QRO Technologies 2500DX Amplifier...HELP...

[W4DNR]

Does the HV go through any shielded cable ?
IF not, can you put a piece of plastic between the hv cable and the chassis where it touches the chassis ?

Cut up pieces of milk jug or coke bottles for insulation.

Don W4DNR

[KA2DZT]

An inrush problem would blow the fuse when you first turn on the HV supply.  Is that what is happening or do you get the bang after the supply is on a few or more seconds??

Fred

[W4AMV]

Hello Fred. I would say instantly. And when first turned on one bang and the fuse blows. The first time today while the HV is being measured and the HV removed from feeding to the plate choke, the FUSE blew after say 500 ms. This was long enough for both of us to watch the front panel HV meter as well the Fluke DVM with a 1000:1 divider and see the HV hit about 1 kV. After replacing the fuse, pulling both tubes, a bang-bang occurs and for that case, hard to get any meter reading. But... direct to your question, for the most part instantly. Recall, when first repaired the HV it sat idle for minutes at 3.3 kV and was tuned up to the tune of 800 W. Then, the big bang and/or bang-bang stuff started occurring and now continues.

[W4AMV]

Sorry, I missed Don's comment on the HV via a shielded cable. Interesting, NO.

The HV wire(s) in the QRO, might be HV wire rated, not sure, but slipped over ANY HV wire, QRO slips a clear plastic tube, does not feel like teflon, and does not feel like heat shrink. However, more like fat spaghetti tubing. We were careful in their dressing, must confess to get these boards out and to do rework we had to cut very carefully the tie wraps for the multiple wire harness and nylon clamps removed then replaced but not all bolted back down to the common wall between RF section and HV supply section. I can't imagine aggravating any HV cross talk among those wires in the harness, as they were all initially in very close proximity of one another.

[W4AMV]

A new input from an outside source says... CHANGE ALL THE DIODES!!

Hard to believe but certainly possible after a decade or so, multiple diodes could be breaking down, despite having 10 diodes in each of 2 strings in a full wave bridge. Gee... is that possible? 1N5408's are cheap.

And perhaps not all 1N5408's are created equally. The ones I used in replacement, not sure what their country of origin or manufacture was?? I did notice upon inserting them, that their lead wire diameter was less than what was present for the others. Do 1N5408's vary that much in quality and can they degrade w/ time? 

[W4DNR]

You are almost there ....

Elminated the RF deck

Elimintated the Transformer

I'd eliminate the capacitor bank next.  One could be internally breaking down.


Don W4DNR

[KA2DZT]

I wouldn't worry too much about meter readings.  There is no way to make any accurate or meaningful readings in that short a time.  No meter reacts that fast to give an accurate reading.  I would take a close look at the HV wiring and all connection points along the HV circuit.  Could be a HV wire is too close to something and it may be arcing, maybe in a harness of wires.  Also, ceramic insulators passing through the metal chassis are another common problem (someone already mentioned this).  They arc inside and you can't see that happen.

You should try disconnecting all load wires right at the HV capacitor output.  If it still blows the fuse, then there is something breaking down is the power supply.  If the 20 amp fuse also fuses other power supplies or filament xfmrs, you may have problem in one of those circuits.

Fred

[KA2DZT]

A new input from an outside source says... CHANGE ALL THE DIODES!!

Hard to believe but certainly possible after a decade or so, multiple diodes could be breaking down, despite having 10 diodes in each of 2 strings in a full wave bridge. Gee... is that possible? 1N5408's are cheap.

And perhaps not all 1N5408's are created equally. The ones I used in replacement, not sure what their country of origin or manufacture was?? I did notice upon inserting them, that their lead wire diameter was less than what was present for the others. Do 1N5408's vary that much in quality and can they degrade w/ time? 

1N5408s are pretty good diodes,  I use them, never had one fail and I've made a lot of diode strings with them.  I always use a 470K resistor and a .01ufd 1KV ceramic cap across each diode.  Many will say you don't need these added parts but I use them anyway.

Replacing all the diodes might be a last resort if you can't find anything else.  I can tell you that once one shorts or breaks down it stays shorted, they don't recover.  A good diode will read open in one direction and read around 900 ohms in the other direction.  You must use a digital meter on the 2k range to make these resistance checks.

Fred

[W4AMV]

I agree on the diodes and I meant to say 5 not 10 in each leg. It is a full wave bridge, so their are 5 series diodes in each of 4 total legs. Anyway PIV is then 5 kV, which at 2.6 kV forward, seems a little light. I would want a bit more headroom. Also, after talking to the fellow ham I am helping on this repair, he indicated we have been using 20 A fast blow, while the manual states, 20 A slow blow. Again, I would like to believe that in the ideal sense, should not matter. However.... just maybe!

