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Earlier this evening, while trying to tune my L7 up on 15 meters, there was a large flash and bang.? The L7 went dead and alot of smoke came out.? Opened it up and found the 47 ohm plate resistors fried and the 2? capacitors exploded. I have ordered a couple of new door knob caps and some resistors. Does anyone have any more ideas as too what else might be effected?? Much thanks in advance.
73? Bob
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Sounds like a plate-grid short. Check the tubes for shorts. Check the plate choke to see if it opened. Look under the chassis and check the grid chokes.
In the power supply check the .82 ohm resistor as it will act as a fuse. Hopefully it didn't take out any rectifiers.
It could also be one of the plate caps shorted. Check RFC6.
Those are the most common failures but these events can be quite catastrophic so hopefully none of the metering got damaged.
--
73 de Mike, W1NR
THAT was the equation. EXISTENCE!... SURVIVAL... must cancel out... programming!
- Ruk -
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Bob,
Could you provide a few extra details?
Were all of the parasitic choke resistors "fried"? ?If so, that might be symptomatic of a self-oscillation or parasitic. ?. ?With the strapping as a DC path it is unlikely that they failed due to a B+ or tube problem.
If the entire amp went dead, then that means the circuit breakers probably tripped. ?
What two capacitors exploded?
73,
Evan, K9SQG
On Friday, April 19, 2024 at 12:47:40 AM EDT, K6OXN Bob <bobk6oxn@...> wrote:
Earlier this evening, while trying to tune my L7 up on 15 meters, there was a large flash and bang.? The L7 went dead and alot of smoke came out.? Opened it up and found the 47 ohm plate resistors fried and the 2? capacitors exploded. I have ordered a couple of new door knob caps and some resistors. Does anyone have any more ideas as too what else might be effected?? Much thanks in advance.
73? Bob
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Both of the Parasitic Choke resistors were kind of burnt. That may be because of parasitics as C39 and 40 were the blown capacitors. I don't think the breakers blew as it seems when the caps went I lost HV. Haven't checked the PS to see if the .82 resistor blew, been cleaning up the area around the tubes.? All the RFC's look ok, no burn marks and check out with an ohm meter.? I have fully grounded grids as I did that not too long ago.? Resistance between grid and filaments check ok. Along with resistance between grid and plate. I really think some type of parasitic oscillation took place while on 15 meters.? I will carefully clean and replace those bad components and then bring it up gradually.
So much for trying to contact Easter Island as they were on strong last night at 221262.00. I could hear them very well about a 58 here and would have liked to have that contact.
73? Bob
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Don't lose hope. They were loud on 10m a little while ago but working EU. I scored Chad for ATNO and two bands yesterday with someone else's L7. This one suffered a flashover in a Chinese 3-500z a couple of weeks ago. That is an event that I don't want to experience again... You would probably know if the safety resistor in the PS went. They go off like a M80 and leave shrapnel inside the PS. On Fri, 19 Apr 2024 08:12:38 -0700 "K6OXN Bob via groups.io" <bobk6oxn@...> wrote: So much for trying to contact Easter Island as they were on strong last night at 221262.00. I could hear them very well about a 58 here and would have liked to have that contact. -- 73 -Jim NU0C
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Bob,
Thanks for the added info. ?When you said the amplifier went dead I thought all power to the RF deck was lost thus the filaments were not illuminated, meter lights out, status indicators out, ?etc. ; that's why I thought the breakers might have tripped.
If one uses the B+ line as a reference, it seems like there are problems on the tube side as well as the RF output side. A parasitic/self oscillation could probably cause something like that, and possibly if the amp is operated as a doubler or tripler (e.g. if the bandswitch was in a band different than the exciter). ?Wonder ?if there is any damage to the RF safety choke across the output or if the bandswitch contacts were damaged.
In any case, I'm curious to learn from your inspections and recommendations of the experts on the Reflector. ?Please keep us posted, we'd all like to learn.
73,
Evan, K9SQG
On Friday, April 19, 2024 at 11:12:40 AM EDT, K6OXN Bob <bobk6oxn@...> wrote:
Both of the Parasitic Choke resistors were kind of burnt. That may be because of parasitics as C39 and 40 were the blown capacitors. I don't think the breakers blew as it seems when the caps went I lost HV. Haven't checked the PS to see if the .82 resistor blew, been cleaning up the area around the tubes.? All the RFC's look ok, no burn marks and check out with an ohm meter.? I have fully grounded grids as I did that not too long ago.? Resistance between grid and filaments check ok. Along with resistance between grid and plate. I really think some type of parasitic oscillation took place while on 15 meters.? I will carefully clean and replace those bad components and then bring it up gradually.
So much for trying to contact Easter Island as they were on strong last night at 221262.00. I could hear them very well about a 58 here and would have liked to have that contact.
73? Bob
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After a serious full inspection it seems that the only damage was the parasitic plate chokes and the coupling capacitors C39 and 40. I have spare parts coming from RF Parts so should have the amp up and running early next week.? Will keep everyone apprised as to final results.? Lots of careful inspection and cleaning of all areas have been accomplished.
