PL-700 OTW

[VSAT88]

Check that. That is what I did. I read and re read the manual then I adjusted both the pots on both channels for DC offset and re checked it again to make sure I did not miss anything. Everything checks fine as far as DC offset. The Bias is what I am haveing trouble with. The left channel Bias will not adjust any higher than .140 mV. The right channel Bias adjusts easily to the correct adjustment of between .300 -.400.

 

[laatsch55]

I was talking about the bias, grab the bias trannt with your fingers and as your fingers warm it up see if bias changes. We are checking to see if the bias tranny has any gain left.. ..

 

[VSAT88]

I was talking about the bias, grab the bias trannt with your fingers and as your fingers warm it up see if bias changes. We are checking to see if the bias tranny has any gain left.. ..

You know I should have known to check that. As much trouble as we went thru last time with that bias transistor with the PL 400. Thanks Lee. I will do that 1st thing tomorrow.

 

[laatsch55]

Coolness....

 

[VSAT88]

Did as you said. Finger did nothing so used a hot soldering Iron underneath each transistor at Q6. I looked on the backplane for the bias resistors and saw that they were not mounted there. The heat on the left channel transistor at Q6 made the meter drop but very slowly. Putting heat on the right channel transistor at Q6 dropped the bias quickly. I am guessing I should replace both with the 2N5088 transistors that you sent me for the 400 ?? The transistors that are at Q6 now are marked Motorola MPS 5172. By the way, I am finding that this thing is a bear to work on as opposed to the PL 400....

 

[laatsch55]

You did notice the bias trannies are on the board and not the backplane..

 

[VSAT88]

Our posts must have crossed/posted at the same time. I found them on the mainboard at Q6, at least I think those are the bias transistors.

Did as you said. Finger did nothing so used a hot soldering Iron underneath each transistor at Q6. I looked on the backplane for the bias resistors and saw that they were not mounted there. The heat on the left channel transistor at Q6 made the meter drop but very slowly. Putting heat on the right channel transistor at Q6 dropped the bias quickly. I am guessing I should replace both with the 2N5088 transistors that you sent me for the 400 ?? The transistors that are at Q6 now are marked Motorola MPS 5172. By the way, I am finding that this thing is a bear to work on as opposed to the PL 400....

 

[oldphaser]

Phase Linear 700 (PL0171 PC Board Comments)

The original Phase Linear 700's are a pain in the ass to work on. I am afraid you will be chasing your tail working on an original 700.

The carbon composition resistors are all going up in value with age and will not provide you with some sort of visual cue as to the fact they are going bad. (NOTE: Phase Linear eventually went to using carbon film resistors (which have better reliability) in place of the carbon composition resistors in later 400's and 700B's.)

The brands of electrolytic capacitors Phase Linear used in the early 1970's on the PL0171 and PL400C pc boards are going bad. (NOTE: Phase Linear eventually went to using different brands of electrolytic's which didn't nearly have as many problems as the early ones did.)

The ceramic disc capacitors in all Phase Linear amps are going down in value with age as well. Albeit, the tolerances on the ceramic disc capacitors were pretty loose and I haven't seen a low value ceramic disc capacitor effect the performance so much as I do the carbon composition resistors going up in value.

By the way, all of the PL0171 pc boards I work on these days, I remove all parts and start anew measuring all parts before they are installed.

With all that being said, the 700 series 1 amp and 400 series 1 amps had the lowest (best) signal-to-noise ratio of all the amps that Phase Linear made. The original signal-to-noise ratio spec was also "unweighted". Later series 2 amps were all "A weighted". Even after the 700B's were produced most recording studio owner's who were in-the-know preferred the 700's over the 700B's.

Lastly, measuring and adjusting DC offset on a PL0171 pc board begs the question.... do you adjust the DC offset when the amp is cold (running at idle after 5 minutes) or after the amp has heated up? The service manual doesn't give any guidance. The DC offset also drifts on PL0171 pc boards.

Ed

 

[laatsch55]

Our posts must have crossed/posted at the same time. I found them on the mainboard at Q6, at least I think those are the bias transistors.

Yep, those are theones...

 

[VSAT88]

The original Phase Linear 700's are a pain in the ass to work on. I am afraid you will be chasing your tail working on an original 700.

The carbon composition resistors are all going up in value with age and will not provide you with some sort of visual cue as to the fact they are going bad. (NOTE: Phase Linear eventually went to using carbon film resistors (which have better reliability) in place of the carbon composition resistors in later 400's and 700B's.)

The brands of electrolytic capacitors Phase Linear used in the early 1970's on the PL0171 and PL400C pc boards are going bad. (NOTE: Phase Linear eventually went to using different brands of electrolytic's which didn't nearly have as many problems as the early ones did.)

The ceramic disc capacitors in all Phase Linear amps are going down in value with age as well. Albeit, the tolerances on the ceramic disc capacitors were pretty loose and I haven't seen a low value ceramic disc capacitor effect the performance so much as I do the carbon composition resistors going up in value.

