Valhalla 2701C schematics?

[forrestc]

I'm in the process of trying to repair of Valhalla 2701C.   At this point, I have lots of theories about what might be wrong with this, based on the description in the manual.

Unfortunately all of the manuals I can find (of various vintages) are lacking (but refer to) the schematics of this product.   I'm hoping someone here has a copy of them or knows where I can find them.

Any ideas?

Thanks.

-forrest

[JFJ]

If you can't find a manual with schematics to download, then Artek can provide one on CD-ROM:

http://www.ebay.com/itm/Valhalla-2701C-DC-Voltage-Calibrator-Operating-Service-Manual-/371715601232

The manuals that I've had from them have been top quality scans (300 DPI text, 600 DPI schematics).

[alm]

You can also order directly from his website. Saves you $1 and should be faster.

[forrestc]

Ended up buying one from artek... and it has everything I need.

Apparently the HV supply is AWOL in this, still trying to narrow it down to the specific Part(s).   Slow going since it's a potentially a 1000V supply and I have no desire to experience 1000V coursing through my body.

[Edwin G. Pettis]

Hi forrestc,

Do you know about how old this unit is?  I also have a 2701C unit, the auto-zero op-amp that controls the high voltage regulator was out to lunch, mine is from the mid-80s.

[forrestc]

Hi forrestc,

Do you know about how old this unit is?  I also have a 2701C unit, the auto-zero op-amp that controls the high voltage regulator was out to lunch, mine is from the mid-80s.

Mid-80's as well, apparently (based on date codes).  It has Bipolars instead of fets for the output chain, so it's an earlier unit apparently.

I believe I found the fault - the HV supply has about 4-5 450V electrolytics.  I finally dug out the ESR meter and discovered one was horribly out of whack (the ESR meter refused to acknowledge it was even a capacitor - which means and extremely high ESR).   I have exact replacements on the way for the entire set, plus others of a similar age in the unit.   We'll see if that fixes it.     I'm fairly comfortable that it will, especially since I was measuring something like 35V ripple across the caps - not exactly any filtering going on.  Wish I would have measured this first, especially since I have had no shortage of bad electrolytics being the cause of problems over the years.

It just occured to me I should also measure the rectifier diodes to make sure they're not shorted before I put those caps in, just in case.....

I'll know sometime next week hopefully.

-forrest

[Edwin G. Pettis]

If your unit has a bunch of yellow precision wire wound resistors in it like mine, those were made by our previous company which made all of the precision WWs for Valhalla at the time.  It has now morphed into Ultrohm Plus years later.

[forrestc]

If your unit has a bunch of yellow precision wire wound resistors in it like mine, those were made by our previous company which made all of the precision WWs for Valhalla at the time.  It has now morphed into Ultrohm Plus years later.

Yeah, if you mean these yellow ones which at first glance one would assume were old capacitors, yep it's full of them.   

Looks like my replacement caps will be here tomorrow - unfortunately I'm headed out of town thursday so I'm not sure I'll get to them before I have to get ready to leave.

[jasonbrent]

@forrestc - sorry to revive a very old thread, but is there any update to your work on your 2701C? I've got a 2701C-LNF I'm attempting to make not-dead...

[forrestc]

Yep, the cap replacement worked.   I pretty much replaced anything electrolytic in the whole thing with identical caps.   It's been a while, but I know I had one definitely bad one, and I think several others were questionable on the ESR meter.   But at the point I'm replacing electrolytics, I'm going to replace them all.

Once the replacement was done, the unit popped up and started working.

I had two additional issues that I can recall:   One of the knobs was difficult to turn and another just twisted on the shaft, I discovered that knob was pushed in mechanically too far on the one and the other was just loose.   I seem to recall that the knob had a cap you popped off to access the screw which tightens the knobs.

In the process of troubleshooting the above, I killed one of the 40 pin IC's (obsolete GPIO of some sort), so I had to procure one off the internet.   Frustrating to kill an instrument you've almost got working, but once I got the replacement everything worked fine.  I've been using it as a calibration transfer source around here (get a DMM Calibrated, then use the DMM to verify calibration the 2701C, then calibrate lower resolution DMMS with it.).

[jasonbrent]

Thank you; my caps all "look" good, but... I've got a full set of replacement ICs on the way from rebay... I've been going through this component by component for a few days now (my first repair...). Mine is the "LNF" variant (low noise floor), apparently. ... Mine outputs voltage in standby mode, always outputs 100mV on 200mv range, 12volts on the 2v range, 40V on the 20v range, and 52 volts on the 120v range at the moment. Only in the 120v range do the knobs seem to impact the output at all, and then by only one volt.

I'll keep plugging away at it.. glad to hear your repair worked!

-j

[wictor]

Closer than you think:
https://www.eevblog.com/forum/metrology/valhalla-2701c-schematics-100832/