Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: Motorola MRF428/PT9790 Question  (Read 2945 times)
TWORLD1000
Member

Posts: 31




Ignore
« on: July 02, 2011, 11:06:50 PM »

I am restoring a HF amplifier's power supply. The amplifier uses 4 pairs of Motorola MRF428 (PT9790) power transistors. According to Motorola data sheet the max Vceo of the transistor is 55 volt. It shows a test circuit with a Vcc of 50 volt.

My question is if it is safe for the amp to operate at 45 - 50 volt Vcc. Well, not because I want to take the risk. The AC power transformer I got may actually result in an output any where between 45 to 50 volt without regulation. It is probably closer to 45 rather than 50 volt. It took me years before I finally found this transformer (1.5KVA) to replace the original which is blown. I don't anticipate to find another that will give me exactly 42 volt, which is the original operating voltage, without costing an arm and a leg. Building a voltage regulator is not trivial in this case either. It needs to spit out 42 volt with a 40 amp capability. I am not sure I can put a regulator of that capacity in the amp. I am not considering more modern switching power supply either at the moment. It will be difficult to find one that fits in the amp perfectly.  

Again the question is can Motorola MRF428 (PT9790) operate at 45 - 50 volt (likely closer to 45)? The amp is a Transworld T-1000 solid state 1kw HF amp. You can see a picture of this amp at:
http://s1179.photobucket.com/albums/x381/tworld1000/My%20Radios/?action=view&current=DCP_1823a.jpg  

The cavity behind the front panel is where the original but blown transformer was located. It has been taken out.
« Last Edit: July 02, 2011, 11:09:07 PM by TWORLD1000 » Logged
M0HCN
Member

Posts: 473




Ignore
« Reply #1 on: July 03, 2011, 03:52:33 AM »

Sensitivity to mismatch will obviously be higher and I would be careful to consider what happens under high line voltage conditions, but my instinct is that (assuming those are DC output voltages not RMS ones) it should work, albeit possibly with reduced reliability. 

Personally I would fit a small buck transformer to reduce the primary voltage seen my the main iron, RF power devices, particularly of the older sort, do not often have much conservatism built into the manufacturers ratings.

Regards, Dan.
Logged
K0BG
Member

Posts: 9833


WWW

Ignore
« Reply #2 on: July 03, 2011, 09:34:30 AM »

There are a few things to keep in mind. The MRF-428 was first introduced in 1976. They haven't been made since about 1986. Other brands labeled MRF-428 will not plug, and play in a Transworld T1000, or most other amps in this era for that matter. So watch your drive levels when you do get it fired up.

To directly answer your question, their rated max Vceo is 55 volts, but at idle the voltage applied was closer to 58 volts in the T1000. I got rid of mine in favor of an Icom 2KL in 1979, which I still own.
Logged

TWORLD1000
Member

Posts: 31




Ignore
« Reply #3 on: July 03, 2011, 11:50:26 AM »

Are you sure it measured 58 volt when idle? Somehow I remember seeing about 50 volt or maybe 48 volt 10 years ago. I remember thinking about to totally dismantle the broken built-in power supply form the broken amp. Then use 4 12 volt deep cycle batteries externally to drive the amp. I abandoned the idea because the actual voltage would exceed 50 volt from 4 12 volt batteries. 

However, the service manual says the unloaded power supply should give 42 volt of DC. This makes me wonder if my memory has gone bad. In audio power amplifiers the operating voltage is normally way below the max Vceo of the power transistors. 42 volt sounds about right for the MRF428s. Why I remember seeing 50 volt in the amp (or 58 in your case)?

Anyone with real experience with these RF power transistors?
Logged
TWORLD1000
Member

Posts: 31




Ignore
« Reply #4 on: July 03, 2011, 11:52:34 AM »

Buck transformer was suggested to me on the other thread. I don't feel having room in the amp to house one. It's a good idea though. Thanks a lot.
Logged
TWORLD1000
Member

Posts: 31




Ignore
« Reply #5 on: July 04, 2011, 12:02:51 PM »

Took a picture of my 2nd T1000. I had to dig my way into my storage. Looks like there is another there. Here is a picture of the T1000s. Not sure if they are all working though. I remember testing the black one on a big dummy load that I yet need to dig out. The black one worked OK with a little drive back then when I got the amps 10 years ago:
http://s1179.photobucket.com/albums/x381/tworld1000/My%20Radios/?action=view&current=T1000Amps.jpg

I may try to take one of them out to measure the actual DC voltage out of the power supply inside. On top of the T1000s is a Halicrafter HT-37.
« Last Edit: July 04, 2011, 12:06:17 PM by TWORLD1000 » Logged
TWORLD1000
Member

