Phase Linear 700 series II restoration

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Phase Linear 700 II excessive offset - cont.

OK, here is the current situation, not that easy: I've put left channel rail fuses. Set main PS at the stepped level of 10-20-30V and measured DC at outputs. Offset voltage was almost equal to PS level. Across both zeners very low readings (1.4-1.8V). Didn't keep the circuits up too long to avoid a new damage. For the same reason didn't want to increase PS voltage - anyway going from 10V to 30V the voltage on zeners was marginally higher (unlike it is the case without rail fuses on). When I took out the fuses, across both zeners the voltage was good = 4.4V-10.2V for the same main PS levels (10-20-30V).

All drivers and outputs were installed. I've checked them with ohmmeter - in line with the service manual - they seem all be good (not shorted).

Can that be one of the faulty drivers, that PL36 is trying to drive and therefore zener's network shows overloading?

Again, please share your views and suggestions. Thank you!
 

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Here is the recent news from my on-going repairs of PL700II (fully comp. model):
  • rail fuses for the left channel in, for the right channel out
  • drivers and outputs NPN/PNP of the left channel (the one with excessive DC on the output) - all temporarily taken out
  • repeated tests to check zener network voltage (at 10-20-30V measured on PS bug caps) - success
  • checked DC at the left output binds - ca. 10mV, seems OK
Then installed ONLY the drivers: MJ21196/MJ21195 (from the pool of taken out transistors, but measured and not shorted). Results:
  • test to check zener network voltage at 10VDC - success
  • checked DC at the left output binds - 8,5V -> bad news.
Unsoldered one lead per D101 and D105 to disable the protection circuit temporarily. Drivers still in the circuit. Results:
  • test to check zener network voltage at 10VDC - success
  • checked DC at the left output binds - 8,5V -> same bad news.
Conclusions?
  1. the protection circuit didn't cause this problem
  2. pre-drivers seem to work properly (only with rail fuses in they are biased)
  3. after adding NPN/PNP drivers and testing, the problem of excessive DC on the left speaker's output remains...
I am looking to the schematic and more getting hypnotized than figuring out what is happening here...

Quick actions on my side: replace the drivers with factory new ones and do carefully (once more) inspection of the TO-3 sockets wiring.

Still I believe there must be another reason.

Dear Phase Linear Experts - please support!
 
Here is the recent news from my on-going repairs of PL700II (fully comp. model):
  • rail fuses for the left channel in, for the right channel out
  • drivers and outputs NPN/PNP of the left channel (the one with excessive DC on the output) - all temporarily taken out
  • repeated tests to check zener network voltage (at 10-20-30V measured on PS bug caps) - success
  • checked DC at the left output binds - ca. 10mV, seems OK
Then installed ONLY the drivers: MJ21196/MJ21195 (from the pool of taken out transistors, but measured and not shorted). Results:
  • test to check zener network voltage at 10VDC - success
  • checked DC at the left output binds - 8,5V -> bad news.
Unsoldered one lead per D101 and D105 to disable the protection circuit temporarily. Drivers still in the circuit. Results:
  • test to check zener network voltage at 10VDC - success
  • checked DC at the left output binds - 8,5V -> same bad news.
Conclusions?
  1. the protection circuit didn't cause this problem
  2. pre-drivers seem to work properly (only with rail fuses in they are biased)
  3. after adding NPN/PNP drivers and testing, the problem of excessive DC on the left speaker's output remains...
I am looking to the schematic and more getting hypnotized than figuring out what is happening here...

Quick actions on my side: replace the drivers with factory new ones and do carefully (once more) inspection of the TO-3 sockets wiring.

Still I believe there must be another reason.

Dear Phase Linear Experts - please support!
First check / replace C111 / 211 (100uF electrolytic)

Let's check some voltage points...
Left output fuses installed, but no driver or output transistors. This will power the pre-drivers (40327 and MM4003). No output load or input signal.
What is the voltage between the base and emitter on Q101 and Q105?
What is the voltage across the 180-Ohm resistor (R127 / R140)?
 
1. First check / replace C111 / 211 (100uF electrolytic)
2. Let's check some voltage points...
Left output fuses installed, but no driver or output transistors. This will power the pre-drivers (40327 and MM4003). No output load or input signal.
What is the voltage between the base and emitter on Q101 and Q105?
What is the voltage across the 180-Ohm resistor (R127 / R140)?

