RCL P1200 Amplifier Fault

[paul_g_787]

I have an RCL P1200 power amplifier and the fault lights come on and after a few seconds all LEDs flicker.

Does anyone have access the the service manual for this amplifier?

I cannot find any documentation for this amplifier or for RCL at all online! Like they never existed! 

When I turn on the amplifier and after approx 5 seconds the +12V rail drops to approx +5.5V and all the LEDs fade on and off randomly / flicker.

Inside is a PSU, 2 power amps and a I/O board at the back. The I/O board has 2 op-amps, I assume to buffer the balanced inputs as there is only a 2-pin cable going to each of the inputs of the two power amps.

Running the PSU board on it's own with everything disconnected outputs very stable voltages on all rails. So it would appear the PSU is good. The same with the rear I/O  board connected so I suspect this is fine too,.

However if I connect either of the power amp PCBs to the PSU, after approx 5 seconds the voltage drops. So perhaps both boards are faulty? Or is it something else I am missing.

I suspect that the amp probably 'switches on' a few seconds after powering up, and whatever the fault is only shows after this attempted switch.
I noted that the relay switches never click on either, I think this is because of the low voltage, they cannot switch on.

Any help is appreciated. Thanks

[paul_g_787]

The part no. on the power amp PCB is LSA-900-1 but I cannot find any info on this board online.

I have noticed that on both power amp PCBs there is a 4148 Diode (Right next to the big metal GND pin) which is soldered the opposite way round to the the silk-screen. It appears that one of these diodes has been replaced at one point as shown in the attached image, however on the other channel's power amp PCB the diode appears to be original and is also the 'wrong way round'. Perhaps the silk screen was printed wrong?

[Audiorepair]

Did this amp used to work, then didn't, or did you aquire this amp not working?

[paul_g_787]

Did this amp used to work, then didn't, or did you aquire this amp not working?

I have just been given this amplifier free from a local band. They told me it just gave up one day during a gig. It just muted and the fault LED on both channels came on. That is all I know.

I have checked the 8 large power transistors (4 per channel) and they are not shorted out.

I am guessing that when this startup protection circuit is attempting to switch, is the same time I see the voltage drop from +12V to +5.5V. However as stated the fault happens with both, or either of the two power amp circuits connected.

The -12V is fine and the two high voltage rails, measuring +/-67V in this case are both stable.

[paul_g_787]

I have fixed it! It took hoirs of head scratching but I worked it out in the end. I noticed that the cover on one of the relay switches was detached on one side and there is a donk in the casing on the corner. So I wondered if it had been dropped.

So after head scratching all evening I decided that as both power amps are experiencing the fault it is most likely the power supply, being common to them both.

So I whizzed out the power supply board to discover a horrible mess of a solder job.  Spatter everywhere and  flux all over the place. Fortunately leaded solder so not very much corrosion but some all over.

I then noticed a teeny-weeny spatter of solder between the GND and OUT pins of the 7812 regulator! I scraped it off with a multimeter probe and BAM! problem resolved! The amp is fully working.

So my guess is that the solder spatter was there from the factory and the donk to the unit must have moved the spatter ever so slightly!

Anyway, I re-flowed the whole power supply board and cleaned the flux off. It now looks like it should have from new.

So conclusion: Don't buy Sh*t brands and don't drop / bash your amp about.

What a silly problem that a few seconds in an alcohol bath at the factory would have prevented!