PL-700 OTW
- Thread starter VSAT88
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- Joined
- Jan 14, 2011
- Messages
- 74,829
- Location
- Gillette, Wyo.
- Tagline
- Halfbiass...Electron Herder and Backass Woof
- Joined
- Jan 14, 2011
- Messages
- 74,829
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- Gillette, Wyo.
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- Halfbiass...Electron Herder and Backass Woof
If you don't want to spend too much money on the 700, but make it "less of a time bomb", you may want to try the following after getting the amp up and running to original specifications and replacing parts such as the carbon comp resistors and other parts that may have drifted (ceramics caps, etc):
In addition, with the DCP installed, your speakers will be protected.
- Replace all the PL909's with newer MJ15024's or MJ21194's devices.
- Solder 100pF silver mica caps across each pre-driver base/collector junction.
In addition, with the DCP installed, your speakers will be protected.
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I would only recommend replacement of the original output transistors in a 400 or 700 series 1 amplifier with faster devices like the MJ15024's , MJ21194's or MJ21196's, if the person doing the replacement has the proper test equipment and knowledge to do so. The test equipment should include the following: signal generator, distortion analyzer, oscilloscope and dummy load. Oscillations can occur at different frequencies and output levels. The largest value "slow down" capacitors Phase Linear ever used on series 1 amps was 33pF. (NOTE: Dean would use the lowest value possible to eliminate the oscillation and on only the positive or negative driver as required.) Any value larger than 33pF will cause slewing of the amplifier at high frequencies. In which case the amplifier will not meet spec (.25% THD at rated output at 20KHz). Adding capacitors where they are not needed can also cause oscillations. My experience with the original 700's in which I installed newer faster devices (like the MJ series of output transistors mentioned above) is that in about 25% of the cases Dean nor I could eliminate the oscillations using the cures; slow-down caps, bypass caps or ferrite beads. In which case we had to re-install the original (slower) output transistors.
NOTE: There was also a service bulletin issued 10-81 "SUBJECT: No more XPL909 or FPL909 output transistors".
".....We are now recommending the NEC brand 2SD555 as a replacement for both pre-Series II (Series I) and Series II amplifiers and the Motorola TP9054/MJ15024 as a replacement for Series II amplifiers only. Be sure to thoroughly test the amplifier on a distortion analyzer while checking for possible high frequency waveform oscillations on either or both halves of the output sine wave. These low level-high frequency oscillations, if present, can usually be remedied by soldering a 10-33pF/100V cap on pre-Series II or 150-270pF/100V cap on the Series II amplifiers across the base to collector junction of the respective pre-driver transistiors (40327-pos., MM4003-neg.) and retesting."
Additionally:
The following is some verbiage in italics taken from the service Bulletin "SUBJECT" Wave Form Oscillation in Models 400, 700 and D-500 Series Two Amplifiers":
On some model 400, 700 and D-500 Series Two amplifiers (or some earlier units retrofitted with the Series Two PL36 drive boards) parasitic oscillations may occur causing excessively high distortion and/or thermal instability of the output stage. During sine wave bench testing these oscillations may show up on either the positive and negative half of the output sine wave.
The series II amplifiers will tolerate much higher value "slow down" capacitors than the series 1 amplifiers. 100pF to 150pF should be adequate values in most instances, however, values up to about 250pF may be necessary to slow down the pre-drivers sufficiently. Capacitor values in excess of this may cause high frequency slewing and the unit should be troubleshot for other sources of the oscillations if a higher value is needed. Capacitors used for this modification should have at least a 100V rating.
NOTE: Phase Linear also used .33uF "bypass" capacitors on later series II amplifiers. They are located on the terminal strip between the positive and negative output stages.
Ed
Since I am not going to be jamming this amplifier anywhere near the full rated power (I rarely drive any amp I have over 40 Watts) could I not with Dons board use the OPT that are in the amp with new caps on the driver board (replace all electrolytes) and any out of spec resistors changed for in spec run the amplifier ? I know that there are other things that should be done such as pull the OPT's and use new mica as well as new wakefield compound. Perhaps I am reading in to this all wrong. I do not want to seem like I am bucking the system, you guys know way more about amplifiers than I will ever know and I do need help but thermal meltdown ? Its not a nuclear reactor. If it does meltdown what am I loosing ? Some old out of spec components that would have to be replaced to do a complete rebuild ? PLEASE do not take this post the wrong way I am just trying to learn. All I really want from this is a half ass reliable old PL 700 that is as close to factory as possible. What I do not want is to be put in that category of the dude that asks for help then will not listen. So, hit me with your thoughts please. How do I go about this and stay as stock as possible. Same as with the PL 400 that Lee and Perry helped me with so much with ? It wound up with all PL 909 OPT's, Dons board, WO main filters, new BR, all new caps on the driver board and reverse diodes. Probably a few more things I cant remember. Perhaps I am deceiving myself but I think that amplifier sounds great ! I am 100% happy with it. It does not get hot...ever....no mater how long or hard I jam it. Its beautiful. I would not take less that 600.00 for it. And I am sure you guys would laugh at that price however if I wanted to sell it I could sell it for that. I do not own any big high SPL speakers. My listening room is so small you would get a good laugh. 8x15. Yes, 8x15. If I turn it up too loud my ears would bleed. That being said what do you all think ? My plan as of now is this. Clean this amp as good as I can. replace any bad bulbs. Two new bias transistors. New BR, Replace any out of spec resistors. Replace all electrolytics. Set bias and DC off set. and Dons board. Then Listen with a fire extinguisher handy (I always have one handy, my house is not that new : ).