Hi THD,

Good to have you around ;)...

Ad.1 - clear, I'll replace the both caps to be sure.
Ad.2 - a question first: at what PS voltage level should I check Q101, Q105 and R127, R140? I think Op-Amps get biased with 40-50V measured on PS caps. Correct? Please advise.

Thank you!
 
Hi THD,

Good to have you around ;)...

Ad.1 - clear, I'll replace the both caps to be sure.
Ad.2 - a question first: at what PS voltage level should I check Q101, Q105 and R127, R140? I think Op-Amps get biased with 40-50V measured on PS caps. Correct? Please advise.

Thank you!
When troublesooting the driver board, you have to use full power supply voltage or close to it. +/- 80VDC should be good (this is what the model 400 uses). Then, when the amp is finally working correctly, you can hit it with the full +/- 100VDC.
 
When troublesooting the driver board, you have to use full power supply voltage or close to it. +/- 80VDC should be good (this is what the model 400 uses). Then, when the amp is finally working correctly, you can hit it with the full +/- 100VDC.

This and other asymmetrical op amp driven power amps will not "center" correctly at low Vcc. If you're still scared to bring it up to full voltage there will be problems. If it takes full Vcc with no rail fuses and passes a signal into a 1k load you're half way home and the amp probably works.

One thing you can do is to install the drivers - but not outputs - and temporarily put 100 ohm resistors across the fuse holders. The drivers cannot be damaged even in the event of a fault (but the resistors could fry). Adjust the bias for around 350mV across them and you're safe to install outputs.

Of couse, it a whole lot easier to just power the darn thing up through a light bulb limiter.
 
1. First check / replace C111 / 211 (100uF electrolytic)
2. Let's check some voltage points...
Left output fuses installed, but no driver or output transistors. This will power the pre-drivers (40327 and MM4003). No output load or input signal.
What is the voltage between the base and emitter on Q101 and Q105?
What is the voltage across the 180-Ohm resistor (R127 / R140)?

Hello THD,
I did the measurements. Testing environment first:

  • C111, C211 replaced
  • The amp was plugged through light bulb limiter only.
  • Left channel rail fuses installed, the right fuses out.
  • No drivers or outputs.
  • No input signal, no output load.
  • PS voltage on main caps was about +/- 100VDC.
Here are the results:
  • Pre-drivers (40327 and MM4003) B-E voltage, left channel: 0,58-0,59V. The right channel: 0,68V for the both transistors.
  • R139 180-ohm (between B-E at Q107 driver): 0,59V
  • R140 180-ohm (between B-E at Q108 driver): 0,60V
  • R239 same location and for the right channel: 0,51V
  • R240 same location and for the right channel: 0,49V
  • R126 180-ohm: 0,33V
  • R114 180-ohm: 0,31V
  • R226 same location and for the right channel: 0,32V
  • R214 same location and for the right channel: 0,32V
And DC on the left output was only 3,2mV...

What you think are the conclusions?
 
Last edited:
This and other asymmetrical op amp driven power amps will not "center" correctly at low Vcc. If you're still scared to bring it up to full voltage there will be problems. If it takes full Vcc with no rail fuses and passes a signal into a 1k load you're half way home and the amp probably works.

One thing you can do is to install the drivers - but not outputs - and temporarily put 100 ohm resistors across the fuse holders. The drivers cannot be damaged even in the event of a fault (but the resistors could fry). Adjust the bias for around 350mV across them and you're safe to install outputs.

Of couse, it a whole lot easier to just power the darn thing up through a light bulb limiter.

Hi Wg-ski, thanks a lot for responding!

I will contuninue the suggested check-ups with drivers in as the next step, drawing/sharing conclusions from THD's advice to perform some more measurements mentioned in the previous post. One point from your recommendation to hit with 100VDC PS voltage is already bringing some promissing news - see my recent reply on testing outcomes.

Looking forward your further comments!
 
