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Dentron Clipperton L 2.5 mh Choke




 
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Author Topic: Dentron Clipperton L 2.5 mh Choke  (Read 862 times)
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KJ7USA
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« on: September 24, 2018, 03:21:11 PM »

I haven't used my Dentron Clipperton L in months and I let the smoke out of her last night. I installed a Harbach power supply board when I first received it and only use it on SSB. The two 1 ohm, 1 watt resistors evaporated and the 2.5 mh choke burned up. I already changed out the two resistors and am looking for a good source for the choke. I see RF Parts has one, but they have a minimum $20 purchase. Please let me know if you know of a good place to get these and what may have caused this fireworks show. Thank you!


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WD5JKO
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« Reply #1 on: September 24, 2018, 05:49:10 PM »



Look for more damage. Your DC plate voltage blocking capacitor to the pi-network looks like it shorted. This would put the HV B+ out to the antenna if it wasn't for that sacrificial RF choke.

Jim
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KJ7USA
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« Reply #2 on: September 24, 2018, 09:32:16 PM »

Thanks, Jim. I am not certain which capacitor you're speaking about. The two resistors that are burned up are R1 and R2. The choke is RFC-3. I've attached schematics of my amplifier. If you could please reference where to look, I'd really appreciate it. Thank you.


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WD5JKO
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« Reply #3 on: September 24, 2018, 09:42:01 PM »



I would be most suspicious that Cap C9 (1000pf / 5KV) either shorted out, or punched through.

Jim
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KJ7USA
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« Reply #4 on: September 24, 2018, 09:47:24 PM »

Got it. I'll take a look at it. Thanks!
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KJ7USA
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« Reply #5 on: September 24, 2018, 10:52:50 PM »

Good call, Jim! The cap was cracked. See the attached picture of it. Here's how I damaged the amp. I turned the amp on and was tuning it on low power and heard a pop and the fuse blew. I opened it up and gave a look around and couldn't find anything, so I cleaned the switches real good and tried again. This time, it made a louder pop (with smoke) just with it powered on and not transmitting. So now, I have found two bad resistors, choke, and capacitor. I hope this will be the last damaged component I find. Any idea on where to find the choke at? The one below is from RF Parts, but there is a $20 minimum and I'm not sure if it's the correct one. The parts list and schematic doesn't list anything more than 2.5 mh.

https://www.rfparts.com/chokes-inductors/chokes-highvoltagerf/chokes-multilayer/4021162.html


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K1JJ
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« Reply #6 on: September 25, 2018, 12:41:04 PM »

Yep, good call, Jim!

Those old door knob caps commonly crack after a few decades.


Now, to check for further potential damage....

Is the burned-up 2.5 mH choke open or closed?   Hopefully it still has continuity so that it clamped the HV at all times.  If it is open, there is a small chance that HV got into the antenna changeover relay and may have arced over into the input circuitry or driver itself.   Just a precaution....  If the 2.5 mH RF choke is still low resistance, even though burned, you should be OK. If open, then check the other circuitry as described, including the tubes.

I usually use  separate relays for the input and output - and use a HV vacuum relay for the output final/antenna circuit.  I have had smaller combination in/out relays arc over across the poles and take out the rice box driver in the past. Nasty biz.

Tom, K1JJ.
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KJ7USA
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« Reply #7 on: September 25, 2018, 01:24:33 PM »

Thanks, Tom. The choke reads open. I checked continuity of the relay and it seems to be fine. I read for continuity on the 572B tubes and only have continuity on pins 1&2 (filament). I don't know of any other tests I could do on these tubes to check that they are still good. Once I get all these components changed, I'm going to be nervous as hell turning her back on.
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WD5JKO
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« Reply #8 on: September 25, 2018, 01:43:35 PM »



   The current path also went through the plate choke. Any evidence of heating there? Should have continuity, but not zero ohms.

Tom,  I seem to recall you using a lower inductance choke across the 50 ohm output; something that won't blow if the B+ is dumped through it. Has to be enough inductance on the lowest band, and NOT have any series resonances on the upper bands. Those 2.5Mh 100ma chokes are not really providing the safety needed since the one here blew OPEN.

As far as the blow up in this email, could this have been started by a bad parasitic? After the repair, leave the plate caps off the tubes for the first power up. Be safe though as the voltage will linger after shutting things OFF.

On another Clipperton Topic, not related:

Many similar vintage (and older amps) put 60 hz AC hum on the RF output. This is very obvious when on AM, when RF QSB Dips, the RCVR demodulated signal has a lot of hum in the audio.

