Phase Linear Model 300 Series Two overheating on one side at low volumes

Thought so. So my plan of attack will be to jumper the C to E from the traces on the reverse side so that I can steer well clear of the power transistor lugs.
 
jtatknox said:
Thought so. So my plan of attack will be to jumper the C to E from the traces on the reverse side so that I can steer well clear of the power transistor lugs.
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Whatever works for you.

  1. Make sure the amp is fully discharged
  2. Bring it up on your DBT
  3. Most important: Don't blow anything up.
 
Jumpering C to E yields a bias voltage of 0.145V, there is no longer any humming, and the DBT extinguishes after a few moments as expected, rather than staying dimly lit during operation.

Does this suggest that the bias transistor is faulty, i.e. not switching properly with correct control input, or that the transistor is fine but is getting inappropriate control input? I'm certain that I can safely replace the bias transistor, but I don't know enough about bias circuits to diagnose beyond that.

BTW, please let me know if there are membership dues or ways to support this forum and community. I don't know anybody else that shares my interests and I'd be lost w/o this group.
 
jtatknox said:
Jumpering C to E yields a bias voltage of 0.145V, there is no longer any humming, and the DBT extinguishes after a few moments as expected, rather than staying dimly lit during operation.

Does this suggest that the bias transistor is faulty, i.e. not switching properly with correct control input, or that the transistor is fine but is getting inappropriate control input? I'm certain that I can safely replace the bias transistor, but I don't know enough about bias circuits to diagnose beyond that.

BTW, please let me know if there are membership dues or ways to support this forum and community. I don't know anybody else that shares my interests and I'd be lost w/o this group.
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That is excellent progress...

It MAY be the bias transistor. Have you used your DMM to determine if it looks like a transistor to your DMM? Use the diode scale to test it out.

With that low bias setting, the heat be gone as well. I suspect your humming was the output stage causing the overcurrent protection to kick in. That can make an oscillating sound.

You know you are in the right neighborhood of the circuit anyway. Anyone have the service manual for the 300?
 
I understand from your documentation in an earlier post that this is an NPN transistor, so with the amp discharged and unplugged, I set my DMM to diode test mode, placed the positive lead on B and then touched the negative lead to C and E.

C: .761, no beep
E: .179, beep

On the good channel, the same procedure yields:

C: .588, beep
E: .588, beep

I'm not sure if this totally diagnoses it as the transistor since it is still in the circuit and I don't know what effects that has, but it's obviously way different. Very handy to have a working example right there – glad we have two ears! I didn't know I could test a transistor this way – thanks for the learning experience.

As an aside, the other 300 example I have turned out to have a minor misbias problem that was making it run a little hot on one channel and with the knowledge I gained here I was able to correct that easily.
 
The only way to know for sure is an out of circuit test. And do the reverse conduction test too. Negative lead on base and positive on c and e...
These tests will show an open or short. They won't tell you if it has no gain left.

This forum started as a place that would not be monetized with those f*&/$% ads. It is totally supported by me. If and when it gets too much for me to handle I will certainly put out a call for help. Costs have gone up quite a bit in the past few years as our storage requirements have grown. I think we're up to 1300.00 a year now..

And let us not forget co-owner jbeckva. Who does all the behind the scenes IT stuff that keeps us up and running ..thanks Jer!!
 
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Ads are the worst.

Reverse conduction test on bad channel is identical to forward conduction test.

Reverse conduction test on good channel is very different. Meter reads open leads between B and E.

Sounds consistent w/ a short in the bad channel bias transistor?
 
Your 0.761v B-C is a dead giveaway that you have a wounded Bias transistor
 
Now we are having fun!

It looks like GE 2N3414 are not that easy to find but NTE289A is listed an equivalent part in a few cross-reference databases. I'm thinking I will order a handful of those and replace both bias transistors and replace filter caps w/ Cornell CGS153U075V5L.
 
NO NO NO!!! No NTE!!
The bias transistor should be a 2N3403...you can sub a 2n5088 but you will have to knock out the 3403 from the p strap and insert the 5088. Pin out is different so download the data sheet...
 
Neat! I'm running music right now through PL300S2 #2 for the moment because the way I jumped the bias transistor was stable but temporary (I didn't trust putting the cover back on).

I will report back next week on adventures in transistor replacement. I was lucky to find the parts I needed one state over in NJ and they have already shipped.

Enjoy the music!
 
Lots of electronics bolt holes in your neck of the woods JT...
 
laatsch55 said:
The only way to know for sure is an out of circuit test. And do the reverse conduction test too. Negative lead on base and positive on c and e...
These tests will show an open or short. They won't tell you if it has no gain left.

This forum started as a place that would not be monetized with those f*&/$% ads. It is totally supported by me. If and when it gets too much for me to handle I will certainly put out a call for help. Costs have gone up quite a bit in the past few years as our storage requirements have grown. I think we're up to 1300.00 a year now..

And let us not forget co-owner jbeckva. Who does all the behind the scenes IT stuff that keeps us up and running ..thanks Jer!!
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Lee, I had no idea. First thank you both and second the formula that has evolved here is the best on the net, feels like real community and the way people help and share with each other is rare and precious. Please consider allowing members to donate to the cause before it becomes a burden? It can be anonymous and should be without rights or privileges to those who donate. I have zero doubt that a great many folks on here would be honored to participate.
 
Well, there have been a couple folks make donations. I will PM my pay-pal address to anyone that wants to donate to the cause. I feel VERY STRONGLY about not wanting to monetize this site...
 
laatsch55 said:
Well, there have been a couple folks make donations. I will PM my pay-pal address to anyone that wants to donate to the cause. I feel VERY STRONGLY about not wanting to monetize this site...
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Absolutely, advertising kills whatever it touches. This is your baby and should remain that way. “ No rights or privileges “ to those that choose to donate!!!
 
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