SMPS Inductor values

[Verne]

Hello all. What brings me to this first post (thanks for letting me join) is I'm repairing a guitar am with a SMPS; I'm needing a bit of help with the inductor.

This snippet from the schematic is for the 6.3v section coming off the transformer, through this circuit then to the main board to power the heaters for the tubes. Aside from the tubes not lighting up there is noticeable burning at the connector for this section, and the PCB under the components in the green box (invertor is in the red box), indicating a voltage jump much higher than the 6.3 volts that should be going through it. Circuits on the main board being fed by other 2 connectors (for other voltages) off this SMPS board indicate they are working.

After going over everything with a meter I'm pretty sure the inductor is gone. My problem is that 2UH drum-type pcb-mount inductors don't appear to be common and/or available. That leaves me with a few questions like:

- can I modify a SM inductor to work?
- is there a way to modify the values of the other components so I can use a more available 2.2 UH or other inductor?

At this point I don't know much about the inductor other than what is on this diagram, nor do I know the current coming out of the transformer into it. I understand inductors and capacitors do similar things, so I'm wondering if it would be possible to balance a new capacitor against a different inductor value.

I can post the entire power supply section if necessary.

Thanks.

 

[BradtheRad]

The image looks like a basic half-wave power supply. All the capacitors perform a filtering task. I suppose it provides 6V 1/2A for one or two tube heaters?

The inductor does a slightly different filtering task. Its value isn't critical. You could even wind your own replacement.

 

[Verne]

Yes, it's for the tube heaters. Thanks.

I found this 2 uH inductor at Mouser that should work (I'm new to SMPS which is why I'm asking here):

https://www.mouser.com/ProductDetail/Sumida/RCH654NP-2R0M?qs=LKl%2FVGYbYtgB9XhJ%252BZdarw%3D%3D

There are lots of 2.2 uH but not many 2 uH.

 

[KlausST]

Hi,

There are lots of 2.2 uH but not many 2 uH.

as already said: the value is not critical.

****
we don´t know the switching frequency, nor the waveform of the current.
But since it is an SMPS it will work with way higher frequency than typical mains frequency.
Then I see those huge 1000uF electrolytics capacitors ... most of them have not good performance on SMPS swithcing frequency (and it´s steep edges).
Thus they may heat up ... and die early.
This would be one of the first things to explore: are all the capacitors healthy?

If you want to replace them: The value may be less critical than their specified frequency range. Be sure they provide low ESR at the SMPS switching frequency.
If it was my power supply design, I´d surely add some fast (ceramics or film) capacitors in parallel of them. Maybe in the 10uF range.

Klaus

 

[Verne]

Hi,


as already said: the value is not critical.

****
we don´t know the switching frequency, nor the waveform of the current.
But since it is an SMPS it will work with way higher frequency than typical mains frequency.
Then I see those huge 1000uF electrolytics capacitors ... most of them have not good performance on SMPS swithcing frequency (and it´s steep edges).
Thus they may heat up ... and die early.
This would be one of the first things to explore: are all the capacitors healthy?

If you want to replace them: The value may be less critical than their specified frequency range. Be sure they provide low ESR at the SMPS switching frequency.
If it was my power supply design, I´d surely add some fast (ceramics or film) capacitors in parallel of them. Maybe in the 10uF range.

Klaus

Thanks for this. I did electronics in high-school, a bit at college and worked as a bench tech fixing tube tvs and stereos for a few years (early 70s) but that was so long ago that I'm pretty rusty and long before SMPS so any help is most appreciated.

The only thing I did differently before it blew was I left the standby switch (SW102) ON which may have caused a surge of current as the likely culprit - it's directly connected to the section that has gone dead. The 320v and +/-17v sections appear to be fine - it's just the 6.3v line that is affected as far as I can tell at this point.

As for trouble-shooting - I'm working backward from the burnt components and plan to replace the inductor, diode and cap in the green box on my original illo above. Anything else I should look for?

These amps (Crate and Ampeg) were made in Vietnam and have had a high failure rate so there's something amiss either in the circuit, the components or assembly. Here's the SMPS circuit if you care to suggest any improvements. It was a great sounding amp when working so it would be nice to get it going again.

 

[KlausST]

Hi,

The diodes (D101, D108) should be rather easy to test:
Both should be low impedance in one direction and high impedance in the other direction.

Use a DVM with diode test feature.

Klaus

 

[Verne]

Reporting in - I ordered the parts from Mouser, got time today to do an R&R and I'm happy to report I got my amp back!

I replaced the L106 inductor, Z101 diode and C115 capacitor. It was cheaper to just replace them then fuss with whether they went bad. Now that they are out of the circuit I'll check to see who the culprit is. For now I'm back to making noise!

Thanks to all for your helpful insights - v

 

[FvM]

SW102 is 115/230 V selection, not standby switch. Operating the power supply with 230V and closed switch will most likely cause damage.

 

[Verne]

SW102 is 115/230 V selection, not standby switch. Operating the power supply with 230V and closed switch will most likely cause damage.

There are 2 switches on the front of the amp - power and standby. There is a slider switch on the back for power selection which is probably what you're referring to.

Here are the other 2 schematics for the amp - pre-amp and power-amp sections. The power amp shows a KPS1 switch so if you're right that's the standby so my bad. But if that's the case I can't see why it would blow the 6v line the way it did since it doesn't touch pins 4 and 5 on the tubes, which are for the heaters.