HF amplifier problem

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KF4SQB
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HF amplifier problem

Post by KF4SQB » Sun Feb 06, 2005 12:43 pm

I'm sure I'm setting myself up for a flame or two, but here goes. I am attempting to work on an old Palomar TX75 HF linear. First off, for those of you in the know, it had a second transistor added long ago. Both transistors (of course) were 70 watt units. I am attempting to "upgrade" the amplifier by replacing the two 70 watt transistors with two 100 watt, but it goes into self-ocillation as soon as RF is applied, and the only way to make it stop and "unkey" is to turn it off. I'm guessing it just needs to be neutralized, but I am not sure how to go about doing that.

BTW, before anyone starts, this will be used behind a low power 10m rig, not a CB!
Brett Kitchens
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Post by Two way repair man » Fri Mar 25, 2005 1:27 pm

Brett,

You brief description makes it tough to provide the correct answer. There are a lot of potential limitations that could be inherent in the original design of the amplifier. Do you know if the manufacturer recommends this modification or have you heard of anyone else doing this specific mod to the amplifier you are working on?

Also, I would like to suggest that you answer the following questions:

1. It sounds as though the amplifier has a spurious output. What type of adjustments does it have (Trimmer capacitors, or potentiometers)? Have you tried detuning any/all trimmer capacitors 180 degrees out from the current adjustments? Have you tried tuning the amplifier for maximum power (on a watt meter) and minimum current (on an inline amp meter)?
2. Is there a mechanical relay on this power amp? If so, is it staying engaged after your release the PTT button on the microphone?

Answers the questions and I can pursue this problem further.

Regards,

Bill

I might be opening up a can of worms but I am willing to take a stab at it. Stab, stab, stab.

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Post by KF4SQB » Fri Mar 25, 2005 8:32 pm

Well, first off, yes it does have trimmer caps in the input and output circuits, which I think I tried to tune, but it has been a while since I last fooled with it and I don't remember for sure. If I did adjust them, I didn't do it the way you are describing, so I will try again that way and see what I get.

Second, yes, there is a mechanical relay in the unit to switch the antenna line either through the amp during transmit, or bypass it for receive. When it oscillates, the relay does stay keyed the whole time.

Third, I have never heard of this particular modification on this particular unit. There is a fairly common mod applied to them, though. These units originally came with one 70 watt transistor installed, and a place to hold another. With some slight modification to the input and output circuits, a second 70 watt transistor can easily be added to effectively double the output power. This particular mod was performed on it long before I aquired it. Whoever did it knew at least somewhat what they were doing, it is not a hack job.

I'll try to report back my results after trying to retune the input and output circuits as you described. Thanks for the suggestions!
Brett Kitchens
KF4SQB

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Of all the things I've lost, it's my mind I miss the most.......

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HF amp problem

Post by Two way repair man » Mon Mar 28, 2005 1:53 pm

Okay, that makes more sense. I would have to suggest that realignment using a watt meter and current meter, after presetting the trimmer caps 180 degrees out of current positions, is your best way to resolve the problem.

Let me know if you still have problems.

Bill

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Post by Ghostwriter » Fri Sep 23, 2005 3:25 pm

Brett,

I'm just trying to understand your own theory why you would take a perfectly good working RF Amplifier that's rated at 140 Watts Output and attempt changing the two output transistor devices to increase the RF Power to 200 Watts Output? This is a +2 dBm increase that can only be detected on a RF Wattmeter but no where else.

I've always heard when increasing RF Power Output Signal, you would want to use a minimum of:

+6 dB = 4 times the Power Output
or
+9 dB = 8 times the Power Output
or
+10 dB = 10 times the Power Output
or
+12 dB = 16 times the Power Output


To detect any signal difference on the receiving end.

