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Author Topic: TESTING DRIVER AND FINAL TRANSISTORS IN HF RIGS-IN OR OUT?  (Read 4334 times)
KA9UCN
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Posts: 92




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« Reply #15 on: October 31, 2017, 02:52:57 AM »

A lot of good advice has been offered. For someone in your situation I would also suggest the finger test. Look for heat or the lack of it on the device with the tip of your finger. It is not a great test by any means but under the stated circumstances. It is easy and sometimes reveals the problem.
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KC4ZGP
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Posts: 1637




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« Reply #16 on: October 31, 2017, 05:20:54 AM »


UCN,

Hear, hear. If it ain't warm, it ain't conducting.

Kraus
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N6QWP
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Posts: 230




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« Reply #17 on: November 02, 2017, 07:30:13 AM »

Thanx all.  Service manual should be arriving today or tomorrow and then the fun begins.  As before, all of the input is greatly appreciated.

Have ordered spare drivers in case they need replacing--giving China a chance (?)....long wait time but much less expensive?  Hope if needed, they don't turn out to be bogus.  

Now have 3 final units (condition unknown) and two non-working rigs=930 and 940.  Hopefully will be able to get the 940 going (have another working 930).  

Since I now have 3 final units that are out of the rigs, wondering if there is a simple way to rig up a separate 28 vdc supply and a signal generator (no scope yet) to run tests to determine where problems might be???



« Last Edit: November 02, 2017, 07:50:24 AM by N6QWP » Logged
AC2EU
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Posts: 1205


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« Reply #18 on: November 02, 2017, 09:01:13 AM »

Thanx all.  Service manual should be arriving today or tomorrow and then the fun begins.  As before, all of the input is greatly appreciated.

Have ordered spare drivers in case they need replacing--giving China a chance (?)....long wait time but much less expensive?  Hope if needed, they don't turn out to be bogus.  

Now have 3 final units (condition unknown) and two non-working rigs=930 and 940.  Hopefully will be able to get the 940 going (have another working 930).  

Since I now have 3 final units that are out of the rigs, wondering if there is a simple way to rig up a separate 28 vdc supply and a signal generator (no scope yet) to run tests to determine where problems might be???


Chinese parts? = Russian Roulette and "out of the frying pan into the fire".
Been there done that with IGBT and MOSFET power devices.
They will either fail immediately under load or will work 6 months or so.
The the job isn't worth paying the money for "real parts", then I don't bother with it.
You know what they say: "Cheap does as cheap is.
Good luck !!!
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WB6BYU
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Posts: 17053




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« Reply #19 on: November 02, 2017, 09:25:19 AM »

Quote from: N6QWP

...wondering if there is a simple way to rig up a separate 28 vdc supply and a signal generator (no scope yet) to run tests to determine where problems might be???



Start with the DC tests. 

Is the idle current the nominal value?  Is there some way you can measure the current on each
stage individually?  If so, that's usually the quickest approach, because a defective transistor
shouldn't draw the normal current (could be higher or lower.)

If the overall current is too high, the finger test should identify the transistor drawing too much
current.  (But check that the bias supply is working properly before replacing the transistor.)

Measure voltage on all three terminals for each device.

Emitter should be 0V.  Base should be around 0.6V (since this is a linear amp, the transistor needs some
forward bias to conduct.)  Collector voltage should be > 24V or so with a 28V supply.

If the base voltage is too low, check the bias network voltages.  (It could also be a base-to-emitter
short.)  If the voltage is too high, you may have an open base.

If the collector voltage is low, look for a voltage drop somewhere in the circuit, or an open circuit.



Going back to your original question,  you're making things way more complicated than necessary
if you aren't using an RF probe for your multi-meter.  This is a simple device - one diode, one resistor,
and one capacitor - that connects to your meter and allows you to measure the relative level of RF in
the circuit.  The parts values aren't critical - you can build it from most junk boxes, or from a broken
transistor radio.  It converts the RF into a DC voltage that your meter can measure.  The circuit has
been in the ARRL Handbook since at least the 1960's.

Using it is simple:  you attach it to the probes on your meter (or put wires on it that plug into the
sockets for the probes) and touch the probe tip to the circuit to measure the RF voltage at that
point.  If you have drive on the base of a transistor but not on the collector, chances are that the
transistor is bad (assuming the proper DC voltages).  If you have RF on the collector but not on the
base of the next stage, something is broken in the interstage coupling.  I built my first one over
35 years ago and used it to find the bad solder joint in the transmitter stage of a 2m HT.

To use it in your case, turn the drive down, key the rig with a steady carrier (CW or AM mode, or
whatever you use) and then increase the drive until you see some indication on the base of the
first stage you want to check.  Then check the base and collector of the remaining stages (and
any other points you want to check to see where the RF disappears.)

