The only thing left to fix of this bench power supply is the second 25V/1A output's display and that has proven to be quite more difficult than I expected. The display is used to read out the voltage and current of the output, but didn't display anything. The display is built around a ICL7106CPL, an 4049 RC oscillator for -5Volt generation and a CD4030 for selecting the decimal points for the voltage and current mode. Just like the powersupply, I've reverse engineerd the schematics with Gimp:
The previous owner had already tried to repair it, but didn't succeed. The negative supply trace was cut and after I fixed that, I found out why: A negative voltage of -2 Volt was pressent on multiple pins of the ICL7106, which was applied to the LCD display and the CD4030. Also the -5V supply wasn't stable and was around -3 Volt. Therefore I could only conclude that the ICL7106 was broken, so I cut it off and installed a new ICL7106 with a DIP socket, just in case if I need to remove it again. This improved the situation some what, now the display still din't display any value, but the negative voltage was now gone and the -5 Volt was stable.
From application note AN052 from Intersil "Tips for using Single Chip 3.5 Digit A/D Converters" I learned that if the display didn't display anything the oscillator wasn't working. With my scope I checked the OSC pins (pins 38-40) and the oscillator wasn't working at all. I checked the components, but they where all fine. I read somewhere that I could use an functiongenerator to apply a 5V clock signal to pin 40. Since I have no functiongenerator, I've used the -5V generator output pin 12 to create a clock signal on pin 40. The -5V circuit operated on 24kHz, while the OSC components are selected for a 48kHz clock. I've added an additional resistor in parralel to R3 to create a clock of ~46kHz.
Now the display shows the following:
From AN052 it can be read that when the display shows -1666 constantly, the OSC isn't working, eventhough i supply a clock of ~46 kHz. So I'm at a loss what could possibly be wrong with the display.
I've checked the following:
- The reference voltage is 1V.
- The -5V is stable.
- The BP/GND pin has no PWM output, it's a DC voltage. The working display on channel 1 has a ~66 Hz square wave on it.
- Ive swapped the LCD displays, both are working fine.
The things that occured to me which might be the cause:
- The integration capacitor C8 must have a good dielectric absorbtion according to the datasheet, but as you can see in the included board photograph the capacitor is slightly damaged. The previous owner must have been waving his soldering iron around and seems to have melted the housing of capacitors C7, C8 and C9. On the integration capacitor the foil inside the housing is visible. The cap still measures 470 nF in circuit with my multimeter, but might this have degraded it's dielectric absorbtion rating that it even stops it's OSC circuit in the ICL7106?
- The C9 cap on channel 1 and channel 3 of the power supply are both marked as 1J50, while for channel two the marking is 1M50. The solder on C9 and C8 is so clean, I don't think the previous owner has replaced them, but it's quite odd that the markings are different from the other channel. Does anyone know what capacitance value this migh be or what the M and J might mean?
I don't have any other ideas as to why the display isn't working with a new ICL7106, any help or comment is appreciated![/list]