zit" on either one of them. I performed the ole charge and "dc" test check on both of them and they seem to drain fairly rapidly so I will replace them both. I also see the bleeder resistors in series. I will replace them also. they seem to be 1 watt 270K and look to be original.
Now for the relays ( one rx and other tx)...It is a 12 volt circuit. And yes there is a pnp transistor in series with one of them. I am guessing it would be the keying transistor..I will pull that and test it.
Question>> Would the electrolyc caps have anything to do with jumping frequency when modulating?
thanks for the help..will post results
Cas
). I don't see any physical "zit" on either one of them. I performed the ole charge and "dc" test check on both of them and they seem to drain fairly rapidly so I will replace them both. I also see the bleeder resistors in series. I will replace them also. they seem to be 1 watt 270K and look to be original.
Now for the relays ( one rx and other tx)...It is a 12 volt circuit. And yes there is a pnp transistor in series with one of them. I am guessing it would be the keying transistor..I will pull that and test it.
Question>> Would the electrolyc caps have anything to do with jumping frequency when modulating?
thanks for the help..will post results
Cas
Nomad! your the man! Thanks everyone for the help...so far....
The electrolyc caps got rid of the humming and replacing the keying relay was able to work on ssb switch only. I know I still have something goofy going on here. Switch it over to am\fm and the realy goes to chattering again when modulating...wierd..in most of the experience that I have had, its usually because the keying relay or circuit is weak but my experience is with solid state...anything different here?
thanks
The electrolyc caps got rid of the humming and replacing the keying relay was able to work on ssb switch only. I know I still have something goofy going on here. Switch it over to am\fm and the realy goes to chattering again when modulating...wierd..in most of the experience that I have had, its usually because the keying relay or circuit is weak but my experience is with solid state...anything different here?
thanks
Uh, well a low-voltage relay/transistor keying circuit will have low-voltage electrolytic capacitors in it, too.
I'd change any of them that are visible before suspecting something tricky. The ones that run at higher voltages will show visible signs when they die, but the low-voltage types often still look perfect when they die.
Sounds as if the filter for the 12-Volt DC relay power is bad. The SSB capacitor serves as an "auxiliary" filter when it's switched in. Maybe.
73
I'd change any of them that are visible before suspecting something tricky. The ones that run at higher voltages will show visible signs when they die, but the low-voltage types often still look perfect when they die.
Sounds as if the filter for the 12-Volt DC relay power is bad. The SSB capacitor serves as an "auxiliary" filter when it's switched in. Maybe.
73
Not sure if it will help but you can get the schematic at CB Tricks if you want to check out the keying circuit a little closer.
Hygain index at CB Tricks
Hygain index at CB Tricks
Most likely culprit is C26, 500 uf 15 Volt electrolytic. A 1000 uf 25 Volt is a more generic replacement, and will do the job.
73
73
Ok...I now have a very excited and happy friend as all works just fine now.
Nomad, i am glad that you said that about the different size because that's what I used. I was hoping it wouldn't hurt anything. Also there was a diode...um I can't think of the schematic number...but it was too, gone.
Thank Ya'll for all the help. this guy is tickled and feels like a big strapper now! Maximum output with original tube's...110 watts! Before the work...40 watts.what a difference...
Nomad, i am glad that you said that about the different size because that's what I used. I was hoping it wouldn't hurt anything. Also there was a diode...um I can't think of the schematic number...but it was too, gone.
Thank Ya'll for all the help. this guy is tickled and feels like a big strapper now! Maximum output with original tube's...110 watts! Before the work...40 watts.what a difference...