Palomar 225 keeps blowing fuses when power leads are hooked up

[Turbo T]

I recently picked up a Palomar 225 amp that blows it's internal fuse when you connect power to it. This amp is a 12VDC mobile amp for 10 meter use.

I took my multimeter and set it to the ohms position. I checked for continuity between the fuse holder and the black ground lead coming off of the amp. My multimeter went to 0 ohms. I repeated the same thing on the red lead and the other side, and got the same thing.

So if I'm not mistaken, this has me believing there is a short somewhere in the amp. Also it makes no difference if the power switch on the amp is on or off.

However I'm not sure where I should check next....should I start checking the resistors? Or a diode? What about the transistors themselves, can they cause an issue with the power circuit?

Does anyone know of where I can obtain a schematic for this Palomar 225 amp?

Thanks in advance.

 

[mackmobile43]

I don't have a schematic in hand but there is probably a protection diode on the power input, check that and if it comes up infinite in both directions then it's probably had the power leads connected in reverse, change that diode then try reconnecting the power.

Pay attention to which direction the diode is connected to ground.

 

[Turbo T]

Thanks for your response. if I may ask, is there a certain resistance a diode should have? Or are they all supposed to be zero resistance one way and max resistance (no continuity) the other way?

Also can a diode be checked for a short while it's still soldered to the board?

And can a diode be checked for a short by probing one side of the diode with a multi meter lead while probing (in my case) the ground wire with the other lead?

 

[137]

Big diode right around were the hot lead goes to the board, no usually you can't check it in circuit because there are caps on the same line. Just cut it and see if you get power, you might get lucky and that be the case, as long as you don't hook it up backwards there is no risk of further damage by doing this.

 

[Turbo T]

Thanks. So is it always reverse polarity that causes this?

As for the diode, you say to just cut it and then hook up the amp and see if it powers up w/o blowing a fuse and arcing when i connect it to the power source?

Does the reverse polarity just short out the diode?

 

[AudioShockwav]

Does the reverse polarity just short out the diode?

Basically, yes, if you hook the wires up backwards, the diode is a direct short, this should blow the fuse ( provided you have the correct size fuse) and not roast the rest of the amp, if you cut the diode and the amp works, REPLACE IT!
It is cheap insurance.

are they all supposed to be zero resistance one way and max resistance (no continuity) the other way?

Correct they allow DC to pass one way, and stop dc the other way
Make sure you always fuse the amp, never run power to a amp with no fuse, it is asking for trouble.

73
Jeff

 

[Robb]

Thanks. So is it always reverse polarity that causes this?

Yup. This also true of CB radios too.

As for the diode, you say to just cut it and then hook up the amp and see if it powers up w/o blowing a fuse and arcing when i connect it to the power source?

If the thing powers up after cutting either end of this diode; then the blown diode was blocking current by creating a dead short and therefore blowing the fuse

Does the reverse polarity just short out the diode?

If it doesn't; then you will have fried a decent chunk of the circuitry. That is why they call it a 'protection diode' BTW.

 

[Robalo]

the transistors are the problem just remove the base and collector and tested with ohm meter warranty is the problem

 

[Robalo]

Connect the black lead of your ohmmeter to the top tab and the red lead to the bottom lead. This is your collector base junction and it should read about 600 ohms. Now connect the black lead to the ground trace (emmiter) and the red lead to the bottom tab (base). This should also read about 600 ohms. It will be common to find open or shorted junctions. This indicates a blown transistor.


copied from the texas star amp repair website

 

[Robb]

I like Robalo's answer the best.
I don't work on amps; he does. Radios have protection diodes. If a linear amp has a protection diode; I wouldn't know. But I wouldn't doubt it either.

 

[137]

I thought it would go without saying that you should replace the diode after the test, I meant there was no harm in testing it that way.

 

[AudioShockwav]

Robalo could correct, sometimes they will short internally....you most often find them open rather than shorted..

Robb, here is a picture of the reverse protection diode in a Sommerkamp PA 100.


73
Jeff

 

[unit_399]

the transistors are the problem just remove the base and collector and tested with ohm meter warranty is the problem

If this is what Texas Star says, then it's no wonder I've never been impressed by their amps. I always check the transistors LAST. Why ? Because it's not an easy thing to desolder without screwing them up, even for an experienced tech. Check everything else first. Then if you haven't located the problem, desolder the transistors and check them. Probably a bad protection diode though.

 

[Turbo T]

I thought it would go without saying that you should replace the diode after the test, I meant there was no harm in testing it that way.

I most certainly will replace the diode if i find it to be shorted. You think Rat Shack still carries them?

 

[137]

Yes they have them._