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Author Topic: TESTING DRIVER AND FINAL TRANSISTORS IN HF RIGS-IN OR OUT?  (Read 4266 times)
N6QWP
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Posts: 228




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« on: October 21, 2017, 01:48:55 PM »

Have rescued two Kenwood hf rigs (TS-930 and TS-940) that have very very low output.

 Looking for a simple way to test both the drivers and the finals to isolate problem.  Would love to learn an easy way to test those transistors--hopefully still hooked up in the final amplifier board--if not, then a simple way, with a digital or analog vom, to lift two or more tabs and find out where the problem transistors are.

It always seems to be EITHER the drivers....OR the finals.  Just looking for a SIMPLE way with minimal test equipment, to find out which pair need replacing.  Can hear the signal in another nearby rig....but very little output shown at the tuner (as in no output).

Please be specific in steps to test.  Thanx and 73
« Last Edit: October 21, 2017, 02:07:49 PM by N6QWP » Logged
KA5IPF
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« Reply #1 on: October 21, 2017, 05:07:10 PM »

There are several ways to test the drivers or finals using an ohmeter that is covered elsewhere. First make sure that is where the problem is. Not all the time is the final unit at fault. The finals themselves seldom go out. Usually it's the drivers and it's obvious, burned bias resistor or holes in the drivers. If no signs of physical damage to the final unit measure the DC voltage on the bases of the drivers and finals when keyed in SSB mode. It should be in the .6-.7v range. DON'T try and adjust it. That will get you started. To check the drive level will require a 'scope or spectrum analyzer. Or if you have a signal generator with a calibrated output you can feed a carrier into the final and look at the power out.

Divide and conquer, don't assume.
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KC4ZGP
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« Reply #2 on: October 21, 2017, 05:16:32 PM »


Check each transistor's rectifying properties.

A signal generator, voltmeter and RF probe for tracing circuit pathes.

Have fun and be safe.

Kraus



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K6AER
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« Reply #3 on: October 21, 2017, 05:17:29 PM »

onitor the current to each stage when driven.
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WB6BYU
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Posts: 17053




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« Reply #4 on: October 21, 2017, 07:18:26 PM »

Make a simple RF probe and measure the RF voltage on the base of each transistor.  If there is
no drive on the base of the finals but there is on the base of the drivers, then the drive transistors
(or something in the collector circuit) is bad.
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N6QWP
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Posts: 228




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« Reply #5 on: October 22, 2017, 08:59:02 AM »

KA5IPF-Please reference articles where rf transistors (drivers and finals) can be tested using a vom.  Tried searching and only response that I could find, indicated that it was not a good indication unless it was a short.

So, is there no simple way-without sophisticated equipment-to just replace the drivers and get the amp section to work???  Was hoping that someone could provide a step by step procedure for those not equipped with the proper equipment or practical knowledge and experience to "muddle" their way back on the air.  Would of course, check reports to make sure that signals were clear and not causing interference.

How about beginner steps starting with the adjustments at far low end of adjustments and slowly increasing until some acceptable output is achieved?  Would like some hope of getting these grand ole rigs back on the air without having to spend hundreds and hundreds of dollars to accomplish that.

I'm pretty sure that others, who are also more "appliance operators" than electronic theorists will want to try some simple way to test, attempt a repair and get back on the air....without derision or disdain.  

The reason we come here, "with hat in hand", is that we are seeking Elmer advise.  Please channel unassuming replies to those of us not having the understanding, experience and equipment that many of you do have.  I, at least, am hoping for practical steps that a beginner can follow--and hopefully accomplish a working repair.  Tnx es 73
« Last Edit: October 22, 2017, 09:18:58 AM by N6QWP » Logged
AC7CW
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Posts: 967




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« Reply #6 on: October 22, 2017, 09:47:29 AM »

I am not at all familiar with those transceivers so this is just general troubleshooting advice: You can't really test transistors in the circuit and unsoldering leads to test them is too much of a bottom-up approach with potential for damaging the pcb they are soldered to. Do a stage by stage output check and find out which stage is failing first. Normally I would do that with an oscilloscope but it's possible to make a very simple peak detector network [diode and a capacitor] to use with your dvm that might give you a good idea about signal levels.
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Novice 1958, 20WPM Extra now... (and get off my lawn)
N6QWP
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Posts: 228




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« Reply #7 on: October 22, 2017, 04:11:05 PM »

Care to share the approximate values of the components necessary and what to look for (and where)Huh

Starting from the input through to the final???  I can hear the signal from the radios in my other receiver....just need to trace that signal until I find the problem transistors and where it ceases to amplify.

