Working on a ZX250

Post by **Jacampb2** » Tue Jun 16, 2009 1:20 pm

I am working on a zx250 I got on ebay a while back. It supposedly powered up, but did not work. Well, I got to it today finally. It had been under water or something at some point. The amp did power up initially, but the fan would take off on powerup, then after a few seconds the green led comes on, and the fan stops... anyhow, the leds were all mangled, so I replaced them, and the next time I powered it up, the same as above happened, but the amp stayed in thermal. The fan should run wide open when thermal is set, but it stops instead. I checked the thermistors, and they both read 108ohms. The traces are in tact from both thermistors back to the PS riser card.

What is the normal resistance of the thermistors?

I am leaning toward there being a problem on the PS riser card. I have thoroughly cleaned the board and all the corrosion off it. The reason I am leaning toward it being a problem with the riser card is mainly that the fan is supposed to run full blast if the amp truly is in thermal, right?

fun, fun, fun. I will get it. Hoping one of you knows the data on the thermistors used.

Thanks,
Jason

Post by **stipud** » Tue Jun 16, 2009 1:23 pm

Might want to PM rolandk about something so specific.

That's a good question about the fan though... I have never thermaled a Ti or ZX amp before. It would definitely be nice if they would keep the fan on, but shut everything else off. Though that would require some extra circuitry I'm not sure many manufacturers would bother with.

Post by **Jacampb2** » Tue Jun 16, 2009 2:24 pm

I had a Zx350 I fixed in the past. It would thermal as soon as the remote was on. That one had a blown ground trace on the board and the thermistors were open. That one, the fan ran full bore as soon as it came on.

That is the only PG amp I have ever seen in thermal, so I suppose it is possible that that one was the odd duck...

Later,
Jason

Post by **Eric D** » Tue Jun 16, 2009 3:30 pm

Jacampb2 wrote:I had a Zx350 I fixed in the past. It would thermal as soon as the remote was on. That one had a blown ground trace on the board and the thermistors were open. That one, the fan ran full bore as soon as it came on.

That is the only PG amp I have ever seen in thermal, so I suppose it is possible that that one was the odd duck...

Later,
Jason

I had the same exact problem on a ZX450.

You may want to pull the PSC riser card and start going over it in detail. I had some bizarre issues with one before which one transistor was bad.

Post by **Jacampb2** » Tue Jun 16, 2009 5:29 pm

I found it a few minutes ago. The comparator on the riser card that handles fan speed, OL detection, and thermal detection was shot. I had a few lm339's in the cabinets, swapped it out and found one of the outputs were blown too. The little guy must have had a hard life. I happened to have some spare outputs, and it is good as new now.

I will probably move onto a ZX450 in the pile tomorrow. I fucked up my back pretty good, and am out of the shop for a bit, so it is down to my mad scientist lab in the basement for a few days

I am waiting on the last few parts for the ZPA 0.8 to come in, and wrapping that one up soon (fingers crossed).

Anyhow, thanks for the pointers.

Later,
Jason

Post by **fuzzysnuggleduck** » Tue Jun 16, 2009 6:55 pm

You are truly the man, Jason.

Post by **ttocs** » Tue Jun 16, 2009 7:27 pm

I have a bunch of 10k thermistors if that should happen to be the right size.....

Post by **stipud** » Wed Jun 17, 2009 6:28 am

Nicely done!

Sorry about your back... but always good to have a reason to work on some more neglected projects anyways