73? Bob
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I think I now know what caused my major fault.? I was trying to tune up on 15 meters because the Easter Island Dx station was so strong. Problem was it was on 21.262 which is not in the proper part of the band for a General Class license. Sometimes karma does get you.
73? Bob
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Bob,
Glad you found the issue which was along the line of a parasitic, self-oscillation, or frequency multiplying. ? Some of the newer amps have circuitry to prevent incorrect bandswitch positions which is almost a requirement for solid state amps. ?Good luck with the repairs and informing us of the causative factor.
73,
Evan, K9SQG
On Saturday, April 20, 2024 at 08:52:40 PM EDT, K6OXN Bob <bobk6oxn@...> wrote:
I think I now know what caused my major fault.? I was trying to tune up on 15 meters because the Easter Island Dx station was so strong. Problem was it was on 21.262 which is not in the proper part of the band for a General Class license. Sometimes karma does get you.
73? Bob
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On Sat, Apr 20, 2024 at 05:52 PM, K6OXN Bob wrote:
I think I now know what caused my major fault.? I was trying to tune up on 15 meters because the Easter Island Dx station was so strong. Problem was it was on 21.262 which is not in the proper part of the band for a General Class license. Sometimes karma does get you.
73? Bob
Pretty sure the radio doesn't really know what frequency you're on!! But that Karma can come in other forms as well, --
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RF Parts and Fedex came thru. Received the replacement Parasitic Choke resistors.? Ordered a spare set also and also 2 door knob capacitors 500 pf 7.5kv plate capacitors. Built up a small 2 strip copper attachement to hold the capacitors and rebuilt the chokes.? Also I forgot of course the .82 ohm fuse resistor in the PS went pop also. Replaced it and after a tenative slow power up checking things the L7 is back amongst the living.? Drake sure built an Armour plated PS and Amp. Is working just fine now on 40 meters my preferred band with 60-70 watts in from a TR7 I get a nominial 1 kw out of the L7.? Thanks for all the info and the help from our files and photo sections are also helpful.? Now to see if Karma has forgiven me.
73? Bob
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The oem Drake plate block caps in the L7 are failure prone. They went cheap, and used a pair of disc ceramic caps.? When they fail, they typ fail shorted.... which will take out the .82 ohm HV fuse.?
You might want to also check the safety choke across the load cap as well. It could have easily failed open during a HV fault condition.?
?The pair of 500 pf @ 7.5 kv rated HEC ceramic doorknob caps you installed is the correct component for the plate block cap assy.??
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Bob,
Thank you for keeping us informed of your progress and success.
73,
Evan, K9SQG
On Sunday, April 21, 2024 at 07:05:45 PM EDT, K6OXN Bob <bobk6oxn@...> wrote:
RF Parts and Fedex came thru. Received the replacement Parasitic Choke resistors.? Ordered a spare set also and also 2 door knob capacitors 500 pf 7.5kv plate capacitors. Built up a small 2 strip copper attachement to hold the capacitors and rebuilt the chokes.? Also I forgot of course the .82 ohm fuse resistor in the PS went pop also. Replaced it and after a tenative slow power up checking things the L7 is back amongst the living.? Drake sure built an Armour plated PS and Amp. Is working just fine now on 40 meters my preferred band with 60-70 watts in from a TR7 I get a nominial 1 kw out of the L7.? Thanks for all the info and the help from our files and photo sections are also helpful.? Now to see if Karma has forgiven me.
73? Bob
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Well thought I had it fixed but again without any notice, large spark and bang.? Lost the .82 in the power supply and I suspect the 47 ohm parasitic resistors in the Amp.? I was using 47 ohm 2 watt carbon resistors in the parasitic choke, should I change them for something else?? When they go they were blackened and fractured. I am open to any suggestions. It happened when I was talking on 40 meters for about 45 seconds, operating at about 750 watts out.? Did not hear an arc.? Will be examining it more better this time. Will have to get some more 47 ohm resistors.? I am beyond my capabilities this time.? Any help or suggestions are welcome.
73? Bob
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Will the L7 do this when just in Idle no transmit? ? Has the power supply been rebuilt ever?
Tim
WB8UHZ
On Tuesday, April 23, 2024 at 03:31:13 PM EDT, K6OXN Bob <bobk6oxn@...> wrote:
[Edited Message Follows]
Well thought I had it fixed but again without any notice, large spark and bang.? Lost the .82 in the power supply and I suspect the 47 ohm parasitic resistors in the Amp.? I was using 47 ohm 2 watt carbon resistors in the parasitic choke, should I change them for something else?? When they go they were blackened and fractured. I am open to any suggestions. It happened when I was talking on 40 meters for about 45 seconds, operating at about 750 watts out.? Did not hear an arc.? Will be examining it more better this time. Will have to get some more 47 ohm resistors.? I am beyond my capabilities this time.? Any help or suggestions are welcome.