Phase Linear 700 series II amps. series
Power caps are absolutely worth of replacing. They also strongly influence the sound when got dried over the years. Sometimes they may well become a source of hum. With the new ones go for twice the capacity than original - ca. 20.000uF @100V at minimum - you can find per 160V at small price difference = better (The originals were 9800uf @100V and they run right around 100V. With caps you buy today you don't get the headroom like you did 20 years ago...). The power supply caps testing: check if they have less than a few hundred mV of ripple at idle, at full load it will be slightly over 1V. Don’t buy power caps older than 3-5 years of shelf storage, if you don’t have time for reformatting them. Watch the cap size to avoid additional mounting problems. Before you begin, remember to discharge the power caps (a resistor 10-25 Ohm/10W would help)!
If you increase the power cap capacitance from original value of 9.800uF to say 20.000uF, you need to take into account the higher stress on your bridge rectifier. Some P/L owners decided therefore to upgrade it for >35A/400V units. In addition bypassing each diode by 0.01uF/1000V cap (ceramic or mylar HQ) will help to cure against RF unwanted interferences. BTW, you need to check the presence of C112=0.22uF/400V as no longer needed. Bypassing filter caps with two mylar caps 0.1-1uF/600V can be considered if you go for larger power capacitors. It will improve the ESR at higher frequencies. For 20.000uF cap, with low ESR, ‘computer high grade’ type, etc., etc. - you may not bother, probably.
I welcome all additional remarks and corrections - nobody is perfect ;-)...
Thanks for the very useful information
I'm New here, but have read the informaion in the past
I just installed 4-caps in the PS for a 400 II 8200uf * 2 = 16,400uf & seemed to work so far & the voltage in way less than 100vdc

Promblem #1 - Voltage to High?
so I did the same in the in the 700II & the voltage was way to high at about 105vdc
I hooked up a variable-tack & about 90vac is the max I want to go so can Computer grade Capitors rated at 100vdc run at 110vdc? since these are snap in cap at 100vdc (Nippon Chemi-con SMH- 8200uf) I might test w another four in their paralle, serial circuit 60vdc per bank

Promblem #2 DC or something funny in the ac sine way output , is this the ESR you are talking about? & can you explain it better on how to fix it, using the term "install a , Across the XXX " anyway thanks to anybody help me fix this problem
its only did it in one channel & the output transistors were gettting hot
I thought I blow something so I put the original PS Capitors back in & everything seemed to work ok
now what? I need some help?
I would like to fix it before determing what to do next or investing in new cap or what value to uses ? 10,000 or 16,400 or 20,000 uf
 
OK, here is the current situation, not that easy: I've put left channel rail fuses. Set main PS at the stepped level of 10-20-30V and measured DC at outputs. Offset voltage was almost equal to PS level. Across both zeners very low readings (1.4-1.8V). Didn't keep the circuits up too long to avoid a new damage. For the same reason didn't want to increase PS voltage - anyway going from 10V to 30V the voltage on zeners was marginally higher (unlike it is the case without rail fuses on). When I took out the fuses, across both zeners the voltage was good = 4.4V-10.2V for the same main PS levels (10-20-30V).

All drivers and outputs were installed. I've checked them with ohmmeter - in line with the service manual - they seem all be good (not shorted).

Can that be one of the faulty drivers, that PL36 is trying to drive and therefore zener's network shows overloading?

Again, please share your views and suggestions. Thank you!
Another troublshooting tip & to try
I found that the "POT" located on the Main board to be easily over looked when troubleshooting & to be the source of the Bia's problem
It also can cause the Amp to run at at Higher than normal Temperatures, which my amp did for years until I finally fixed it! (computer fans are the best & Cheap)

I have Replaced Many of the pots now & have add them to the list of Parts. Another temporally solution would be to clean them w "Deoxit" which I never tried, I just replace them now

I did take one apart & it was in pretty Bad shape!

if you thing about it the pots are really old & one good thing about the Phace Linear amps is that they do not have very many pots & switchs wow the older Pioneer SX have all kinds of them & can be a source of all kinds of problems my cheaper Sx-550 sounds great cuz it has less of them! LOL
I hope this help somebody just another thing to add to the list after replacing the Bias Transistors
 
Phase Linear 700 series II restoration - summary

If you are looking for more advice regarding the PL700 II restoration, please have a look there:

- Any Phase Linear Experts out there? - Page 17 - AudioKarma.org Home Audio Stereo Discussion Forums -> lessons learned from my repairs discussed here earlier on;

- Phase Linear 700 II recapping and more - AudioKarma.org Home Audio Stereo Discussion Forums -> specifically you may read "PL700II quick repair advice.pdf" file and subsequent remarks in the thread.