STILL LOOKING FOR A SIMPLE PROCEDURE AFTER REPLACING THE FAILED TRANSISTORS (DRIVERS?) TO BRING THE RIGS BACK UP TO OPERATING PARAMETERS--WITHOUT HAVING LOTS OF TEST EQUIPMENT???
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AC2EU
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« Reply #8 on: October 22, 2017, 04:17:13 PM »

KA5IPF-Please reference articles where rf transistors (drivers and finals) can be tested using a vom.  Tried searching and only response that I could find, indicated that it was not a good indication unless it was a short.

So, is there no simple way-without sophisticated equipment-to just replace the drivers and get the amp section to work???  Was hoping that someone could provide a step by step procedure for those not equipped with the proper equipment or practical knowledge and experience to "muddle" their way back on the air.  Would of course, check reports to make sure that signals were clear and not causing interference.

How about beginner steps starting with the adjustments at far low end of adjustments and slowly increasing until some acceptable output is achieved?  Would like some hope of getting these grand ole rigs back on the air without having to spend hundreds and hundreds of dollars to accomplish that.

I'm pretty sure that others, who are also more "appliance operators" than electronic theorists will want to try some simple way to test, attempt a repair and get back on the air....without derision or disdain.  

The reason we come here, "with hat in hand", is that we are seeking Elmer advise.  Please channel unassuming replies to those of us not having the understanding, experience and equipment that many of you do have.  I, at least, am hoping for practical steps that a beginner can follow--and hopefully accomplish a working repair.  Tnx es 73

How can I say this in the best way possible?
All I can come up with is an analogous question like:
Why can't I fix a jet engine by using a sledge hammer and screwdriver?
It just doesn't work that way!

Doing in-circuit "diode checks" on the transistors is not very accurate. You would have to lift the base tabs ( not recommended).
I'd try checking the base bias in SSB without the mike( no carrier but Amp is turned on).
Then check to see if there is a significant current increase on the 12V line while in SSB tx.
If so, the outputs are probably working It is possible that only one is still alive. You would know by checking against the mfr bias current spec.
Whatever you do; DONT ADJUST ANYTHING without the "sophisticated test equipment" or the knowledge to use it!

That's about all I can suggest. At least you can get an idea if the finals are alive or not.
However, without a way to test the drive, etc, you're dead in the water.
Tweaking stuff in a dead circuit will do nothing but double the work to repair it.

If you bought those units for nothing, and you don't care if they ever work again, then proceed with plan A.
« Last Edit: October 22, 2017, 04:19:15 PM by AC2EU » Logged

KD0REQ
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Posts: 2018




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« Reply #9 on: October 22, 2017, 04:34:55 PM »

get a scope, use a 10x probe, do a little math to see what the voltage drive to the finals is, set the input attenuator to the next highest setting (safety setting,) and key up into a dummy load.  if the screen is barely showing a trace, click down one, now you should be reading a full sine wave.  if you fill the screen with a clean sine, it isn't the drivers.  assuming nobody has been monkeying around with the bias.

the finals will load it down.

crappy waveform and/or low voltage, driver transistors.  at that point, you can see if you can get replacements.

assuming your dummy load is something like a Cantenna with a takeoff point for monitoring a lower output, put a power meter on the lead to the dummy and the scope on the takeoff jack. creepy results, it's the finals.

if that's too scary or you're not equipped for this, find a local Elmer to assist.

personally, I would put a voltmeter at the start between radio chassis and ground lead to the scope.  if you see significant ground voltage difference, something needs an isolation transformer. sparks are always a bad sign, and smoke a worse one. 
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KC9UOD
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« Reply #10 on: October 28, 2017, 12:41:49 PM »

In and Out transistors can always be tested with a VOM with the Diode Test, How ever most of the time you need to remove the transistor from the  board first the easier way is to get full Prints and test voltages at test points! Bias Resistors are a common problem with power drivers in any circuit! I find it much easier to test with a Scope for correct displays witch are included with any detailed schematics! any more Questions?
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K1HMS
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Posts: 463




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« Reply #11 on: October 29, 2017, 07:03:53 AM »

There is a lot of good advice here already. Here is the service manual http://www.radiomanual.info/schemi/TS940_serv.pdf the amp is on page 90.

