Phase Linear 700B blowing mains fuse

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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
 
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
 
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.
 
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.
 
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.
 
m6tt said:
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
 
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.
 
To keep from using the dreaded NTE parts I think the Series II versions used silicon instead of the 2N1304/5 germaniums. I'm trying to track the Series II schematic now, it's been awhile. I do remember the main difference is the Series I front end is discrete where as the Series II was an opamp. Here's some bad news, I had several bins full germaniums, just gave them all away a few months ago, I know there were some NOS 2N1305s in there. Had them for years never needed any.

Craig
 
m6tt said:
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.

Hmm...either you fix it good (needs a skilled tech) or you'll end up fixing it constantly, certainly if you are going to use it in a PA environment. This is not the kind of amp to tweak easily or give a quick repair with parts lying around. Avoid NTE at all cost.
 
"Avoid NTE at all cost"

Considering the relative un-availability of the germanium transistors used in the limiter circuit, what are you going to do?

In general, anything (within reason) with the correct Be voltage drop at about 10mA (0.35V max) will do. It does not bother me to use NTE102 and NTE103 parts for this application.

http://www.nteinc.com/specs/100to199/pdf/nte102.pdf

There are some guys selling 2N1304/05 on eBay for about $13.50 a pair, but I can get the NTE locally for half that price.
 
I finally got around to working on this thing again. There were a few blown outputs, I scavenged from the left channel and matched for beta (diode drops to base closet matches) and installed them. I had amplification, 20mv offset, everything. I had rewired the meter range control and power switch to heavy duty toggles, and I turned it on and listened. Then I tried to put it back together, while on (idiot!), and forgot the preamp board wasn't bolted down. KABLOOEY!

I wiped out the input pair, the second pair, and the RCA40412V1 in the fancy heatsink-boat. Now for the noob questions. I have a pile of 2sc1815, can I use these for the input pair? They look to have the Vceo and gain I'd need, I think. I have two BF421, matched at hfe 120...would these work ok for the second pair? I'm kind of lost on a substitution for the RCA, the only data I can find is NPN 250v 1w min 10 hfe. If 1W, why the largish heatsink? I have some 2sc2625 and a video output NPN with similar ratings, but higher Ft...can I use these but put say 100pf between emitter and base?

Sorry for the noob questions, but after hearing this thing start to sing I am terrified I'll never hear it again! I have a bunch of circuit boards from old monitors & switching supplies etc. with likely subs, so if you guys know something common I can probably dig it up. Thanks!
 
I understand that the germanium protection circuit is the strange part of an already strange beast, those transistors are surprisingly OK, as is the 2N3403 phase splitter & the quasi-comp predriver arrangement. As per the service manual it D9 & D10 have been lifted currently, removing it from the amp in any case. The input pairs and class-A driver seem to be parts of this amp that I could conceivably repair with parts lying around, provided q was limited. Thanks for any help, I'd rather learn, fix and improve rather than just solder in replacements.
 
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