EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: Spike on October 25, 2015, 05:50:01 am
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Hi guys,
I just picked up this Valhalla 2500-E current calibrator on eBay that was for parts or repair. I had half hoped that it was simple operator error and that it actually did work, but alas, I didn't get that lucky. I'll post my request up front before I get too far. If anybody has a schematic for the Valhalla 2500-E (NOT 2500EP or 2500EN) I would greatly appreciate getting a look at it. I found a couple of manuals for the 2500EP/EN at Valhalla's site as well as KO4BB, but that seems to be a slightly later model and the schematic is just different enough to make me unsure of myself (did I mention I'm fairly new at this). The 2500E is the older 10A one with the pushbutton range switches shown in the attached pics.
The guy claimed that no repairs had been attempted, and I'll give him the benefit of the doubt, but somebody was clearly in there as can be seen. Sharp eyed folks (and even dull eyed ones) will spot an issue right away. I know what you're saying, "well there's yer problem, there's a giant hole in yer turbo encabulator." Well, yes, there does seem to be a hole where a power transistor used to go, and a missing wirewound resistor, too. Not sure what happened here. An aborted repair, a failed repair, or maybe he just scavenged the transistor (hoping it's this one).
Not sure if this is the only issue, but I'll start here. I found some replacement transistors on eBay after a fruitless search locally, and I think I have some 1 Ohm power resistors I can use, but I would be grateful if someone has or can point me to a schematic for this unit in case I have to troubleshoot this further.
Thanks in advance.
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It is basically a power amplifier using current feedback, so treat it as a plain amplifier. The missing transistor is easy to change, though it will probably be a TO247 packaged device in it's place, and replace all 3 on that side when you do, as they require rough matching and a device made in a different process will be different enough to cause issues later. As is just check all power devices that they are not shorted, it will run at low power if one of them is missing, it will just not get to the peak 14A current. Check supply voltages, and gently wriggle all socketed IC's to clean the pins in case they are not making good contact, check the supply voltages, probably around 30V on the main rails, and the opamps will generally have supplies of 15V both positive and negative. Use those nice convenient test points marked on the board
If all voltages are there then you can start, check all switches are working electrically, and that there are no hot components. Check the voltage at the output, it should be close to 0V with the input shorted, and for most testing it can stay that way. If the 2 red LEDs light equally then the output stage is probably healthy. Depending on how it is connected the output stage used those 3 transistors as a compound darlington, one at the front driving the 2 rear ones.
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Thanks. Once I get the transistor replaced I will run through the items you listed. I did check the two rails shown on the schematic, the +-15V rail is fine, but what is shown as the +-25V rail is reading about +-19.5V.
I managed to find some NOS RCA TO-3 package replacements on eBay. They seem fairly plentiful. There's actually six of these, three top and three bottom, plus six more NPN 2N3055 on the other side of the heat sink for the positive side.
I've attached the schematic I do have. It is for a later model which is similar, but I've notice several differences already (ignoring the range switch differences). I haven't really looked thoroughly. The missing transistor corresponds to TR12 on this schematic. R27 is also gone.
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Hello,
The AC-DC Current calibrator 2500 I use it with very good results recently without applying signal to the input I have an overload indication if we apply input signal responds normally. It has been observed, there is an idea of this problem.
Best Regads,
Giannis
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Check the two tantalum capacitors aren't short, they do this a lot after time.
Best replace them anyway with 105 degree rated electrolytics before they do, cheap and easy insurance.
They are the 2 green blobs next to the heatsink and the two smaller transistors standing up.
(they are polarity sensitive, there should be a small + marked showing the positive terminal)
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A very common problem with 2500's is somebody applied more than 2V to the input and blew the input op-amp.