Since there is very little feedback from the community I will continue this thread as part of my therapy 😕

Anyway I seem to have found the source of the problem! While investigating if the controller PCB #40 was alive I noticed that the !RESET signal is always low on connector P50 pin 4. This signal is generated from the transistor circuit seen below on the power supply PCB #60. If disconnect the connector P50 on the PCB #40 and manually short pin 4 to ground briefly, then the unit goes to stand by and lights the standby indicator diode. Shorting the TEST MODE connector under the glass panel enters the testing routines. I also get the CD door to open (though it needs some TLC, it doesn't move smooth) and the cassette door.  

So I'm going out tomorrow to get some new BC577B (TR15 & TR16) transistors and the C12 (2.2uF). Hopefully that will allow me to fix the problem. 

Stay tuned!

Sounds like a good place to start.
What kind of voltage do you have around TR15 ?

Martin

Hi Martin,

I have done some measurements:

Conditions: connector P85 (containing pin 8 !RESET signal to MCU PCB #40) connected

Measurements:

TR15:

Base 6,75V

Collector 4,97V

Emitter 1,3V (RESET)

IC4:

Pin V1 11,5V

TR16:

Base 158mV

Collector 15,5mV (!RESET)

I assume that the circuit should work normally so that while the system voltage is on the rise the TR16 allows current to pass until TR15 becomes saturated? So would you say that the TR16 is the problem (always energized)? I'm a bit rusty in the analogue domain. 

Since the MCU board comes alive when I disconnect the connector from the PSU board and manually ground the !RESET pin briefly I'm thinking the problem should not be there?

Mikael

Mikael wrote:

TR16:

Base 158mV

Collector 15,5mV (!RESET)

Sounds like either TR16 or C12 is shorted.

Hi,

I'm not so sure it is shorted. There is approx 150kOhm across TR16 collector and emitter when !RESET is connected to the MCU pcb #40. If open connector then i read several MOhms on the same spot.

/Mikael

Hi Mikael,

as mika said check TR16 and / or C12. The collector of TR16 should be around 0V only for a short while after power on. Then it should rise to nearly 5 Volts. You said that you get 15,5 mV at the collector that's logical a '0'. The RESET line is low-active due to the line above RESET. and a logical '0' is low. so your processor gets a constant reset signal and cann't start working it's propgramm. If you disconnect the reset line as you done before and connect the RESET line briefly to ground the µP can start as you stated.

Ralph-Marcus

Hi,

I'm leaning towards replacing TR16 and maybe the C12 while at it.

/Mikael

TR15:

Base 6,75V

Collector 4,97V

Emitter 1,3V (RESET)

The TR15 emitter should be held at a constant 5V by the voltage supply.
I suggest you check this.

Martin

Ok, back from my little workshop. I've replaced both TR15 and TR16. That gave no improvement at all. More or less the same voltages as before. The TR16 collector is 50mV now so still a logical zero. 

Then I replaced C12 as well. That also gave no improvement.

Checking R36 and R37 gives resistance according to the schematic. 

Sigh, I am a bit out of ideas right now...

I guess moving on to check the mcu PCB which btw still starts up if I manually reset it.

/Mikael