2 PL 700B amps one to repair the other to listen to

[GolfBum]

Good evening new member here.

I picked up a couple of phase linear 700 B amplifiers from an old recording studio. One amp is in great shape and works perfectly hardly any DC offset (10 mv) produces a nice sine wave and no issues whatsoever. It sounds great.

The second unit had all of the outputs pulled from the rear of the amp with a big sticker that said “do not connect DC on outputs”. Well, that is a challenge I would love to tackle. When it was brought up to voltage on the variac with no load, no signal, and no output transistors, the fuses held but the amp puts out about 76 V DC on left channel, and 54 on right. The filter capacitors each put out about 100 volts and I'd like to know where to start as the manual says to start with replacing Q1 and Q2 on the PCB and go from there. No issues with smoking, buzzing when powered up.

Second there are yellow and blue wires that control the meter sensitivity switch on the face plate. Can these two wires be reversed on their pins so that the attenuation from 0DB to -20 DB reverses to -20DB to 0DB? I purchased a new face plate off of eBay and it's backwards from what the existing amp is. I have a variac, DMM, signal generator and oscilloscope to work with. Both units have the face plates off, and I can take measurements from the good amp and compare them to the bad.

Thank you

Doug

 

[mlucitt]

Welcome to the forum. With 2 PL700Bs, you fit right in. Let's address the first amp, the "good" one. If it is original, you have one immediate upgrade to do. Go to https://www.whiteoakaudio.com/estore.aspx
and order the PL700 Capacitor kit. Your bulk power supply capacitors are at least 45 years old. The life expectancy of those capacitors is 15-20 years, so yours are on borrowed time. The "kit" has two capacitors with much better specs than the original capacitors. You may notice a tightening of the bass, or no change in the music at all, but at least your capacitors will not fail catastrophically.

 

[mlucitt]

Now, on to the second unit. I would recommend a new control board. You can try and replace Q1 and Q2, many here have started down that trail. The bottom line is that the original Phase Linear control board is junk by today's standards. The White Oak Audio Design Control Board is so much better by design and component selection. So essentially, you have a shopping list for the second unit:
(1) White Oak Audio Design Rev G1 Control Board Kit
(1) White Oak Audio Design Capacitor Kit
(20) MJ21196G Output Transistors (people here can supply you with the 48 screws needed for the transistors)
(4) RCA 410 Driver Transistors (the bottom row below the Output Transistors) There are people here on the Forum that have these.
(24) White Oak Audio Design - Bergquist TO3 Transistor Insulators
(1) DC Protection Relay/Delay Board (saves your speakers from the DC voltage on the outputs) Available from Watts Abundant here on the Forum.

The procurement, assembly, and installation of these parts will reward you with a very nice sounding amplifier, ready for another 45 years of use.

The faceplates of the PL700B amplifiers changed when the Meter Sensitivity switch design changed. There are people on the forum that have the proper switch for your amplifier to keep it original and proper. You will have to tell them the wording (-20 DB on the left or right) on the Faceplate to get the proper switch.

Ask questions, post pictures, and enjoy the ride on the Phoenix Forum.

 

[NeverSatisfied]

Welcome GolfBum, you have arrived at the center of the Phase Linear universe. Anything and everything about these wonderful amps can be found here as well as some of the nicest and down to earth characters to be found anywhere. Just like the gentlemen above ^

 

[J!m]

Welcome!

 

[Vintage 700b]

Welcome GB,
There is a wealth of information here and some of the nicest people you will meet!

 

[NavLinear]

Welcome to Phoenix. Good things come in pairs. Here's a welcome Nav babe.

 

[GolfBum]

That is quite a pair, indeed!

 

[D-500 Flogger]

Doug,

I'm one of the newer members here -- after lurking a while decided to join up after realizing that the members here are steeped in real-world hard-won knowledge on Bob Carver's creations. Gepetto is the engineer behind all the Phase Linear control modules & back planes that bring the performance & reliability forward from the '70s to today. Not to mention all the guys in here that have repaired/upgraded/hot-rodded these amps to meet their needs.

Speaking for myself, it's such a relief to find a place where, when you ask how to best go about fixing one of these amps, the next 5 replies don't ask you why on earth would you want to fix one of those? :0)

Anyway, welcome to the forum! The more you dig, the more you will find...