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Any linear pro's?


only putting 12W in, the pills dont get hot but the 1000PF cap gets 80C pretty fast.
I also changed C23 it was a tiny wimp and tested bad.

2fpFmOw.jpg
 
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357 said:
only putting 12W in, the pills dont get hot but the 1000PF cap gets 80C pretty fast.
I also changed C23 it was a tiny wimp and tested bad.

2fpFmOw.jpg
Click to expand...

You talking 12 watts PEP on SSB? If your on AM what's the dead key input and peak swing?
 
Not an expert by any means, but I have worked on a radio or three.
First blush thoughts:

I'd agree with tba02, you may well still be over-driving it. Unless you have a wattmeter that has 'true peak' reading; then you are not getting an honest reading. Don't expect a Dosy meter to be giving empirical data; not gonna happen. You might be putting in 20w and not know it. That cap is at the input stage; so it might caused by the input feed signal.

Those transistors were spec'd for 10w peak/5w ea @12.5v to operate as designed.
Look up the spec sheet for the 2SD1446 online and read it for yourself.

If the radio has been modded by some dude who fancies himself as a real tech; then his work might be questionable. The radio itself - because of improper modding - could also be adding/feeding lots of harmonics in that amp too. That could well be adding heat where it wasn't meant to be due to those same harmonics present. That larger silver/mica cap is part of the input matching circuit with that coil designed to get a proper match to the transistors @ 27mhz only; not the harmonics of 27mhz.

When in doubt, use a stock radio and then see if it still heats up. Bur you will need to to turn down the output to 2w.dead key in AM mode. If it does; replace it.

Seldom does a ceramic disc cap go bad. Never seen it myself, but I know it is possible. They usually are used to couple a circuit to another, as part of a filter circuit, or sending noise/ripple from a DC circuit to ground in radio circuits. Dunno much about amps.

Seen that silver/mica cap on the input coupling of that very same amp you have - get blown up, due to just too much input power applied.

Improper or faulty biasing could also be to blame, as well as feedback/oscillation. Just keying up an amp for too long can heat them up too. Heat being conducted from their proximity to the finals.
 
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There also appears to be a burnt or partially burnt trace at the 470UF cap that is located in the small circle with the marking
??0UF, and it also appears that where your intputs for the large transformer, or whatever side is near the cap that has the burnt trace, also appears to have been heated a lot. Check the solder joints on the other side of the transformer and also the area around that cap. And these amps don't like a lot of drive. Even 12w pep is about all you really need. More realistically about 8-10w pep on SSB would be better IMO. The POS knock off palomar I have doesn't have the same board as yours, but it does have 2 sd1446's. It also doesn't like much drive. 8-10w pep on SSB is about where I would run it at. 12w is right on the edge of overdriving them IMO. Just what I have experienced with the one I have. And it doesn't get hot with the lower power input. Might loose a bit of wattage on a meter, but no one will notice it on the receiving end. JMHO. Here are a couple of pics of the inside of mine. IDK if it's even a amp that is truly a class C or what amp, but it does work on SSB with low drive, and gets good audio reports. Ill let the amp gurus tell me what it is and if it should be used on SSB or not. And the radio I use is a Uniden 980 with this amp. Power is set at 8-10w SSB and on AM I set the DK to about 2w swinging to about 6-8w. Maybe I paid too much for it, but at the time I was in nowhere land and just wanted an amp. So for $80 this is what I got. I also scored a magnaforce 350hd for the same price, just a different location. Anyways, some pics of my amp and please let me know what the heck this little thing is??
 

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Man Robb I swear I didn't read your entire post the first time, but what you stated backs up what my amp does when I over drive it. Gets hot and doesn't sound very good on air either. Keep the input PEP wattage low and the DK at 2w or less and it will last a lot longer. One reason I really like the Uniden 980, hard to over drive an amp with it LOL!! I've de-tuned the SSB power even below stock levels to keep it cool and it really has helped a bunch! At any rate, great post and info as well!!!
 
I'd pull the pills first and test them with a transistor tester. It's making power so you know they aren't dead but were looking for a weak one. Then start pulling and testing all the mica caps. While youre doing this keep your eyes open for solder splatter or flux that's where it shouldn't be.
 
Thanks for the replies.
The 470uf cap is the delay cap that had a broken trace and had no ssb delay, I did my best to replace that.
I got the amp off ebay and it had the keying diode and heated resistor next to it already cause I think someone ran a 2510 into it. http://www.ebay.com/itm/Galaxy-Palomar-HD-250-Mobile-Bi-Linear-Amplifier-/291664823233?nma=true&si=HPlC7a0e0nOqX%2BBrs2ijn%2B7n8Zk%3D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557

I was hoping it was a AB biased like 222's amp, but it wasn't.
222's is the better of the 2.
I guess the only thing I can do is get 2 trimmers and mic it out.
I was hoping for a simple answer but I dont think I'm gonna find it.

The radio is my Centurion, some wannabe tech did work on it and recapped it.
Maybe he didn't know what he was doing.../sarcasm
 
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Sorry 357. And I didn't know mine was AB biased. Wild. Guess it wasn't a bad buy after all!! It works well when you keep the input low!! The Uniden 980 is a great radio for driving it. The outside of the case is a bit rough. Nothing not easily fixed! But the inside is clean I suppose. I haven't used the amp in probably 2 years. It just sits on a shelf now along with my magnaforce 350hd, an older TNT 600hd with Toshiba 2879's. When I say older, it's over 10yrs old I would venture to say. None see use anymore since getting a good radio and antenna setup. 100w is plenty enough for me these days. At any rate, I still have them, and all work well. The magnaforce 350hd is a little work horse man. 2 Toshiba 2879's. It made me a lot of contacts while in the mobile!! Anyways. Thanks for telling me what I had here 357. I didn't know mine was a class ab bias amp. Even class B would be fine with me for GP SSB use. Good radio in front of one and they work great. At any rate. Again, thanks for reply and have a good one sir. Hope you get you're amp issues resolved!!
 
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Thanks, I had a shooting star 225 that was the same as yours, it was really clean.
Only thing you gotta watch is the blue chiclets by the relay.
I had the output one die and it had no output but still drew power.
I replace the output one (not sure which on left or right) with one I found in a telephone believe it or not, lol desperate for the part.

The one I have now I made some progress.
I took the 750PF off the input, put it on the output and put a 47PF on the input.
SWR's and the heat from them has dropped to an "acceptable" level but I'll have to hunt down some variable caps, 300-1000PF or somewhere in between.
 
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Variable caps is what I would try to use as well . . .

You can improve the bias circuit by putting the diode (think it is on the bottom of the board) across the top of the finals. Then use some heat sink compound between that diode and the pill, so that the heat of the pill will change the value of the diode. You might have to put a bypass cap (.010uF) on the leads to that diode, in order to keep RF from affecting its function. Can't remember the details ATM.

There is a real great thread on this forum that shows you how to use a simple relay, a voltage regulator, and a few resistors to make a nice bias circuit. Use the forums search engine and see if you can find it, worth the time to look. Learned a lot from that thread - recommend it.

Ahh; found it:
http://www.worldwidedx.com/threads/quick-and-dirty-class-ab-mod-for-kl300p.93982/
 
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