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Old 9th July 2009, 06:37 PM   #1
m6tt is offline m6tt  United States
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Default Phase Linear 700B blowing mains fuse

So I found a Phase Linear 700b amp from the seventies recently. All of the rail fuses are in tact through visual examination, hopefully meaning the power transistors are OK. The only blown fuse is the mains fuse, and the power cord has been cut. I've worked with more tube equipment than big solid state power amps...before I just pop a fuse in it and turn it on, does anyone have any experience with these? I figure I need to check continuity on the transformers and caps, but beyond that I'm somewhat lost. Thanks
-matt
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Old 9th July 2009, 07:39 PM   #2
llwhtt is offline llwhtt  United States
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Hey Matt,

First locate a schematic and download it, make sure you get the correct version, there are at least two or three. Check the bridge rectifier and filter capacitors for shorted condition. The two good fuses you found protect the output stages only. If something is bad on the circuit board I can't imagine it would blow the line fuse without blowing itself up first. There's mucho info on these amps online if you look. I think I still the service manuals for one of the 700s if you can't locate one. Let me know.

Craig
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Old 9th July 2009, 07:46 PM   #3
m6tt is offline m6tt  United States
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So the bad news is someone put 15A fuses in the power supply 5A rail positions. Yuck! I can't stand when people do that. There's at least one carbon comp resistor that is split open attached to one of the power transistors. I think I may have some blown transistors on my hands. I think the power supply is most likely OK, at least the caps seem to be good everywhere.
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Old 9th July 2009, 07:50 PM   #4
jaycee is offline jaycee  United Kingdom
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Put a 60W light bulb in series with the Live wire. Replace the fuse and power on. If the bulb lights brightly and remains lit, there is a problem.

The power supply capacitors will certainly need replacing at this age. If I remember right they also use a speaker output capacitor - that will also need replacing. To do the job right, you should replace ALL electrolytic type capacitors in the unit.
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Old 10th July 2009, 05:24 AM   #5
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Link to schematic:
http://www.audio-circuit.dk/Schemati...0B-pwr-sch.pdf
Hopefully the right version
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Free Schematic and Service Manual downloads www.audio-circuit.dk, Company: www.dupont-audio.com, Joint venture: www.DupontMantra.com
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Old 10th July 2009, 06:04 PM   #6
m6tt is offline m6tt  United States
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So with the rail fuses removed, and the mains fuse still in place, she lights up! 98v across the f!#%ing giant capacitors. VU meter for the left channel seems to be experiencing spurious noise, although the inputs are not currently grounded...oscillation?

Somehow I don't think I'm checking the transistors right. Do they need to be disconnected from the circuit first? The readings so far don't look good, i.e. not one infinity on my meter in any direction.
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Old 10th July 2009, 07:04 PM   #7
jaycee is offline jaycee  United Kingdom
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Yes it is best to remove them from circuit to test properly
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Old 10th July 2009, 07:22 PM   #8
Netlist is offline Netlist  Belgium
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Quote:
Originally posted by m6tt
There's at least one carbon comp resistor that is split open attached to one of the power transistors.
Check all outputs out of circuit. There are easy enough to remove but make sure to do a continuity check when you put them back in place. I'm pretty sure some will be broken. Check the drivers and ALL other semiconductors if you happen to find broken outputs.

/Hugo
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Old 10th July 2009, 11:14 PM   #9
m6tt is offline m6tt  United States
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B+ fuse stays solid on R channel, B- fuse blows. Left channel looks terrible, they're at least two shattered carbon comps I can see. I see flame linear hit the asymptote. If I can scrounge enough working transistors from each channel to redo one channel, is that wise?

I am pricing out MJ15024 and 2SD555 etc. It sounds like the MJ15024s are the way to go. I don't mind mismatched channels, since I'll probably be using it for some mismatched speaker stacks as a PA. I plan on changing most of the electrolytics and any faulty preamp transistors.
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Old 10th July 2009, 11:21 PM   #10
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don't forget to check and replace any bad emitter/collector resistors.
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