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PDM Generator question




 
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K1JJ
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« on: January 11, 2020, 11:29:22 PM »

My PDM generator pulse died on the REA revision C board.  I ordered a new UCC25701N gen chip, but want to check the circuit operation first.

Is there a schematic with measured voltage points of chip pins for the gen board, etc?  

Even the 14 pin readings of the working PDM chip would be very helpful.  I am especially not sure what pin 9 of the PDM chip should read during shutdown enable and disable for the audio and pulse train...

Tnx.

Tom, K1JJ
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« Reply #1 on: January 12, 2020, 01:30:23 PM »

Tom,
Have you looked at the datasheet for the UCC25701N?  There might be enough info to get you in the ballpark.  Datasheet attached.


* slus293c-3.pdf (979.94 KB - downloaded 88 times.)
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Bob
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« Reply #2 on: January 13, 2020, 12:17:23 PM »

Tnx, Bob -

The voltage enable/disable I am getting at the enable pin does not match what the data sheet suggests, but may be enuff to work, I dunno.  So I'll just be a tube jockey and replace the chip when it arrives and go from there.

BTW, I found an excellent way to check transistors in circuit with no power is to use the "diode" junction test of the VOM.  If the junction is open or shorted, it shows up quickly.
So far it all points to a bad PDM chip.

This is for the Dual Quads, Rico Suave PDM rig and has me excited to get it back on.

T
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« Reply #3 on: January 13, 2020, 08:48:29 PM »

Hi Tom...

Have built 4 of Steves PWM generators and they all performed well with 100% reliability.
2 were dead dugged and the other 2 from his Kit...
All gave similar results.
The UCC chip reminds me of the MC1496 mixer......capacitors and resistors everywhere and all part values important for good operation...
Never had a UCC chip failure ... not to say it could not happen though....

Wonder if the audio is getting through the filter to pin 9....
Is the enable/disable circuit working ....
At least this could be checked very easily and only take about 2 minutes of your time....
So you get no pulse train from pin 4 ?


Wayne

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K1JJ
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« Reply #4 on: January 13, 2020, 10:02:13 PM »

Hi Tom...

Have built 4 of Steves PWM generators and they all performed well with 100% reliability.
2 were dead dugged and the other 2 from his Kit...
All gave similar results.
The UCC chip reminds me of the MC1496 mixer......capacitors and resistors everywhere and all part values important for good operation...
Never had a UCC chip failure ... not to say it could not happen though....

Wonder if the audio is getting through the filter to pin 9....
Is the enable/disable circuit working ....
At least this could be checked very easily and only take about 2 minutes of your time....
So you get no pulse train from pin 4 ?
Wayne



Hi Wayne,


I'm getting audio to pin 9.   The enable/disable appears to be working fine giving good DC swings at the transistor output and also working its way to pin 9 of the PDM chip.


So, the chip is enabled with a plus voltage but no pulse output at pin 4.


The unit may have been turned on for days at a time when not in use, so maybe there was a voltage spike or something.


Thanks for the suggestions.


Tom, K1JJ

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« Reply #5 on: January 13, 2020, 10:36:29 PM »

OK.....

Maybe the simple thing is to wait until your new chip arrives and try that....hopefully that's it and away it goes....

I couldn't help myself but with a fine cutter would snip the base leads of the 904 and 906 transistors just to see if they have an effect....

You could easily drop a small drop of solder back on them afterwards if your soldering is steady enough.....


Wayne
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WD5JKO
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« Reply #6 on: January 13, 2020, 11:20:20 PM »


Tom and Wayne,

The 904 and 906 transistors at PDM output were not there on the Rev C schematic that Tom built to.

I'd be curious about a cold solder joint that over time caused a fault. Give that board a good magnified eyeball under a lot of light. In case of doubt, reflow the connections. If the PDM chip is in a socket, remove the IC and reinsert several times.

The Ramp capacitor, 220pf going to pin 10 might be flaky as well. Is it Mica?

Jim
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K1JJ
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« Reply #7 on: January 14, 2020, 12:07:16 PM »

Good suggestions, Jim.

Troubleshooting down to the component level will be the plan if this chip arrives and the dreaded no-go happens.  It's an easy enuff circuit to fix, but the problem is when lacking the right parts on-hand.

I noticed that pin 14 (not used, N/C) had a melted look to it. That is odd.

I'll report back.

T
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« Reply #8 on: January 14, 2020, 02:05:25 PM »

Although my pdm generator is different, I based it along the kjnes of Steve's with the same chip. Mine has a DC coupled positive peak stretcher allowing to 420 pct positive modulation.  However, the pdm itself is really close to Steve's

I've had a couple unexplained crap outs on the UCC device.  Was working fine, then didn't.

Replaced the ic, right back to life.

