PPI A200 wont power up

[Dr Zeus]

I've got this and a working A600.2 if its worth referencing parts for fixing this A200. This amp will now power up on the bench. No LEDs, no drive voltage to the fets. Non of the fets nor outputs are shorted. All outputs are measuring exactly 47 ohms across legs 1&3. It has a SG3525A:

SG3525A
Pin 1: 0.287
Pin 2: 2.568
Pin 3: 0.023
Pin 4: 0.338
Pin 5: 2.058
Pin 6: 3.631
Pin 7: 2.047
Pin 8: 0.005
Pin 9: 5.98
Pin 10: 0.001
Pin 11: 0.009
Pin 12: 0.001
Pin 13: 13.21
Pin 14: 0.010
Pin 15: 13.21
Pin 16: 5.16

 

[1moreamp]

how about the voltage on the fets leads ? what do they read ?

 

[Dr Zeus]

Fets are IRFZ44. This amp is 100% original never been worked on.

Leg 1: 0.009
Leg 2: 13.77 (+B ~0 ohms)
Leg 3: 0.001 (Ground ~0 ohms)

 

[ivkata]

u don't have any output of the SG3525A
so i think its replated no power

 

[Dr Zeus]

Are you thinking the sg3525a is defective?

 

[ivkata]

there should be an op amp as well near the sg3525a maybe u should check for power on it and output as well.In the past i had units fail to power due to bad op amp .Im newbie in this as well but hey 2 heads are better than one

 

[Dr Zeus]

Hmm this amp is different that what you are thinking, or a different rev. There are only 2 opamps in the whole amp and only in the audio section.

 

[ivkata]

how about the rp1 ic in the past i had orrion that failed to power due to the ic not getting power .have u tested transistors near the 3525a

 

[Perry Babin]

What's connected to pin 8? If nothing is connected to it, preventing it from going to 5v, I'd suggest replacing the IC.

If you short pin 8 to 5v (pin 16), does the IC produce output?

 

[Dr Zeus]

Pin 8 is connected to the 3rd leg of an MPSA06 at Q40.

Q40:
Leg 1: 0.001
Leg 2: 0.685
Leg 3: 0.005

Shorting Pin 8 & 16 only change Leg 3 of Q40 to 5vdc

Shorting Pin 8 to Pin 16 allows the amp to power up with regulated PS, but after about 10 seconds the amp flickers on/off with power and low-imp LEDs flickering. During <10 seconds regulated voltage look OK, but rail voltage drops from +-24v down to +-22v. Once it hits +-22v everything flickers.

 

[Perry Babin]

What's driving the base of Q40?

What happens if you remove Q40?

 

[Dr Zeus]

Theres a 220 ohm resistor from ground, connected to a 10k resistor, connected to the base of Q40. Between the 220ohm and 10k resistor is a 2n5064 at U4. The cathode (Leg 1) of this thyristor is connected b/t 220ohm and 10k resistor.

I'm not sure if I can test a Thyristor using diode check.

With Thyristor 2n5064 removed
Red on Leg 1 (Cathode)
Leg 1-2: OL
Leg 1-3: OL

Red on Leg 2
Leg 2-1: 0.641 (beeps)
Leg 2-3: OL

Red on Leg 3
Leg 3-1: OL
Leg 3-2: OL

With both of these TO92 transistors removed, and the jumper b/t Pin 8&16 removed on the SG3525, the amp powers and stays powered without flickering, no voltage drop.

The MPSA06 measures good off the PCB. I put it back in (With the Thyrister still out of circuit and the jumper still removed) the amp powers on forever.

I'll trace what this thyrister is connected to after I get back from an errand

 

[Perry Babin]

The SCR is likely being used as a latch. With DC, if you charge the gate of the SCR, it conducts and continues to conduct until voltage is removed from the other 2 terminals.

Yes. follow the circuit back to see if you can determine what's driving it.

With it out of the amp and everything clamped down, does it produce clean audio into a load up to clipping?

 

[Dr Zeus]

I think I figured it out. I had noticed the two 50v caps close to the thyristor were a tad loose before, but now they are broken of the board. They may not have been connected up before. I will replace them and glue the new ones down but need to buy these new. I don't think amp is gonna work otherwise.

 

[Perry Babin]

'Thyristor' is a type of semiconductor that covers multiple components. You have an SCR (a type of thyristor).

 

[Dr Zeus]

Much to learn

I carefully put a pair of temporary caps in place of the 50v/1000uF units. The temp ones are 50v/2200uF. They are way to big to fit under the cover, but they allowed the amp to power up with all components installed. Time to order-up some of the proper size.

 

[sonus movens]

Hi there,
I have a PPI 2200M, dual mono. I have an anomalous absorbtion on the left channel. I put a 21w bulbe and a 3A fuse, on both +12v inputs in order to prevent irfz44 burning. When I turn on the amp, the left bulb lights up, the right one not. Left channel sounds lower and without dynamic, the right one is fine. All mosfets are ok, all final trnsistors are ok too. After inspection, I found some problems on the PWM chip, a SG3525A: higher DC values if referred to the right one. I checked the chip, reversing the left one on the right side and viceversa, and the problem remains on the left channel, so SG3525A are both ok. I checked also the transistors near to SG3525A but everything seems ok. Have you any idea?