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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
Follow Ups:
A 30+ year old component cannot possibly have good ones.
I spend most days restoring classics and most of the problems I encounter are due to dried out or shorted electrolytics.
With gear that has not been powered on for a long time I go right to changing out all of the electrolytics.
Also inspect the boards for cracked solder joints. There are usually a few bad ones here and there, Test all the diodes and transistors. Remove the excess flux which can itself cause corrosion.
With restorations if you want it to work well, everything must be checked/inspected and changed out in the case of parts subject to "wear". Good luck!
which transistors burned? if it is the output transistors, try this first..... with nothing connected to the speaker terminals, 1> remove the output transistors, 2> replace the output transistors with 1k/2w resistors across the base-emitter connections, 3> turn the bias control all the way to the left (all the way down), 4> while monitoring the voltage across the 1k/2w resistor that is in the place of one of the burned transistors (it should never go above 0.6v) slowly bring the amp up on a variac. if the voltage across the resistor goes above 0.6v before the amp is brought up to full voltage, shut off the amp power immediately, since this indicates a bias problem. if you get the amp up to full voltage, and the resistor voltage is still less than 0.6v, connect an oscilloscope to the output terminals, and apply an audio signal to the amp input. you should see undistorted audio on the oscilloscope. if the signal is distorted, you have other problems.
I have a Phase Linear 700B. A shorted speaker cable took out two of the right channel's output transistors(#phase linear XPL909). NPNs. I guess I have to change out all 10 but I don't have an equivalent replacemnet transistor number. And can I just stickem'in and take my chances or do I have to do the bias procedure thing?
Many thanksI agree with your proposition
I will do thatConcerning the resistor replacing the transistors, should i use one resistor for each transistors ( 10 output transistors per chanels ) or for the whole transistor ( one for each side of the output + rail and - rail )
Should i conserve the driver.When my transistors that i've replaced have burned, i've seen that the bias was adjust in this side with minor resistor than the OK's'side.
It could be the problem and the reason of the transistors burning.I haven't got variac but i will use the solution with bulbs in paralell to reduce the amp when applying voltage.
Many thanks
Do you think that it could be the bias adjust that cause the transistors to burn, knowing that when there wasn't any load (loudspeakers) connected, the fuses don't burned
excuse my bad english.
many thanks
Loïc
first, you want the lamp in series with the amp, so if there's excessive current, the line voltage will drop across the lamp and limit the voltage going to the amp.the bias control will not always be set the same for both channels, but on the channel you are repairing, you should turn the control fully counterclockwise (to the left) before applying power, and only turn it up after you are sure that there isn't a bias problem. ther are two proper ways to adjust the bias, the simplest being to attach a voltmeter across an emitter resistor and adjust the bias for between 10 and 25 milliamps (across a 0.47 ohm resistor, this translates to between 5 and 12.5 millivolts DC). the better method is to attach the amp to an 8 ohm load and a distortion analyzer, apply enough signal at 20khz to drive the amp to one watt, and monitor the distortion analyzer residual output with an oscilloscope. you would then adjust the bias until the crossover spikes in the distortion residual signal just barely disappear. if you don't have the distortion analyzer, audio oscillator, and oscilloscope, the first method will work perfectly. just try not to let the current go over 25 milliamps
you are actually very fortunate that only two transistors went out. Phase Linear amps have a reputation of catastrophic failure when the output transistors short, which is where they get the nickname "Flame Linear".
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