I have worked for many years with the VME team in Knutsford. Last time I popped in there were 4 Lab Gruppen power amplifiers on the bench looking a little forlorn.
Two were an easy fix, some new audio pots in one, new 400V caps primary side of the switched mode in another after a 3 phase fault wiped a pair out, surprisingly the bridge rectifier was fine.
That left two, a FP1000Q, a four channel unit which seemed fine visibly, the output stages seemed fine, but had no audio output on any channel, the other an FP6400 took a while to repair, and only this evening am I happy it is fully functional.
Fixing the FP1000Q : March 2020
The FP1000Q has a relatively straightforward, accessible design, but I did spend hours absorbing the service manual that I have marked up and provided here.
The block diagram is the only place some of the most important control chips are listed, for example, U414 the chip controlling the channel MUTEs, the actual cause of the mute output.
The short story is that the Atmega16L chip had failed, shorted internally 5V to 0V ( check 5V at test point Pin 2 ISP header port ) This was an odd and unexpected fault as normally control circuitry is well protected by at least two levels of power regulation, primary PSU and further linear regulation to 5 / 3.3 V as in this amplifier.
A quick clue ( for any uP ) being the crystal was not running which I find is one of the easiest things to check. I took the crystal out and it worked fine on a spare Arduino board I had ( 3.28MHz )
It seems a little odd to loose a uP at all, stranger that it should cause the failure of all audio, read on.
I will start the conclusion first.
All components refer to Channel 1, eg R29, thus channel 2 equivalent is R129 / Chan 3 = R329 etc
With reference to Page 21 of the PDF. If the uP is NOT running, pin 15 of U414 is LOW, and thus Pins 11-14 signals MUTE channels A/B/C/D are not enabled and thus 'high Z'.
Page 22/3/4/5 : MUTE A/B/C/D lines float up to 2.4V via R29 ( for channel 1 ), enough pull up for Q23 and Q4 to turn ON.
Q23 Collector pulls opMute LOW, which under normal conditions would hold at around 15 Volt via R62 and Zener D17 (15V ) detailed on Page 1
Further, Q4 pulls pin 2 of U3 low, the opto isolator limiter ,thus limits the amplifier output significantly even if Q23 is removed ( Channels 1 & 2 were both faulty ).
This was a strange fault. I had tested each of the amplifier boards powered from a +/-32V power supply. The MUTE signal is pulled high to ~14 V from the 160V +VRail, dropped through R62 and regulated by D17 15V Zener.
The relevant parts of the MUTE signal ( which changes name from MuteA to Mute to OpMute ) over three pages
Replacing the Atmega : Removing the chip was straightforward. Before placing the new one, I checked that the power rails were working with no chip present, they were and were good once the new chip was installed.
The ATMega16L ( L for low power / low voltage , you will note that the chip does not receive 5V from Q404, the emitter follower providing about 3.5V to pins 5/17/38)
More on that here which was more of a traditional, large amplifier repair.
I can always be emailed with the hex file for this chip at w e b 2 0 a t i w e u a d 0 t com
From factory the Low Byte is 0xE1, the chip running from internal oscillator. From the manual it looks like the Low Byte low nibble ( bits 0-3 ) needs to be set to 1111 for 3.28 MHz ext crystal.
Reading the Low, High and Lock bits from another FP1000Q LOW : 0xBD HIGH : 0xD0 LOCK : 0x3C ( LB1 & LB 2 set to Zero so you can't copy a good chips....shame... )
To read fuses AVRDUDE + a USBtiny or USBASP, the Sparfun Pocket AVR Programmeris mycurrent choice for support and avrdude -c usbtiny -p m16 -P usb -U flash:r:-:i
The terminal mode is a straightforward method of writing to fuses directly
avrdude -c usbtiny -p m16 -P usb -u -t
>>dump lfuse // you can just type 'd lfuse' then to write to the fuse >>write lfuse 0 0xEF // this will set all bits of CKSEL to 1 ensuring the external Crystal will work >> d lfuse >> quit
So that is as far as I got. I removed R29/R129/R229 and R329 And I shorted across the LED diode ( Pins 1&2 ) of the limiter opto U3/U13/U23/U33 to prevent limiting. This brought the amplifier fully up, but far from factory restored as the control side was not working, which for this application had never been used anyway, so no great loss.