I've probably built 5 or so, and had 2 crap out within a year or so.

I passed it off to China fab.

--Shane
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W1RKW
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« Reply #9 on: January 14, 2020, 05:05:16 PM »

Shane brings up a good point, is the UCC25701N a bonifide TI device and not counterfeit?  How does one determine a genuine product such as a semiconductor device?   What assurances are there when buying from a "reputable" source.

Those are rhetorical questions and not intended to divert or hijack the thread.

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Bob
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K1JJ
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« Reply #10 on: January 14, 2020, 07:35:18 PM »

Interesting on the crapout potential of the chips, Shane and Bob.

I've used this same 2007  Rev C board  on and off for about 13 years.  I've had about three bad UCC chips in that time.

I was surprised that they cost over $9 these days. I figgered they would be 70 cents each by now like some op amps... :-)


T
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« Reply #11 on: January 14, 2020, 07:42:17 PM »

On a similar subject...

Every time I get a big plate modulated 4-1000A rig running right, I get the urge to convert it over to PDM.  I did this at least two times.

But floating the RF final filament/cathode and screen/grid above HV to modulate the cathode is really a lot of work and dangerous.

Has anyone seen the technology to PDM modulate the plate circuit (like conventional plate modulation)  using a fiber optic pulse input (or whatever) for isolation -  keeping the RF final cathode at ground?     The PDM modulator would have to float, so maybe that is the drawback.    Or maybe it is more complex than that.

T
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« Reply #12 on: January 14, 2020, 08:34:31 PM »

Yes and on a lesser issue you have your Output Filter with a say 8000 ohm impedance input.....

You can build your own PWM generator very easily ..... have uploaded a photo.....
Another photo of the triangle waveform..... sorry have to say its a very nice waveform  Smiley
Only really 4 areas .... pulse generator crystal locked...triangle waveform generator .... Comparator and an offset amplifier ...
Also the circuit has feedback for anti jitter as well.....

Hope my spelling is Ok its a very hot humid day today......


Wayne


* PWM Gen Board.JPG (394.12 KB, 1600x1200 - viewed 200 times.)

* TriangleWave.jpg (102.54 KB, 800x480 - viewed 115 times.)
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K1JJ
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« Reply #13 on: January 15, 2020, 08:09:40 PM »

Hooray!

The good news and the bad news:

The Bad News: I plugged in the new UCC PDM chip and it didn't work.  I also discovered that I had a few UCC chips in stock, so I ordered more for nothing.
I reflowed the bottom of the board, but still no pulse.  It's usually not that easy.

The Good News:  I pulled out the UCC chip and using the ohmmeter, I made careful resistance measurements and compared them to the schematic. I found everything matched except pin 12 of the UCC chip. C403, the tiny 4.7 uF electrolytic was reading 1.5K in both directions. It had been installed backwards 15 years ago (by me) and took this long to crap out.  I replaced it and the pulse came back perfectly.


I've often thought capacitors were probably the hardest thing to troubleshoot. I am kinda proud of myself that I still have the skill to get down to the component level.


Jim/ JKO, you were very close in your guess suggesting it was pin 10, the ramping capacitor.

Now I can fire up the 4D32 PDM rig and see what else has acted up.

Thanks for the help, everyone.

T


For future reference, here are the chip readings for the PDM UCC25701N:


Pin1 0V   Pin2  0V   Pin3  14.35V     Pin4 Disable 0V   Enable +9.73           Pin 5  0V    

Pin6 1.3     Pin7 1.32     Pin8  7.05     Pin 9 Enable +.66V  Disable -0.72V         Pin 10 5.66V

Pin11  0V      Pin12  5.65   Pin 13  0V      Pin 14 2.5V (not used)

I really should make measurements on the other chips and transistors too. This is probably the most complex board of the whole classE rig, so is worthwhile.
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« Reply #14 on: January 16, 2020, 01:51:21 AM »

UPDATE:   Tube rig, Quad 4D32s PDM modulated by Quad 6LF6s success -

Rico Suave aka "Dual Quads" is running well and back on the air.   I have it doing about 350 watts carrier with 140% modulation.  It seems at least as clean as the 4-1000A rig, maybe better. I will run some IMD and triangle wave tests tomorrow.

I've always liked tube PDM rigs.  It's just that they are dangerous with the floating supplies and RF final...  something like a Harley compared to a Honda.  It will make a great summertime rig.

I really like 4D32s.  They look like 813s but 1/2 the size. They are great tubes to substitute into 100 watt boat-anchors.

T
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« Reply #15 on: January 17, 2020, 06:08:51 PM »

Hey Tom....congrats on finding the reversed cap.  If it was a tube piece that cap would have failed in milliseconds with the higher volts.  Grin Grin   A good method of finding it with resistance checks.  See you on the aiya.....

Pete

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