Precision Power 2150AM Repair

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#24
You can't have any FETs installed without a gate resistor. If the gate resistors are out, you, at the very least need to install something across the gate and source to keep the FETs from conducting. It can be a wire, one of the old gate resistors or virtually anything else. If it's easier to remove the FETs, you can do that as well. The FETs are easy/quick to remove if done like this (sorry about the quality, 20 years old):
https://www.bcae1.com/temp/removingpartsfromolderorionamplifiers01.avi
 
#25
Ok, with only 2 Fets and 2 100 ohm resistors in it seems to be taking Q46 out and blowing a 10A fuse.

Drain to source read .2 ohms on the removed FETs
 

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#28
Have you checked the FETs, out of the circuit to see if any are defective?

Recheck the output transistors to confirm that none are shorted.

Do you have anything (like a large 2 ohm resistor or a halogen lamp) that you could use for current limiting. All of these blown fuses are getting expensive.
 
#29
Yes the FETs have been blowing and testing bad when the fuse blows but only putting 2 known good ones back in at a time, all the output transistors read the same, most 47 ohms pins 1 to 3 few others in the kohms.

Bout out of good fets though, have some others coming monday MTP25NO6. Yes i could get a big 2 ohm resistor and a scope if that would help as well.

Thanks
 
#30
Don't buy a garbage scope that won't do the job. Can you borrow one? Bear in mind that I don't know your budget. I don't know if you're going to have to miss meals due to buy parts of this is absolutely nothing to you.

If you buy one, I'd recommend a 100w, 2 ohm version of the attached type. I'm not 100% sure that these will power up with a limiter but it would save fuses and can be used a limiter for other amps.

Are you going to have to order more parts or are the ones you ordered expected to be the last?

If you're going to have to make any more parts orders, I'd suggest buying the IRF3205 and some 47 ohm gate resistors.
 

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#31
Sounds good, i can get a scope from work, we have good ones there. Parts not really an issue, would reqlly like to get this old school stuff working for my son vs. the new stuff.... i did see that a few output transistors are less then 3ohms between pins 2 and 3. Seems low?

Shud maybe just pull em all and work on getting the supply up and running

Thanks
 
#33
I removed Q12 wich was shorted pins 2 to 3.. now have power light and things are looking good with the 2 FETS and the 100 ohms.

1 fet (Q55) still seems to be getting quite warm after 60 secs or so

Where are the rectifiers?

What do you think i should do for next steps?



Thanks for the help
 
#34
Now you have to order output transistors. You need to order enough to replace all in the channel that had one fail. You need (at the very least) matching date codes so order extras. The same goes for the PS FETs.

60 seconds is an eternity. Some power supplies will idle forever (exaggeration) without getting hot and some will begin to heat immediately (even if there are no problems.

Clamp the transistors you have down and check for audio on both channels.

Do you have more of these that you need to repair or is the only one?
 
#35
Just this one.. my A300 is good to go!

So i looks like there are 10, 2N6487s.

Shud i replace all or just the right 5?

Seems that one FET is getting a lot hotter than the other one, but may have been damaged as well.

Sounds like i shud get the IRF3205s. Ill get these ordered and see what happens. Thanks again

Update: audio on both channels, good news!!!
 
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#36
There are 5 2N648x in each channel and 5 2N649x. The 87/90 will work but if they're not available, the 6488 and the 6491s will work. I'd replace all in the channel that had one fail. Since yu have to replace all of the ones we know about, I'd likely recommend replacing the outputs in both channels. That's up to you.

Where are you ordering parts from (NOT eBay or Amazon. PLEASE)?
 
#38
The ones in parallel with the blown output absolutely need to be replaced. The next minimum (not quite as critical) would be to replace the others in that channel. Replacing the ones in the other channel isn't critical but they're 30(?) years old. I wouldn't recommend replacing any of the outputs if none failed but it's not entirely useless to do so.

I'd use the 3205s. A purist or collector could be against it, thinking that the originals worked perfectly for all of these years and would prefer to have it back to close to original. I'd use the 3205s.

DigiKey is perfectly fine, as Mouser.
 
#39
Ok, so im gonna just replace all output transistors. See what i can find on digikey for the parts you mentioned, also go with the the 3205s.. u said these need or shud have 47 ohm gate resistors? That all sound correct?

Thanks for the help!!
 
#40
The higher gate capacitance of the 3205 (more than 3000pF+) means that it takes more current to properly control the gate voltage. The 47 ohm is typically a good compromise for the 3205 drive. Lower resistance puts more strain on the drive circuit. Higher resistance will not get them turned off quickly enough to prevent cross-conduction (shoot-through) which can make them run hot at higher duty cycles.
 
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