Laney dp100 bass amp disaster

[nosaj]

Ok so now the power amp without the preamp connected power resistors not getting hot. When connecting wires to preamp power resistors get really hot. Pulled the zeners to test next. With zeners pulled I get a power and eq light still pulling very little current. Will post results next

nosaj

[g1]

Yes, 3 tip31, one tip32.
Clip black to base lead (not case!). Check B-C and B-E.
Now clip red to base and again check B-C and B-E.
Now check C-E, with black to E, then reverse and check C-E with red to E.

1st meas. (black to base) will show approx. .6V for both b-c and b-e for PNP, open for both for NPN.
2nd meas. (red to base) will show open for PNP, .6 both ways for NPN.
C-E should show open both ways for both types of transistors (except in rare cases).

Darlingtons will have B-E double of standard transistors, which some meters will have trouble with, good meters will show 1.2V instead of .6V

[nosaj]

Can I do this in circuit or does it need to be out?

Oh yea those answers I provided were from a diode test on the DMM.


Thanks,
nosaj

While I was waiting i decided to see if it was a preamp or output issue .Both work. From the input it also works Sounds good too. Tested those 12v Zeners all 4 are shorted. I'm Thinking Someone put header cable on wrong before and shorted them? All lights and switched appear to work. But when noise gate switch is flipped, dim bulb goes full on. Thinking its has to do with the zeners and the 15v rail going to ne571.

[nosaj]

Ok, update here. Got 12v zeners replaced. With the volume up on the dial 8 or 9ish playing will fade away. It also does this if I input into FX return it does the s/ame. I'm thinking filter caps possibly.


Other things for later

pre shape switch is noisy possible tlo72s?

Limiter switch when flipped makes a wild noise and ammeter starts to rise.

So far that's it for now I think we're supposed to work backwards from speaker forward so I'm gonna check filter caps. Unless someone has a better idea.

Thanks all,

Nosaj

[Enzo]

If you have some amp, and the main filter cap breaks off the board, what happens? Do you get no sound? Or do you get a lot of loud hum? I don't think a filter cap is causing the sound to fade away.

[nosaj]

If you have some amp, and the main filter cap breaks off the board, what happens? Do you get no sound? Or do you get a lot of loud hum? I don't think a filter cap is causing the sound to fade away.

A lot of hum because your loosing dc filtering resulting in more ripple?

Ok, looking at his amp further it looks like there is a fx loop on the power board and one on the preamp board. I cleaned all the jacks and pots reflowed the solder on the jacks.
Still cutting out when hooked to the input or flipping any of the switches around.
This time I plugged into the fx return on the power board and could not make it cut out. Even inputted 1kz tone from hp200 ramped up to full output on hp no cutout. So I think we can say output is healthy and that the issue is on the preamp board.

Ok so I read the manual for the amp http://pdf.textfiles.com/manuals/STA...0,%20DP300.pdf


And I'm getting the idea this is all related to the limiter switch cause one way I get bad noise and the other the sound cuts out when played hard.

I can pull voltages and match them up with what listed on the schematic and report back.


Am I on the right track?

Thanks,

Nosaj

[J M Fahey]

Ok, there is a bit of a problem here, I think it started as a typo:


Quote Originally Posted by J M Fahey View Post
Each of them needs to pass 6 diode tests:
on diode scale:
* with black probe on base, Vcb and Vce
* same with red probe on base
* base open, Vce with black on emitter
* base open, Vce with red on emitter



Where he said "black probe on base, Vcb and Vce" (same with red probe on base), I think it should be Vcb and Vbe.

But looking at your results, it appears you may have been measuring from base to each of the other legs anyway.
Of note is that BtoE will almost always measure a bit higher than BtoC.
Just to be 100% sure, can you verify that your higher numbers where BtoE (not CtoE).

Yes, you are right, I mistyped: when one probe is always on base, measurements possible are Vbe and Vbc .
Thanks for catching that

[nosaj]

Pins 1, 10, 14, 16 are showing abnormal. I'm thinking for pins 10 and 14 I need to checkout c47.
Pin1 r75 and c52 need to be checked
Pin16 I'm going to check r65.

Any thoughts?

Thanks,
nosaj


Voltage reading schematic listed

pin1 1.05vdc .07
pin2 1.82 1.8
pin3 1.80 1.8
pin4 -0 ground
pin5 1.84 1.8
pin6 4.72 4.7
pin7 7.93 7.8
pin8 1.84 1.8
pin9 1.84 1.8
pin10 7.50 5.1
pin11 1.82 1.8
pin12 1.85 1.8
pin13 14.91 15
pin14 1.72 - 1.92 2.3v
pin15 2.38 2.3
pin16 .03 1.8

[nosaj]

C47 C52 both test ok on ESR
r75 2.2 meg meters wouldn't check it correctly subbed in a new
r65 330r test ok.

Only other thing I can think of is maybe C49. But I don't quite understand how this works.

I can pull voltage at R70 and R69 voltage at pin 10 is 7.5vdc and should be closer to 5.1.

When limiter switch is flipped it's a nasty DC noise.

Any ideas would be greatly welcomed .

Datasheet sa571 or ne571 http://www.mouser.com/ds/2/308/SA571-D-99895.pdf

Thanks, nosaj