trying to repair a phase linear 700B

[leloga]

Hello,
I'm French, so please escuse my bad english.

I've got a phase linear 700B that have been rebuild by a pro t( done by the old owner ).

After a bad manipulation with the "normal/direct coupled" switch, one side has broken down.
two trans are in short circuit.

I've test all the power transistors and the drivers.
The preamp board seems to be good ( good trans, and other components ).

First i've replaced the two broken transistors.

I've try the amp without charge ( speakers ).
the fuses have not burned.

Then i've tryed the amp with the speakers.

First nothing.
unplugged preamp and repluged: then just one side OK.

two transistors have burned

Many questions:

Have you an idea, why the transistors burn?

Is there a problem with the bias? ( why now? )

i've got 8 other trans and i would like to find a solution to repair this amp.

Do you have a technique?

Can i plug just two transistors ( with the drivers ), to limit the problem possibility?


Many many thanks for your help

[djk]

So which transistors are failing?

http://hometown.aol.com/phasetek2/pl20.jpg

If you only replaced two of the 30 year old transistors (out of ten per channel) you were asking for problems.

[leloga]

Hello,

Thank you for your help.


The last owner have sell me the amplifier after having done a bad manipulation (with the direct-normal switch ).
This manipulation has damaged the amp.

but before this, he has give this amplifier to a pro who has rebuild the amp with new transistors. ( MJ 15024 motorola ).

In order to repair this amp, i've buyed 10 transistors ( MJ15024 from ON semi ex motorola ).

It's the reason of my questions in the first post.


DO YOU THINK THAT IF I REPLACED ALL THE TRANSISTORS THERE ARE MORE CHANCE THE AMP WILL BE OK?

IS THERE PROBABILITY THE PREAMP BOARD OR THE BIAS COULD BE RESPONSABLE?

Many many thanks

PS:
The many questions in the first post are more for me to understand the amplifier principes...

One more time excuse me for my "bad english" and thanks you for your help.
In trying to repair this amp, it's because i love this vintage model and more because i love learn how to do.

[djk]

I posted the schematic.

I asked which transistors failed.

I still need to know.

Did the previous rebuild replace all the outputs?

What about the drivers?

Are the protection transistors still germanium (won't protect with silicon)?

Do you have the proper AGX series fuses (I see a lot of AGC fuses forced into the fuseholders)?

[leloga]

Hello,

Many thanks djk

I've already had the schematics

Yes the previous rebuild replace all the outputs

i think the drivers to but i'm not sure ( now, they are model 15020 motorola )

I don't know if the protection transistors are still germanium ( do you want a photo of the preamp board ? )

The fuses are 6*32mm 4A 250V quick blow glass fuses


Do you think that if i replace all the outputs transistors with the new ones and the
( just two of the new have been used when i first tryed to repair it ) it will be OK.
Perhaps i have to do something with the bias ( maxi resistor? )

many thanks

[Gibbi]

I also had the problem that you describe but on the original model 700. I had replaced all output transistors in a rebuild/refurb operation (used OnSemi 21196, but did not replace the drivers) and at some point after I replaced them (amp worked fine for a period) I blew a couple of output transistors. Now when I replaced the bad transistors the amp continues to attempt to fry the outputs (I put a lightbulb in series with the power in to prevent frying). Anyhow, my assumption was that the problem was on the main board somwhere, and I began to get out of my league in electronic problem solving, so I sent it out for repair. I have not received it back so I cannot tell you what the source problem is, but our problems do indeed sound about the same. I would be interested to know what the source of your amp problem is - when the tech finishes mine I will report to you what the root cause was for mine.

[Original Burnedfingers]

I learned the hard way many years ago that replacing the drivers and bias transistor needed to be done along with the outputs as a common practice. This is the minimum step that I take. To go thru an amplifier and relpace ALL the transistors/semiconductors is the way to go. In addition one should take a hard look at the caps and resistors.

Typical shop repair thinking is to replace only what is bad and hope that the amplifier will make it thru the 30 day warranty.

[anatech]

Typical shop repair thinking is to replace only what is bad and hope that the amplifier will make it thru the 30 day warranty.

Don't forget they charge 150 ~ 200 to do this. I hate those people.

-Chris

[amp-guy]

yeah IMHO burnedfingers is right replace all semis in this amp

back in the 80s I repaired a PL 700 for the rock band Foghat.
their drummer thought it was cool to do some banging on one of his microphones with his sticks , which flammed the 700b and then set the woofer in the klipsch lascala stage monitor on fire!
needless to say it was a basket case.

[poynton]

Originally posted by burnedfingers
To go thru an amplifier and relpace ALL the transistors/semiconductors is the way to go. In addition one should take a hard look at the caps and resistors.

In my PL amp, I replaced all the resistors in the output as well as the semiconductors.
The R' were cracked through stress.

Andy