EDC4600 AC standard

[nssouza]

Good Afternoon! I am an electronic technician, more than 40 years working with electronic instrumentation. I recently acquired in an auction site, this AC Standard, that is not working. I search the internet but no information found. In this site I found the 4601 model, but it is almost totally new. If someone have or know who has such equipment, I will appreciate any help.
It is powering up, the operation mode indicate that is working, but no output, in any frequency or output voltage. Thanks

[Zucca]

Post some pictures and maybe jump in the metrology sections

https://www.eevblog.com/forum/metrology/

[nssouza]

Ok, Zuca!

I will assemble and close the cabinet, take some pictures of it and post it there asap.

Thanks

[ebclr]

1St check those internal fuses near the heat sink

[nssouza]

Hi, ebclr!

The fuses, as well as the voltages at power supply, are OK (+/- 16 V, +/- 51 VDC, at the output of power supply.
I suspect of the AC/DC converter, there are 'custom ICs' possibly made by EDC, and also an AD Converter AD534... But without documentation, it will be very hard to find the problem...

The overall aspect is reasonable, see pictures...

In another topic, I found some people that was working the EDC 4601 (I downloaded the operation/service manual, but there are too much differences...

Thanks

[pelule]

It looks quite close to the Krohn Hite (now EDC) 4601.
So it may worth to use that as a guide.
/PeLuLe

[nssouza]

Hi, pelule

I have already downloaded the 4601 SM 9from this forum), but it has a lot of differences. I really need the service manual of the 4600. As I red in another topic here in this forum, the 4601 was originally made by EDC, but I believe the EDC was sold to KROHN-HITE. In their site, this model (4600) is obsolete, as well as 4601. One possibility can be any Laboratory of Calibration that makes calibration & repair of such equipment...
I have already follow the boards and recreate some of the circuits. But is a huge task, I don´t know it worth the work. Anyway, let´s cross the fingers

[PTR_1275]

Apart from the output / control connectors on the back (and the colour), this looks the same as my edc ac voltage standard. Mines a ac1000 from memory. I emailed krohn hite and they emailed me a copy of the manual. I'll see if I can find it

[PTR_1275]

Found it. Mines definitely a ac1000, but the front panel layout is the same as yours.

I changed the transformer taps for 240vac input and the fuses, fired it up and I have output on the 1000v range at all frequencies, but 3 other ranges are dead. Haven't gotten any further.

Send me a private message and I'll get your contact details and email it through for you

[nssouza]

Hello PTR_1275!

I understood that you have EDC4600, like mine. And the manual you have is of the model 4600, right? If true, it will help me a lot!.

Unfortunately, I´m new in this forum, and I don´t know how to send a private message for you. If you can help me, I appreciate.

Thank you!

[PTR_1275]

Hi,

Mine is the AC1000, undortunately I could find no information on that model when I searched and searched. I'll send you a private message and you can reply to that.

[nssouza]

to PTR_1275

Sorry, I did not understand that AC1000 was a model of your instrument. But OK, if you can send me a copy, I really appreciate, even if it´s not exactly the model I need. BTW, is it possible that Krohn-Hite have the manual of the 4600, and can send me a copy? If so, please inform the address of KH, and I will send them a message, OK?

Thanks, again

[schenkzoola]

In case you hadn't seen it, there is an EEVBlog video with a 4601 teardown.

[nssouza]

Hi, schenkzoola!

Thank you for your link! Unfortunately, this instrument has a lot of differences relative model 4600. I even downloaded the operation/service manual from the link informed in that post. Anyway thanks again. All information is important!

Thanks

[nssouza]

good afternoon, ebclr and all the friend that are following this thread!

Problem (partially) solved. Some resistors were replaced, with wrong values, in the PA board, that was not working correctly (no AC output). Also, the reference diodes in the AC Converter board, were replaced by ordinary zeners (1N4735), so the reference voltages varies continually, the same occurring in the AC output. The same problem occurs in all frequencies. I am using an HP 34401A DVM to monitor the AC voltage in the output of the instrument, and for example, in the range 10V, the variation begins to occur in the 4th decimal.

Besides, the IC4 (in dc reference amplifier), the DC offset of this IC cannot be zeroed. As this IC is custom made by EDC, and has a different pin out of normal OPAMP ICs, it will be hard to find it to replace. Probably the most indicated replacement will be an OP07 OPAMP, but I´ll have to modify the original circuit.

I send an e-mail to Krohn-Hite (sucessor of EDC), and they kindly send me a PDF copy of the operation and service manual of this model. There are some differences, however it was possible to follow the manual in most part. I will ask their permission to upload this manual to a site like BAMA/Edebris, to make it available in the internet.

I thank you all for the help. But I intend to keep trying to maximize the performance of this instrument. To begin with, I have to replace the zener diodes with the originals, I found it in the irnternet, but it is very expensive. Maybe it will be better option to replace it with more recent IC´s, like, for instance LM329, that is a 6.9V V, with changes in the original reference circuit. Any suggestions will be appreciated.

[schenkzoola]

I have never worked on an instrument like this, but I will offer my two cents anyway as food for thought.

Since this is a precision instrument, I would try to find the original reference zener.  It will have very low drift compared to normal components.  It is not likely, but it is possible that EDC included temperature compensation circuitry to account for the temperature coefficient of the original reference.  It would be a good idea to check before modifying the circuit.  Even if you can find the original reference, there may be some resistor swapping necessary to calibrate to that particular zener.  Hopefully the procedure is in the manual.

If you can't get the original reference, perhaps something like the LM399 would be a better choice.

Is it possible your DC offset can't be zeroed due to a different reference voltage or current than original?  The IC may be fine.

[nssouza]

H, Schenkzoola!

Thank you for your suggestion. I believe that LM399 will work, with the advantage of simplification (and elimination of) the reference section. I have to check if this IC is available here, in Brazil.

And about the defective IC, I put an oscilloscope in the output of the OPAMP, and observe an small signal, that cannot be eliminated with the adjust of the balance trimpot.  This signal is not a noise, it looks like a triangular waveform, very low amplitude (I don't remember the frequency), but this amplitude is not affected with the balance trimpot. Don't look like an spurious oscillation!

When I replaced it with another IC of same type (3 of this ICs type are used in this board), this signal disappears. But,  if I install the defective IC in another position, the same signal appears in the output of this IC.

I asked to the service personnel of Krohn_Hite, if this particular IC is still available, or the internal circuit of it (to make another with discrete components), but no answer to the moment.