Blew my 503

[sonoma]

if you blew the mosfets it would be very cheap to replace them. I think it takes 8. been too long since I saw the inside of this unit.

A buddy blew a KL203p a few years back. was using a 2950 radio turned down and he cranked the power up and forgot the amp was on and POP went the mosfets. it can happen to any one.

 

[TheRATT]

So here’s a pic of the internals!! I can’t find any obvious damage.

 

[Robb]

It should be on the underside somewhere along the centerline where all those brackets are.

 

[Handy Andy]

Well it's got Fuses...


BUT, what they look like are what I would call Slo-Blo types...

Remove the fuses and check them ohmic - should be a short - or just swap out with a known good one...

Determine the input side of the fuses to the output the side that they protect.

Easy enough when you have a schematic and DVM - just one of the fuse terminals should got to the power supply feed + - you should find one if not all three as sourced this way. Be sure to put the power switch to on.(Not connected to power of course...)

IF you don't get any conductivity with them ANY TERMINAL tested to power supply positive - check foil ground "bolthead" to negative - then determine - because the amp has prolly' blown traces/switch/parts or even relays, and can cost more to repair than it's worth.

Best to check from terminal to the "bank" they feed...you have 3 sets of fuses - each one goes to protect one area from the others. So more than likely they are in "banks" one is a main fuse, the other two (should be lesser rated) would work the banks of MOSFET as "pairs".

 

[Handy Andy]

I'm suprized guys. No one noticed that the amp drives two MOSFETS in parallel(Push-Pull) in two banks, looks like one driver so 9 total MOSFET count - 1 X (2 X 4) setup

 

[TheRATT]

OK, so nothing looked outta place so I put the cover back, on and plugged it in. Low and behold, it works!

Not sure what would have tripped it after I took it off my table but maybe it was just a self-protection circuit?

I'm pleased as punch!

 

[TheRATT]

I'm suprized guys. No one noticed that the amp drives two MOSFETS in parallel(Push-Pull) in two banks, looks like one driver so 9 total MOSFET count - 1 X (2 X 4) setup

Andy,

I'm am totally new to radio electronics. Is the 1 X (2 X4) a good set up or super cheap?

And are we looking at the 9 solders to the far right of the circuit board?

 

[Handy Andy]

I'd be taking it back out and looking at it some more...

Why?


There was some waveflow soldering going on, but some work was done by hand - and these are dual sided - if you don't meet one side to the other. Well, can you trust open holes when you know you got "bricked" by a mistake before?



Don't be so sure...go over it and reheat those joints - reflow some solder. Beef up the connections - you got a warning shot across the bow, next time it may be on target...

 

[Handy Andy]

Andy,

I'm am totally new to radio electronics. Is the 1 X (2 X4) a good set up or super cheap?

And are we looking at the 9 solders to the far right of the circuit board?

Actually it is because the items are cheap, simple and act like switches - only very very fast.

The amp is built with a clean layout - just make sure you seat it right - torque DOWN not to one side or another - the MOSFET are cheap yes, but not the fire extinguishers used to put out the burning Bunsen of a PCB board and epoxy, what is left behind is a stench that fills your nostrils and stays for a long-long time.

You'll be fine - just note the foil traces and routings - the error could happen again, and it may have been arcing that caused the fault - only banging around the amp may have jostled the PCB to loosen the spot weld that occurred underneath that overhead shot.

The "bars" are torqued too - not overboard, just firm enough to keep it in good thermal contact - it's important to spend the time aligning the board so you don't twist work that once it gets hot - the solder although may still be holding in place, is exposed to RF heating to greater degree than you may realize. So with that remember that with spot welding and RF heating going on - that board may have solder that is weak or too little to keep parts in place when that RF heating event occurs. IF that were to happen POP - open circuit and whom knows what type of mis-match and cascaded event may take place - see "fire extinguisher" event above for a "scratch 'n sniff" moment...

 

[sp5it]

Do you have RF probe? If not, replace MSFT1 for a start.
Mike

 

[Handy Andy]

The layout is;

far right - is the driver - the pairings are between the two core combiner pairings...are the output...Buss feeder is the fuses to the far left upper.

Bottom left is the RF keying relay routing - sends RF from RFX in to front panel,

and main rear left upper white box is main power relay.

If you look closer you can see the MOSFET pairings are two to each side of the core combiner - so it's a pushpull design, but uses twin MOSFET instead of "single pill" each side arrangement.

 

[743SEPA]

Just started running a 503hd behind my Stryker 955hpc. So far I'm liking it and they seem to play well with each other. This is my 1st amp and figured I'd go with the hd since I'd only be running it with exports. I really like the adjustable output,although if you read the instructions that's not really what it is.

 

[TheRATT]

Just started running a 503hd behind my Stryker 955hpc. So far I'm liking it and they seem to play well with each other. This is my 1st amp and figured I'd go with the hd since I'd only be running it with exports. I really like the adjustable output,although if you read the instructions that's not really what it is.

I run it on setting 1 only, with the input dialed down to near nothing. Just remember to not crank your RF out while you're half asleep or you'll let the blue smoke out!

 

[9C1Driver]

Just started running a 503hd behind my Stryker 955hpc. So far I'm liking it and they seem to play well with each other. This is my 1st amp and figured I'd go with the hd since I'd only be running it with exports. I really like the adjustable output,although if you read the instructions that's not really what it is.

Be careful, a Stryker 955 is good for 90-120 watts. Even dialed back some it will still push that 503 pretty hard. Many times they stamp HD on amplifiers because they know they will sell better that way.

 

[Slowmover]

if you blew the mosfets it would be very cheap to replace them. I think it takes 8. been too long since I saw the inside of this unit.

A buddy blew a KL203p a few years back. was using a 2950 radio turned down and he cranked the power up and forgot the amp was on and POP went the mosfets. it can happen to any one.

Ah, ha. “Asking for a friend . .”