PL 700B blown fuses and transistors

[OldPLguy]

PL 700B blows 5AGX fuses on one channel. Checked resistance ( only have volt ohmmeter for testing ) of all output transistors ( removed from unit to test ) which showed 4 ohms except 2 on blown channel which showed 0 ohms. I assumed the two 0 ohm transistors were bad, so replaced two with some 4 ohm ones from good channel. Powered on, had reduced, scratchy volume on “bad” channel fading to no volume, blown fuses and one “replaced” transistor now showing 0 ohms. Thinking control board bad, but don’t want to just start buying parts, ie board, output transistors. Can do some repairing, but not sophisticated testing. Ideas? Shop to repair my unit? Located SE PA. Thanks

 

[mlucitt]

Something is killing your output transistors. There are several possibilities but a new White Oak Audio Design Control Board will likely solve the problem, https://www.whiteoakaudio.com/estore.aspx
It is a much improved design with the best components available. Your original control board is almost 50 years old and some of the components are past "end of life" such as the electrolytic capacitors and the resistors that may be out of tolerance due to heating cycles.

The website also lists the MJ211996G output transistors that would replace the 20 output transistors that you presently have. While you are on the website, I would strongly recommend purchasing a PL700 Capacitor Assembly. These are better than stock, and your original capacitors are probably not doing you any good now after almost 50 years, they have a 15-20 year life expectancy.

Good Luck!

 

[OldPLguy]

My output transistors are marked F 722 PL909 Hongkong, does that correspond to the MJ211996G ? Someone else suggested checking the emitter resistors. Thanks

 

[mlucitt]

Those original equipment transistors were made by children in Hong Kong sweatshops under contract by Fairchild Semiconductor for Phase Linear in the late 1960s and early 1970's (yours date to February 1972). They have no where near the Safe Operating Area (SOA) of the ON Semiconductor MJ21196G devices. Additionally, if you measure the gain of your PL909 devices, you will see numbers all over the chart. The MJ2116G devices are extremely consistent and have much higher gain - MUCH.

Due to the wide deviation of gain, the emitter resistors sometimes had to work hard and it is not unusual to find them cracked or broken by heat, especially in amplifiers used by professional sound reinforcement folks. They were rather inexpensive carbon composition resistors, with tolerances of 10 or 20%. Today we use metal oxide resistors which are more reliable and have a tolerance of 2% (Mouser P/N 660-MOSX3CT631RR33G). They are only $0.25 each and you need 22 of them. I can send you some if you want.

 

[MarkWComer]

Someone else suggested checking the emitter resistors. Thanks

Blown transistors usually mean that the corresponding emitter resistor is blown too. You mentioned “scratchy” sound after replacing transistors, this could be evidence, possibly asymmetrical clipping from those blown resistors.

 

[MarkWComer]

A blown transistor…

 

[mlucitt]

Mark,
Good one... I'm going to borrow that picture of a blown transistor.
Do you know the transistor number or the source of the picture?

 

[MarkWComer]

Mark,
Good one... I'm going to borrow that picture of a blown transistor.
Do you know the transistor number or the source of the picture?

Either a 21196 or 21195, one of the ones that failed in my 400.

 

[George S.]

A blown transistor…

View attachment 52768

Hey Mark W., can you tell if those wires are soldered or welded? So that a Onsemi?

 

[MarkWComer]

Hey Mark W., can you tell if those wires are soldered or welded? So that a Onsemi?

I don’t know how the transistors are manufactured, so whether the base and emitter leads are soldered or welded is unknown to me. I’d think they’re welded due to the high current and voltages they handle, as well as the heat- but I can’t authoritatively state one way or another.

Yes, the transistor was made by On Semiconductor.

 

[OldPLguy]

I don’t know how the transistors are manufactured, so whether the base and emitter leads are soldered or welded is unknown to me. I’d think they’re welded due to the high current and voltages they handle, as well as the heat- but I can’t authoritatively state one way or another.

Yes, the transistor was made by On Semiconductor.

Those original equipment transistors were made by children in Hong Kong sweatshops under contract by Fairchild Semiconductor for Phase Linear in the late 1960s and early 1970's (yours date to February 1972). They have no where near the Safe Operating Area (SOA) of the ON Semiconductor MJ21196G devices. Additionally, if you measure the gain of your PL909 devices, you will see numbers all over the chart. The MJ2116G devices are extremely consistent and have much higher gain - MUCH.

Due to the wide deviation of gain, the emitter resistors sometimes had to work hard and it is not unusual to find them cracked or broken by heat, especially in amplifiers used by professional sound reinforcement folks. They were rather inexpensive carbon composition resistors, with tolerances of 10 or 20%. Today we use metal oxide resistors which are more reliable and have a tolerance of 2% (Mouser P/N 660-MOSX3CT631RR33G). They are only $0.25 each and you need 22 of them. I can send you some if you want.

Sounds like control board, emitter resistors and possibly a set of output transistors could be needed. I’d rather not just buy parts without some confirmation of the actual condition of the unit, but I haven’t yet been able to locate someone local to test the amp. I’m located in southeast PA.

 

[OldPLguy]

Sounds like control board, emitter resistors and possibly a set of output transistors could be needed. I’d rather not just buy parts without some confirmation of the actual condition of the unit, but I haven’t yet been able to locate someone local to test the amp. I’m located in southeast PA.

Those original equipment transistors were made by children in Hong Kong sweatshops under contract by Fairchild Semiconductor for Phase Linear in the late 1960s and early 1970's (yours date to February 1972). They have no where near the Safe Operating Area (SOA) of the ON Semiconductor MJ21196G devices. Additionally, if you measure the gain of your PL909 devices, you will see numbers all over the chart. The MJ2116G devices are extremely consistent and have much higher gain - MUCH.

