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Author Topic: CCI EB63A - Occasional White Noise  (Read 3515 times)
W7ASA
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Posts: 208




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« on: April 20, 2013, 01:01:13 PM »

A question for those you who might have some experience with this problem.  I just built my EB63A from Communications Concepts Inc (CCI).  It's a good experience building from their kitted materials. this amplifier takes my signal from QRP levels to over 100 Watts easily. I am operating using the standard CCI output filter for 20 meters and really having a good time with my -now- high powered home brew CW station. 

Here is the 'difficulty':  Occasionally, the amplifier begins to generate RF 'hash' - that is white noise. I hear is between dits while sending, and immediately kill power (no harm to the amp so far). 

Here is my best thought:  The RF HASH is bale to continue (until I hit the power switch - as fast as I can) after the exciter signal stops.  Because it continues after keu UP (no signal) from teh exciter, it is not the 'exciter'.  The occurrance of RF haSH increases with drive level/output power. The input to this amplifier has NO resistive pad, but only an L/C formed by the input to the first binocular transformer at the input. 

I believe that this is caused by RF feedback. What are your thoughts?



73 de Ray ..._ ._
W7ASA 
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TANAKASAN
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Posts: 933




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« Reply #1 on: April 20, 2013, 01:12:28 PM »

If the signal continues after you release the key then the power amplifier is starting to oscillate, maybe at a frequency higher than HF. Start by checking your work, especially decoupling capacitors C6 and C7. Ferrite beads on your power supply leads may help as well.

Tanakasan
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W7ASA
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« Reply #2 on: April 21, 2013, 11:19:06 AM »

Hajime Maste, Tanaka-san,

I will take a look at the bypass capacitors as you pointed-out and also likely upgrade the ferrites I have on the DC power leads.

>>> I like the CCI amplifier and at some future date, might build another one of different power.




>Ray
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WB2WIK
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« Reply #3 on: April 22, 2013, 09:49:21 AM »

Is the amp generating any measurable RF output power when it's doing this, or is the "hash" just something you observe on a receiver?
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W7ASA
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Posts: 208




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« Reply #4 on: April 22, 2013, 04:34:01 PM »

Yes - it is generating high power, broad spectrum RF hash.  Mean power output is roughly the same as when it's operating properly.  However, when the RF hash begins, the SWR goes through the roof. 


Thank You for Asking,


Ray

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K6AER
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Posts: 3483




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« Reply #5 on: April 22, 2013, 08:50:12 PM »

If the amplifier is oscillating. It will be drawing more than the idle current when you are not driving it.
Besides the bypass capacitors the others have mentioned, look for the following.

Is you bias set equally and is the current draw upon keying set to high.

Look for bad grounding both on the circuit board and in the input and output transformers.

Make sure the current is equal on you push pull devices.

Without an input pad your amplifier might have a bit too much gain. An input pad would stabilize the amplifier a lot.

MAKE SURE THE COMPONENTS ARE WIRED IN THE RIGHT SPOT.

You did not mention if this problem just started or has the amplifier always exhibited this problem?
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W7ASA
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Posts: 208




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« Reply #6 on: April 22, 2013, 10:18:05 PM »

This amp was completed last week.  I am STRONGLY suspecting RF feedback at this point.  I can often hold an entire sked with no RF hash.  Because it's still my prototype, it's not fully enclosed and the cables are often in different positions. Bias checks before the 'smoke test' were uniform and well within spec.  The power transistors -when operating normally- are not hot, even after long skeds (no hash). Warm - as normal.

I'm a big fan of resistive input networks for this reason. I'd rather have a little less gain, but better stability.  I'll do some calcs on a good resistive pad and likely add that as well.  

I do NOT see this as a design or other fault from CCI.  Their materials, board and etc. have been quite good.

I do believe that it's RF feedback, occasionally causing the amp to oscillate. I have power off as soon as I hear it in the RX (and believe me I hear it and the SWR meters show it). so it's only done this for a few 1 second bursts since I built it.

I wanted to pass this idea by the fellows here for a 'sanity check'.

I do agree, better enclosure, bonding the entire station and accessories together and GROUNDING the station more effectively.  All things I should have done anyway.

I'll take it from here and post my findings in a week or two after testing.


Thanks!

73 de Ray
W7ASA ..._ ._


Ps.  Sssssh! I'm a QRP'er, so DON'T let this get around about me building a 140 Watt AMPLIFIER!!!    Shocked
« Last Edit: April 22, 2013, 10:22:45 PM by W7ASA » Logged
N0NZG
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Posts: 115




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« Reply #7 on: April 25, 2013, 09:32:35 PM »

Thanks for all the great info on your build. The last qrp rig I built I did note that you have to be very careful about the way you dress your wires in a high RF field.  You know the amp works. I would suggest you focus on getting it into a metal case of some sort and trim the internal wires down to as short as you can while still running the along the ground plain of the case. You can put toroids on the internal wires also to provide a high impedance path for any VHF oscillations. Extra bypass caps never hurt anything. Some times I have found the need to bypass everything that connects to a control like pots, switches, mic lines ,key lines and incoming power, etc…  Covering your control lines etc over the top and securing them to the case with wide copper or aluminum tape as a shield works well to, just be sure you have a good ground connection on the tape.  The aluminum HVAC tape is cheap and works great for an impromptu shield.

Good Luck,

Jeremy
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WB2WIK
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« Reply #8 on: April 26, 2013, 08:30:09 AM »

The design is not by CCI, it's by Motorola (Engineering Bulletin #63).

However, after reading your writeup, I'd strongly recommend you package the amplifier properly and that alone may make the feedback path causing the oscillation to completely disappear.

