Overload Board

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Re: Overload Board

Postby K9ACT » Fri Oct 29, 2010 10:03 am

steve_qix wrote:This is very odd.... Ok, Jack - if you adjust the 100k current gain resistor, such that it is all the way at full resistance across the op-amp minus input and output (this will be maximum gain) - AND if you then adjust the current zero resistor so you have 0 volts at TP2 (or very close to it - you probably can't get it at perfect 0), what do you get at TP2 when you transmit - AND how much current are you running when you get whatever reading you get?



Not sure which way is what but with the gain pot fully CW, there is no zero. The range of the zero pot at TP2 is between 2.1 and 2.8 volts.

>If you then put the current gain pot all the way to the other extreme - re-zero and transmit again, what do you get?

With the gain pot fully CCW and TP2 near zero, the board faults when the transmitter is keyed so I can't tell you what the current is.

Step one should give you something to ponder.

js
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Re: Overload Board

Postby steve_qix » Sat Oct 30, 2010 9:32 am

K9ACT wrote:
steve_qix wrote:This is very odd.... Ok, Jack - if you adjust the 100k current gain resistor, such that it is all the way at full resistance across the op-amp minus input and output (this will be maximum gain) - AND if you then adjust the current zero resistor so you have 0 volts at TP2 (or very close to it - you probably can't get it at perfect 0), what do you get at TP2 when you transmit - AND how much current are you running when you get whatever reading you get?



Not sure which way is what but with the gain pot fully CW, there is no zero. The range of the zero pot at TP2 is between 2.1 and 2.8 volts.

>If you then put the current gain pot all the way to the other extreme - re-zero and transmit again, what do you get?

With the gain pot fully CCW and TP2 near zero, the board faults when the transmitter is keyed so I can't tell you what the current is.

Step one should give you something to ponder.

js


Ok, good - this is more or less what I expected. Ok, now - if you put the gain pot about 25% away from minimum resistance across the op-amp, and re-zero, what do you get when you transmit?

I would expect the board to generate a fault under maximum gain, and it is. So, it APPEARS as if all is working correctly.
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Re: Overload Board

Postby K9ACT » Sat Oct 30, 2010 10:58 am

steve_qix wrote:
Ok, good - this is more or less what I expected. Ok, now - if you put the gain pot about 25% away from minimum resistance across the op-amp, and re-zero, what do you get when you transmit?


Before I answer that, please tell me how to determine min/max resistance of pot and save me some typing. It's not obvious to me without knowing seeing the empty board.

1. 25% from full CCW position, with zero zeroed the board faults under load.

2. 25% from CW posit and zero I get 2.1v under load.

If I attempt to tweak the gain to get 3v, I can't get any closer than 2.8 and the no load zero goes to the 2.1 to 2.8 range with no hope for zero.

For the record, these tests are into a 5 ohm dummy load as I blew a fet last night in the middle of telling someone I think I solved the fet problem.

And finally, have you shipped those Transorbs that I ordered?

js
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Re: Overload Board

Postby steve_qix » Sat Oct 30, 2010 11:16 am

K9ACT wrote:
steve_qix wrote:
Ok, good - this is more or less what I expected. Ok, now - if you put the gain pot about 25% away from minimum resistance across the op-amp, and re-zero, what do you get when you transmit?


Before I answer that, please tell me how to determine min/max resistance of pot and save me some typing. It's not obvious to me without knowing seeing the empty board.

1. 25% from full CCW position, with zero zeroed the board faults under load.

2. 25% from CW posit and zero I get 2.1v under load.

If I attempt to tweak the gain to get 3v, I can't get any closer than 2.8 and the no load zero goes to the 2.1 to 2.8 range with no hope for zero.

For the record, these tests are into a 5 ohm dummy load as I blew a fet last night in the middle of telling someone I think I solved the fet problem.

