Ameritron AL-82 Loud Bang

[N4OGW]

It should be easy to tell the difference between sheet metal and a HV arc- they usually sound rather different.

I am curious what you find because I am having a similar problem with my AL1500:

After about 10 years service, one of the HV caps failed: there was a bang and the fuse blew. When I checked the capacitors, it was obvious one had vented. I replaced all the caps and bleeder resistors.

After the cap replacement it ran fine for a couple of months. Then completely at random there was another arc somewhere in the HV supply. I did see a flash through the vent holes on the left side of the amp, but with the cover on I couldn't locate the exact spot. Very loud, definitely not a sheet metal noise. This time the fuse didn't blow, but the protection circuit took the HV off line.

I took it apart again, but couldn't find any obviously damaged capacitor or bleeder resistor. I also discovered the arc had destroyed one transistor in the QSK circuit. I replaced that and put everything back together. As soon as the HV turn on, another big bang So now I need to take it apart again.


Maybe I have a wire out of place that created the arc. One clue is that when it arcs the fuse does not blow like it did when one of the caps failed. But testing is difficult, because every arc takes out more components in the QSK board!

Tor
N4OGW

[W1QJ]

Tor, did you replace the caps with the short cap or the longer ones?  The longer ones require insulation between the caps and the chassis.  Generally a rssitive type HV arc to ground would not take out a 15 or 20A breaker, it will pop every once in a while if there is leakage to ground.  I find shuch arcing by leaving the cover off and turning off all the lights in the room so it is very dark.  A minor arc will show up very plainly.  A big one is a no brainer.

[N4OGW]

I'm not sure if they are "short" or "long". They were exactly the same size as the ones I took out. There is plenty of space between them and the chassis in the AL-1500, so I doubt that is the problem.

It is not a minor arc, the first time it went off I almost fell over in my chair I need to check again carefully to see if I can see evidence of where it arced.

Tor
N4OGW

[K8AXW]

Bill:  I hope you have indeed found your problem.  If it was a sheet metal "bang" then there is one thing left to do.

When you reassemble the cabinet, etc., make sure there isn't any binding or pulling by the screws.  As long as it goes together without any stress, the problem shouldn't reappear.

[N1DYX]

Bill:  I hope you have indeed found your problem.  If it was a sheet metal "bang" then there is one thing left to do.

When you reassemble the cabinet, etc., make sure there isn't any binding or pulling by the screws.  As long as it goes together without any stress, the problem shouldn't reappear.

Yup will do.  Lets hope it was as simple as that.  Boy it really must have a flex point somewhere because it's pretty loud.  But definitely no arc that I can see.  I ran it tonight without any problems, cover tomorrow.

Tor, sounds like yours is much different than mine, a lightning bolt like that will surely get you attention.  Hope you can find where it's coming from.

[W8LAD]

High Voltage short...!

[W8JX]

I still think he has a tube shorting out and the bang it the transformer surge.

[N4OGW]

Bill:  I hope you have indeed found your problem.  If it was a sheet metal "bang" then there is one thing left to do.

When you reassemble the cabinet, etc., make sure there isn't any binding or pulling by the screws.  As long as it goes together without any stress, the problem shouldn't reappear.

Yup will do.  Lets hope it was as simple as that.  Boy it really must have a flex point somewhere because it's pretty loud.  But definitely no arc that I can see.  I ran it tonight without any problems, cover tomorrow.

Tor, sounds like yours is much different than mine, a lightning bolt like that will surely get you attention.  Hope you can find where it's coming from.

I think I found the problem. After checking the HV supply carefully the only potential problem I could see was a solder joint on one of the HV leads going to the filter capacitor board. It is a stranded wire and the connection didn't quite have enough solder. I fixed that and it started up without arcing.

Tor
N4OGW

[N4ATS]

Rule of thumb with MFJ equipment , ALWAYS inspect and repair the common cold/bad/no solder joints prior to plugging in , even if it is new and right out of the box.

[N4OGW]

Rule of thumb with MFJ equipment , ALWAYS inspect and repair the common cold/bad/no solder joints prior to plugging in , even if it is new and right out of the box.

No, this one was my fault! I had to unsolder/resolder the connections to the HV filter board when I replaced the caps.

Tor
N4OGW

[K8AXW]

Tor:  Did you notice ANY kind of indication that this poor solder joint/connection was arcing?  I'm having a difficult time getting my head wrapped around your "solution."

[N4OGW]

Tor:  Did you notice ANY kind of indication that this poor solder joint/connection was arcing?  I'm having a difficult time getting my head wrapped around your "solution."

No- I know the arc was somewhere in the power supply because I could see the flash through the left vent holes. But I never ran the amp without the cover off so I could not pinpoint exactly where it was coming from.

Tor
N4OGW

[K8AXW]

Tor:  Personally I wouldn't set too easy for awhile.  The 'snap' one hears from HV is it jumping a gap.  If you have several strands of a wire making good contact I have a difficult time visualizing voltage jumping causing a 'snap.'  It might do this with a heavy current draw but would then expect to see evidence at the ends of the broken wires and PCB.

I wish you good luck.