Tek 485 Vertical Amplifer Board Removal

[ignilux]

All-

I recently picked up a 485 scope that was listed as not powering up. After hearing the dreaded ticking noise of an angry SMPS, I found a shorted tantalum cap, C696, on the vertical amplifier board. Great! Hard part is done, right? Well, I can not for the life of me figure out how to get the board free from the chassis. The service manual lays out the relatively simple procedure of removing boards on page 4-12 (or page 41 of the link above), mentioning that exceptions to this process would be mentioned below (vertical amplifier board not subsequently listed). After disconnecting all wiring harnesses, comb connectors, and screws, the board feels loose, but it's getting hung up somewhere around U660, which is the large metal can adjacent to the coil form.

In my research I've found this thread from the Tektronix groups.io forum, which mentions that the CRT anode lead needs to be pulled in order to free the board, but there is no mention of this in the service manual, and even if I were to do so, I cannot see how that would help. Is there anyone out there who can give me some advice on how to get the board out?

Thanks in advance!

[pbarton]

You should be able to replace the tantalum capacitors on Tektronix 485 Vertical Amplifer Boards, without removing the board.
Yes, you only have access to one side, but its easy enough.

[ignilux]

I had considered that as well, but the service manual goes out of its way to mention that the vertical amplifier board substrate is particularly sensitive to heat. Funny that it says to use a low wattage iron, when that would actually wind up transferring more energy to the board.

[med6753]

You'll have to trust me at my word. Do NOT attempt to remove the vertical board from your 485. I have experience here it isn't pretty. Can't go into detail now.

Change the shorted tantalum from the top side. It's possible and with reasonable care you won't damage the board.

[ignilux]

Well, that's two votes for removal from the top. On hand I have some nice Panasonic aluminum electrolytics with the right specs, but I'm thinking I'd have to put a smaller ceramic in parallel to maintain the high frequency performance of solid tantalum. The 485 has a switching supply, making this even more important. Has anyone tried aluminum polymer caps? Seems like they're uniquely suited, with the strengths of Al and Ta, and none of the drawbacks.

[ignilux]

Heck, it's a 6.8 uF cap. Could an X7R ceramic be just fine? 10 uF cap to make up for capacitance loss under DC bias?

[tggzzz]

Look at ceramic capacitors' capacitance vs DC bias voltage.

Don't worry too much about the precise type of capacitor. Nichicon and Panasonic 85 or 105C capacitors rated for switching PSUs are OK for most purposes. But don't use 15V bead tants on a 13V rail!

[ignilux]

The dead cap is C696, which appears to bypass the +24.7 V rail. It was rated at 35 V, which is clearly not enough. What I have is a Panasonic EEU-FR1H100B, which is a 10 uF, 50 V, 105 deg C rated aluminum electrolytic. It's rated at 250 mA ripple current, which sounds like enough, based on the 50 mA ripple spec for the +25 V rail (which feeds the +24.7 V rail). What concerns me is the ESR at 100 kHz, which is an order of magnitude higher than a solid tantalum at the same frequency. I'm not against buying new caps, but using what I have on hand would be great. That, and I'm not keen on using MnO2-containing tantalum in anything if I have the choice.

[tggzzz]

I can't see C696 on the schematics, so I can't make any specific recommendation.

A general question is if you replace C696 with X:

• how will you be able to tell if X is or isn't sufficient
• if X fails, what's the penalty above replacing it again

[ignilux]

It isn't in the vertical amplifier schematic for whatever reason. It's on the power distribution schematic, which should be the last or second-to-last schematic in the service manual.

In any case, you make good points and I decided to stick with the cap I have on hand. The scope has been up and running for a couple of hours, and I've been working my way through the calibration procedure. Now to figure out what's wrong with the 475 on the other bench!