HELP! Rescuing Dad's old PL400 S2 (pics)

[gadget73]

The op-amps are the two small black chips towards the top of the board, each has 8 legs. It stands for operational amplifier. Its basically a small nearly complete amplifier circuit in a handy-dandy package. Add a few external parts depending on what you need it to do, and off you go.

 

[lightfall]

With 8 legs I see why it would be the last, most complicated, thing to test! I have to install a handrail today with my Dad, I will show him the progress, and then come back later to finish testing the transistors and diodes.

 

[gadget73]

The other handy thing about op-amps is that they rarely go bad unless you zap them with too much voltage. if the zener diodes didn't go open circuit, chances are good that the ops are fine too.

 

[lightfall]

I just tested the two half dome transistors with 3 legs along the top edge of the board and I got 1.9 testing one way and .67 the other way.

Testing the four small black square square transistors with 3 legs across the middle of the board I get .18 testing one way with no conduction on the other prong and .6 the testing the other way but I have no idea where EBC is on these and I do not hear a beep with any combination.

 

[lightfall]

The two large stone-like resistors along the left edge that show 7.5k test at no conduction for the top one in 20k mode and 17.4 on the lower one. No visible signs of damage.

 

[lightfall]

The four transistors with heatsinks on the bottom of the board test like the following:

+ middle prong negative left prong .6, + middle negative right prong nothing. Negative middle + left nothing, negative middle + right 1.5.

 

[lightfall]

I just noticed my parts diagram says the left most square resistors should be 1.8k 5w and mine say rockwood 7.5k 3w

 

[lightfall]

Also, the two large metal square surrounded transistors (q106/206) check out .56 one way and no transmission the other way.

 

[laatsch55]

Brad , you must identify the Base, Emitter and Collector leads and whether they are NPN or PNP. And when you post your results say something like " Pos lead on Base, Neg lead on Emitter--reading .454 volts DC..

 

[lightfall]

OK, I suppose I will go back and try to determine the E B C of the four black square bottom transistors Q102/202 104/204

 

[lightfall]

I have to ask for a bit more insight: I spent quite a long time determining which output capacitors had went, there were 2, and the plan was made to replacing all of the output transistors. Now I am testing each transistor and then resistor; but if I find a bad one will it be replacement PCB time or will it be an individual component replacement situation? Will I still be needing expensive testing equipment like an oscilloscope and tone generator? What is the reasoning behind the current actions; if I determine any particular transistors are blown will that tell me exactly what went wrong, and then how can I apply that knowledge? Just some more insight into the reasoning behind these steps and where they are taking me and why would help keep me going here.

 

[lightfall]

Q103/Q203:
Pos lead on Base, Neg lead on Emitter--reading 1.847 volts DC. Pos lead on Base, Neg lead on Collector--reading climbs to 1.99 then NOTHING. Neg lead on Base, Pos lead on emitter -- reading 0.69. Negative lead on base, Positive lead on Collector--reading 0.67.

Pos lead on Base, Neg lead on Emitter--reading 1.86 volts DC. Pos lead on Base, Neg lead on Collector--reading climbs to 1.99 then NOTHING. Neg lead on Base, Pos lead on emitter -- reading 0.69. Negative lead on base, Positive lead on Collector--reading 0.66.

 

[laatsch55]

You stated in the beginning you didn't want to do upgrades, ie, the White Oak boards. So we are trying to systematically test all components that may have failed. It will then be up to you to determine if it's worth fixing..you do not have to replace all the output transistors. You do have to replace the 2 blown ones with ones of the same type, IE, On-Semi-MJ 15024's. You have re-branded MJ-15024's as outputs , except for the 2 GE outputs, which should be replaced regardless of what you decide to do with the output section.

You will not need any expensive testing equipment to get it up and running. A dim-bulb tester is a must in my book. It will keep you from damaging components when powering the amp up after any work being done on them...

 

[lightfall]

Thanks Lee, that is helpful and motivational. Frankly, with the amount of time that I have into this unit even if I did have to do a WO PCB at this point it would be worth it to fix it. I would like to continue trying to do things the right way, I am just big on knowing how and why as I go. I will now try to determine the lower four transistors and post their results in a more clear format.

