465 Scope AC Gnd DC Coupling Switch


JJ
 

Gentlemen,
My 465 has a stuck coupling switch - it developed during a project that I
am working on. It's stuck in the AC position. I don't want to force it. I
haven't taken the unit apart yet but with a Hail Mary Pass I sprayed a
little DeoxIT through the front slot. No glory! I checked through the
message archives but didn't find anything.
Before I open it up and break something, I was wondering if anyone else has
had the problem and whether the switch is repairable. I didn't see any
switches for sale on eBay.
I'm now using my 7904 so I'm not pressed to get the 465 functional.
Merry Christmas and Happy New Year to everyone.
Best,
John Justin


adesilva_1999@...
 

I am assuming you have just 1 of them stuck. If you remove the cover and also remove the shield on the vertical amp of that channel, you will be able to see the contacts operated by that switch. Make sure they are not bent in someway which is highly unlikely anyway. Beyond that, it seems like a mechanical problem to me. Getting to that involves removing the vertical board out of the scope which takes some doing. There is good info in the service manual on how to go about it but there are also people on this board who have done this. In my case, the only time I had to fix one of them it did not involve pulling the board out. I am sure one of the experts will chime in soon. You might have to wait since it is the holiday weekend.


JJ
 

OK. Thanks for the heads up. It is mechanical, the coupling switch on the
other channel works fine.
Best,
John


Dave Peterson
 

John,
I'll chime in:

I'm new to the 465 world of repair myself, but do have some experience in electronics. I understand the trepidation of digging into your scope. Despite being pretty comfortable with a soldering iron I didn't feel like it was a small thing going into it. Bottom line is there is no way to remove the vertical assembly without some soldering iron work. That said, it's not bad. I'll leave it to you to decide how invested you're into taking a soldering iron to it.

Before beginning review the "Vertical Preamp Assembly Removal" section of the 465 Service Manual in the "Maintenance" section. Between the circuit description and calibration sections.

The key thing that enabled removal for me was that only C3 (CH1) and C53 (CH2) need be removed from the attenuator assemblies to access the 1/4 inch nuts behind them. Everything else is pretty straight forward.

There's a small ground strap on the bottom on the A9 board, and disconnection of the small red wire and the delay line toward the rear of the scope are also needed. I suspect the assembly could be pulled away enough without disconnecting them, but once you have the iron ready you might as well. The delay line is soldered to the board by one of the shield pins. The other pins of the delay line are plug-in and do not need to be unsoldered.

The ground strap on the bottom is between two push-button push rods for INVERT and BEAM FINDER. I removed those so I wouldn't touch them with the hot soldering iron.

I was able to disassemble C3/C53 and the delay line shield with solder wick. For C3/C53 (these are under the attenuator shields) there is one connection to R1, C1, and C2 that's the bundle of leads all soldered together. After wicking the majority of the solder away it was easy to use tweezers and the soldering iron to separate R3 from the bundle. (same goes for CH2). The other side of C3 is connected to tiny 1/4w (or 1/8w?) R3 and is easy to separate. The capacitor body is mounted to the board via a plug-in lead. It just lifts out after unsoldering the two electrical leads. You may notice the cap pivoting on this plug-in lead as you disconnect the R1/C1/C2 bundle.

The delay line shield also responds well to wicking of the majority of the solder first. Then a small pair of tweezers or pliers pulling away from the board while holding the iron to the soldered shield will release it.

I took extensive pictures while doing all this to keep all the connections straight. All other mechanical connections are per the procedure in the manual, and are pretty obvious. The entirety of A1, A2, A3, and A4 come out as an assembly. The CH1/CH2 BNCs and associated 10x components come out with the assembly. Oh, right, there's two signal wires for CH1/CH2 10x lights at the CH1/CH2 attenuators. Those have to be disconnected too.

I haven't posted pictures yet, but have them saved off. I could send key ones if that helps. Let me know if there are any questions, I'll try to help.

Dave


JJ
 

Hi Dave,
Wow! I thought removing and repairing my 7904's power supply was tough!
This really sounds tricky. I'll need to spend a day or so going through the
465 service manual.
Thanks for the terrific detail, I'll be referring to it as I move along.
Pictures would be great!
I won't be getting to the 465 for a few days, I'm in the process of chasing
down signals in my Carver stereo receiver that I purchased new some 35
years ago. I just saw that the big filter caps that I need to replace are
glued down to the PC board! What Fun!
Thanks again.
Best,
John


adesilva_1999@...
 

That is a great write up indeed! if one follows this with the instructions in the manual, it should be a piece of cake.