And finally, have you shipped those Transorbs that I ordered?

js

Ok, you've got it figured - now it just needs to be adjusted. The CCW position is full gain. The CW position is no gain. So, currently, you need MORE GAIN.... so adjust the pot a bit more to the CCW direction and re-zero - then see what you get under load. Not 3 volts yet? Keep going with the gain further CCW, zero, etc. until you get what you want.

It won't take too many iterations to get it!!
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Re: Overload Board

Postby K9ACT » Sat Oct 30, 2010 12:52 pm

steve_qix wrote:


And finally, have you shipped those Transorbs that I ordered?

js



I am leaving that at the top so you don't miss it.

>Ok, you've got it figured - now it just needs to be adjusted. The CCW position is full gain. The CW position is no gain. So, currently, you need MORE GAIN.... so adjust the pot a bit more to the CCW direction and re-zero - then see what you get under load. Not 3 volts yet? Keep going with the gain further CCW, zero, etc. until you get what you want.

Bingo!!! That works.

What I seem to have missed it that I can't adjust to 3v while under power because it grossly overshoots the zero stuff.

What seems to work is to look at the voltage under power; unkey; make a small adjustment; zero and look again.

I suppose you said all that but I didn't quite get it.

That last blown FET was another heat sink problem. The back side was scorched brown and the silipad imprinted over the whole face. I thought I solved this problem by torqueing them down more but there is still something else going on. This time I used grease along with a new pad.

Can I assume that even set up correctly, the Overload Board would not recognize an over heating problem?

Thanks for your help.

js
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Re: Overload Board

Postby kf1z » Sat Oct 30, 2010 1:24 pm

My GUESS is that by the time a FET heats to the point of failure, the FAILURE of the device would trip the overload.

A FET heating to this point is a sure sign of something very wrong.

Is it the same device every time? ( same posistion on the heatsink)


From your pictures I saw last, it looks like you do not have a really good, wide, low-impedance drain buss.

IF I saw this correctly, and there is just a 'wire' connecting the drains together, I believe this could be one possible
source of a problem.

If the drain buss presents too much impedance ( capacitive or inductive ?) it could cause the device that is closest to the transformer connection to "do all the work".
Thus causing that FET to overheat.

I would use at least a 3/8 or 1/2" wide piece of copper, or brass... even PCB material for those drain busses on a transmitter like yours.


Just some thoughts... :P
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Re: Overload Board

Postby K9ACT » Sat Oct 30, 2010 1:54 pm

Unfortunately, now for the rest of the story.....

After replacing the FET and a little retweaking I get near zero and 3v under load.

The bad new is that just about any amount of modulation faults it.

I went as far as 3.75v and it made no difference.

I checked the Vcal and Vthresh and they are nominal.

So, something is still not right.

js
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Re: Overload Board

Postby K9ACT » Sat Oct 30, 2010 2:05 pm

Bruce....

Roger on your buss comments. Seems like the last few have been the same FET and it is the one closest to the transformer.

I haven't been keeping very good records but the grease on this one should be a clue on the next failure. If it is a different fet and the symptoms are all the same I will grease them all before reworking the board. I am planning on building another one from scratch so I will keep this in mind.

For what it's worth, last night's failure was less than a minute into a transmission so there was not much chance for heat buildup. I have run this continuously for 15 minutes under modulation so it's a rather illusive problem.

What I am working on is using two CPU heat sink/fan combos and one for each pair of fets. This way I can grease them up and screw them right to the heat sink. It's a little tricky laying it out but I think it can be done. I may need a third one for the drivers but that's a little easier.



Thanks for the tips.

js
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Re: Overload Board

Postby kf1z » Sat Oct 30, 2010 2:38 pm

That does sound tricky!

I would worry about not having one, solid ground plane.

Could start a whole new batch of issues.


The gates should have a similar buss, BTW.
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Re: Overload Board

Postby steve_qix » Sat Oct 30, 2010 5:06 pm

K9ACT wrote:Unfortunately, now for the rest of the story.....

After replacing the FET and a little retweaking I get near zero and 3v under load.

