Old Model 250 ac/dc HF Problems

[ryanjones2150]

Since it works perfectly in AC, like you said, I'd think the transformer is probably unscathed, by and large at least.

[BukitCase]

Digital meters can be tricky, I'm wondering if your other three diodes are actually OK - I would check those 3 again, this time setting your meter to NON autorange, and check each one on several different MANUAL ranges - it's possible you might get a reading of some kind, also if your meter has a "diode" setting THAT's the one I'd use... Steve

[Aeronca41]

I agree with Steve--I'm not sure you have four failed diodes. While I have more digital meters than any reasonable person needs (I'm like that with welders, too.....), when I'm testing diodes I like to grab one of my trusty old Simpson 260s. Love to see that needle swing. Not that you can't do it with a digital meter--I can and do, but being an old guy I still like some of the old ways, and this is one of them.

How did you test the ones that gave no indications? Here's what I would do: Don't use the diode test/beeper range on the meter--use ohms. Connect the leads solidly with alligator clips, and get a resistance reading in one direction. If your meter has autoranging, turn it off as Steve suggests. You may have to switch scales until you get a reading. Then do the same thing with the leads reversed. I'm guessing you only have one bad diode. I think you are going to find the other three have low resistance one way and high the other.

[Jmcghee]

You guys were right... the beeper selection has a ceiling below the resistance I was getting (300 I believe). I selected a higher range, and presto. 3 of them check out & one is trashed. Should I just replace the one, or all 4 & the resistors while I’m at it?

[Aeronca41]

I have to say--another old guy thing, I guess--I never use the diode test/beeper unless I'm simply testing for wire continuity and I need to hear the beep from many feet away. Don't think I've ever used it even once to check a diode.

I'd just change the one--others opinions may differ. Can you find a part number on it?

[ryanjones2150]

I'm in the camp of if it ain't broke, or obviously about to break, don't fix it.

[Aeronca41]

Yep--what he said.

[Meltedmetal]

Internet craziness today, ignore me.
---Meltedmetal

[BukitCase]

"I'd just change the one--others opinions may differ. Can you find a part number on it?"

I agree - IMO, the three that survived are proven TOUGHER :=) -

on part #, watch out for an "R" in it - a lot of bridge rectifiers on old stuff only used TWO heat sinks (save money) - so HALF of the diodes had the CATHODE on the end that BOLTS on, the OTHER HALF had the ANODE on the bolt.


Other diodes may just have different part # for straight or reverse polarity, so...

Lacking an "R" in the part #, if your machine has 2 heat sinks and 4 diodes, make SURE you check the pn# of the one that's BAD, not the one that's easier to read... Steve

[Aeronca41]

Good advice.

[Jmcghee]

The diode came in yesterday. I swapped it out, fired it up, and I'm back up & running! Thanks again everyone for walking me through it

[BukitCase]

Cool; to quote the guy on the A team, "I LOVE it when a plan comes together" :=)) ...Steve

[ryanjones2150]

That would be colonel John "Hannibal" Smith.

[Aeronca41]

Fantastic! Success is sweet.