BIG problems

[Michael F]

Hey Michael,

Going with that post you made earlier (major negative on the right channel??) and from the pics, look very carefully at that power buss wire to the far right on the channel strip of transistor sockets next to the speaker outputs. Although I can't say positively, it sure is very close to the transistor screw threads securing the right side of the transistors? Is it making any contact with the screw threads? Could it have when the DCP was installed?

Do you have a ohm meter? If you do, pop all 4 fuses, then measure across the 4 diodes that bridge across the center tabs of the chassis. If you have a dead short (less than 3 ohms)... you have problems. Also measure with one lead on the chassis itself (I normally use a screw hole with an alligator clip on it), the other across the collectors (should be the larger center contact on the back of the transistor sockets). That could be an indication of a mica insulator failure.

Hi JB, thanks for chiming in. I think I know which rail wire and TO3 mounting screw you are referring to: while close, there is no penetration, there is 1.3M between all XPL 909 and RCA 410 collectors to chassis.
I`m not sure which diodes you are referring to, the 1N4004s?.
I wish I had a schematic for my particular amp

 

[laatsch55]

I thought you did have a schematic?? Did you have AGX or AGC fuses in?? The reason I ask is if ypou put AGC's with Don's board they will short out on the bottom of the DCP to the tip of the fuseholder...

 

[laatsch55]

PL 14 board, what do you have in DC volts between the +/- speaker posts??

If it's less than 100 volts DC +/- and it's not blowing fuses, there's a 50% chance it's Q1-Q4

 

[Michael F]

I thought you did have a schematic?? Did you have AGX or AGC fuses in?? The reason I ask is if ypou put AGC's with Don's board they will short out on the bottom of the DCP to the tip of the fuseholder...

I`m not sure which schematic to use for my serial #
Fuses are AGX

 

[Michael F]

PL 14 board, what do you have in DC volts between the +/- speaker posts??

If it's less than 100 volts DC +/- and it's not blowing fuses, there's a 50% chance it's Q1-Q4

Dumb question: should this be tested with the bulb in circuit? If you look back a few posts of my findings after replacing 5a fuses, all tests were made with bulb in place.

 

[laatsch55]

That's not a dumb question and ABSOLUTELY. The DBT will save you weeks of trouble from finding shorted or open board components...

 

[laatsch55]

Any schematic showing a PL14 board. Phase did not keep that close of documentation as to what serial numbers had what changes, at least not at the level you could COUNT on. The changes made to the 14 board do not preclude separate troubleshooting procedures for each one.

 

[Michael F]

Developement

Both channels D14 1N4004 are shorted

 

[Gepetto]

That is an unusual failure mode. Not seen often.

 

[Gepetto]

A shorted upper transistor device will look like that too...

 

[Michael F]

That's not a dumb question and ABSOLUTELY. The DBT will save you weeks of trouble from finding shorted or open board components...

OK, so unless specified to the contrary, the bulb shall remain in. I just discovered both channel D 14 diodes are shorted, do you still want me to put fuses in and measure DC on the output posts?
You think it would be wise to disconnect the meters? I do not wish to ad to the casualty list.

 

[Michael F]

A shorted upper transistor device will look like that too...

The diodes in question are visibly charred

 

[laatsch55]

I've hever had a meter fail due to a board problem but it's not impossible. D14,15 can look horrible and still be good. The only certain way to check them is out of circuit.

 

[laatsch55]

Not even close to powering up yet. Got lots more stuff to check. D13 will check .79 each direction, so it has to be out of circuit to check also( one leg will do...)

 

[Michael F]

I've hever had a meter fail due to a board problem but it's not impossible. D14,15 can look horrible and still be good. The only certain way to check them is out of circuit.

I stand corrected, out of circuit they measure good

 

[Michael F]

Not even close to powering up yet. Got lots more stuff to check. D13 will check .79 each direction, so it has to be out of circuit to check also( one leg will do...)

D13 measures like a normal diode out of circuit and not .79

 

[laatsch55]

Meter in low ohm scale,negative lead on collector bus bar, positive to emitter resister of each output. You're looking for 10 ohms, a shorted output will show in the tenths of ohms.

 

[jbeckva]

Both channels D14 1N4004 are shorted

And that is with all 4 fuses removed, correct?

 

[Michael F]

Meter in low ohm scale,negative lead on collector bus bar, positive to emitter resister of each output. You're looking for 10 ohms, a shorted output will show in the tenths of ohms.

Auto scaling Fluke 75. ready for this?

*Left Bank 1
All outputs measure 11R, driver measures .8R

*Left Bank 2
All outputs measure .5R, driver measures .5R

*Right Bank 1
All outputs measure .7R, driver measures 11R

*Right Bank 2
All outputs measure .7R driver measures 190R

Measurements taken on emitter leg and not through resistors

HTF did that happen?

 

[jbeckva]

A shorted upper transistor device will look like that too...

Yep. emitter to collector on any of the five. Trick is now being able to see which of the five.. pop 'em out one by one, etc...