Fuse blows when putting rig on standby?

[AA8A]

I have a HB rig that I built using 4-400A's mod by triode connected 813's. I have separate power supplies for both stages. Unconnected to loads both supplies work on and off without a problem. When just the rf deck is powered, it works just fine on and off very reliably. Its only when the mod deck is activated that the fuse in the rf power supply will blow when going to standby(will blow after one to three or four cycles). The whole rig seems to be working and modulating 100% but then blows the fuse as the ptt is activated to standy. The mod tranny is a 300w Thordarson multitap that I protected with the K6AD's protective circuit (ER #154, March,2002). The rf deck runs at 350-450w so I believe the 300w mod tranny is ok. I drive the mod with 6B4's and the rf deck with 6AG7/2E26.

I've looked for arching but see nothing. I'm worried that this old mod tranny is shorted out somehow so I put it on a piece of plywood so that the case is not grounded that didn't work.
Could the 813's be at fault? Any ideas would be appreciated...Thanks
Bob, AA8A

[W1RKW]

813's drawing gobs of current in stand by?
Switching transient? 
Fuse not strong enough or not the right type?
Got a schematic of the rig?

[flintstone mop]

You might want to put the P.S. choke on a piece of wood also. Just as a test to make sure it's not breaking down from this transient from tansmit to receive. Is your bleeder resistor for the power supply the proper value? I forget the formula for the number of ohms for each thousand volts of power supply voltage. Ex. (correct if wrong) 3000 volt PS and a 100,000 ohm bleeder, 200 watt resistor.
What is the amperage size for the fuse? K6AD's protective circuit defective?

Fred

[Ed-VA3ES]

I'll bet the mod transformer is dumping a transient when you go to stand-by.  Sounds like it's dumping it's  collapsing field all at once and dumping it's current into the supply.   

My Apache does that, and dumps it's field into the RF PA on stand-by - the RF PA takes 2 seconds  to stop transmitting! 

If not the mod transformer, then a choke or plate xfmr is doing that.

[WQ9E]

Bob,

Is all of this happening with a dummy load or have you had this on the air and had problems?  If you are using it with a T/R switch then you may have a problem with the transmit antenna relay releasing before the rig is in full standby.  Many of the larger AM rigs (my Desk KW for example) specify a delay on release for the antenna switch to prevent such problems.  If this is happening on a dummy load then I have no suggestions beyond what has already been posted.

Rodger WQ9E

[2ZE]

To expand on what Rodger has said (he brings up a very good point):
If the antenna relay is releasing before full HV on the modulators is dissapated, and with triode connected 813's which are running zero bias, the modulators are drawing an infinite amount of current for a split second. A step start circuit AND step stop circuit are in order. Someone makes a kit for those but I can't recall who (someone else know?), I bought 4 at Rochester a couple of years ago, and put 1 in the 833 rig and they work supa' pissa'!
Hope this helps.

Mike, ZE

[WA1GFZ]

you need to drop the DC or turn on bias before you open the antenna.

[flintstone mop]

Hi
There are some good thoughts here. Someone hit on a "soft-stop" when going to RX.
How would a sequencer work for this? To bring the TX to the TX mode and to return to stby when receiving? BUT what happens to the finals if they lose RF excitation?
I think we're making this sound complicated, when a relay may work to apply a cut-off voltage (bias) to either the RF PA or the modulator to stop the transient or runaway condition. Sounds like the RF PA is going crazy with the excitation being removed while there is still HV on the pubes.
Fred

[2ZE]

Someone hit on a "soft-stop" when going to RX.

Not a soft stop, a step stop. A certain process for turning things on and off. The antenna relay should be energized first and de-energized last, then bi-ass, then HV.

The problem he is having is the modulator, not the RF deck or loss of drive. That is a completely seperate animal.

[AA8A]

Hi,
Thanks for all the advice!

Since the rig works FB in CW. I tried turning on/off the mod deck
while the rig is transmitting. It liked that. Seems to go on and off
without blowing the fuse if I first shut off mod deck then rf deck.
I also discovered that the preamp may break into oscillation and
put a surge on the mod deck. I have to check this out but turning down
the gain seemed to work now have to see exactly where 100% mod is.

Bob, AA8A

[flintstone mop]

Time to break out the 'scope and see what is happening in there! You need something to show you how modulation is doing. Watching meters used for tuning and general monitoring of transmitter parameters are not meant to indicate modulation. It might be some big spike of audio as the rig makes it's transition from TX to STDY.

Fred

[W1ATR]

I make all my external t/r relays with a cap parallel on the coil. Just big enough to hold the relay for a half second so the plate has time to turn off. Each time, I have to experiment with the size of the electrolytic because the relay's are all different. Once it's right, it'll come on immediately, and hold the antenna on for a half second when you let go.

On a side note, I put everything on wood, the mod trans, reactor, and the damn cap.

[K1DEU]

Dear Bob, AA8A; I replied to your posting here at end of my post at http://amfone.net/Amforum/index.php?topic=11418.0

In Choke input supplies (perhaps your rf deck and or Modulator) when we remove the AC Mains its very normal to have problems with the primary surges (which can pop a primary fuse) from a collapsing choke in a choke input supply. Consider shunting the transformer primary as I describe and or placing a diode across the choke to snub the collapse. When ordering two half wave solid state rectifiers I sometimes order a third to place across the choke input filter.  73  John, K1DEU


http://www.spiritualpeaceproject.com/  and  http://hamelectronics.com/k1deu/