I've built EB63 amps before and never had any problems with them; however, they're built inside aluminum enclosures with short leads to the input and output coaxial receptacles, bypassing and ferrites on the DC power (+) lead, panel-mounted fuseholder and 20A fuse, etc.  The aluminum enclosure (with tightly fitting cover, held in place by 8 screws) helps -- a lot.

If you don't have an enclosure, I'm guessing you have a heatsink fitted to the two output transistors.  Since the mounting surface for this amp board is very close to the backside of the PC board, make sure not a single component lead protruding through the bottom of the board can come in contact with (or arc to!) the heatsink (or chassis/enclosure when you start using one).

Although the highest DC voltage in the amplifier is 14V or so, a sharply pointed component lead protruding through the board can possibly have much higher RF voltage on it than that and create an intermittent arc to anything grounded that's close by.
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WB8VLC
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Posts: 116




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« Reply #9 on: April 29, 2013, 08:16:50 PM »

Ray,

I have one of Helge's first prototype boards in my dog pile collection for that engineering bulletin along with his wavetex generator, scope, freq counter and other equipment. When I left Motorola RF back in1998 I was given some of his old engineering  boards and bags of development transistors so I may have some original dated but still fine Motorola 454's in bag at one of my qth's.

Does your amp use MaCom devices or original dated Motorola parts?

What I found when the transition from Motorola to MaCom came about was the MaCom devices have higher gain and depending on the collector voltage and the load Z there were some spurs visible during load pull test that are dependant on the input voltage and the temperature of the heatsink.

Operation above 13 volts was stable for my board with original Motorola devices but spurious using MaCom MRF454'S during some of my load pull tests.

My fix was to parallel 2x 10 ohm resistors across the original R5 and R6 locations in place of digging deeper and adding C-B feedback networks.

Since iv'e now moved up to freescales HV6 and HV8 devices I never really sat down to play anymore with any of my older bipolar amps since about the 2000 to 2001 timeframe.

But I also agree with others that should also check your grounding and enclosures if the input resistor changes don't resolve it.

Mike
WB8VLC
Prescott,AZ. Or Salem, Ore.


Edited to fix frigging android auto spelling BS and other android annoyances.
« Last Edit: April 29, 2013, 08:27:05 PM by WB8VLC » Logged
KB1GMX
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Posts: 713




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« Reply #10 on: May 02, 2013, 03:44:31 PM »

<Yes - it is generating high power, broad spectrum RF hash.  Mean power output is roughly the same as when it's operating properly.  However, when the RF hash begins, the SWR goes through the roof. >


if there is power out its oscillating.  The problem is it may not be exactly RF.  Most RF power devices
have way more gain at audio and low RF and if the bypassing, grounding, and lead dress is not top notch
they will take off.

I'd definitely use a PAD on the input even if its 1db though more is better. Make sure your grounds are good.
most all RF power amps I do have the edges wrapped with copper and plenty of wires through if there are
no plated through holes.  Especially around the emitter leads.  At 15-20A ground has a habit of looking less so.

Allison
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ZS5WC
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« Reply #11 on: May 03, 2013, 06:40:09 AM »

 Smiley Negative feedback from collectors to Base--this will improve linearity, kill unwanted oscillations and keep the amp stable.
Something like a 220 Ohm or 150 Ohm 5W MOF resistor in series with 100nF 250V cap. from each collector back to the base.

73 de William
ZS4L
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W7ASA
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Posts: 208




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« Reply #12 on: May 15, 2013, 09:36:02 PM »

UPDATE -

I placed the set into a prototype cabinet so that I do not unnecessarily drill several holes into my 'new' case which is it's intended destination.

1.  Thank you ALL for excellent advice.  Tough I am a very experienced builder and moderately successful amateur designer,  is my first high powered amplifier, so it's good learning experience.

2.  Reading through some of the less exciting text in the application notes, there was a little gem about keeping the input and output coax cables as far apart as possible.  This made a terrific difference in stability, allowing a few successful tests at full power. I have a VERY small ham 'nook' and keeping wires apart at QRP levels in unnecessary - at 140Watts, with maybe a little stray RF... problem.

3.  to: WB8VLC:  All ver useful information - thank you. The transistors are Motorola and I've been VERY careful with the through hole leads on the bottom side near the heatsink and actually lightly sanded them before final assembly all for JUST THE REASON YOU MENTIONED - potential for arcing from pointed leads. I am leery of so much gain in a single stage... but I'm a very conservative old school guy when it comes to design and we used to sat 10dB or maybe 13dB max in a single stage RF amp for fear of feedback, but I was third-string in such things  - at best! ha ha

The power is good, the amp is quite usable except for these spurious signals on my SDR up and down the band when I use the amp into the dummy load.  I did hat it on the air, but the SWR meter indicated multiple peaks, which told me that I had multi-frequency emissions wide enough to dull my normally sharp tuning peak! Dummy load testing revealed the spurs.  Suspecting my switching PS might have super imposed these waveforms into  the am at load, I swapped to a large analog PS: no change...

It appears that the amp itself is generating spurs.

We're close to being able to put this puppy on the air once these spurs are eliminated and when I have time, I'll do some experiments to determine whether a resistive input might help.  The negative feedback is very much on my mind //tnx ZS4L//

I'll CONFIRM and if necessary retrofit more through board ground plane through wires, especially on the emitter pads.

Now all that I need it time and a little more application of these excellent insights. I am also considering an in-box separation plate, to try to give a good path for srtay RF/currents back to ground, rather than finding entrance (line of sight) back into the input.



73 de Ray
W7asa ..._ ._

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