Will PL 400 mod also benefit a PL 700 (both Series I)?

[rtbeemer]

I am refurbishing a Series I PL 700 with PL0171 drive board, replacing all electrolytic caps. A recent thread described a mod for a PL400 Series I drive board: replace C6 47uf/40V cap with a 470uf/50V cap to flatten lower frequency response and bridge this cap with back to back 1N4148 diodes to prevent latch up of rail voltage. My question is : will this same mod work in similar fashion for the PL 700 Series 1? The cap to replace would be C11 (47uf/40V) on the PL0171 board, which is in the same relative location as the PL400C drive board bias regulator transistor (Q3 on PL 400 and Q6 on PL 700). Thanks in advance for any help, input or questions.

 

[laatsch55]

Yes, I believe that's been done before
..

 

[oldphaser]

I would suggest also replacing all of the "carbon composition" resistors (which go up in value with age) on the PL0171 pc board with at least "carbon film" resistors. Or you may want to take it one step further by replacing them with metal film resistors.

https://www.percyaudio.com/Catalog.pdf

https://www.audioasylum.com/cgi/t.mpl?f=tweaks&m=11260

 

[rtbeemer]

Are some stages on the PL0171 board more affected by carbon composition resistor drift than others? Also, are back plane resistors as critical as driver board regarding drift? Planning to do refurbish/repair a step at a time, checking functionality along the way to more easily isolate/prevent fubars.

Back story on this PL 700 amp: Bought from original owner in late 70's, no prior mods/repairs, 2 months ago the left channel faded to nothing regardless of level control setting.

Extensive internet searches, service manual downloads, and tracking down links (thank you for the two above!) has turned blissful ignorance into cautious optimism for refurbishing Phase Linear amps. The knowledge so freely shared on this site is especially appreciated because the backup amp is an Ebay PL 400 Series I which works fine (speaker fuses added in meantime) and next in line for refurbish.

Hopefully we all share the characteristic of carbon composition resistors, increasing in value as we age (but not becoming obsolete and in need of replacement).

 

[laatsch55]

As I age it appears the thresholds of certain things drops, not increase...

 

[MarkWComer]

As I age it appears the thresholds of certain things drops, not increase...

As I age my insanity becomes more difficult to hide...

 

[oldphaser]

Are some stages on the PL0171 board more affected by carbon composition resistor drift than others? Also, are back plane resistors as critical as driver board regarding drift?

Unfortunately, Dean and I didn't keep any data on the carbon composition resistors that were going up in value (greater than +10% of nominal). If my memory serves me correct, it was the lower value resistors under 1K ohm we had issues with. In some cases the values doubled. It's just better to go ahead and replace them all.

When I work on most Phase Linear amplifiers, I remove the output transistors and heats-sinks. This allows me to carefully lift the output stage with all the emitter resistors and bus wires and carefully examine the back-side of the emitter resistors for bulging or cracks. You might be surprised to see how many still measure good but are near failure.

As for the emitter resistors (i.e. .22, .27, .33) located on the output stage (a.k.a. backplane), measurement errors can occur at low values. As an example, the effects of your hands touching the resistors can effect your results. Reversing the leads may change results as well. (NOTE: I need to investigate this further.) Unfortunately, I do not have a (4) wire terminal DMM which may help to reduce some of the errors. My DMM's are (2) wire terminal only. If you can, be sure to zero out the effects of your lead lengths before taking measurements.

"The main measurement issue with the two-wire method, as applied to low resistance measurements, is that the total lead resistance (RLEAD) is added to the measurement. Because the test current (I) causes a small but significant voltage drop across the lead resistances, the voltage (VM) measured by the meter won’t be exactly the same as the voltage (VR) directly across the test resistance (R), and considerable error can result. Typical lead resistances range from 10mΩ to 1Ω, so it’s very difficult to obtain accurate two-wire resistance measurements when the resistance under test is lower than 100Ω because the resistance of interest will be completely swamped by the lead resistance. In fact, lead resistance will be the dominant source of error. For example, using test leads with a 100mΩ combined resistance to perform a two-wire resistance measurement on a 500mΩ resistor will result in a 20% measurement error in addition to that of the instrument."

NOTE: 10mΩ is = .01 ohm, 500mΩ is = .5 ohm

Ed

 

[laatsch55]

Can a guy run a say 5 ohm resistor in series with a lead , then go lead to lead , get a baseline then measure the emitter resisyors??

 

[rtbeemer]

Ed, thank you for the detailed advice on resistor replacement, Mouser is sending carbon films for the PL 700 drive board. I used a Fluke DVM to check output stage resistors, as you pointed out, it is incapable of accurately measuring a .27/.22 ohm resistance (resolution is .1 ohm and displays .4 ohm with leads shorted ....) The output resistors all have uniform continuity readings with a .2 -.4 ohm increase over "zero" so appear to still be functional (thanks to "Wired by Esther") . Will focus on driver board and update with results when available.

Safety side note: replaced power capacitors with 18,000 uf 100 V Chemicon. Mounting to the anodized aluminum ground plate is a little frightening, having to center the hot side 10-32 screws through a 3/8 hole. Used a -012 oring for a liner to keep the mounting screws centered, now have uniform .090" gap isolation to prevent Armageddon.