 

[laatsch55]

Q103/Q203:
Pos lead on Base, Neg lead on Emitter--reading 1.847 volts DC. Pos lead on Base, Neg lead on Collector--reading climbs to 1.99 then NOTHING. Neg lead on Base, Pos lead on emitter -- reading 0.69. Negative lead on base, Positive lead on Collector--reading 0.67.

Pos lead on Base, Neg lead on Emitter--reading 1.86 volts DC. Pos lead on Base, Neg lead on Collector--reading climbs to 1.99 then NOTHING. Neg lead on Base, Pos lead on emitter -- reading 0.69. Negative lead on base, Positive lead on Collector--reading 0.66.

Those 2 are good....correction....they do not appear to be shorted or open. A transistor can test good on the diode test, whether or not it has gain is another question. Gain is it's ability to actually do the job that it is in there for. It may seem frustrating that there seem to be no hard andf fast answers...that's because every component in the circuit affects and influences every other component. Thats why you get the funny reverse bias readings on Q103,203. Q103, 203 are PNP TO-92 (form factor) transistors.

 

[gadget73]

The original board can be repaired if there are bad parts on it. I rebuilt one for a friend, and I've done a little service work on my own. If it doesn't look like someone took a blowtorch to it, its not that big of an issue if you just need a couple parts. If it seems like everything is cooked, then you really should consider a replacement.



About the big sand resistors, the value on those changes depending which op amps are installed. If it doesn't look like they have ever been changed, it probably worked with them and I wouldn't worry about it.

http://home.earthlink.net/~dimlay/sitebuildercontent/sitebuilderfiles/seriesiiopamp.pdf has the specifics on which set of resistors goes to which op-amp type.


A scope isn't absolutely needed, but its a nice tool if you have one.

 

[lightfall]

Q101/Q201 (NPN):
Pos lead on Base, Neg lead on Emitter--reading .60 volts DC. Pos lead on Base, Neg lead on Collector--reading .61. Neg lead on Base, Pos lead on emitter -- reading 0. Negative lead on base, Positive lead on Collector--reading 1.55.
Pos lead on Base, Neg lead on Emitter--reading .60 volts DC. Pos lead on Base, Neg lead on Collector--reading .61. Neg lead on Base, Pos lead on emitter -- reading 0. Negative lead on base, Positive lead on Collector--reading 1.55.

Q105/Q205 (PNP):
Pos lead on Base, Neg lead on Emitter--reading 1.53 volts DC. Pos lead on Base, Neg lead on Collector--reading 0. Neg lead on Base, Pos lead on emitter -- reading 0.64. Negative lead on base, Positive lead on Collector--reading 0.65.

Pos lead on Base, Neg lead on Emitter--reading 1.53 volts DC. Pos lead on Base, Neg lead on Collector--reading 0. Neg lead on Base, Pos lead on emitter -- reading 0.64. Negative lead on base, Positive lead on Collector--reading 0.66.

 

[laatsch55]

Those look good too Brad..

 

[lightfall]

Q102/Q202 ():
Pos lead on Base, Neg lead on Emitter--reading 0 volts DC. Pos lead on Base, Neg lead on Collector--reading 0. Neg lead on Base, Pos lead on emitter -- reading 0.6. Negative lead on base, Positive lead on Collector--reading 0.66.
Pos lead on Base, Neg lead on Emitter--reading 0 volts DC. Pos lead on Base, Neg lead on Collector--reading 0. Neg lead on Base, Pos lead on emitter -- reading 0.58. Negative lead on base, Positive lead on Collector--reading 0.618.

Q104/Q204 ():
Pos lead on Base, Neg lead on Emitter--reading ,61 volts DC. Pos lead on Base, Neg lead on Collector--reading 61. Neg lead on Base, Pos lead on emitter -- reading 0. Negative lead on base, Positive lead on Collector--reading 0.

Pos lead on Base, Neg lead on Emitter--reading ,60 volts DC. Pos lead on Base, Neg lead on Collector--reading 62. Neg lead on Base, Pos lead on emitter -- reading 0. Negative lead on base, Positive lead on Collector--reading 0.

 

[laatsch55]

It looks like you have the wrong transistor numbers for the test. Q102, 202 are NPN, and your test results reflect good PNP's....