Good luck!
 
Hi Folks;
Christmas came early! I just had to snap up these (future) beauties! I have been a Phase Linear masochist/fanatic since my teens. Neither amp is functional. I hope to create at least one "WOPL". One amp was plugged in and the first led on each channel lit. The other amp is missing the main PS caps, and some outputs, that one did not even get plugged in.
I will re-read this thread and the link to the other forum.
Regards, Peter in Canada
 

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Before sharing the progress and results of my current Phase Linear renovation project let me post here a wrap-up of many valuable ‘lessons learned’ gathered from various DYI and Audiokarma threads for those who wish and can restore their Phase Linear 700 series II amps. Originally I posted such list on Audiokarma.org, but here is reedited version with some new thoughts embedded . These are all key points written in one place for possible further use, thanks to very useful contribution of many experts here and Phase Linear enthusiasts over the last years!

So, the main purpose of the list below is to help in planning and performing the restoration of 30+ years old Phase Linear 700 Series II amplifier (P/L700II), and certainly not to re-engineer the gear. Replacing a vast number of components, that were working well so far and are not specifically exposed to aging, is not recommended. No doubt however that a temptation to implement some advisable upgrades is in everyone of us. After all you decide at your own risk …

GENERAL RULE: Use extreme caution while performing any replacements/upgrades due hazardous voltage presence in P/L700II power amplifier. Be sure that before starting any replacements the line cord is unplugged and power supply caps are discharged. Strictly follow ALL check-up and repair procedures as written in P/L700II Service Manual and subsequent service bulletins. Wear safety glasses while taking data from powered circuits and soldering.

