Crate PA-4 help

[mresistor]

+ I've got a Crate PA-4 on the bench.  I messed with it about a year ago, in the process I installed IC sockets.  The problem is that the -/+ 16vdc to the preamp gets loaded down when the preamp is connected. I have injected a signal into J1 pin 4 at the power supply board and it makes music. so I consider the power amp to be working. I also can plug in a guitar to the Line in and it also works.


I have removed all the NE5532 ICs and the +/- 16v is still getting loaded down to -2.5vdc and +2.1vdc. The reverb tank is also disconnected. 
I am starting to think one of the input jacks is shorting out. There are 4 of them. The are the combo type.


Could someone with fresh eyes take a look and maybe you can think what might be the problem or point me in a positive direction to discover the problem.

[sluckey]

Look at the stuff on page 1 locations G3 and G4.

[mresistor]

Thanks Sluckey   I was also thinking it might be one of those capacitors..  I'll check that area out..  the power led lights up   

[mresistor]

Resistors R27 thru R31 check good.  The caps all look good physically I will have to take a lead up to check them I guess. They all read 30nf or so on my Fluke 87.


I'm going to start checking some 1N914 diodes at the inputs..

[mresistor]

Ok a little perplexed.  I have every diode with one leg lifted.. they all check good.  And I have C26, C27, C28 and C29 removed from the board.  Neither one of my Flukes measures these little 10uf capacitors.


I hook the preamp up and turn the unit on with all the above, and I still get 2.4v and -2.4v where there should be 15v and -15v.

[sluckey]

What do you get where you should have 16v and -16v?

[mresistor]

Sorry 2.4v +/- is what is at the wiring harness that leaves the power board when it is plugged in to the preamp board..  and it is 16.6v and -16v when the harness is unplugged from the preamp board.  I said 15v by mistake..

[shooter]

the harness is unplugged from the preamp board.

when "that" plug is un-done, do an ohm check + to ground and - to ground on the preamp side wires

[mresistor]

Shooter   are you saying  leave it connected to the power supply side and the measure ground to +/- 16 pins on the preamp end.. ?


The harness is presently not connected to the PS.. it IS connected to the preamp..

[mresistor]

harness connected to PS not to pramp..  checked pin 1 at preamp end to pin 2    - reads 2.3k   
pin1 to pin 3   it's like it's reading a capacitor..

[shooter]

I misunderstood, thought there was a main power distribution cable on the PS board.  my thought;
if you have a "load" that sucks B+/- from 16 to 2, you have current that increased by 80ish%, something should be getting real warm, if not flaming.  If you have a bad PS, as soon as you "add" a load, it goes limp.

the way to "test", come up with a dummy load R to put on the PS, if the supply is good, should stay at 16vdc

[mresistor]

I had it on for a bit testing the voltage and the 120 ohm 10W resistors on the PS board were getting pretty toasty..   


I was thinking to try to put a load on the PS to simulate the preamp but I haven't figured out what to use yet..   I guess two 5 watt resistors  should work, but what value?


I have also started to think that the PS has the problem not the preamp board, just as you say. 

[mresistor]

OK  I hooked up two 470 ohm 5w resistors as a load substituting for the preamp load on the 16+/- volt supply. The PS 16+/- outputs held steady.

[shooter]

I've been burnt couple times with bad PS, so now you know what direction, something that shorted should fail the knuckle test.  touch each IC, and transistor, CARFULLY, somebody should be getting hot, associated R's also
 

[mresistor]

Well Sluckey was right.  C31 was bad..   I replaced it with a physically much larger film cap of same value,  the stock one is not much bigger than a pin head.   The +/- 16v now holds steady with the preamp board plugged in..   I will re-solder all the diodes and caps I disconnected and re-install all the ICs and see if this thing is going to work.

[mresistor]

Works great    thanks Sluckey and Shooter for the help