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Tom,

This is where I found mine. It is not a service manual in the traditional sense with schematics but it does have some useful information on removing boards and voltage test points. Despite the name it does include the 684A.

www.dennlec.com/images/manuals/tek-tds-784a-service-manual.pdf

Roger

Tom,

Do you mean a TDS 684A? I have one of these and it has serious problems
with leaking of the SMD electrolytic capacitors on the acquisition board
and subsequent corrosion. There are not too many to change and you can
find the service manual which details the procedure for removing the
acquisition board. Look very carefully around the other SMD components
near the attenuator assembly, this is where I have the most obvious
corrosion rather than near the electrolytics. Best to fix these before
they cause damage to the PCB tracks and vias.

Roger

Hi Harvey,
Thanks for your email. It was very helpful. I was working with the scope
for a while, turning it off and on a few times, and finally the screen came
up. I suspect that since the scope hadn't been powered up in a long time,
it might take a while to initialize, and it seems fine now. Thanks very
much. I appreciate your help. Now I have two more Tektronix scopes to fix,
and one could be a project.

Tom

On Sun, Aug 12, 2018, 9:22 AM Harvey White <madyn@...> wrote:

On Sat, 11 Aug 2018 20:31:33 -0700, you wrote:

All the lights ought to come on, then the power up sequence starts.
You should hear relays clicking, and you should see different lights
on the front panel. If not, then you have a possible power supply
problem, or a possible processor problem. There are diagnostic modes
that you should be able to use to check out the processor.

However, another possibility is that the display has failed. If so,
then you can plug in a computer display to the scope and you should be
able to see something. I'd do that first, then check power supply
voltages, then check the processor, etc.

Harvey

On Sat, Aug 11, 2018 at 8:27 PM Thomas Dodge <tdodge2404@...>

wrote:

Hi,
My name is Tom, and I haven't sent any emails in a while. I just bought

a

Tektronix TDS 648A scope. I plugged it in for the first time, and all

the

lights come on, but no intensity at all. Does anyone know how to

approach

this issue? Thanks a lot.

Tom

I have recently become the proud owner of a 475 that I found abandoned at the side of a road! Other than a couple dents in the casing, it appears to be in pretty good condition.

After replacing the fuses, I was greeted with what appeared to be a very fuzzy trace when turning the power on. However, very soon afterwards, a resistor directly adjacent to the J4 connector on board A9 (listed in the manual as 'interface circuit board assembly') began to smoke, at which point I powered it off. One of the leads of the resistor in question is connected to the positive side of a orange dipped (tantalum?) capacitor, which leads me to suspect that it may be the culprit, given all that I have read so far regarding worn out and failed capacitors on 400 series scopes. When testing the capacitor in the board, it certainly seems like it's shorted, which I presume would explain the smoke from the adjacent resistor.

I'm hesitant to power it on again to check all the rail voltages in case I further damage the resistor. I suppose I should start by verifying which capacitors are good and bad, and replacing the failed ones? In this case, is there a definitive criteria for determining which capacitors are good and which need to be replaced? Or should my attention be directed elsewhere first?

Any advice and suggestions would be greatly appreciated.

Hi Mark,
That is an interesting point you make about using aluminum electrolytics
with a higher diameter to length ratio.

I have been using Kemet T494 series SMD -55 C to +125 C Low ESR Tantalum
for a very long time in aviation products and have not had issues. They
claim to use "conflict free Tantalum". I have also used high quality
electrolytics with higher diameter to length ratios if I needed really big
capacitance.

I don't share your disdain for tantalum electrolytic capacitors. Their
dramatic failure modes to happen when they are operated near their rated
voltage. For this reason, I de-rate their voltage application by more than
4:1. That seems pretty safe by most accounts. Also, I chose capacitors
with an ESR equivalent or less than the aluminum capacitors they replaced
(smaller ones were available at higher ESR). I know nothing of the
"ethics" of tantalum capacitors or their vendors.