Driver board PL36

  1. P/L700II driver board (PL36) requires some closer attention. There are components that over last decades could loose resistance /capacitance desired characteristics or get open, even impacting other elements, not to mention about shorted diodes or signal transistors. Testing 'in circuit' as stated in the service manual is mandatory the very first step. I know many people who anyway replace all diodes and transistors from the driver board 'en block' - mainly if restoring unknown gear with visible badly performed repairs in the past. Resistor candidates to consider for replacement, as they may drift from heat and age, are the following:
  2. R1, R2 = 7.5K/5W 5% (these dropper resistors get so hot to even discolor the PCB card and affect nearby parts as well),
  3. R101 = 7.5K/2W 5% (applicable per both channels),
  4. R103 = 2.7K/0.5W 5% (applicable per both channels),
  5. All power outputs emitter resistors = 0,33Ohm/2W (if you replace them, pick high grade with 1% tolerance 3-5W, coated wirewounds are nice).
  6. Capacitors to cure possible oscillations, usually after replacement of power outputs: 22~47pF caps usually work well to damp out any such tendency (see specific P/L service bulletin on the issue). The factory recommended 150~270pF value might be too high, and can cause high frequency slewing at high output power. Ceramic or silver mica types are recommended with at least a 100-Volt rating.
  7. Another point to consider is to replace input coupling caps (.47uF) with a high grade equivalents - worth of experimenting (limited space as they are mounted closely to RCA sockets).
  8. All electrolytic caps replacement is highly advisable. Try to stick to high class brands designed for audio applications (e.g. Panasonic FC, Nichicom KZ, Rubycon ZA). No need to exaggerate though - please bear in mind that Phase Linear power amps were direct coupled and have no electrolytic caps in the signal path. So the replacement is a rather a precaution to avoid caps aging problem in the power supply, feedback network and bootstrap circuit.
  9. The Op-Amp IC LF356 replacement with LF351 was recommended by the factory, so is the replacement of R1, R2 from 2.4K/5W to 7.5K/5W (the LF351 op-amp has better specs and helps with turn on/off thump). I don’t want to open a separate debate on whether LF351 is fine for front-end applications or not, in view of many options available these days. The op-amp in the P/L design had one purpose: to provide high gain and it was used to replace a traditional differential input pair - much more refined solution known from P/L 400-700 Series I. This had few advantages back in the 70's: all the transistors inside the op-amp could be closely matched (important for a differential amp) and production / testing was more efficient since matching of transistors was no longer necessary. LF351 is not at all a bad op-amp (in later complementary models there were also TL071P Low Noise used as the op-amp). So, you may leave it as is. However, for All who like experiments the use of BB OPA134 op-amp as direct replacement to LF351 might be sound quality beneficial upgrade.
  10. Regardless whether you stay with LF351 or not, there are some common practices you may want to consider for the op-amp working conditions improvement. First is to add decoupling 0,1uF (e.g. per 100V) high grade film type cap on the +/- 15V rail supply and mounted directly on the supply pins for each op-amp (diagram here Any Phase Linear Experts out there? - Page 5 - AudioKarma.org Home Audio Stereo Discussion Forums). The solution helps in removing power supply noise and possible distortions at high frequency. Second would be to add 4,7uF/35V electrolytic cap ('reservoir') in parallel to each D1, D2 Zener diode. Be careful though - too much capacitance increases the turn-on thump.
  11. Bias transistors 2N3404, located in-between power transistor banks, are obsolete. Two options that I saw for considerations: replacement of 2N3403 with TN6705A with reused “P” type heatsink and pinouts connection inverted (CBE) and even better – with NTE192A, already in TO92HS type package and compatible pinouts (BCE).
  12. Final review of the PCB card from the traces side is maybe a simple advice, but very important at the same time. Apparently over time the PCB material gets more ‘fatigue’ and you need to re-solder anything that looks questionable. Cold/open solders are most time consuming cause for the amp malfunctions to find.
Output stage
  1. Outputs and drivers should be reinstalled even if not destroyed. The reason are aged thermal compound or rubber insulators causing failures in time when overheated. If the amp had silicon pads on the outputs always get new ones - never reuse them! Don't over-tighten while mounting transistors. If you use mica insulators as preference, go easy with term compound grease. Keep it out of the sockets. Heat sink plates must be well cleaned before re-installation of outputs and drivers.
  2. Following P/L service bulletins, output transistors in each channel should ALL be of the same type (e.g. MJ15024 or MJ21196 for quasi-comp. stage, or MJ15024/25 - MJ21196/95 respectively for fully comp. stage). MJ2119* are probably today’s most advanced high power TO-3 outputs and have higher SOA than MJ150**. In P/L700II fully complementary amp, the drivers and output transistors are the same. For example – the driver for the MJ15024 output transistors is a MJ15024. Same comment for the MJ15025 (PNP match for the MJ15024). In other words DO NOT mix different types of output transistors in a single channel. The previous statement is very important when doing service or repair work – not only with P/L equipment, but any kind / brand of amplifier. In P/L700II quasi complementary amp there are installed obsolete RCA410 as drivers. Leave them, if not shorted, for better stability and sound (subjective impression). Last but not least, avoid buying the output transistors from unknown sources, counterfeited ones are often found. They won’t be of any use in P/L power amps’ severe working conditions.
Wiring