It's quick, it's cheap, it's easy, and it's intuitive.
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N6QWP
Member

Posts: 230




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« Reply #20 on: November 03, 2017, 06:17:04 PM »

Thanx, will get the necessary parts and try to follow your instructions.....but would like to do so using the probe with a rf generator and external 28 vdc supply......before the power supply problems are finished.  Can that be done?

From online accounts, the power supply can sometimes become an unending chain of problems and nightmares.....and that gives me pause.  Would like to test the various final units (have the 3=two extra that I purchased but don't trust now) before getting bogged down in the supply and then finding out that another nightmare awaits.  Reaching the end of the amount of money that I am willing to invest--if there seems to be no end in sight.

Lack of scope and practical knowledge will prevent me from properly adjusting bias....if it turns out that the drivers are blown.  Would like to know that before going any further.
« Last Edit: November 03, 2017, 06:37:43 PM by N6QWP » Logged
WB6BYU
Member

Posts: 17053




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« Reply #21 on: November 03, 2017, 08:40:02 PM »

Certainly you can run the DC tests - that should only require the power supply attached to the amplifier
module (and perhaps some control lines).  That's the first step anyway, as it can catch a number of failure
modes (particularly if you can measure the currents individually for each stage.)

Then if you haven't isolated the problem already, you certainly can try driving it with a signal generator, but
I don't know how much drive it requires to get enough RF that you can see it with a sniffer.  Certainly if you
can measure the output of your generator using the RF probe on the most sensitive meter setting, then you
should be able to trace the RF through the board to see where it suddenly drops out.
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N6QWP
Member

Posts: 230




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« Reply #22 on: November 04, 2017, 09:20:24 AM »

Thank you all, once again.  The service manual finally arrived and I am off to get the schematics blown up to where I can see and follow them.

Am going to purchase all of the parts recommended to replace in the power supply and AVR so that, with those working properly, I should be able to move forward with the testing of all three of the final units that I have.  Praying that one of them will have drivers that survived.  

If one of the three final units still has good drivers, I think that I can get this rig back on the air.  The "wall" will be reached if none of them survived and since I will not be able to properly adjust bias after replacing those drivers,  guess that I will then again be searching for a complete and working final unit ;-(

All this great information that you gents have put forth is a great archive for other owners with similar problems.  It will survive and be available by anyone who needs to it.  The saga continues.
« Last Edit: November 04, 2017, 09:40:36 AM by N6QWP » Logged
N6QWP
Member

Posts: 230




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« Reply #23 on: November 05, 2017, 03:52:07 PM »

I can confirm that the output from the power supply and AVR is indeed 29 volts.  This leaves either the final unit or more likely, perhaps the aforementioned input that controls the bias Huh  OR NO RF INPUT TO THE FINAL UNIT???

Since I have tried several final units....and there is no output from either of them, I am leaning more toward that circuit's lack of input for the bias....OR NO RF INPUT?  If anyone is familiar with that circuit that goes either high or low (?), I would appreciate any pointers on troubleshooting there.  

Will still pursue the drivers and finals using the suggested methods of tracing rf to and through the final unit if this turns out to be a dead end.  Thanx for all the great help.
« Last Edit: November 05, 2017, 04:12:57 PM by N6QWP » Logged
N6QWP
Member

Posts: 230




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« Reply #24 on: November 06, 2017, 06:37:30 PM »

Just had schematic blown up to where I could read it.  "txc" output from rf board to "txc" input on final board is, I presume, the control voltage to enable bias on the final unit.  I am confused by the voltages shown in the schematic of both units.  "rf" board shows 0 voltage-but no other voltage (for transmit) is indicated.

On the final board, if the "txc" line is followed to the first transistor that it connects to, it shows 12 volts +/- (on transmit).  If I want to check out if the keying voltage is actually working.....at plug 1 on the final board, shouldn't I see 12 vdc +/- in transmit there to indicate that it is correct and should provide bias???

If not there, that seems to me, that it would eliminate the final board (s) as the problem and give me the symptoms that I am experiencing?

My other thoughts concern RL3 and RL4.  If the control voltage mentioned above is working properly.....wouldn't one of those not making contact, prevent any rf signal from getting to the final board and giving me the lack of rf that I have Huh

Sorry that this will necessitate someone looking up the TS-940 schematic, but it would sure help me to isolate the problem and eliminate possibilities.  If someone experienced in working on the 940 can answer these two questions, I think I should be able to solve the puzzle.
« Last Edit: November 06, 2017, 06:58:09 PM by N6QWP » Logged
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