-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
Skywalker Base Amp Repair
- Thread starter 9C1Driver
- Start date
It's only doing about 300 watts on SSB and seems to draw way more amperage from the power supply then it should. I'm going to have a local guy remove and test the finals first. He has something that is connected to his computer that can test electronic parts to see if they are still working.
Yech... that unregulated power supply makes me cringe.... it also needs proper biasing on the bases of transistors... it's running class C in its current state, so if you're wanting to run SSB on it, it will need some serious reworking.
You should just sell it to me.
~Cheers~
You should just sell it to me.
~Cheers~
Ok, on a serious note. I do see some things that will need to be changed out/reworked.
1) Proper biasing on the bases of the transistors for class AB. This needs to be around .6V.
2) There's no input tune at the input combiner, add a 465 Arco variable trimmer capacitor at the input combiner to ground.
3) the 1000pf silver mica caps on the output transformers should be 1000pf metal-clad caps... far more durable and are more stable.
4) The 4 disc capacitors that go from the collectors of each transistor to ground, replace them with good quality 120pf mica capacitors instead, for reasons stated in #3.
5) I'm betting your output tune is not correct either... since you don't show a picture of the entire amp, I'm not able to see what kind of capacitance or type of caps there are at the output combiner.
6) There is an outside chance that the transistors have gone low gain, due to being over-driven. But they are not shorted, if they were, the little 10 ohm resistors that go from the base of each transistor to ground would be toasty.
Hope this helps.
And yes, I'd be serious about buying the amp, even if the transistors were gone out of it and the power supply was removed.
~Cheers~
1) Proper biasing on the bases of the transistors for class AB. This needs to be around .6V.
2) There's no input tune at the input combiner, add a 465 Arco variable trimmer capacitor at the input combiner to ground.
3) the 1000pf silver mica caps on the output transformers should be 1000pf metal-clad caps... far more durable and are more stable.
4) The 4 disc capacitors that go from the collectors of each transistor to ground, replace them with good quality 120pf mica capacitors instead, for reasons stated in #3.
5) I'm betting your output tune is not correct either... since you don't show a picture of the entire amp, I'm not able to see what kind of capacitance or type of caps there are at the output combiner.
6) There is an outside chance that the transistors have gone low gain, due to being over-driven. But they are not shorted, if they were, the little 10 ohm resistors that go from the base of each transistor to ground would be toasty.
Hope this helps.
And yes, I'd be serious about buying the amp, even if the transistors were gone out of it and the power supply was removed.
~Cheers~
The input is tuned. But I agree with metal clad across output transformers. The single DM19 fails often. Even a pair of 500pf DM19 across each would be a big improvement. Looks like the ferrite transformers are all 43 as well. A lot of loss with those.
That amplifier should sound like dog doo-doo on sideband., It really was not built for it. The extra complexity needed for SSB requires more parts, and boosts the cost of building it. Won't make it sell any better, so they leave that stuff off.
A switch marked "SSB" on the front consists of two parts. The switch and a capacitor that makes the relay "hang" so it won't chatter. And that's all. Has nothing to do with how it will sound if you use it for SSB. If it has that switch, it's a fraud pure and simple.
The power supply probably runs around 20 Volts. The transistors were designed to be stable running from about 2/3 that much.
Adding bias to a "competition" AM-only amplifier like this one raises the risk that it will become unstable and blow out the expensive RF transistors.
Reducing the power supply voltage to a level that's safe for sideband is not simple.
Always expect around half the peak power on sideband that you can get by pushing the amplifier with large AM peaks. The distortion you get by pushing the amplifier hard in AM mode won't make your AM signal sound all that bad.
Sideband is different. The distortion will be pretty high unless you hold your peak drive level down to roughly half or two-thirds what you can use for AM.
Pushing it with excess drive on sideband just makes it sound bad, even it the amplifier is properly set up for clean-sounding SSB.
73
A switch marked "SSB" on the front consists of two parts. The switch and a capacitor that makes the relay "hang" so it won't chatter. And that's all. Has nothing to do with how it will sound if you use it for SSB. If it has that switch, it's a fraud pure and simple.
The power supply probably runs around 20 Volts. The transistors were designed to be stable running from about 2/3 that much.
Adding bias to a "competition" AM-only amplifier like this one raises the risk that it will become unstable and blow out the expensive RF transistors.
Reducing the power supply voltage to a level that's safe for sideband is not simple.
Always expect around half the peak power on sideband that you can get by pushing the amplifier with large AM peaks. The distortion you get by pushing the amplifier hard in AM mode won't make your AM signal sound all that bad.
Sideband is different. The distortion will be pretty high unless you hold your peak drive level down to roughly half or two-thirds what you can use for AM.
Pushing it with excess drive on sideband just makes it sound bad, even it the amplifier is properly set up for clean-sounding SSB.
73
One of these days I gotta write up a how-to for wiring a "buck" transformer into one of these "competition" base amplifiers.
It will reduce the 120 Volts feeding into the power transformer and allow reducing the power supply voltage to the transistors to a safer level.
Consists of mounting the right Antek toroid transformer and wiring it into the amplifier's power transformer's 120-Volt input, dropping the AC voltage from the wall socket.
Thought I had a pic in my ImageShack account showing this mounted in a typical "DFX" base amplifier. Ah, "DFX" as in "Dave-Fat-Xforce".
Can't find it. I'll track down the original next time I'm back at work.
73
It will reduce the 120 Volts feeding into the power transformer and allow reducing the power supply voltage to the transistors to a safer level.
Consists of mounting the right Antek toroid transformer and wiring it into the amplifier's power transformer's 120-Volt input, dropping the AC voltage from the wall socket.
Thought I had a pic in my ImageShack account showing this mounted in a typical "DFX" base amplifier. Ah, "DFX" as in "Dave-Fat-Xforce".
Can't find it. I'll track down the original next time I'm back at work.
73
Since your using on sideband. Really the power supply needs to be regulated to keep signal linear. Same with bias. It should be regulated and include temperature tracking.
Why not build a regulator or use a regulated power supply? One of the issues with an unegulated supply is poor stability. It seems like adding another transformer would make this even worse.