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Texas star/outcom Dx-600 repill

Enterprise312ok

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Jun 12, 2022
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I have an Outcom dx600 with 1 0r 2 blown mrf454 pills. What do all you amp experts think I should put back? 2 hg454s or 4 hg2879s?
I have a 667v faceplate and variable that could be added but I just want to get the thing back up to full output or a little better.
I have recently started working part time at a local shop that makes "cw keying units" so I have access to teflon wire to rewind the transformers if needed and hve some 2879s on hand but would have to order the 454s. I am new at this so any advice on amp tuning would be much apreciated.
20220731_073340.jpg
20220731_073348.jpg
 

nomadradio

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Apr 3, 2005
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If you want it to work like it used to before it broke, replacing the blown parts with the same type number is the safest bet. The capacitor values and transformer ratios are already optimized for that type number. The MA/COM parts with the original Motorola type numbers on them are a legitimate licensed component and should perform like the originals.

Then again, if what you'd like to do is experiment and see what happens using substitute transistors, I would recommend using the listed component values in the Texas Star diagrams that cover the newer models.

Changing transistor types nearly always causes the optimal circuit setup to change. Simply swapping one type for the other usually doesn't get you the full power that component should deliver.

I need to post a quick guide to the "suicide resistors" in Texas Star amplifiers. The 10-ohm resistor soldered to the base lead of the left-hand transistor of the pair nearest the front is a clue. It's visibly darkened at the center in this pic. This resistor smokes because of surge current coming OUT of the transistor's input lead after the transistor shorts out inside. There is also a low-value resistor on each pair of 454s, R15 and R23 on the DX667V schematic. Not exactly the same as the 600, but close. These two resistors serve to set the transistors' "idle" current draw when there is no drive power. Any time you see one of the 10-ohm resistors visibly damaged, this bias resistor is almost certainly blown or damaged. Simply dropping in new transistors with either of those two resistors damaged may easily blow out your new transistors in the second or third key of the mike. A damaged bias resistor can cause the RF transistor to run away and pull all the DC current your power supply can pump into it. Until it blows.

Bad juju.

And you'll never guess how I discovered this back in the Jurassic era.

73
 

nomadradio

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Apr 3, 2005
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Um, okay.

I learned about the bias-resistor hazard by blowing up two new Toshiba 2879 transistors. The first key of the mike. Not a particularly divine thing to do.

Years ago an old friend complained to his dad about the pain from injuries he got from his first motorcycle wreck. His dad's reply was "Stupid should hurt". The pain does improve the odds you'll remember and skip doing the same stupid thing next time it comes up.

But that's the real message, don't do the same stupid thing I did.

73
 

Enterprise312ok

Active Member
Jun 12, 2022
154
165
43
34
If you want it to work like it used to before it broke, replacing the blown parts with the same type number is the safest bet. The capacitor values and transformer ratios are already optimized for that type number. The MA/COM parts with the original Motorola type numbers on them are a legitimate licensed component and should perform like the originals.

Then again, if what you'd like to do is experiment and see what happens using substitute transistors, I would recommend using the listed component values in the Texas Star diagrams that cover the newer models.

Changing transistor types nearly always causes the optimal circuit setup to change. Simply swapping one type for the other usually doesn't get you the full power that component should deliver.

I need to post a quick guide to the "suicide resistors" in Texas Star amplifiers. The 10-ohm resistor soldered to the base lead of the left-hand transistor of the pair nearest the front is a clue. It's visibly darkened at the center in this pic. This resistor smokes because of surge current coming OUT of the transistor's input lead after the transistor shorts out inside. There is also a low-value resistor on each pair of 454s, R15 and R23 on the DX667V schematic. Not exactly the same as the 600, but close. These two resistors serve to set the transistors' "idle" current draw when there is no drive power. Any time you see one of the 10-ohm resistors visibly damaged, this bias resistor is almost certainly blown or damaged. Simply dropping in new transistors with either of those two resistors damaged may easily blow out your new transistors in the second
Thanks Nomad glad to know the macom 454 perform like the originals. I have removed the two bad pills and also 4 motorola ssl100a pills from an old varmint 1x4 that the shop owner gave me he said he is 80% sure they are Varmint part numbers for 454s. May be a low cost fix. I should have a component tester tommorow if it gets delivered.
 
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AudioShockwav

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Thanks Nomad glad to know the macom 454 perform like the originals. I have removed the two bad pills and also 4 motorola ssl100a pills from an old varmint 1x4 that the shop owner gave me he said he is 80% sure they are Varmint part numbers for 454s. May be a low cost fix. I should have a component tester tommorow if it gets delivered.
Off topic, can you post a picture of the varmint 1x4?
I would like to see the input and output transformers.
Solid state varmints are few between.
Thanks.
Jeff
 
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nomadradio

Analog Retentive
Apr 3, 2005
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Louisville, KY
www.nomadradio.com
I should have a component tester tommorow if it gets delivered.
Good deal! More than once I have tested a high-power RF transistor like the 2SC2290 and seen a backwards result on the screen. Tries to tell me that it's a PNP transistor.

Not. It's a NPN. Best I can tell the tester's DC voltages are just enough to make the transistor oscillate at some RF frequency. The long wires on a typical tester have enough inductance and capacitance for this to happen. I'm guessing that a circuit meant to measure DC voltage/current gets confused if you feed it a stream of RF voltage.

It's a valuable tool, but be skeptical if it tells you something that seems off the wall.

73
 
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Enterprise312ok

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Jun 12, 2022
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Off topic, can you post a picture of the varmint 1x4?
I would like to see the input and output transformers.
Solid state varmints are few between.
Thanks.
Jeff
Next time I get to the shop I will get you some pictures of the intact 1 pill varmints on the shelf and maybe the shop owners 4 pill varmint, he was good freinds with the owner of varmint amps. I will see if he has any old service manuals.

Oklahoma has a long history of making these boxes , I run an oklahoma made kenrich eagle 500.
 

AudioShockwav

Extraterrestrial
Staff member
Apr 6, 2005
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Nor Cal Sierra Nevada
Here is whats left of herView attachment 60077
Thanks, yes that is what I wanted to see. The output transformers used large cores, the other one I have seen also used large cores.
It was 2 of the amp you have built into one case and combined.
I dont want to hijack the thread.
I will bump another thread.
Back to programming...thanks

73
Jeff
 
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Enterprise312ok

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Jun 12, 2022
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Ok due to the cost of the macom 454 and the fact that hg 454s would also require tuning I have decided to go with hg 2879s does anyone have the component values for the newer 667s that run the hg or dei 2879s?
 

Enterprise312ok

Active Member
Jun 12, 2022
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I think I am leaning more towards hg 2290s have been getting some real good numbers out of 2 pills when I match them with my component testers, probably going to try them with the transformes the way they are and add some variable capacitors and try to tune it.
 

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