The bad new is that just about any amount of modulation faults it.

I went as far as 3.75v and it made no difference.

I checked the Vcal and Vthresh and they are nominal.

So, something is still not right.

js


Oh, make the current show 3v under load (at TP2) and the voltage and the voltage (TP1) about 3.3V or so. That should give you enough lee-way.
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Re: Overload Board

Postby K9ACT » Sat Oct 30, 2010 6:08 pm

steve_qix wrote:js


Oh, make the current show 3v under load (at TP2) and the voltage and the voltage (TP1) about 3.3V or so. That should give you enough lee-way.[/quote]



That is actually what I did do. With 3.7v at TP1, it faults around 50%. I didn't go any farther because your instructions say "up to 3.5".

js
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Re: Overload Board

Postby steve_qix » Sun Oct 31, 2010 7:16 am

K9ACT wrote:
steve_qix wrote:

Oh, make the current show 3v under load (at TP2) and the voltage and the voltage (TP1) about 3.3V or so. That should give you enough lee-way.




That is actually what I did do. With 3.7v at TP1, it faults around 50%. I didn't go any farther because your instructions say "up to 3.5".

js

RIght, 3.5 is about as high as one would NORMALLY want to take it. Under experimentation, you could run it up to 4 volts. The big question here - are these real overloads (or nonlinearities) or false triggers? How much TOTAL capacitance is at the RF deck, or from any other sources, AFTER going through the overload board. If the overload board is looking into a lot of capacitance, you will get false triggers.

Does the drain current "kick up" or rise at all with modulation?

If the voltage and current do not track linearly, the overload board will detect a problem - and this is what you want, of course. If the board looks into too much capacitance, the capactive reactance will cause problems.
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Re: Overload Board

Postby K9ACT » Sun Oct 31, 2010 9:46 am

steve_qix wrote:RIght, 3.5 is about as high as one would NORMALLY want to take it. Under experimentation, you could run it up to 4 volts.


Makes no difference.

> The big question here - are these real overloads (or nonlinearities) or false triggers? How much TOTAL capacitance is at the RF deck, or from any other sources, AFTER going through the overload board. If the overload board is looking into a lot of capacitance, you will get false triggers.

It faults at exactly the same level with a 5 ohm resistive load 3 inches from the board.

As a point of interest, what is the purpose of the V thresh adjustment. Sounds important but moving it around the nominal doesn't seem to have any effect.

js
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Re: Overload Board

Postby steve_qix » Sun Oct 31, 2010 5:24 pm

K9ACT wrote:
steve_qix wrote:RIght, 3.5 is about as high as one would NORMALLY want to take it. Under experimentation, you could run it up to 4 volts.


Makes no difference.

> The big question here - are these real overloads (or nonlinearities) or false triggers? How much TOTAL capacitance is at the RF deck, or from any other sources, AFTER going through the overload board. If the overload board is looking into a lot of capacitance, you will get false triggers.

It faults at exactly the same level with a 5 ohm resistive load 3 inches from the board.

As a point of interest, what is the purpose of the V thresh adjustment. Sounds important but moving it around the nominal doesn't seem to have any effect.

js


Set Vthresh to give 3V at TP3.

I know this is a really silly question, but I have to ask it. The overload board is after the complete PWM filter, including the output capacitor, and the only thing after the filter is the board itself, and the only thing after the board is a resistor... Just checking to make sure.
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Re: Overload Board

Postby K9ACT » Sun Oct 31, 2010 6:59 pm

steve_qix wrote:
I know this is a really silly question, but I have to ask it. The overload board is after the complete PWM filter, including the output capacitor, and the only thing after the filter is the board itself, and the only thing after the board is a resistor... Just checking to make sure.


The 5 ohm resistor is a dummy load on the modulator to eliminate all the cap and inductance problems you suggested.

It follows directly, the overload board. It defaults at the same point as the RF deck with antenna or 50 ohm dummy load.

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