By the way, all of the PL0171 pc boards I work on these days, I remove all parts and start anew measuring all parts before they are installed.

With all that being said, the 700 series 1 amp and 400 series 1 amps had the lowest (best) signal-to-noise ratio of all the amps that Phase Linear made. The original signal-to-noise ratio spec was also "unweighted". Later series 2 amps were all "A weighted". Even after the 700B's were produced most recording studio owner's who were in-the-know preferred the 700's over the 700B's.

Lastly, measuring and adjusting DC offset on a PL0171 pc board begs the question.... do you adjust the DC offset when the amp is cold (running at idle after 5 minutes) or after the amp has heated up? The service manual doesn't give any guidance. The DC offset also drifts on PL0171 pc boards.

Ed

Mr. Ed. I agree 100% with what you said. I know that everything will need be replaced in this amp. In fact the only two things that I will not wind up replacing would be the face plate/meters and the transformer. I will WOPL with this one however (that means but, but , but. LOL) my intention for now is to get her up and running with a minimum of parts time and effort if I can. I want to listen to it and I have some old speakers that I care not for to do so (if possible). I will order a bit at a time the parts from White Oak to WOPL and Watts Abundant to put the relay in then put them aside building those parts as I go. If this thing goes nuclear in between I do not think I have really lost anything as all the internals will be replaced by WO parts in time anyhow. What do you guys think about that. As it stands right now all I think I need for it to just work are two : 2N5172 transistors to sub for the stock Motorola MPS5172 's at Q6. It will be doubtful that if this does not work that I would go any farther as far as trying to "fix" this amp.

 

[Bradrock]

I dig the historical significance of the PL700. That's why I want a nice looking one or more on my pile.

 

[VSAT88]

I dig the historical significance of the PL700. That's why I want a nice looking one or more on my pile.

And I agree. Perhaps I would go in the wrong direction to WOPL this particular amp. Perhaps I should try to rebuild it part for part keeping it as stock as possible. Lee, Perry, Ron, Bradock, Marco, Joe, Ed, Gentlemen please give me me your opinion. I have plenty and I mean plenty of gear to listen to. I do not have to try and bring this one back up at this time.

 

[Netfly]

..With all that being said, the 700 series 1 amp and 400 series 1 amps had the lowest (best) signal-to-noise ratio of all the amps that Phase Linear made. The original signal-to-noise ratio spec was also "unweighted". Later series 2 amps were all "A weighted". Even after the 700B's were produced most recording studio owner's who were in-the-know preferred the 700's over the 700B's.
Ed

Any idea what caused the difference in S/N Ed ? I'm guessing the ratings change may have been to conform to the then current norm, but if you can tell a difference I'm curious as to why. Thanks,
Bill.

 

[WOPL Sniffer]

And I agree. Perhaps I would go in the wrong direction to WOPL this particular amp. Perhaps I should try to rebuild it part for part keeping it as stock as possible. Lee, Perry, Ron, Bradock, Marco, Joe, Ed, Gentlemen please give me me your opinion. I have plenty and I mean plenty of gear to listen to. I do not have to try and bring this one back up at this time.

Can't remember (cuz 'm old now), but I don't think you can WOPL the original 700....

Trade it to get one you can... Otherwise it's a ticking time bomb.... (that is of course that it can't be WOPL'd)... Historical significance??? Maybe not.....

 

[WOPL Sniffer]

And I agree. Perhaps I would go in the wrong direction to WOPL this particular amp. Perhaps I should try to rebuild it part for part keeping it as stock as possible. Lee, Perry, Ron, Bradock, Marco, Joe, Ed, Gentlemen please give me me your opinion. I have plenty and I mean plenty of gear to listen to. I do not have to try and bring this one back up at this time.

Did you buy it to collect dust??? I don't have that luxury.... Good score though.

 

[VSAT88]

Did you buy it to collect dust??? I don't have that luxury.... Good score though.

I would just as soon fix er. That's why I am asking what you guys think I should do. Just sit it to the side and buy all WO for it if I can or restore as close to original as possible.. Hell, I don't have a clue at this point.

 

[WOPL Sniffer]

I would just as soon fix er. That's why I am asking what you guys think I should do. Just sit it to the side and buy all WO for it if I can or restore as close to original as possible.. Hell, I don't have a clue at this point.

What I'm saying is, I don't think the WO Backplanes or Control boards fit.... So WOPL'ing may be out of the question

 

[VSAT88]

OK, now the noodle understands. I had bought this with WOPL in mind. Now I know what must be done. I will restore it as close as I can to original and get a board from Don and hope for the best. Thanks Cool Cats.

 

[oldphaser]

Any idea what caused the difference in S/N Ed ? I'm guessing the ratings change may have been to conform to the then current norm, but if you can tell a difference I'm curious as to why. Thanks,
Bill.

Rather than spending a bunch of time writing a long winded reply:
(NOTE: Some of the terms and definitions as well as techniques and methods noted below have changed over the years.)


