Repairing a power supply...

[KD8TUT]

Hi guys,

I've got three Alinco DM-330MV 32 amp power supplies. One of them is an early serial number (1023). The others are in the 10000s.

The low serial number PSU always performed weirdly. First, when it hits a heavy load the voltage does not sag, it goes up by .75v. Second, this may have been a pre-production unit since it only had a two prong plug and no provision for a ground.

I've corrected the ground problem by getting a schematic and comparing to my other units. It's been re-wired and tested for smoke. We're good.

But- the irregularity of the voltage is a concern. There is a pot on the board, which based on my read of the schematic, should allow me to change the character of the output voltage at load. I'll give it a try as soon as I find my plastic screwdriver.

The question is this: Have any of you seen an over-voltage behavior like this and what was the cause?

[N8YX]

Output boosting.

Current sensing resistors at the output side of the pass transistors are used to control the regulator which drives said transistors. The idea is to attempt to keep output voltage constant under heavy load conditions, and the process should be gradual in a properly designed and adjusted supply. The fact there's a threshold which causes a jump makes me think you have a bad component somewhere in the feedback loop.

Do you have a link to the schematic for your supply?

[KD8TUT]

Output boosting.

Current sensing resistors at the output side of the pass transistors are used to control the regulator which drives said transistors. The idea is to attempt to keep output voltage constant under heavy load conditions, and the process should be gradual in a properly designed and adjusted supply. The fact there's a threshold which causes a jump makes me think you have a bad component somewhere in the feedback loop.

Do you have a link to the schematic for your supply?

Thanks! Here's the schematic:

http://www.radiomanual.info/schemi/ACC_powersupply/Alinco_DM-330MV_DM-330MVT_DM-330MVZ_serv.pdf

[N8YX]

It's a switcher.

This is definitely a "boost" function. If you look at the TS-930 Upgrade thread, you'll see mention of the replacement switching power supply which comes in Compudigital's kit. That supply is capable of 20A continuous @ rated voltage, with a boost of 26A for short periods of time.

Without comparing schematics, I suspect they're similar in nature.

Look at Pin 11 of IC1 with a scope when the supply is under heavy load. See a waveform characteristic changing?

Next, have a look at D4. What's the drop across it under heavy load?

[KD8TUT]

It's a switcher.

This is definitely a "boost" function. If you look at the TS-930 Upgrade thread, you'll see mention of the replacement switching power supply which comes in Compudigital's kit. That supply is capable of 20A continuous @ rated voltage, with a boost of 26A for short periods of time.

Without comparing schematics, I suspect they're similar in nature.

Look at Pin 11 of IC1 with a scope when the supply is under heavy load. See a waveform characteristic changing?

Next, have a look at D4. What's the drop across it under heavy load?

Thanks John.

I'll try adjusting the thing this week... but over the weekend I'll go to a friends and use his scope.

Thanks for the pointer. Very helpful.