HP25C repair, help needed

[teenix]

(01-18-2019 01:48 PM)albertofenini Wrote:  We will be more than happy to hear from anyone that may have additional suggestion, on how to check the components

Thanks again for the precious tips !

Resistors are easy, get a color code chart from web and check against the component. Read with an ohmmeter. Remove components from the circuit first or you will get a false reading, unless you know what to expect.

Capacitors are best tested with a capacitance meter, or you can still use an ohmmeter. Some have capacitor checkers built in.
They should read a brief short circuit followed by open circuit as they charge up. Reverse the leads and the same should happen. You may not notice the short, depending on how big the cap is, the important thing is being open circuit.

You mentioned, the capacitors were hot, so they are probably destroyed. These will only get hot because of higher than normal current flow through them. They should only pass current with an AC voltage unless they are shorted. They can also be destroyed, quite literally, if they are of a polarised type and the applied voltage is reversed, such as the tantallum types in the power supply. I would replace these regardless.

Google "check transistor with analog multimeter" for checking transistors unless you have a proper checker. You mentioned yours were hot, so replacement might be a good idea, considering their age. Jacques Laporte mentioned for the HP25, 2N3704 and 2N3904, both of which seem available from Mouser.

Diodes should have a low ohms reading in one direction only. The cathode is marked with a line at one end of the component case, the other is the anode. An ohmmeter black lead connected to cathode, red to anode should pass current. The other way around should be open circuit. Some ohmmeters have a diode setting, or use around the 2K range. In the HP25 circuit, it looks like 2 different types, but IN4002 types might do.

One of the diodes in the HP25 circuit, connected to the base of a transistor and the 6V output, is a zener diode. These can still be tested as per a normal diode, but they are used to provide voltage regulation. You can test it by placing a 1K2 resistor in series and apply a rising voltage. Test with the +ve supply via a series resistor to the Cathode, and the anode connected to ground. Measure the voltage across the zener and when it reaches its breakdown voltage, it should stop rising, probably somewhere around 3-5V. I'm not sure what this voltage will be, so don't apply too high a voltage, say < 7. This diode should conduct almost no current until the breakdown voltage is reached, then any extra voltage will be dropped across the resistor.

The toroid coil has 3 coils wound on it, all with low resistance, so hard to read with an ohmmeter, but probably won't be shorted. Make sure no coils are open circuit.

If you remove the chips, de-solder the power supply pins last. If the chips have inbuilt protection from static, the power supply circuit will help absorb nasties. Solder these pins first when fitting back to the board. Best not to touch the pins with your fingers either unless you are protected against static build up. Store them wrapped in foil.

Your Pico Scope should help identify if the power supply is working. If it is similar to HP25, and if you have it isolated from the rest of the circuit, then you should read around 4V, 6V and -12V with respect to ground.

You could load up each supply output with say a 470ohm resistor for 4V, 680ohm resistor for 6V and 1200ohm (1K2) for 12V, all connected to ground. These will all draw about 10mA of current.

Be careful pulling out components when de-soldering them. Some of the vias in old double sided PCBs that connect one side of the board to the other can pull out with the component lead. These can be repaired with wire links, but messy.

cheers

Tony