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Ed,
When I recommended the 100pF caps, I was thinking of the Series ll version. However, you are right, the Series l should use a smaller cap on the pre-driver base to collector predriver transistors.
The PL service bulletins were filled with good information, however, one item I find interesting, that was never published from PL, was the addition of output stage base stopper resistors. This was well known at the time to reduce output stage oscillations. This revision alone can stop oscillations, especially with the long lead wires from the driver board to the hand-wired output stage. This should have been included in the PL production design.
VSAT88,
I understand your dilemma. I would get the 700 running back to original, then add the 2.7 Ohm base stopper resistors for good measure. These will not cause oscillations (not supposed to), but it more than likely will suppress the urge for the map to go wild anytime in the future with various loads and output levels. Almost all paralleled output stage designs nowadays use base stopper resistors, especially in a triple emitter follower (EF3) configuration.
Also, if your amp does not have film bypass caps such as the 0.33uF / 100V caps as in my attached schematic, you may want to add those too.
When I recommended the 100pF caps, I was thinking of the Series ll version. However, you are right, the Series l should use a smaller cap on the pre-driver base to collector predriver transistors.
The PL service bulletins were filled with good information, however, one item I find interesting, that was never published from PL, was the addition of output stage base stopper resistors. This was well known at the time to reduce output stage oscillations. This revision alone can stop oscillations, especially with the long lead wires from the driver board to the hand-wired output stage. This should have been included in the PL production design.
VSAT88,
I understand your dilemma. I would get the 700 running back to original, then add the 2.7 Ohm base stopper resistors for good measure. These will not cause oscillations (not supposed to), but it more than likely will suppress the urge for the map to go wild anytime in the future with various loads and output levels. Almost all paralleled output stage designs nowadays use base stopper resistors, especially in a triple emitter follower (EF3) configuration.
Also, if your amp does not have film bypass caps such as the 0.33uF / 100V caps as in my attached schematic, you may want to add those too.
25 transistors for 7 bucks??? Pretty cheap if you ask me.
https://www.ebay.com/itm/MPS-5172-N...716111?hash=item2a63e7194f:g:k2wAAOSwGvhUAhZo
https://www.ebay.com/itm/MPS-5172-N...716111?hash=item2a63e7194f:g:k2wAAOSwGvhUAhZo
The original 700's already have 4uF 150V bypass capacitors installed. (They were made by Mallory and were either brown or orange in color). They are located on the terminal strip between the positive and negative output stages. I don't recall Dean ever removing the 4uF caps and replacing them with .33uF film bypass caps or adding .33uF's to the 4uF caps to an original 700 series 1 amplifier in an effort to remove any oscillations after I installed newer faster output transistors like the MJ15024's, MJ21194's or MJ21196's. At that time, little else could be done to the series 1's other than to add (up to) 33pF "slow-down" capacitors.
SIDE NOTE: If my memory serves me correct........The addition of "slow-down" capacitors will more than likely also reduce the original bandwidth of the amplifier. Particularly in series 1 amplifiers. As an example, lets say a 700 series 1 had a bandwidth around 27KHz and 33pF "slow-down" capacitors are installed. The bandwidth will be reduced very close to 20KHz. Using any value greater will typically reduce the bandwidth below 20KHz. This is all with respect also to the amplifier being driven to rated output into a 8 ohm load and meeting the .25% T.H.D. specification.
Ed
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In all the original 700's I have worked on over the years, I have replaced the MPS5172's (which are located on the pc board) with 2N3403 p-strap bias transistors with red, black and brown leads I added which I then soldered to the pc board. I drill (2) holes on the back of the amplifier (one on the left channel and one on the right channel) and mounted the 2N3403's. This is then like all the other Phase Linear amps that followed.
Only the original 700's did not have the bias transistor mounted on the output stage where the heat is being generated by the output transistors located on the heat-sinks.
Others here on the forum have some ideas on what they are using as a replacement for 2N3403's and I am sure someone will chime in.
Ed
Only the original 700's did not have the bias transistor mounted on the output stage where the heat is being generated by the output transistors located on the heat-sinks.
Others here on the forum have some ideas on what they are using as a replacement for 2N3403's and I am sure someone will chime in.
Ed
Last edited:
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