Posts: 31




Ignore
« Reply #6 on: July 04, 2011, 12:35:41 PM »

Here is the circuit diagram of T1000's amp module:
http://s1179.photobucket.com/albums/x381/tworld1000/My%20Radios/?action=view&current=T1000AmpDiagram.jpg

I can't believe how simple it is. I have never seen an audio power amp that is not far more complex than this. You can see the DC supply is marked 40 volt which feeds through the center tap of the RF output transformer to the collectors of the MRF428s. The emitters are grounded. It is that simple. I have no idea why people are messing with tube amps of the caveman age these days. I only need to deal with 40 - 50 volt DC only.
Logged
KE3WD
Member

Posts: 5694




Ignore
« Reply #7 on: July 04, 2011, 02:10:09 PM »

I'd be willing to bet that you could run those things off a surplius Telco supply...

Don't give up on the buck transformer option.  It can standalone since it would be on the primary side of things.  Consider winding your own secondary to handle the current.  Might even work with a large microwave oven xformer where you strip out the secondary and wind your own for the buck purpose using heavy gauge wire to handle the current. 


73
Logged
TWORLD1000
Member

Posts: 31




Ignore
« Reply #8 on: July 04, 2011, 02:29:18 PM »

Thanks for the buck transformer suggestion. I believe I have seen small ones in electronic surplus stores before. I just did not know what they were. I do have a question about using one for my power supply. If the 2ndary winding is 10 volt AC at 10 amps. That means it requires the primary winding to be 100 volt AC at one amp. This buck transformer will be quite sizable. I think it has to be outside of the amp. I should be able to find a regular transformer of 125/10 ratio with needed KVA rating. I do prefer to be able to install it inside the amp though. I will see what I can find. Thanks for the suggestion. 
Logged
M0HCN
Member

Posts: 473




Ignore
« Reply #9 on: July 04, 2011, 04:42:53 PM »

The buck transformer can be smaller then you think as it does NOT have to handle 1.5KW.

If you want to buck say 10% off a 120V line at 1.5KVA nominal load  you need a buck transformer that can handle 1500/120 = 12.5A @ 12V, which equals only 150VA. Something like a 120V primary to a 12V secondary at 150VA should do the job.

Incidentally that PA circuit does not by any means tell the whole story for a solid state PA, in particular the output filters (Vital) and (all important) protection circuitry is missing, as are the power combiners and splitters, TR switching. Also I have my suspicions about the linearity of that circuit, doing a really linear PA takes rather more work then that!

Regards, Dan.
Logged
TWORLD1000
Member

Posts: 31




Ignore
« Reply #10 on: July 04, 2011, 07:22:06 PM »

Dan, thanks for commenting. Yes, the buck transformer needs 100 - 150 VA. This implies 120 V 1 amp primary and 10 V 12 amps secondary. This is going to be a fairly large transformer that probably will be difficult to fit in the amp. A DC regulator will be much smaller.

You are absolutely correct that the amp module circuit is a small part of the whole story no doubt about it. But it is the real circuit in the T1000 amp. It has 4 such modules. There sure is an input splitter and an output combiner too. They look very simple too. I have no doubt that they only look simple but to copy and implement one it is not a trivial challenge.

As far as linearity goes it is compensated by the filters following the combiner. The linearity issue causes high level of harmonics and is common among solid state amps. The T1000's output filtering is a 7 pole elliptic low pass filter (one for each band) that is spec'ed to a -50db attenuation on 3rd order harmonics. The filter board doesn't look complicated. But the service manual does not provide inductance numbers of all toroidal inductors on the filter assembly. So the filters will be a challenge too for anyone to copy.

There is a built-in VSWR metering circuit in the amp. It also provides VSWR protection for the amp modules. There are heavy thermo protection for the amp too. It regulates the bias for the power transistors and controls the cooling fan speed. There is also a DC current protection circuit that will shut off the DC supply when the supply current exceeds a certain level.

In my opinion T1000 is a well designed rock solid amp. it is capable of full power transmitting at 1kw all day according Rudi here:
http://www.qsl.net/zs6dx/station.htm

I am amazed by how simple the basic amp modules are. I am determined to fix the power supply that meets the original specs so that the amp will be safe to operate as spec'ed by the origimal specifications.
« Last Edit: July 05, 2011, 12:26:46 AM by TWORLD1000 » Logged
KB2CPW
Member

Posts: 304




Ignore
« Reply #11 on: July 04, 2011, 08:00:11 PM »


  Maybe you can retrofit the supply from another amp like the ALS 600, there is a linear and switching supply option for that amp ( I don't recall what the finals use but it is easy enough to check.

