开云体育

Hi all,
On the heels of success fixing our 2712 (thanks again, Jaap!) I was doing some digital circuit design using all four channels of the 2465 (although I doubt this fact played any role in the failure) and all of a sudden I heard a sound like an electrolytic cap venting and smoke poured out of the back of the scope, propelled by the fan.

Being very familiar with it after fixing lightning damage seven years ago I immediately pulled it apart, hoping to find a still-smoking part...and I cannot find anything visible..no cap vent residue, no overheated parts. I measured many (but not all yet) electrolytics and they measured in spec with low D and ESR. All the power transistors in the supply measured good...what the heck was the noise and smoke?

I had read about the X and Y caps in the AC input filter failing, but mine was cold to the touch making me doubt any heat could have evolved from any internal components. Has anyone had this experience with any of the 24xx series scopes? I plan to pull the AC input module out and check it later, but for now I am baffled.

Thanks in advance for any clues!

Cheers & 73,
Howie / WA4PSC

On Tue, Sep 17, 2024 at 07:48 AM, Howard Hoyt wrote:

s? I plan to pull the AC input module out and check it later, but for now I am
baffled.

I had similar in my HP 8904A: it was (probably RIFA) cap in line filter, integrated inside C14 mains socket.

Howie usual for the VDE caps to burn up or explode on a 240V mains but seldom on 120V.

No overheating as the event is rapid and the caps are inside the EMI / IEC modules.

Also check the LV PSU board for blowing rectifiers and lyrics as well as tantalum caps.

See thé EEV blog 75 pages on 2465B teardown.

Bon chance

Jon

My 2445B blew the RIFA caps exactly as you described. Good news is that this is a simple fix. Replace Rifa caps on the AC line input filter. Clean up the mess they left. Search the forum. It has been explained and explored in depth many many times. 2445, 2465, etc. They all have it.

If you think you have it bad with a couple of these things, I feel your pain. I'm working on a Studer A80 right now. The motor control cards are COVERED with Rifa caps of the type that fail. I have to replace them all. I think they are on about ten different plug in card assemblies.

There are RIFA condensers in the power supply on the primary side of the transformer as well. Replace them. I used 630V film types and the X1 types to replace them.

Mark

It's not only the 2x68nF on the A2.
On the A3, you'll find another 4 of these to be replaced.

See the overhaul here:


They're all included in my updated overhaul kit.



I do not sell on Ebay anymore.

Menahem Yachad

Hi Keith, all,
Indeed it is the RIFA cap issue in my 2465 as well. I have a few thoughts about this, and Keith I feel your pain. I was responsible for 20+ Studers of A80 (QC-0.150"/?"/1"/2") and A820 (?"/1") vintage, all with RIFAs all over the place, but the only ones I had fail are the ones exposed to HV/Mains AC. I am postulating it is the dielectric dissipation which heats and slowly degrades to the point where it fails. It may not be necessary to replace the ones in the control cards, unless you are interested in prophylaxis or really bored......just my 2 cents worth.

Another thought I had about the failed resistors in series with the mains EMI filter RIFAs: the original 30 and 68 ohms 1/2 watt resistors were carbon composition resistors, which makes sense given those types are non-inductive. Mouser/Digikey/Newark(Farnell) all have discontinued that type for the most part, all those vendors had in those values were wirewound or 1/4 W metal film, all inductive but I was able to find old carbon IRC ones on eBay. Given they are in an EMI filter role it could potentially make a difference to the filter shape. Probably not a big deal, just a thought.

Cheers & 73,
Howie / WA4PSC