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AMP QUESTION ? SKIPPER BY PALOMAR AB1

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Old 07-12-2008, 08:21 AM
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Default AMP QUESTION ? SKIPPER BY PALOMAR AB1


The amp in question is a 2 x 2879s Skipper by Palomar box AB-1 (mobile) everthing works on the box except the High and Max side of the box ? With 1 watt DK to 18 watts pep on AM , Low does about 10 to 40 and medium does 15 to 60 , and does the same on SSB . The numbers only change very slighty, if any, on High and Max but it still shows major amp draw on those two settings ?

The amp looks very clean within (for what good that helps me,although the screws on one of the 239s look longer then the others so it's obvious to me that somebody was in there before ? ) Could 1 Transistor be bad on this or could it be something else ? Anything I could look at on this one ?


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Old 07-12-2008, 08:37 AM
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sorry about the second thread , no idea how that happened ?

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Old 07-12-2008, 10:45 AM
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I've been looking at the picture for sometime, and only see the too long bolts on the one intput connector as you did, someone changed that along the line...


The amp is reasonably classic in design, as most of these are buildt in the same configuration, with or without bias or class C or class AB1.
This one has the bias, so clearly an classs AB1 for SSB.
It has an rudimentary Pi filter, so it will be relative clean if it is not overdriven.

Looking at the resistor network for selecting the drivelevel, i see no burnt resistors there, just check if they still have the value as read out from the coloured rings.

2SC2879, 120 watt output with 7watt input gain 15 dB at 30 mHz

You should see with adequate drive 200 watts output with an carrier of 12 to 20 watts bypassing the resistor network.

If both transistors are good, but the amp doesn't have the output it should have, but does pull the amperage you have an faulty C or an fault in the output circuit.

In there the HF partly goes to mass through an faulty leaky C or other faulty component.

Without physically putting it on my testbench, you need to check the components.
The amp looks clean, no ugly repairs or "mods" i can see in the picture.
So mostly an component failed, relais has burned contacts, or bad solderjoint.

Make sure the in and output connector make good ground.
Rebuilding my KL500 to amateur use amp, the extra grounding straps there gave an almost 3 dB gain.

Sometimes the in/output transformers are at fault.
The copper tubing through the ferrites must be thoroughly soldered to the pcb wich is then soldered to the collectors and output or basis and input.

The (teflon) wire through the transformer should not make contact with the tubing, check for nicks in the isolation there.

Looks like an decent little amp, hope you get it sorted.




The network from collector back to the basis , C, R and the L form an feedback from collector to basis depressing too much gain, and stabilising the amp.

Checking the transistors is relative simpel, lift the basis flaps from each transistor, use an multimeter in diode measurement and seee if the transistors have about 600 Ohms in one direction, basis to Collector, basis to emittor, and thee other way round it should be almost infinite resistance.

If you have that, the transistors are good, and the problem is elsewhere.

73,

Cor

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Old 07-12-2008, 12:23 PM
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"2SC2879, 120 watt output with 7watt input gain 15 dB at 30 mHz"

db gain = 10LOG(POut/PIn)
db gain = 10LOG(120/7)
db gain = 10LOG(17.142)
db gain = 10(1.234)
db gain = 12.340

the 2SC2879 is rated at 100WPEP output with 4.75W in @ just over 13db. gain at 28 Mhz.

db gain = 10LOG(POut/PIn)
db gain = 10LOG(100/4.75)
db gain = 10LOG(21.052)
db gain = 10(1.323)
db gain = 13.233

http://www.firecommunications.com/trdbase/2sc2879.pdf

"use an multimeter in diode measurement and seee if the transistors have about 600 Ohms in one direction, basis to Collector, basis to emittor, and thee other way round it should be almost infinite resistance."

the diode test function does not provide resistance measurements.

Last edited by freecell; 07-12-2008 at 12:39 PM.

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Old 07-12-2008, 01:34 PM
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I know my dB stuff, but didn't want to complicate things.

dB gain by an manufacturer is nice, in real life you have losses to compensate, in/output circuits, etc.

If he uses an multimeter as i suggested he also has resistance ranges on that, and can use the diode test and/or resistance test.

Finding an datasheet is for nobody an problem, using an Smithchart might be an challenge for some.

Since i don't know Switch Kit's knowledge about repairing the amp, i tried to keep it to the point and simple, otherwise he wouldn't have asked help if he had thee knowledge to do the job himself.

Trying to show off that you know how to calculate dB is stuff here for the novice license wich i passed in spring 1977

Soon after that the full license.

Main thing is here to help him to repair his P.A.

My nick is not chosen like it is for nothing, Justme.

I have no need to show off, i'm past that stage an long time.

Have an nice day

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Old 07-12-2008, 01:48 PM
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"I know my dB stuff, but didn't want to complicate things."

no you don't because if you did then you would know that at 7 watts of input @ 15db. of gain the transistor output would be 224W.

db gain = 10LOG(POut/PIn)
db gain = 10LOG(224/7)
db gain = 10LOG(32)
db gain = 10(1.505)
db gain = 15.051

"dB gain by an manufacturer is nice, in real life you have losses to compensate, in/output circuits, etc."

the manufacturers live in the real world, you live in lala land. if you bothered to look at the spec sheet you'll see that's a real live test circuit they used to obtain the specifications for the device. i think everything you were worried about was pretty much covered there.

"If he uses an multimeter as i suggested he also has resistance ranges on that, and can use the diode test and/or resistance test."

if he uses a multimeter in the diode test mode he's not measuring resistance and neither are you.

"Finding an datasheet is for nobody an problem, using an Smithchart might be an challenge for some."

not a problem here, although it's rather apparent that understanding a data sheet is a challenge for many. better save the smith chart until after you master the simple stuff.

"Trying to show off that you know how to calculate dB is stuff here for the novice license wich i passed in spring 1977"

then maybe you could get it right next time.


Switch, how much current draw are you looking at?

Last edited by freecell; 07-12-2008 at 02:04 PM.

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Old 07-12-2008, 03:58 PM
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Thank you Justme for your input on this. Amp draw is roughly 24 to 27 on SSB (ballpark) with 18 watts from radio off a Astron 50 amper on the max side of the box. (I honestly do not know how accurate the meter is on the Astron ?)

You are right Justme , I honestly DO NOT know that much about amplifiers and how and why they work or don't work. I do know this is a fine well made AB1 amplifier and I know Freecell tends to feel the same way about these boxes.

At times I look for an easy fix that I can do myself before I attempt to pay somebody else to do it. It also can be a learning experiance along the way. I surely appreciate all help given.

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Old 07-13-2008, 03:02 AM
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@ freecell

Your social abileties lack severely

The TEST circuit used by an manufacturer on ONE frequency has nothing to do with the broadbanded circuit the transistors are in now.

So, who is living in lala land now?
Gain is lowered to stabilise the amp by the network from collector to basis, or didn't you read that conveniently?

It was an answer not directed to you, nice to butt in, but it was directed to Switch Kit, who obviously didn't have the background to follow that.

KISS
Now, go back to your shop, and leave real life to real people.

For Switch Kit, don't worry too much.
The rest we dealt in private messages.
Have fun, and experiment.

Cor

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