Due to the wide deviation of gain, the emitter resistors sometimes had to work hard and it is not unusual to find them cracked or broken by heat, especially in amplifiers used by professional sound reinforcement folks. They were rather inexpensive carbon composition resistors, with tolerances of 10 or 20%. Today we use metal oxide resistors which are more reliable and have a tolerance of 2% (Mouser P/N 660-MOSX3CT631RR33G). They are only $0.25 each and you need 22 of them. I can send you some if you want.

Those original equipment transistors were made by children in Hong Kong sweatshops under contract by Fairchild Semiconductor for Phase Linear in the late 1960s and early 1970's (yours date to February 1972). They have no where near the Safe Operating Area (SOA) of the ON Semiconductor MJ21196G devices. Additionally, if you measure the gain of your PL909 devices, you will see numbers all over the chart. The MJ2116G devices are extremely consistent and have much higher gain - MUCH.

Due to the wide deviation of gain, the emitter resistors sometimes had to work hard and it is not unusual to find them cracked or broken by heat, especially in amplifiers used by professional sound reinforcement folks. They were rather inexpensive carbon composition resistors, with tolerances of 10 or 20%. Today we use metal oxide resistors which are more reliable and have a tolerance of 2% (Mouser P/N 660-MOSX3CT631RR33G). They are only $0.25 each and you need 22 of them. I can send you some if you want.

 

[OldPLguy]

The PL schematic DWG 402461 that I have shows for each channel output resistors as follows - 5 .33 ohm 1W, 5 .27 ohm 1W, 2 10 ohm, and 2 more shown as R36 180 and R 37 180 ( don’t know what those 2 are ). Is that what you have available or is something done differently these days? Also, if I replace all 20 output transistors should I also replace the 4 transistors ( two per channel ) at the bottom of the stack that are shown on drawing as Q11 SJ2741 and Q12 SJ2741?

 

[laatsch55]

Only if the SJ 2741's are bad. They don't carry much load being drivers...

 

[mlucitt]

The PL schematic DWG 402461 that I have shows for each channel output resistors as follows - 5 .33 ohm 1W, 5 .27 ohm 1W, 2 10 ohm, and 2 more shown as R36 180 and R 37 180 ( don’t know what those 2 are ).

Phase Linear used the .33 Ohm and the .27 Ohm resistors for Emitter resistors and we used to duplicate that setup in our rebuilds. Now we just use .33 Ohm 3W 2% resistors for all the the Emitters. The two 10 Ohm resistors are there to provide the voltage drop necessary to read the bias voltage (normally about 350 mV) and the 180 Ohm resistors provide a bias voltage for the base of the Pre-driver transistors on the bottom row. The original carbon composition 10 Ohm and 180 Ohm resistors typically do not fail, but will drift in tolerance over time due to heating cycles. Much better to replace them with modern metal film 1% 1/4W resistors because they are inexpensive.

 

[OldPLguy]

One pair of resistors R47/R48 10 ohm 2W between + and - output terminals for speakers shows signs of being overheated ( burned looking ).They read 5 ohms just like the other pair, but seems like it makes sense to replace all 4 ( and maybe the 2 C19 .1 capacitors that go with them ) along with the 4 180 ohm, the other 4 10 ohm, and the 20 .33/.27 ohm resistors. What do you think and can you supply these items and how do I proceed to acquire them? Phase Linear 700B, Serial Number 6415.

 

[George S.]

We buy most of our electrical bits and parts from Mouser Electronics and some from Digikey. The websites are easy to use once your aquanted with them. For "antique parts" like old out of production output transistors, many members here may have what you need in old used parts. Most, but not all of us ditch the old point to point back plane wiring, control board, transistors, etc, and put in all new circuit boards and parts from White Oak Audio Design.
There are technicians who frequent the forum who can repair it for you if you don't want to do so yourself.
If you really want it to last and be trouble free then invest in WOPLing it. Make it White Oak Phase Linear.

 

[George S.]

Here's what the backplane kit looks like. There are two circuit boards in that bag, they replace that old almost 50 year old point to point wiring with over engineered modern components.

 

[laatsch55]

One pair of resistors R47/R48 10 ohm 2W between + and - output terminals for speakers shows signs of being overheated ( burned looking ).They read 5 ohms just like the other pair, but seems like it makes sense to replace all 4 ( and maybe the 2 C19 .1 capacitors that go with them ) along with the 4 180 ohm, the other 4 10 ohm, and the 20 .33/.27 ohm resistors. What do you think and can you supply these items and how do I proceed to acquire them? Phase Linear 700B, Serial Number 6415.

The R/C network on the speaker posts are the" Zoebel" network. If they are burnt lookin, replace them. Caps too. The 4 180 ohm usually are not wounded, if they read right leave em. The 10 ohm are the bias resistors, order some 1% Metal Oxide or Metal film resistors. The .33, .27 ohm emitter resistors should be replaced with 1% metal oxide or metal film also. All can be ordered as George has said from Mouser.com.

 

[laatsch55]

We buy most of our electrical bits and parts from Mouser Electronics and some from Digikey. The websites are easy to use once your aquanted with them. For "antique parts" like old out of production output transistors, many members here may have what you need in old used parts. Most, but not all of us ditch the old point to point back plane wiring, control board, transistors, etc, and put in all new circuit boards and parts from White Oak Audio Design.
There are technicians who frequent the forum who can repair it for you if you don't want to do so yourself.
If you really want it to last and be trouble free then invest in WOPLing it. Make it White Oak Phase Linear.

Or old NEW parts...