I repair rigs. I haven't seen a TS-940 but have been into many types.

I find 90% of the problems with a VOM, many with the power off. Just make sure you measure on the DC side and not the RF of the circuit. A typical VOM is useless above a few kHz. Looking at the schematic you can see chokes, inductors, or resistors and bypass caps or other circuitry that define the RF and DC sides.


People run these rigs into bad matches including open and short circuits which can cause the following issues due to high RF voltage or currents due to the mismatch.

Your symptoms could be power control/ALC. The forward tx detector or related circuit blows. It is the circuit that sets the power to the commanded value and to 50 watts during ant tune.

Bad RF relays. Look at the schematic, work backward from the ant connector, figure out which ones are in the TX path.

Inductors in the antenna tuner get fried and caps shorted, bypass the tuner. The problem could be the bypass relay. Bad RF relays generally result in very low RF output with slightly more output at higher frequencies.

And some times it IS a driver or output device. It is usually easy to tell. For the MRF-422 If the collector has near 0 volts check collector to ground (Vce) resistance with power off. If not shorted turn on the power and probe back toward the power supply to find the problem.  Do the same on the base to ground (Vbe).  If Vce is near the PSU voltage and Vbe is near 0V the -422 shouldn't be drawing current, as the -422's Vbe approaches 0.7V it should draw current and by 0.85V it is hard on drawing a lot of current. Don't forget V/R=I.  With the power off you can measure resistance from the PSU to a "DC" point in the RF output or driver stage. Let's say you measure .9 ohms with the probe tips together and 1.0 ohms between these points so the circuit is about 0.1 ohms. There are a lot of variables but if I measure the voltage across the same circuit and see 1.5V when Tx is keyed I know there is around 1.5/0.1 or 15 Amps, I expect to see the voltage drop when not keyed.


A 100MHz HP "lunch box" O'scopes with a LCD display is around $200 on eBay, and very usefull.

I have a full lab with spectrum analyzers, counters, etc, etc but find most problems with power off resistance measurements and then voltage measurements with a VOM or a O'scope if one is handy.

So, look at the schematic, and ask questions as you make measurements or don't understand the function of the circuit.

Don't turn internal pots and don't unsolder power transistors.  Neither are helpful.

Hamilton
K1HMS

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KC4ZGP
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Posts: 1637




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« Reply #12 on: October 29, 2017, 07:31:06 AM »


Until one of us has your radio on our bench, we can't help you further than what's here.

You asked questions, we gave answers.

Have fun.

Kraus

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K1HMS
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Posts: 463




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« Reply #13 on: October 29, 2017, 01:19:15 PM »


 Just make sure you measure on the DC side and not the RF of the circuit. A typical VOM is useless above a few kHz. Looking at the schematic you can see chokes, inductors, or resistors and bypass caps or other circuitry that define the RF and DC sides.


I thought I should add an example of a "DC side" from the schematic; if you look at page 90 of the schematic and find C29 and C30. These are both a .47uF cap. or less than 0.2 ohms at 1.8 MHz.   (Xc = 1/(2pi * F * C)) thus making this point a RF ground and a good place to measure DC with a typical VOM.  The other side of L14 or L15 at C24 is even better due to the series inductance of L14-15 and the bypass caps C22-24.

If measurements suggested a bad transistor I would lift one end of L8-9 and L14-15 to isolate the collectors to test them before ever trying to remove the transistors.  The two transistors are still in parallel but a bad device can still be verified. It is much cleaner than pulling the devices.

The TS-990, 590SG and TS-450 have a small coax connector where the input drive comes onto the "Final Unit" board. Removing this connection removes the RF from the board for testing. It is also a good place to inject a test signal or connect a Network Analyzer if desired.

Example of the O'scopes are the HP TDS-210 and TDS-1002. At under $200 it is a nice scope. Some of the HP 54XXX scope are nice as well.

73

Hamilton
K1HMS





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K1ZJH
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« Reply #14 on: October 30, 2017, 03:05:36 PM »

Quiescent PA idle current should indicate whether the PA or driver transistors are damaged. This assumes the bias was set correctly before a device failed.
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