The following was taken from http://www.rane.com/note145.html (written by Dennis Bohn, former Phase Linear design engineer):
S/N or SNR. Signal-To-Noise Ratio

[FONT=&amp]What is tested? This specification indirectly tells you how noisy a unit is. S/N is calculated by measuring a unit's output noise, with no signal present, and all controls set to a prescribed manner. This figure is used to calculate a ratio between it and a fixed output reference signal, with the result expressed in dB.[/FONT]
[FONT=&amp]How is it measured? No input signal is used, however the input is not left open, or unterminated. The usual practice is to leave the unit connected to the signal generator (with its low output impedance) set for zero volts. Alternatively, a resistor equal to the expected driving impedance is connected between the inputs. The magnitude of the output noise is measured using an rms-detecting voltmeter. Noise voltage is a function of bandwidth -- wider the bandwidth, the greater the noise. This is an inescapable physical fact. Thus, a bandwidth is selected for the measuring voltmeter. If this is not done, the noise voltage measures extremely high, but does not correlate well with what is heard. The most common bandwidth seen is 22 kHz (the extra 2 kHz allows the bandwidth-limiting filter to take affect without reducing the response at 20 kHz). This is called a "flat" measurement, since all frequencies are measured equally.[/FONT]
[FONT=&amp]Alternatively, noise filters, or weighting filters, are used when measuring noise. Most often seen is A-weighting, but a more accurate one is called the ITU-R (old CCIR) 468 filter. This filter is preferred because it shapes the measured noise in a way that relates well with what's heard.[/FONT]
[FONT=&amp]Pro audio equipment often lists an A-weighted noise spec -- not because it correlates well with our hearing -- but because it can "hide" nasty hum components that make for bad noise specs. Always wonder if a manufacturer is hiding something when you see A-weighting specs. While noise filters are entirely appropriate and even desired when measuring other types of noise, it is an abuse to use them to disguise equipment hum problems. A-weighting rolls off the low-end, thus reducing the most annoying 2[SUP]nd[/SUP] and 3[SUP]rd[/SUP] line harmonics by about 20 dB and 12 dB respectively. Sometimes A-weighting can "improve" a noise spec by 10 dB.[/FONT]
[FONT=&amp]The argument used to justify this is that the ear is not sensitive to low frequencies at low levels (à la Fletcher-Munson equal loudness curves), but that argument is false. Fletcher-Munson curves document equal loudness of single tones. Their curve tells us nothing of the ear's astonishing ability to sync in and lock onto repetitive tones -- like hum components -- even when these tones lie beneath the noise floor. This is what A-weighting can hide. For this reason most manufacturers shy from using it; instead they spec S/N figures "flat" or use the ITU-R 468 curve (which actually makes their numbers look worse, but correlate better with the real world).[/FONT]
[FONT=&amp]However, an exception has arisen: Digital products using A/D and D/A converters regularly spec S/N and dynamic range using A-weighting. This follows the semiconductor industry's practice of spec'ing delta-sigma data converters A-weighted. They do this because they use clever noise shaping tricks to create 24-bit converters with acceptable noise behavior. All these tricks squeeze the noise out of the audio bandwidth and push it up into the higher inaudible frequencies. The noise may be inaudible, but it is still measurable and can give misleading results unless limited. When used this way, the A-weighting filter rolls off the high frequency noise better than the flat 22 kHz filter and compares better with the listening experience. The fact that the low-end also rolls off is irrelevant in this application. (See Digital Dharma of Audio A/D Converters)[/FONT]
[FONT=&amp]Required Conditions. In order for the published figure to have any meaning, it must include the measurement bandwidth, including any weighting filters and the reference signal level. Stating that a unit has a "S/N = 90 dB" is meaningless without knowing what the signal level is, and over what bandwidth the noise was measured. For example if one product references S/N to their maximum output level of, say, +20 dBu, and another product has the same stated 90 dB S/N, but their reference level is + 4 dBu, then the second product is, in fact, 16 dB quieter. Likewise, you cannot accurately compare numbers if one unit is measured over a BW of 80 kHz and another uses 20 kHz, or if one is measured flat and the other uses A-weighting. By far however, the most common problem is not stating any conditions.[/FONT]
[FONT=&amp]Correct: S/N = 90 dB re +4 dBu, 22 kHz BW, unity gain[/FONT]
[FONT=&amp]Wrong: S/N = 90 dB

Additionally, Rod Elliot has this to say about "A-weighting For Audio Measurements"
http://sound.whsites.net/project17.htm

Ed[/FONT]

 

[laatsch55]

I would just as soon fix er. That's why I am asking what you guys think I should do. Just sit it to the side and buy all WO for it if I can or restore as close to original as possible.. Hell, I don't have a clue at this point.

Backlanes won't fit, the Series 1's have a different bolt pattern for the outputs. You have to drill two holes to mount the driver board. You can, however, buy a new chassis and heatsink. The faceplate will fit...