One other clue, when first turned on the unit gave out a single snap or bang. The unit had been idle for week, cap bank fully discharged. Then, the second time turned on, we do NOT get a single bang, but a set of multiple bangs. The difference, the cap bank was partially charged. This would have had an effect in the magnitude of the inrush. Just thinking out loud...

[KA2DZT]

If you have 5 diodes in each of the four legs of the FWB circuit, that is 10 diodes in series.  With that high a voltage (2-3KV) supply, five diodes in series would short.

Fred

[KA2DZT]

If the caps are partially charge, that would lessen the inrush current.  The fuse should be a slow blow, quick acting fuses could blow just from the sudden inrush current.  But, this doesn't explain the banging noise you're hearing.

One thing to consider are the filter caps,  never liked strings of electrolytics, I only use HV oil caps.  Electrolytics, even though exactly the same, don't always have the same internal resistance or charging rate.  The equalizing resistors, if are too high of a resistance, will not divide the voltage equally across each cap.  Each resistor is in parallel with differing resistance of the caps, so the voltage does not divide equally.  The lower the resistance of the equalizing resistors. the better the voltage division occurs at any instance in time.

Maybe the caps are breaking down,  They probably do recover from an arc over internally.

Fred

[W4AMV]

Yes Fred, that is possible. On the PIV, the requirement for the bridge is just the magnitude of the secondary V, ~ 2 kV. The 5408 is rated at 1kV, so 5 puts us at 5 kV, about 2.5 x... I would personally like more and usually go 4 x. In any case, not sure I understand the comment on the diode short. Yes, the bang is really quite loud...i.e. when the fuse blows, why do I hear as well as FEEL vibration although slight from the chassis. I failed to mention that earlier, but my colleague ham who threw the switch, said he actually felt a slight vibration delivered to the ON/OFF rocker switch front face.

[KA2DZT]

Quick comment on the diodes, with 5 diodes in each of the four legs of the FWB, you do end up with a string of 10 diodes. You have to think about it and look at the FWB.  On each half of a cycle there are 10 diodes conducting, they are just not next to each other in the series circuit of the xfmr winding, the load, and the diodes.

The vibration is not unusual, most likely the plate xfmr under strain of a heavy overload.

On the PIV,  remember the 2KV xfmr voltage is the RMS voltage.  The peak voltage is 1.414 times that number.  The diodes are 1KV PIV (peak inverse voltage).  You always design for a big margin of safety with the PS diodes, more is better.

Fred

[DMOD]

Does this amp have a 2.5mH Choke to ground at the output of the PI net?

It could be the output coupling capacitor(s) are leaky or shorting.


Phil - AC0OB

[W4AMV]

Does this amp have a 2.5mH Choke to ground at the output of the PI net?
Hello Phil. No, this unit uses a 4:1 xmfr as the safety choke on the output. He Z matches the tubes using a PINET to 200 ohms instead of 50. The 4:1 has a double duty function. In any case, we have determined by pulling HV from the RF plate choke and pulling both tubes, that the problem is unlikely in the RF cage (area).
It could be the output coupling capacitor(s) are leaky or shorting.


Phil - AC0OB

[W4AMV]

If you have 5 diodes in each of the four legs of the FWB circuit, that is 10 diodes in series.  With that high a voltage (2-3KV) supply, five diodes in series would short.

Fred

Hi Fred, I understand your comments on the PIV, and I believe you are saying it is unlikely that the PIV is the problem, as 5 diodes would HAVE to short before an inverse breakdown would possibly occur.
So the bad diode(s) idea is unlikely.

[W4DNR]

Yes, the bang is really quite loud...i.e. when the fuse blows, why do I hear as well as FEEL vibration although slight from the chassis. I failed to mention that earlier, but my colleague ham who threw the switch, said he actually felt a slight vibration delivered to the ON/OFF rocker switch front face.

As you and your friend have observed, a High Voltage arc is really a High Voltage SHORT.

3000 volts    Lets say the arc was a 10 ohm short.   I=E/R          3000v / 10r =  300 amps

Power =  Volts x Amps    3000volts x 300amps = 900,000 watts  ( however briefly )

Enough to blow a 30amp 220vac primary fuse   ( 6,600 watts )  and make the plate transformer emit a huge
pulse of energy into the chassis  ( observed as a slight vibration ).

At this point, I'd use the lightbulbs in the primary  or change the primary 220vac  breaker
to something like a 5 or 10 amps.

Reconnect the diodes ( if you cant read a dead short on each diode with your meter, they are good )

Disconnect the capacitors from the diode bridge but leave everything else connected.

Make sure the transformer is connected to the diodes correctly  ( just saying )

and turn it on again.     if it still arcs, you have ( at least ) eliminated the capacitors ...
and one source of the energy in the arc.