  1. There is an issue here, known to all P/L users. In P/L 700II amp the RCA input ground is correctly tied to the input ground of the amplifier's input circuits. Then the right channel's speaker ground is directly tied to the RCA ground... So ground current flowing into the right channel speaker ground is injected into the 'quiet' input ground (unwanted loop deteriorating the sound). Grounding is critical to both stability, measured performance and the amp sound quality. Star topology should be observed. At least one ground wiring should be added between a solid copper bare inside your amp and the common point on the power supply caps. Here are the most common problems and remedies:
  2. Issue #1: RCA input ground tied to the input ground of the amp input circuits (not a star topology). Solution: Leave it as is.
  3. Issue #2: Speaker terminals ground tied to the RCA and driver board input ground. Solution: Cut the grounding connection of speaker terminals, draw instead a new grounding wire to the PS caps common point clad. Move the driver board ground to the power supply ground, at the junction of the two PS caps.
  4. Issue #3: 230VAC, B+/- DC and low level audio wiring run in a bundle all together. Solution: Unbundle all the leads carrying AC, DC and audio. Shorten the DC power leads by directly routing them to the capacitors from the output stage. Shorten the ground leads in the same manner. Reroute the audio wiring from the rear apron directly to the front panel and add shielding on the input wires between RCA sockets and PL36 board.
  5. To conclude on re-wiring, a piece of electromechanical advice: replace all RCA inputs and speakers posts for something 'more sound neutral' and solid than it was 30 years ago… A variety of solutions can be found in other DIY threads. Also don’t forget about the power cord – better to replace the old one if suspected condition. It should be non-polarized and not wall socket grounded (even for EU standards) – otherwise very likely hum problems appear. Keep the 'earth ground' connected closely to your source of music (CD/DAC/Pre-amp).
Power Supply
  1. Power caps are absolutely worth of replacing. They also strongly influence the sound when got dried over the years. Sometimes they may well become a source of hum. With the new ones go for twice the capacity than original - ca. 20.000uF @100V at minimum - you can find per 160V at small price difference = better (The originals were 9800uf @100V and they run right around 100V. With caps you buy today you don't get the headroom like you did 20 years ago...). The power supply caps testing: check if they have less than a few hundred mV of ripple at idle, at full load it will be slightly over 1V. Don’t buy power caps older than 3-5 years of shelf storage, if you don’t have time for reformatting them. Watch the cap size to avoid additional mounting problems. Before you begin, remember to discharge the power caps (a resistor 10-25 Ohm/10W would help)!
  2. If you increase the power cap capacitance from original value of 9.800uF to say 20.000uF, you need to take into account the higher stress on your bridge rectifier. Some P/L owners decided therefore to upgrade it for >35A/400V units. In addition bypassing each diode by 0.01uF/1000V cap (ceramic or mylar HQ) will help to cure against RF unwanted interferences. BTW, you need to check the presence of C112=0.22uF/400V as no longer needed. Bypassing filter caps with two mylar caps 0.1-1uF/600V can be considered if you go for larger power capacitors. It will improve the ESR at higher frequencies. For 20.000uF cap, with low ESR, ‘computer high grade’ type, etc., etc. - you may not bother, probably.
Other
  1. When you performed all the upgrades, installing the output protection circuit is to be seriously considered to eventually save your precious loudspeakers... There are few options for P/L700II to follow - just Google the topic. But one common mistake here is to forget about reconnecting the RF Zobel network (C108 & R135) from speakers’ terminals to the protection unit inputs. You will experience oscillation problems if you forget the precaution.
  2. Volume pots can be sometimes in a really bad shape – replace them with low noise equivalents, watch out for dimensions and axis profile.
Testing & fine-tuning
  1. For powering up, before the check-up procedure starts, or/and for performed modifications testing, use light bulb test set (Error | Audiokarma Home Audio Stereo Discussion Forums). Start with a 40 or 60 Watt bulb. Step up to 100 Watt or 200 Watt as your confidence builds. You can't have a load connected when you use the bulb. The amp puts out several volts DC as it starts up. If a load is attached, the bulb limits the current, as it is supposed to, and the AC input never gets above 30-40 volts. Using variac and bulb test circuit is probably the safest. Variacs do not limit the current and may cause burned all the output transistors (painful when you just replaced them…). If you still want to use the variac only, change AGC fuses to much smaller values to control excessive current.
  2. In the end bias setting is always needed. See related procedure in the P/L700II service manual. Here is a short summary - not to forget about the step. So, when all the changes to your amp are finally done, use the bulb testing device to start the amp with no I/O connections. When successful, use a voltmeter to measure Voltage across the zener diodes. You should see +/- 15 VDC. Typical is around 14.5 VDC. Then with full input voltage check for about 300~350 mV DC on the 10 Ohm/0.5W resistors on the output buss (R109). Adjust the pot (R107) for the corresponding channel as needed. Recheck in ten minutes. Verify if less than 30 mV. Be careful when taking data. A slip of the probe will cause major damage. Insulate the probe tip with tape or heat-shrink.
I welcome all additional remarks and corrections - nobody is perfect ;-)...
Hello, I'm FRENCH I have a small question about the capacitors of 4.7 UF to put on both diodes D1 and D2 1N4744 Zener 15v can you make me a diagram to do the installation. Thanks for an answer
Regards LIMPERIAL
 
