Global Specialties PW-2120 - Page 1
Forum section: electronics / repair

author: staze
author_rank: Frequent Contributor
timestamp: July 17, 2015, 02:36:22 am
content: All,Picked up a broken PW-2120 locally, and after repairing the rotary encoder (pcb had just broken off), I can't get the unit to output anything.controls all mostly work (couple LEDs seem defective, or their driver is mis-behaving), but other than that, the unit seems okay on visual inspection. I've contacted the company for schematics, but their initial response was just the plain manual (disheartening).If anyone has one of these, I have a couple questions.One, is "U1" populated, and if so, by what? It seems odd it would be missing, as it's labeled "U1". =/Two, on the front PCBs there are a few jumpers. Is there anything on those? Also, the 7805CV on the unit gets pretty darn warm pretty darn quickly. Making me think something is drawing excess current. The only thing I see heating up is the CLC522, which is socketed, on the main board. Appartently that's a variable gain amplifier. I don't see anything really coming in, or out of it on the scope.Thanks for any help. I'm going to tear it down more this weekend and see if anything jumps out at me.Interestingly, the Frequency counter aspect of the unit works perfectly. So, I can assume the oscillator (where ever that is, I can't find it) is working.

author: staze
author_rank: Frequent Contributor
timestamp: July 17, 2015, 03:33:06 am
content: Seems like there's a near short in the 5V rail. I'm seeing less than 500ohms between 5V and ground... so now I just need to figure out where it's at. Guessing the 7805 or the output cap for it...

author: staze
author_rank: Frequent Contributor
timestamp: July 17, 2015, 04:24:08 pm
content: Well, finally got the schematics (low quality). Looks like U1 is a MAX038CPP, which is missing. Which is a 0.1hz to 20mhz frequency generator. Which is why it no workie. Sadly, not made anymore.Just ordered one on a slow boat from china. =/ We shall see.

author: staze
author_rank: Frequent Contributor
timestamp: July 21, 2015, 07:03:47 pm
content: All,Just curious, am I crazy thinking 500ohms between 5V and Ground is too low, or do you think this could be due to the massive number of decoupling caps (like, a TON of Tantalum caps) on the 5V rail causing erroneous readings?Though thinking about it, if there WAS a short to ground, that would discharge all those decoupling caps, and I'd get a much lower reading... 500ohms-ish just seems too low in my book for 5V to ground (I generally expect 1-2k, at least)... or am I crazy?

author: staze
author_rank: Frequent Contributor
timestamp: August 21, 2015, 04:05:54 am
content: So, U1 didn't help. I think things are borked.One point of interest... there's only 272R between the -5V and +5V rail. That doesn't seem right to me. Any thoughts at all on this, short of shotgunning and removing every IC that's on both those rails.Using my FLIR, I don't see anything getting hot fast. So, I'm at a bit of a loss. If anyone has any tips, or if I'm crazy about the resistance between rails (though, checking between the -15V and +15V, I get a few Kohms, and charging. That seems reasonable. =/Thanks!

author: staze
author_rank: Frequent Contributor
timestamp: August 24, 2015, 05:21:30 am
content: Okay, my measurements were being thrown off by the U8, a U441 (dual jfet), that is attached to both the -5V and +5V rail. Guess the ohms voltage was enough to turn it on (at least partially) and result in super low readings.Also, the U1 (MAX038cpp) I got, was bad (probably fake, never worked). I tested it on a breadboard by itself, and it appears to have a partial short from -5V to ground, causing it to get super hot, and no output. So, I've ordered another one on ebay from another seller, so hopefully that one shows up and works.Once that comes in, we'll see how it goes. Also replaced the socket it was in, as the previous socket was pretty damaged.

author: staze
author_rank: Frequent Contributor
timestamp: September 22, 2015, 01:37:45 am
content: Got this badboy all fixed. Second MAX038 worked great. Also had to replace a couple LEDs, the encoder, and a few other bits.Now to see what the heck I'm going to do with it (probably sell/donate).Hopefully this will be helpful to someone.