Don  W4DNR 

[W4AMV]

Understood Don, thanks. Will try this approach in addition go over to slow blow fuses. And finally, need to look into the value of the resistance the AC side of the xmfr is presented.  Had not checked this and would like to think this is a reasonable size resistance. This would involve both cards, CAP and DIODE card present.

[KA2DZT]

Understood Don, thanks. Will try this approach in addition go over to slow blow fuses. And finally, need to look into the value of the resistance the AC side of the xmfr is presented.  Had not checked this and would like to think this is a reasonable size resistance. This would involve both cards, CAP and DIODE card present.

Most likely the DC resistance of the plate xfmr primary is only a few ohms.  You're almost right about the diodes.  You have a total of 10 PIV for each half cycle and you don't want any of them to be shorted.  If it's not too difficult you can recheck them to be sure none are shorted.

Don made a good point, try disconnecting the caps from the diode bridge.  This leaves just the xfmr and the diode bridge, no caps or any other load.  If you don't blow the fuse the problem is the caps or a short in the HV circuit somewhere.

Fred

[W4DNR]

A cheap Hi-potter.


At this point, to save the  $$$ transformer, it might be worth a trip to the local neon sign shop and
beg for an older ( non solid-state )  neon transformer that they have taken out of service.   

A small beer commercial window logo transformer will do,.      small transformer .... not the beer.

I'd still run it through a variac and add a couple of microwave oven diodes in series with the output.

Hook the output of the diodes to the output of the amp's diode quad.

Another way would be to lift the transformer's leads and let the diode quad rectify the neon transformer's HV AC.

Connect the neon transformer case or ground wire to your amp's negative returm or the capacitor negative wire.



The neon transformer is self-limiting  ( low amperage ) , but it should start arcing at your problem area fairly steadily.    Makes a pretty mean "Jacob's Ladder"  to impress the grandkids too.

This may give you enough time to spot or hear your failure point,
 

[W4AMV]

Tnx, Fred, Don and others for additional thoughts!

We measured the HV xmfr secondary winding R, it's about 150 ohms. The HV cards total Rin (cold...no Vin) appears to be a high Z, no real shorts here. There is a second winding on the same HV xmfr and so we will measure the screen supply bias card just to be sure its Rin is also high, no shorts. Next step, it is easy to clip out the CAP bank from the DIODE bank. The tubes are pulled, the HV is off the plate choke, will fire this up and see if fuses go. And with that will report back. Hopefully will find its either the diode card OR the cap bank card breaking down.

[W4AMV]

Problem found.

First, thanks for all the advice from the forum.

Lesson learned, when replacing the CAPS in the cap bank for HV supplies, don't stop with just the caps. Check and replace if needed the equalization R's. Obvious after the fact. HALF OF THE 5W 51k flame proof metal film ? R's were open or in the several meg ohm range. The result of this event was that two of eight replaced caps went to dead shorts. The remaining 6 of 8 have high leakage! Either a result of the open R event, or all poor caps from the start.

I suspect the HV read while the unit was in operation is possibly traceable to the meter shunt and its value change. Of course double check of the R's in the voltage divider network of the supply will be done to be sure that is not the culprit for the slightly higher HV reading.

Alan

[flintstone mop]

Problem found.

First, thanks for all the advice from the forum.

Lesson learned, when replacing the CAPS in the cap bank for HV supplies, don't stop with just the caps. Check and replace if needed the equalization R's. Obvious after the fact. HALF OF THE 5W 51k flame proof metal film ? R's were open or in the several meg ohm range. The result of this event was that two of eight replaced caps went to dead shorts. The remaining 6 of 8 have high leakage! Either a result of the open R event, or all poor caps from the start.

I suspect the HV read while the unit was in operation is possibly traceable to the meter shunt and its value change. Of course double check of the R's in the voltage divider network of the supply will be done to be sure that is not the culprit for the slightly higher HV reading.

Alan

That was unusual...I guess resistors wear out too. What is the wattage rating of the failed resistors? Too much stress?

Fred

[Jim, W5JO]

Your failure was similar to some of the Ameritron amplifiers of years back.  The equalizing/ bleeder resistors were seven watt WW @ 50 K.  I have an AL 1200 that I use regularly but not contesting.  It began to snap which indicated a HV problem so I removed the capacitor deck and checked the resistors after checking the diodes. 

One resistor was open and after removing the capacitors from the board, the one that was under the open resistor had spewed some of the internal paste. Ameritron changed the resistor supplier and, to my knowledge, have not had the problem since.

Had I continued to use the amplifier, I would have had several capacitors do the same, you did good in finding your problem and probably saved the transformer.