PL 700 II Clair Bros Rising from the Ashes

Peter S

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Jun 29, 2019
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#2
Quick update; one supply fuse was blown in the Left chan. I found two shorted output transistors, one in the B+ side, the other in the B- rail. I removed all the drivers and outputs and replaced the fuse. Powered up via variac, put a 1k load on the chan with no drivers or outputs and "TA-DAH"; signal output!
I will now try to check all the emitter resistors, I believe I can do this without having to loosen the PL 36 board (why stress the bias transistor leads at this point?) It looks like I can just measure from each emitter pin socket; to the speaker output for one bank, to the B- power supply for the other bank.
Then, I will reinstall the RCA410 drivers and see if the fuses still hold.....then maybe a couple of outputs in each bank?
If I am committing a mortal sin, please jump in before I mess it up!
Thanks, Peter in Canada
 

MarkWComer

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Victim of the record bug since age five
#3
I had a transistor failure on my 400/II a while back. From that point on I will assume that the associated emitter resistors will need replacement as well. Asymmetric clipping!
 

Peter S

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#5
A little slow in updating; you may have noticed the cover is off my scope. It's been acting up, the trace just disappears, (like no high voltage). I really think it is a bad connection somewhere, it just instantly, randomly comes and goes. Could give myself another present, but I would hate to throw this one out; old school CRT but nice 100 MHz multi channel Leader.
Back to the amp; With the drivers and one output in each bank, the Left channel produced a clean 1 kHz sine wave into an 8 ohm speaker. The other (more or less) functioning channel had a small HF ring on the (neg) swing of the signal. Each channel was only driven to about 5 V P to P. I have had this "fuzzy" oscillation before, Joe from White Oak Audio spent considerable time and patience with me to eliminate this from my 700B. Joe would not even sell me the "WOPL" upgrade until we fixed that problem. After a week of emails and experimenting, it turned out to be an under-size capacitor in the Zobel network. Incredible Customer service! I think Joe added the note regarding "all products are RTM; "read the manual" after dealing with me!
 

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Peter S

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#6
I suspect the Zobel nework again. These amps have some very fancy aluminum finned power resistors in the network and one of the resistors has an un-matching screw---something fishy has gone on there!
The adventure continues but stalled now, my scope died again.
 

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mlucitt

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#7
What are you using to drive the input signal into the amplifier and how much RMS voltage are you using? Sorry to hear about your O'scope. Leader LBO-516 is a good scope, have you tried to find service or parts for it?
 

Peter S

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#8
Hi mlucitt,
I think I would be further ahead to by a less elaborate new scope than to have this repaired, I cooled off some warm transistors mounted to the rear frame, with compressed air. The trace came back for about 5 seconds, then went out again. The scope still shows 'synced' when connected the calibration lug, so the front end still functions, even when the trace blanks. I don't see leaky electrolytics and would guess that a cap problem would not come and go?
I have a Leader Audio Generator (I like leader!) not sure what level I was driving the input, but low, WAY below clipping (sig gen switch at -20 and pot at 25%) the input level was not changed as I moved to the other channel.

PS; The scope is working again after a 45 minute cool down. I will play with it some more tomorrow, if the Boss (Jefa) lets me!
 

mlucitt

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#9
Don't mess with Jefa. I was curious what input level would generate a ringing output due to a failed or out of spec capacitor. The Phase Linear manual I have says 1.75V will give rated output but some people will drive them up to 2.0V. I would think clipping would ensue above that level with the stock circuitry. The WOPL upgrade will result in greater sensitivity or more power output for a given input, clipping is way past the threshold of pain as you will probably discover soon. What do you have for speakers?

Mark
 

Peter S

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#10
Hi Mark;
Glad you asked! I have a 3 way system electronically crossed over by a mini DSP. HF is handled by 8 planar tweeters driver by a Yamaha P2200. MF speakers are self-designed line arrays, 16 drivers per cabinet, 5 1/4" each. The drivers are very slightly over-lapped to reduce the centre to centre distance. I feel that my design has some merit because the over-lap allowed use of larger drivers that could then be crossed over at 150 Hz. The over-lap also allowed an upper limit of 2 kHz, before the line effect was lost and the drivers behaved like point sources with the associated interference patterns. The MF line arrays are currently driven by a PL 700B with a White Oak board. I hope to rebuild the PL 700 series II full-complimentary for the MF's.
My goal was to build a system that is High Fidelity, but can reproduce rock-concert levels. I built LF cabinets which are basically copies of JBL cinema subs. 1" MDF was used, as well as another minor improvement; a front protruding duct. I hesitate to suggest that I could improve on a JBL design..............however; a ducted port cabinet tends to develop a different (momentary) static pressure, (and resultant cone off-set) due to 'pumping', or the difference in resistance to air motion (in verses out). This is caused by one end of the duct terminated in free air, and other end terminated at the cabinet face. I believe that a 1 & 1/2" forward protrusion may help. I could then use a larger diameter (and subsequently longer) duct which would reduce 'whistling'.
I used RCF 18" drivers which have a maximum P to P cone excursion of 50 mm (2" !) The RCF's had very similar parameters as the JBL's used in the cinema subs, but claimed 97 db/ 1 Watt @ 1 Meter, 2 db less than the JBL's. This would prove to be expensive. The LF's sound and feel great, the lower tuning point is 19 Hz, but my old PL 700 was not able to get the maximum 'punch' out these woofers. From my limited research, it seemed that the Crest CA-18 had respectable specs---and monstrous power: 1000 Wpc into 8 ohms.....so when the wife was not looking, I bought one. Pink Floyd can now almost blow out candles at 20 feet! There may some finger wagging here, at having a class H amplifier in a so called "HI FI" system, but this amp is crossed over at 150 Hz and down. It did however, take a few months to get this amp civilized enough for home use. The S/N ratio was good, but the amp itself was noisy. It took 20 dips of the toroid transformer in electrical varnish and much lower cfm fans to be able to live with the amp in my listening room.
 