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About 45 years ago, I repaired a number of 400s and 700s and came to the conclusion that the Phase-Linear op-amp based circuit was fundamentally misguided and unstable, resulting in oscillations and failure. I managed to stabilize them by adding local feedback around the op-amp. But these were my least favorite amp since they were so badly designed. Of course, I never/rarely had to repair better amps. If I had one of these today, I would gut the PCB and replace it with a standard ~"blameless" circuit.
 
About 45 years ago, I repaired a number of 400s and 700s and came to the conclusion that the Phase-Linear op-amp based circuit was fundamentally misguided and unstable, resulting in oscillations and failure. I managed to stabilize them by adding local feedback around the op-amp. But these were my least favorite amp since they were so badly designed. Of course, I never/rarely had to repair better amps. If I had one of these today, I would gut the PCB and replace it with a standard ~"blameless" circuit.
Hello, thank you for this quick response, if you have an idea of the meaning to connect the two capacitors 4.7UF 35v on zener diodes 15v (schematic)
I fully agree with you for the remark on the 700 II


Regards LIMPERIAL
 
Hello, I'm FRENCH I have a small question about the capacitors of 4.7 UF to put on both diodes D1 and D2 1N4744 Zener 15v can you make me a diagram to do the installation. Thanks for an answer
Regards LIMPERIAL

Hi, from the perspective of time I should say it is the least important intervention to get your PL Series Two gear back alive and kicking. If want to try then just keep the caps polarity vs Zener diodes correct as they go in parallel. That's all. As for IC's decoupling, attached is a simple drawing. More info here: Phase Linear 700 Series II restoration advice - updated | Phoenix Audio Community Forums
Cheers!
 

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Be careful with op amp rolling in this type of amplifier. THE reason the LF35x was used is because it will tolerate the amp sticking to the rail under fault conditions - putting out the full + or - 75V DC though the feedback resistor into the minus input. Turn on transients can put the same kind of peak into the input stage. Not all op amps will take this, even some premium FET input ones will die (ie, OPA260x).

The most important things to ‘fix’, other than shorted transistors, are the zeners and their bias resistors (the components that discolor the board), and the boot strap capacitor. The zener regulator components really needed to be stood off the board using spacers on the leads so that they can convection cool from all sides. At a minimum, resolder all joints in the area, even if they look good. The boot strap cap is insidious. If it is leaky, you will get a very high offset voltage that makes absolutely no sense - according to all your readings it shouldn’t be happening. If you have one that has high offset or sticks to the rail, the very first thing to do is pull bootstrap cap if the problem goes away youve found it. This is peculiar to the PL amps because the bootstrap is returned to the driver stage and not the output. If the bootstrap were returned to the output, this wouldn’t happen even if the cap is bad, but crossover distortion would be horrible when running fully class B per factory settings.

The fully complementary versions can be run class AB (with at least some air flow on the undersized heat sinks) but the quasi’s will oscillate uncontrollably.
 
caps aging potential problem in the bootstrap circuit

The most important things to ‘fix’, other than shorted transistors, are the zeners and their bias resistors (the components that discolor the board), and the boot strap capacitor. The zener regulator components really needed to be stood off the board using spacers on the leads so that they can convection cool from all sides. At a minimum, resolder all joints in the area, even if they look good. The boot strap cap is insidious. If it is leaky, you will get a very high offset voltage that makes absolutely no sense - according to all your readings it shouldn’t be happening. If you have one that has high offset or sticks to the rail, the very first thing to do is pull bootstrap cap if the problem goes away youve found it. This is peculiar to the PL amps because the bootstrap is returned to the driver stage and not the output. If the bootstrap were returned to the output, this wouldn’t happen even if the cap is bad, but crossover distortion would be horrible when running fully class B per factory settings. The fully complementary versions can be run class AB (with at least some air flow on the undersized heat sinks) but the quasi’s will oscillate uncontrollably.

Yes Sir! It was exactly what I experienced while trying to repair my first PL700S2 amp. Going for fully-comp cures the distortion. I read somewhere that you may actually set any BIAS and no oscillations arise because the circuit gains more stability margin, provided your drivers and power output transistors are kept within the specified by Lynnwood factory range of 4-7 MHz...
 
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