I have a 4 power dx 400 .. no output ... on inspection looks like a 100 ohm resistor maybe 2 watt not sure is blown And the ferride thats wrapped with a green and red wire is burnt as well... next to them is a mica capacitor that has small crack in it ... may be bad as well.. anyone no the values of said parts and where I might get them... I'll figure it out but thought someone might have a schematic of first hand knowledge of these amps..pic of what I'm speaking of
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Bi lateral 4 power dx 400 amplifier
- Thread starter RICKY DUNCAN
- Start date
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The second set of pics explains a lot. The toroid coil is the combiner transformer. It puts together the power from the front and the rear circuit boards. So long as both circuit boards each deliver the same power level, this part runs cool. The resistor attached to it absorbs only the difference in power between the two pairs of transistors. If they are both delivering the identical power level, it will continue to look like the one in the first pics. The difference will be zero.
But when one of the two pairs of RF transistors fails, the difference is now a couple of hundred Watts, more or less.
The resistor now has all of the power difference pumped into it. Turns it into a crispy critter pretty fast.
The burned combiner and resistor are only a symptom of the real problem. NOT the cause of the problem.
RF Parts used to sell the combiner and the capacitor, both. Can't find the combiner there any more. Barkett has it here:
https://stores.goldeneagleradios.com/texas-star-large-toroid-combiner/
And when they are installed, the amplifier will immediately blow them both to the devil like the first ones. It's a long shot to expect all four RF transistors to be okay. If a capacitor on one pair of transistors comes loose, this can unbalance the power between the two pairs. The calibrated eyeball and a bright light would reveal this.
73
But when one of the two pairs of RF transistors fails, the difference is now a couple of hundred Watts, more or less.
The resistor now has all of the power difference pumped into it. Turns it into a crispy critter pretty fast.
The burned combiner and resistor are only a symptom of the real problem. NOT the cause of the problem.
RF Parts used to sell the combiner and the capacitor, both. Can't find the combiner there any more. Barkett has it here:
https://stores.goldeneagleradios.com/texas-star-large-toroid-combiner/
And when they are installed, the amplifier will immediately blow them both to the devil like the first ones. It's a long shot to expect all four RF transistors to be okay. If a capacitor on one pair of transistors comes loose, this can unbalance the power between the two pairs. The calibrated eyeball and a bright light would reveal this.
73
Thanks for the info... will look into the actual issue ... just was seeing the obvious... great info explaining the why and now I have to locate the reason for the failure...I'll reach out if I have more questions... take care
If you had a good clear pic of the entire amp (both boards only), people here would tell you what else might be wrong. The caps on both input transformers have to match, in fact, everything on each board has to match the other. I've had one with a bad cap on one of the input transformers, and the output combiner got very hot, replaced each cap on both input transformers, and the problem went away.
It would be best to unsolder one end of each of the matching caps between each board and test them to see if they match each other, resistors can be checked for proper values.
The main thing when you go to test it out is to first make sure that the bias is correct on both boards, which would mean instant death to the transistors if wrong. There are many videos on youtube where a lot of this is explained.
RF parts do have SRF2072 pills (transistors) in stock, but they are not matched, but must be matched as close as possible to work properly. There are other pills that can be used, but the amp may have to be re-tuned for them to work properly. Whatever you do if any of the pills are blown the replacement would have to match the good ones, or all four would have to be replaced with a "matched quad" (4 pills that are matched), or the same thing could happen.
You may also have to take both boards out to inspect all the bottom traces, at the same time put clean out and replace the heat sink compound under the pills, if they are still good. Putting clean fresh heat sink compound would have to be done with new pills anyway, and highly recommend it for the old ones.
Looks like a fun project.......
73
It would be best to unsolder one end of each of the matching caps between each board and test them to see if they match each other, resistors can be checked for proper values.
The main thing when you go to test it out is to first make sure that the bias is correct on both boards, which would mean instant death to the transistors if wrong. There are many videos on youtube where a lot of this is explained.
RF parts do have SRF2072 pills (transistors) in stock, but they are not matched, but must be matched as close as possible to work properly. There are other pills that can be used, but the amp may have to be re-tuned for them to work properly. Whatever you do if any of the pills are blown the replacement would have to match the good ones, or all four would have to be replaced with a "matched quad" (4 pills that are matched), or the same thing could happen.
You may also have to take both boards out to inspect all the bottom traces, at the same time put clean out and replace the heat sink compound under the pills, if they are still good. Putting clean fresh heat sink compound would have to be done with new pills anyway, and highly recommend it for the old ones.
Looks like a fun project.......
73
Hey Nomad the combiner you linked is for the DX1200/DX1600.
This is the one for the DX400/DX500/DX600/DX667. https://stores.goldeneagleradios.com/texas-star-small-toroid-combiner/
Now I know you can use the large one in place of the small one because my 667 has had one in it for almost 30 years, I'm just pointing it out.
Yes but he got me to the right place lol... no worries... also found a blown resistor in front of the push button switches ... there's 2 there .. the one on the right side was blown and the one following it around the corner looked like it was about to be ... Def some issues... no worries.. appreciate all the info