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Peter S

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#11
Here is my rack with the 2 Phase linears. I bought the old Phase from Triumph in 1978. I heard a rumor that Triumph acquired some equipment from a system used by The Who, so that PL 700 could be a collector's item! It has a 7 & 1/2" face plate rather than the standard 7". The house was designed around the living room/ listening area. The room is approx 32' x 24' with 17' ceiling. The ceiling is not treated acoustically but I believe reflection to not be a serious issue, as the line arrays have controlled vertical dispersion. There are 7 non parallel walls to limit standing waves. There are 4 separate 15 amp circuit breakers supplying power to the rack (all on the same phase of the power panel) One breaker for each power amp, and one for processing equipment My wife tells me I get 'get carried-away' sometimes......she just doesn't get it!
 

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Peter S

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#12
I will now attempt to investigate the oscillation (ringing) in the right channel....if my scope works! Did you notice the mixed output types in that channel? BRB with any new discoveries.
 

mlucitt

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#13
Your speaker building abilities are amazing. I never would have thought to stagger the mids for overlap! I built some LF boxes, although I would not call them subs. I used JBL 2226 15" drivers (I am a JBL guy from some time in Pro Sound Reinforcement). I selected a driver with lower excursion (7.6mm) so I did not have to vent the box, I made it just big enough to act as the air spring at the the resonant frequency (Fs) of 40 Hz. My passive crossovers were the JBL 3110's at 800 Hz. Geez, I could talk about speakers all day...

You have a great listening room, I wish mine was even close to those dimensions.

The PL 700 II you have will be more than adequate for the MF line array after it has been WOPL'd. I did notice the wide variety of output transistors you had. The real magic is when you have a complete set of new ON MJ21195G/96G outputs. They are just so much cleaner and more powerful than anything else out there. The PL WOPL board is particularly well-suited for those outputs.

I would like to hear more about your 3-way electronic crossover. For me (many years ago) the active crossovers colored the sound to such a degree that the sound was artificial and out of phase at different frequencies, I'm sure they are better now days.
 

Peter S

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#16
Thanks Mark;
I am not a 'golden ear' type, but I truly believe that electronically crossed over systems are "night and day" I believe damping factor goes out the window when the inductor in a passive cross-over is inserted between the woofer and amplifier. I use a laptop as my main signal source, the signal remains digital until the entire process, ( EQ and Cross-over ). The DSP has 8 D/A converters, I use 6. I also need the DSP because the steepest slope stand-alone crossovers I could find were only 24 db per octave. The DSP has (adjustable) 48 db slopes. This is a huge advantage because it keeps more LF out of the mids, allowing higher overall SPL, and reduced LF/MF and MF/HF interference patterns near the crossover frequencies.

Thanks Fishoz;
I have had a love affair with Phase Linear since my teens. I saw the series II's in magazines and was kind of disappointed when the 'fugly' original 700 showed up. I had 2 (still have one) 700B, nice to listen to and look at! I finally got a chance to get my hands on these series II's, but was put off by the dark face plates initially but have since realized that there are some real improvements to the Clair Bros versions.

Below is a shot of the listening room
 

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Peter S

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#17
I could be on to something;
First, I couldn't resist 'getting the kinks out'. Not a priority, but the bent chassis bothered me! Looks like the amp, in a rack, possibly, fell forward. I rigged up this arrangement to carefully straighten it.
 

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Peter S

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#18
After tickling the HV supply in the scope, the trace appeared and I could continue with the oscillation in the right chan. Lo and behold---a leaky electrolytic! I guess I should do what everyone says and replace all of them.
 

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mlucitt

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#19
Your bent chassis is common for a 700, there is just so much weight with that transformer, and the heat sink fins do an excellent job of creating a strongback across the other side of the chassis; it can only flex in the middle.

I would not waste my time on that board and the associated caps. Just buy the WOA Control Board and never look back.
 
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