Burning amp (unintentionally!)

[Buhl]

Hi Everybody - once again I could use a bit of help. I have finally managed to solder a beast together that did not work at first flick of the powerswitch - guess everybody runs out of luck sometime

This is a P1.7 preamp, boards by KK-pcb. Powersupply alone works fine, and measures close to the specified 60 volts. Connected it this evening to the main board, and measured 0 volt on the output of the psu - after a few seconds the regulator FET on the PSU blew up (litteraly - there's hole in the plastic where it used to say IRF610!) in a mushroom cloud of nasty smellin' smoke.....damn!

Of course, I have checked the main board for solder blobs in the wrong places (lead free solder is a pest to deal with I think), capacitor polarity, short circuits between FET legs and FET heatsink - everything looks fine.

Can I measure the resistance of a preamps "power-inlet" (my physics says "NO!" - but I gotta ask) - any other idea of finding out what's wrong before I run out of IRF610 FETs?

Kind regards
Hans

[Mooly]

Run it up with a low wattage bulb in the primary side to stop excess current.
If the PSU output was zero and it blew then it sounds like a dead short that it's working into, particularly if nothing it was supplying blew/smoked etc

[Buhl]

Thought of something like that - will something like a 15W 220 volt bulb do the trick?

Kind regards
Hans

[Mooly]

Should be OK for faultfinding.

Measure on ohms the DC input to the preamp PCB... make sure it's not reading short.

Is this the amp
PASS LABS CLONE - ALEPH P 1.7 DIY Preamplifier PCB layout

[Buhl]

Hi Mooly
Yes it is - I have low ohm resistors right after the dc input, so I have a few ohms on the ohmmeter - if I recall I can not measure resistance in order to check if a FET is blown?

Kind regards
Hans

[Mooly]

I would run it up with the bulb... check for overheating of the reg all the time.
Looking at the circuit there is too much resistance in the preamp circuit to blow the FET like that unless those 1000uf caps were in wrong way round or faulty.

[Mooly]

How low ohms are you reading ?
You can't easily test FET's on a meter unless you know what to expect. They can read "short" or low ohms due to charge on the gate turning them on.

[Buhl]

I've got 2.2 ohm on the dc input (two channels in parallel) - each of the four big 1000uF caps measure 5.4 ohms across. I only have one PSU though, I know the original uses two, but one should do - the circuit draws about 25 watt total, incl. the loss in the regulator.

Kind regards
Hans

[Magura]

Could you post a pic of your setup?


Magura


EDIT: First thing that comes to mind, when blowing the reg, would be inverted polarity of either a cap or inverted polarity of the PSU>amp connection.

[Buhl]

Will try and get a picture online later tonight or tomorrow - double, triple and quadruple checked power caps polarity, and circuit polarity. Working on getting a bulb socket up and running, then going to test the two sides of the circuit once at a time.

Kind regards
Hans