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Post by KF4SQB » Sat Sep 24, 2005 12:06 am

Well, I would agree with you, but it's not a matter of taking a 'perfectly good working amplifier' and replacing the RF power transistors. One of the original transistors was damaged (I goofed and applied a little too much drive to it :oops: ), and I just happened to have the pair of 100 watt transistors laying around. The question now becomes 'why would I want to go buy a set of 75 watt transistors when I already have a set of perfectly good 100 watt units'.
Brett Kitchens
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Of all the things I've lost, it's my mind I miss the most.......

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Post by Ghostwriter » Sat Sep 24, 2005 2:15 pm

Hello Brett,

Now this project makes perfect sense to me. Ooooh, I hate when RF over driving semiconductors occurs as it normally releases that little puff of smoke in the form of Greenback $$$$$.

I was wondering if you were able to stabilize the RF devices from the spurious / self oscillation / flying condition and if you have the schematic for the original 140 Watt amplifier circuit? The original RF 140 Watt Amplifier design was from 1976 and it used a pair of /\/\ MRF454 as the Finals.

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Post by KF4SQB » Sun Sep 25, 2005 5:02 pm

To be prefectly honest, I'm not even sure where the thing is right now. I do need to find it and get back on it, though. Bad thing about overdriving it, I only applied about 10 watts to it. I was running it behind an old Icom IC-701 with a blown PA board. The drivers were supplying anywhere from 5-10 watts, depending on which band it was tuned to. BTW, the blown PA in the Icom wasn't my fault. That one was a lightning strike. Strangely enough, it only popped the finals, and a relay in the power supply. The receiver is just as hot as a match.
Brett Kitchens
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Of all the things I've lost, it's my mind I miss the most.......

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Post by Ghostwriter » Sun Sep 25, 2005 6:12 pm

Hi Brett,

Lightning Strike, Yes I know first hand at what that can do to electrical and electronic equipment. At the end of April, 2005, we had an early morning thunderstorm with air to ground strikes. I have a Rohn 120' SSV tower in my backyard and although it's grounded with 10' Copperweld stakes, one on each tower leg, it took a direct hit.

The Lightning came down the tower to ground and back up on my Primary 220V AC and Telco lines blowing several breakers in the Electrical Panel and surging my computer's Modem, Power Supply, 3.5" Floppy Disc Drive and the main printed circuit board but the Hard Drive was okay. And I had a /\/\ Cordless Phone that took the hit too. I knew that was toasted as it had the distinctive smell of burnt carbon resistors and ICs. I took that off the wall, walked it to the garbage can and inserted ... it was trash! Well my computer went there too!

Lightning is a strange problem to deal with in electronic equipment, you never know exactly what and where has been affected by the surge and most times different problems will continue to appear long after the hit.

So I understand your frustration with the IC-701 Xcvr ... RF PA & PS Relay BURNS UP to SAVE Receiver section. Oooh, that makes perfect sense for Lightning! That radio has to be in the neighborhood of 25 years old and probably ICOM discontinued the replacement parts or they're so expensive you could purchase another radio for the same cost.

Let me ask this question again, Do you have the schematic for the original 140 Watt amplifier circuit? If not, I can make a photocopy and drop it in the U.S. Snail Mail so you would have a schematic with component reference values.

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Post by KF4SQB » Mon Sep 26, 2005 1:43 am

As you say, lightning is a very strange creature. I've seen lots of electronics, televisions mostly, that there is no damage in the RF section, no damage in the power supply section, but the vertical reference section and audio section are toast. Makes no sense, but that's the way it happens sometimes. So far, the PA and PS seem to be the only thing affected in the 701. As I said, the receiver works great, along with the control sections, synthesiser, and exciter. Hell, I've made some pretty long-range contacts with it with the drive from the exciter routed directly to the antenna, and about 7 watts on 40 meters.

As for a schematic for the amp, I'm pretty sure I have one somewhere around here that, while not an exact match, I think is close enough to use as a reference if I need it. I do greatly appreciate the offer though.
Brett Kitchens
KF4SQB

kf4sqb "at" wetsnet "dot" com

Of all the things I've lost, it's my mind I miss the most.......

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