   Also look at the TS 950SD PA circuit, they use two transistors with a 2sc2922 to regulate a pair of 429's (similar voltage) Maybe you can copy the circuit and make what you need using several and run it from a similar but more hefty transformer. I recall the voltage to the circuit was about 77 volts before regulation by the 2sc2922. Just an idea.. Richy N2ZD
Logged
W8JI
Member

Posts: 9304


WWW

Ignore
« Reply #12 on: July 05, 2011, 04:17:05 AM »

You did OK except this part. This part is totally off:

As far as linearity goes it is compensated by the filters following the combiner. The linearity issue causes high level of harmonics and is common among solid state amps. The T1000's output filtering is a 7 pole elliptic low pass filter (one for each band) that is spec'ed to a -50db attenuation on 3rd order harmonics. The filter board doesn't look complicated. But the service manual does not provide inductance numbers of all toroidal inductors on the filter assembly. So the filters will be a challenge too for anyone to copy.

Linearity in a solid state power amp is a gain or amplification transfer function between input and output. It is not a harmonic issue, and cannot be cleaned up with filters. People seem to think throwing a filter on a dirty amp cleans it up so it doesn't splatter, or that lack of an output filter makes it splatter, but all the filter does is attenuate harmonics. The does not narrow a wide in-band signal, nor does lack of one cause a wide in-band signal.

Bandwidth problems are caused by the output power not tracking the input power. If we plotted drive power vs. gain on a graph, changes in the slope of that line would determine the what intermodulation products are created and how bad they were. It doesn't need to be a straight line. Well-designed tubes have a slope, but the slope is rounded or gradually curved with a shape that discourages odd-order IM products that cause signal bandwidth issues.

Bandwidth is determined by several things, but all of those things come back to the transfer function of the system. Harmonic filtering does not change the transfer function (unless it modifies something else important, which has nothing to do with "filtering") it has no effect on bandwidth.

Matching the load impedance to optimum loadline for the device, having a stiff power supply, having a stiff bias supply, having a device that has uniform gain with drive level variations or a proper curve to any variations, and any controlled negative feedback all help determine odd-order IMD that causes bandwidth issues.

By the way, the IM is called odd-order because it is something like 2Fa-1Fb (2 + 1 equals three, an odd number)  or 1Fa - 2Fb (again third) or 2Fa-3Fb (this one of the two problematic fifths) , and so on. Nothing to do with filtering because they fall next to the desired signal, and thus cannot be filtered with L/C filters in our applications.

73 Tom



73 Tom
Logged
TWORLD1000
Member

Posts: 31




Ignore
« Reply #13 on: July 05, 2011, 10:23:52 AM »


Linearity in a solid state power amp is a gain or amplification transfer function between input and output. It is not a harmonic issue, and cannot be cleaned up with filters.

Tom, I am afraid I don't agree. Harmonics distortion is caused by non-linearity of an amplifier. It causes 2x, 3x, 4x, 5x.... multiples added to the fundamental frequency at the out put of the amp. You can filter the harmonics out if the filter is properly designed. To meet FCC regulation the harmonics suppression needs to be greater than -50 db.  

By the way, the IM is called odd-order because it is something like 2Fa-1Fb (2 + 1 equals three, an odd number)  or 1Fa - 2Fb (again third) or 2Fa-3Fb (this one of the two problematic fifths) , and so on. Nothing to do with filtering because they fall next to the desired signal, and thus cannot be filtered with L/C filters in our applications.

Keep in mind, if Fa is the transmitting frequency and Fb is the audio (say a 400 Hz test tone) the harmonics products caused by IMD of the amp will impact the fidelity of the audio at the receiver end. The frequencies of such harmonics are not much greater than the Fa frequency itself. So it is true the amp's filter has no effect to them. But they don't cause splatter either. The harmonics are within the base band of the signal. It is the multiples of the base band signal that will cause splatter.

« Last Edit: July 05, 2011, 10:26:19 AM by TWORLD1000 » Logged
KB8E
Member

Posts: 24




Ignore
« Reply #14 on: July 05, 2011, 12:24:24 PM »

Tom is correct. Any nonlinearity in the transfer curve causes distortion products. Depending upon the signal applied and the actual shape of the transfer curve, both harmonic distortion and intermodulation distortion products can be created. The harmonics and even-order intermodulation products can be filtered out, but odd-order intermodulation products can appear outside the immediate bandpass of the desired signal, but close enough that they can't be filtered out; this is splatter. Consider your example of a carrier with a modulation tone at 2.5 kHz. Due to a nonlinear amplifier, a third-order intermodulation tone could be created at 5 kHz away from the carrier. Audio distortion, yes, but also a signal outside the desired passband. Two-tone SSB is similar.

Sam
Logged
Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!