Regulated linear power supply repair

[sirlemonhead]

Hi,

I'm a newbie but not sure if this belongs here or in beginners..

I have a plug in AC/DC wall adapter that broke a few years ago and has been sitting in a box. It's a Roland/Boss power adapter for powering guitar effect pedals. I believe the original problem was that output was intermittent but checking it now, I'm getting nothing from the DC output lead.

I think the problem is due to a broken ceramic capacitor. I'm trying to identify a suitable replacement. I have the capacitor removed from the circuit and I tested it with my multimeter (a Brymen BM257s) in capacitance mode. Normally with the probes not touching anything, the display shows all zeroes. When I test it on this capacitor, the zeroes disappear and I just see a decimal place.

When reading another ceramic capacitor, I see the expected 22uf value show on the multimeter. So I think I'm doing things correctly

The PSU itself is the BOSS PSA-240 and is the older non-switching model. It's a basic enough job that takes in 230 volts AC and puts out 9v DC, 200ma. Uses a transformer, bridge rectifier (4 diodes), 3 caps (two electrolytic and the ceramic in question) and then a 7809 regulator.

Can anyone help determine what replacement value to use for the ceramic cap? The markings on it are tiny and faint - it looks like 'KDE' (or KRE?) with '040-2' written below. From looking at the PCB, It's situated between the live/hot output of the transformer and the bridge rectifier. I've not seen this configuration in any of the standard PSU schematics online.

The other two caps are a 10v - 100uf located after the output of the voltage regulator, and a 25v - 2200uf  located before it.

I've attached a picture of the capacitor and also the PCB with the parts marked as best I can.

Thank you

[ArthurDent]

It is not a capacitor but probably a thermal fuse. The numbers are probably an inhouse part number.  If it is good you should read continuity between the two leads and very high resistance if it is bad. If bad you need to find why it failed which could be shorted output wires or a shorted component.

[shakalnokturn]

Where it's placed it's a fuse ("polyswitch" basically a PTC thermistor with a very sharp slope and high hysteresis)
It's there to protect the supply from overloading.
Often the transformer will have a built in thermal fuse on the primary.
The first things you want to check are the primary resistance (at the plug) and the output lead's continuity.

[sirlemonhead]

Thanks you! that makes sense it should be a fuse based on where it's located

It seems I'm actually getting continuity on it now, which is interesting. I'm wondering was I just measuring it incorrectly the whole time and it's actually been fine all along.

I suspect the original output wire got shorted, right where it enters the adapter casing. I hear it's a common point of failure with these adapters. I've cut off a few inches of wire at this point and the wire is now testing good.

The two output points on the adapter PCB are not shorted.

I'm getting about 320ohm between the primary points of the transformer, and about 3ohm on the secondary transformer outputs.

Does this sound OK?

[shakalnokturn]

Yes, sounds good...
Should work now.

Please be aware that a freshly de-soldered Polyswitch can measure open just because the heat has triggered it.

[ArthurDent]

Also make sure that you get the two output wires soldered back in the correct orientation. It would be bad to fix the power supply then damage what it powers. Check for proper voltage polarity before connecting the supply to the device.

[sirlemonhead]

Finally got back to this - I put the thermal fuse back in, and soldered back on the output wire making sure to do it centre negative and it's working now! Outputting 9v as expected.

Thanks for your help! 

[shakalnokturn]

Thanks for the feedback.
Am I losing it or a post has been deleted recently? Not one of OP's.