73? Bob
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The power supply has the Harbach mod done and has been working great. Of course when this happens only the .82 fuse resistor blows. I do notice some possible arc points on the cover of the L7 and will place some Fish paper in that area.? After the first repair, I left the amp on in idle with HV at 2700+ and had no problems for over 4 hours in that state. Now it seems that the arcing is taking out the 47 ohm parasitic resistors when I am modulating in SSB.? Only tested on 40 meters at this time and cannot see any parasitic oscillations on the scope. Replacing the parasitic resistors now along with the .82 in the PS.? Will bring it up slowly and may try SSB with the amp in CW mode unless that is not recommended. Otherwise, I will reduce the TR7 to about 50 watts and see how it is doing.
73? Bob
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Wish you had another pair of 3-500Z tubes to put in. I had that problem with what looked like a new set of Eimac tubes, the red ink was perfect, the guy that sold them to me said he had just used them. I checked them with an ohm meter, no issue. I put them in the amp(L7) and in low power no issues in high power lighting struck fuse resistor fried. I took the tubes out of course and put in my old tubes and all was OK. The seller did after I found him refund my money.
73 Tim
WB8UHZ
On Tuesday, April 23, 2024 at 05:54:31 PM EDT, K6OXN Bob <bobk6oxn@...> wrote:
The power supply has the Harbach mod done and has been working great. Of course when this happens only the .82 fuse resistor blows. I do notice some possible arc points on the cover of the L7 and will place some Fish paper in that area.? After the first repair, I left the amp on in idle with HV at 2700+ and had no problems for over 4 hours in that state. Now it seems that the arcing is taking out the 47 ohm parasitic resistors when I am modulating in SSB.? Only tested on 40 meters at this time and cannot see any parasitic oscillations on the scope. Replacing the parasitic resistors now along with the .82 in the PS.? Will bring it up slowly and may try SSB with the amp in CW mode unless that is not recommended. Otherwise, I will reduce the TR7 to about 50 watts and see how it is doing.
73? Bob
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Check the plate tune capacitor for any signs of arcing. Also check you coax and antenna system. A load mismatch can cause unpleasant things. 73 -Jim NU0C On Tue, 23 Apr 2024 14:54:29 -0700 "K6OXN Bob via groups.io" <bobk6oxn@...> wrote: The power supply has the Harbach mod done and has been working great. Of course when this happens only the .82 fuse resistor blows. I do notice some possible arc points on the cover of the L7 and will place some Fish paper in that area.? After the first repair, I left the amp on in idle with HV at 2700+ and had no problems for over 4 hours in that state. Now it seems that the arcing is taking out the 47 ohm parasitic resistors when I am modulating in SSB.? Only tested on 40 meters at this time and cannot see any parasitic oscillations on the scope. Replacing the parasitic resistors now along with the .82 in the PS.? Will bring it up slowly and may try SSB with the amp in CW mode unless that is not recommended. Otherwise, I will reduce the TR7 to about 50 watts and see how it is doing.
73? Bob
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The tubes may have been gassy. That can happen if they are babied. You might have been able to recondition them by running them for a while at low voltage with some red in the plates. They need to get that hot for the getter materiel to scavenge the gas. That's why they can fail early if they have been babied. 73 -Jim NU0C On Tue, 23 Apr 2024 22:06:45 +0000 (UTC) "amfone via groups.io" <amfone20000@...> wrote: Wish you had another pair of 3-500Z tubes to put in. I had that problem with what looked like a new set of Eimac tubes, the red ink was perfect, the guy that sold them to me said he had just used them. I checked them with an ohm meter, no issue. I put them in the amp(L7) and in low power no issues in high power lighting struck fuse resistor fried. I took the tubes out of course and put in my old tubes and all was OK. The seller did after I found him refund my money.
73 Tim
WB8UHZ
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Bob, your exciter is a TR7 on 40M, correct? I wonder if the nasty "40M oscillation" that has been batted around here may be in play at this juncture.?
After you get the amp back to an operational condition, I would try a different radio even if you have to borrow one. Make sure any other rig especially?solid state,?can either handle the voltage and current of the L7 or use an interface. I mentioned this as I am not familiar with the keying arrangement of the L7.
Bob K9JU
On Tuesday, April 23, 2024 at 06:21:22 PM EDT, Jim Shorney <jimnu0c@...> wrote:
The tubes may have been gassy. That can happen if they are babied. You might have been able to recondition them by running them for a while at low voltage with some red in the plates. They need to get that hot for the getter materiel to scavenge the gas. That's why they can fail early if they have been babied.
73
-Jim
NU0C
On Tue, 23 Apr 2024 22:06:45 +0000 (UTC) "amfone via groups.io" <amfone20000@...> wrote: >? Wish you had another pair of 3-500Z tubes to put in. I had that problem with what looked like a new set of Eimac tubes, the red ink was perfect, the guy that sold them to me said he had just used them. I checked them with an ohm meter, no issue. I put them in the amp(L7) and in low power no issues in high power lighting struck fuse resistor fried. I took the tubes out of course and put in my old tubes and all was OK. The seller did after I found him refund my money. > > 73 Tim > WB8UHZ
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Jim Shorney