My Gonset GSB-201 had this problem. My old 10m buddy Ray, W2XC has a web page on this issue pertaining the Clipperton L.

http://www.w2xc.com/Clipperton_L_Hum_Mod.htm



Jim
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KJ7USA
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« Reply #9 on: September 25, 2018, 01:51:54 PM »

The plate choke checks good. There is a black spot from where the resistors burned up, but did not cause any damage to the choke. I ordered the 2.5 mh, 500 ma choke from RF parts along with a new doorknob cap. Thanks for the article on the hum. I only use this amp on SSB and have never heard of any complaints of a hum on my audio. Once I receive these parts and get them installed, I'll let you know how it goes. Thanks for all your help!
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K1JJ
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« Reply #10 on: September 25, 2018, 02:36:32 PM »

Interesting that the 2.5 mH choke opened up, thus removing its protection. FAIL!  :-)   Those little 2.5 mH chokes are probably good for 500 volts across them, not 3KV. But the bigger issue is current, which can be several amps or more for an instant, depending on the PS filter capacitor size and will open up the choke quickly.

Take it step by step as Jim says and you will be OK when firing it up again.

Jim, yes you remembered correctly. I'm an advocate of heavier wire on the protection choke.  I wind my own single layer protection choke on a plastic rod that uses #18 or #16 wire or heavier.  They always take out the fusing/breaker protection without opening up. Since this heavy-duty choke is used to protect you from HV on the 50 ohm line, the lighter-duty  plate choke feeding it may burn up before the fusing and breakers go, but that's the risk.  The 10-25 ohm fusing HV resistor will help here.

The good thing is that since this is a ~50 ohm spot on the tank output, the inductance can be up to 50 times less than the plate choke. (example: plate choke tap = 2.5K vs: C2 tap = 50 ohms)      As shown in the 2nd picture of my pair of 4X1s in RF GG with regulated DC screen and grid voltages, the brown Teflon wire plate choke is MUCH larger than the smaller black taped wire protection choke. I use this amp up to 160M and the protection choke does not get warm or affect tuning.

T




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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
K1JJ
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« Reply #11 on: September 25, 2018, 04:37:02 PM »

BTW, it is a good idea to do the AC fil xfmr mod. What a cop-out using no CT!  Shame on Dentron. On ssb it is very difficult to detect hum since it intermods with the voice. However, on AM or a solid carrier, the hum is very easy to see on a scope or hear.

There is another version of fil hum created in linears….  Years ago Frank/WA1GFZ had hum on his 4X1 GG amp. He looked at all the obvious stuff, but it persisted. Finally he determined it was coming from his filament choke. His fil leads were wound on separate cores - NG -   instead of both in a bifilar manner on one choke. He made the bifilar change and the hum disappeared.

So, just for the record, hum on an amplifier can sometimes come from the filament circuit itself.

One other version: The above examples were for directly heated cathodes/filaments. The indirectly heated cathode, like an 8877, etc.,  has a separate floating cathode that uses a trifilar choke to minimize hum.  The bifilar choke will work, but the trifilar choke circuit minimized hum for me.

T
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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
WD8BIL
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« Reply #12 on: September 26, 2018, 03:32:10 PM »

If you need a 1000pf doorknob I have one you can have Jim.

Is your address good in QRZ?

Buddly WD8BIL
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KJ7USA
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« Reply #13 on: September 26, 2018, 05:51:10 PM »

Not sure if you meant me, but I really appreciate the offer. I already bought one from RF Parts. Those things are expensive and the one I got is not new!
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MikeKE0ZUinkcmo
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« Reply #14 on: September 26, 2018, 06:56:20 PM »

I had close to 20% modulation/AC Hum on my Dentron amp, and as said in the referenced article, the hum was suddenly gone.

I mounted the transformer on the center divider between the power supply and AMP areas.

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Mike KE0ZU

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WD8BIL
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« Reply #15 on: September 27, 2018, 12:43:05 PM »

Quote
Not sure if you meant me, but I really appreciate the offer. I already bought one from RF Parts. Those things are expensive and the one I got is not new!

yes John..... sorry I was thinking John and typed Jim! Sad

Glad to hear you got one.
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KJ7USA
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« Reply #16 on: September 29, 2018, 03:37:03 AM »

I am glad to announce that the Clipperton is back in action! Thank you all for your support and guidance with the repair. It was a little nerve racking when I fired it up since I'm not one who likes to be startled by a bang and smoke. She is performing great.

John
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