HP 6220b use

[oldsoul]

Hello,
First time poster and complete novice. I purchased this HP 6220b with the hopes of using it for guitar pedal and modular synth projects I've been working on. I got it from eBay so I know that can be a cause for concern. I think part of this might be my lack of understanding of how to operate it but had a few questions. It seems that upon power up I can't get the voltage on the meter to go up at all. I tried measuring the voltage with a multi meter and only got low mV values. Can someone direct me how I can perform some basic tests to see if this unit needs servicing or I'm just not operating it properly? Again I'm completely a novice and pretty confused by some of the stuff in the manual. In the attached PDF page I can't seem to get step d of 3-1 to do anything. The meter stays at 0.

[TimFox]

Make sure that the current limit control is not fully counter-clockwise (zero current), as that will normally override the voltage control even into a high-resistance load.

[oldsoul]

Nope that doesn't help. When I set the meter switch to either 1.2A to .12A I can adjust the current control to get a reading. But When I put the meter switch in either 60V or 6V range the voltage control does nothing and the meter stays at zero.

[macboy]

Ensure that the straps are installed exactly as in the manual page you posted, Fig 3-2. These are important connections.

With nothing connected to the power supply, the current meter should not rise above zero (nothing is drawing any current). The voltage output should respond to adjusting the voltage control. Your symptoms indicate something is short-circuiting the output.  This model does not have a crowbar (a type of over-voltage protection circuit) but it does have a reverse-biased diode across the outputs. This diode is intended to protect the supply from reverse voltages, such as if you connected another voltage source like a battery backwards across the terminals, or under some conditions when using two supplies in series. I suspect this diode is failed short. It appears to be CR34. Remove it, test and replace it if necessary. I'd use a 3 A diode like 1N5401 (5404,5407, etc).

[oldsoul]

Well that diode is indeed fried. What a pain to try and get access to the bottom of that board. Holy moly. Would you recommend any other preventative maintenance like replacing the electrolytic capacitors? Is that diode fry indicative of something else wrong?

[macboy]

Well that diode is indeed fried. What a pain to try and get access to the bottom of that board. Holy moly. Would you recommend any other preventative maintenance like replacing the electrolytic capacitors? Is that diode fry indicative of something else wrong?

The fried diode was probably due to user error.  HP used top notch components back then, don't replace them unless needed. That said, if you have the board out and you have an ESR tester, now might be the time to check them out.

[oldsoul]

Hello,
I replaced the diode and checked the electrolytic. They tested fine. Still can't get the voltage above .85 V. Anythought on what else this could be?

[alm]

Since it appears the diode at the output got fried by reverse voltage, I would check the other transistors and diodes in that path. For example, the pass transistor might be damaged by reverse voltage at the b-e junction. I would measure all diodes and b-e and b-c junctions of all transistors with the diode check function of a DMM (observe polarity for npn and pnp).