TDS-460A - Have to hold the ON/Standby button to power up

[abeasley]

I know I saw this addressed somewhere but I can't find it again for some reason. I have a TDS-460A where I have to hold down the ON button for it to power up. Once I release the ON button it turns off as if I pressed it again. Any ideas what I need to look for to address this issue?

Thanks!

[nidlaX]

Have you cracked it open and checked the switch for continuity / contact issues?

[tautech]

Welcome to the forum.

Thinking about your problem, by virtue of it having a standby mode it must have a standby PSU much like a modern PC PSU.
If the ripple was excessive on that PSU it might explain your symptons.
Time for some study of the Service manual, and get the Tek troubleshooting pdf in the first post in this board.

As with most repairs, the PSU is mormally the first suspect. 

[abeasley]

Thanks for the replies.  I messed around with it some and was able to get it to boot up without holding the button, but now the button will not turn it off.  I've checked the button for continuity and it's right in line with the other buttons.  I'll try checking the power supply to see if anything seems off.  For what it's worth the scope boots without any errors being reported.

Thanks!

[nidlaX]

That's pretty odd, do the same symptoms occur if you short the switch contacts / pads? If it's a PSU problem, I imagine it would be independent of whether or not you press the switch.

[abeasley]

I messed around with it some this evening.  When I first turned it on it powered up without pressing the ON/STBY button.  After using the scope for a while I pressed the ON/STBY button and it powered off.  However, once I press the ON/STBY button again it doesn't power on unless I hold the button down. 

Do you happen to know if the Backplane Assembly controls this?  I was scanning through the troubleshooting guide and it was mentioned with regards to power issues but I didn't see anything directly related to this issue.

I checked all the voltages and everything looks great with no noticeable ripple.  I also ran all the diagnostics for the scope without any issues either.  Definitely perplexing.

[tautech]

Can you post or link the schematic?
Sounds like the toggling of the switch between it's 2 states is not latching.

Is it built with discretes of an IC?

[W3DRK]

Hah, I just acquired a TDS-420A with the EXACT same issue.  Everything else works perfectly, nice bright and clear CRT too.  At first the Power/STBY botton did nothing, and the scope was always on.  But after poking around, I now must hold the button in to keep the scope running.

I've been digging around in the scope for a few hours, using the service manual for a reference.  According to the service manual, the STBY button signal is passed through from the button membrane "cable" to the FRONT PANEL PCB, where it passes through to the ribbon cable that connects the FRONT PANEL PCB to the CPU board.  Unfortunately, TEK doesn't provide schematics and it's appears to be a 4 layer PCB making it pretty difficult to follow the power button signal.

Did you make any progress with your TDS-460A?

[abeasley]

I haven't made any more progress.  I ordered a replacement power supply which I will try as soon as it arrives.  If that doesn't help I'm going to try to retrace everything.

[abeasley]

I have installed the new power supply and I'm having the same issue.  Actually now it's not reading the suspend button at all.  When I first powered it up I had to press the on/suspend and then it came on.  I wasn't able to turn it off with the on/suspend button though and each subsequent try it just powered on when the power was turned on.  I'm going to try rechecking the button and make sure the wires are all conducting properly when I get some more time.

Let me know if you have any luck.

[Shock]

Was that a new power board you ordered? Check the backup battery. Look for leaky surface mount electrolytic caps (hazy look on the PCB). Check for bad/intermittent solder joints, clean and check the interconnecting cables. Especially on the aux power PCB (A07).

Check all the voltages from the service manual write them down here so perhaps others here can compare, check and follow the troubleshooting steps. Trace out the circuit from the soft power switch and especially check caps for ESR on that circuit.

Using a scope if possible capture the button presses to capture odd behavior. With the scope off check the resistance to ground on the switch contacts. Measure the resistance of the switch circuit to each contact. Disconnect any interconnecting cables and recheck.

Check also voltage regulators on all boards and their accompanying caps ESR. Check ripple.

Bump that post that you have on the Tektronix forum.

https://isurplus.com.au/manuals/Tektronix%20TDS430A%20Oscilloscope%20User%20Manual.pdf

[abeasley]

Thanks for the reply Shock. 

The power supply was a custom one made by Gianni Lisini, eelectronic.shop@gmail.com, and works perfectly.  I had installed one from my TDS-420 while waiting for this one to arrive.    I replaced all the backup batteries as I thought that might be the culprit and have examined all the caps closely and they are pristine.  I have another A07 board which I can try as well.

I'll pull the faceplate off this evening and trace out the buttons.  Hopefully whatever the problem is with show itself.

[abeasley]

I'm stumped.  I replaced the auxiliary power supply, the front panel, the back plane and the cables going to the back plane to the auxiliary power supply and the cables from the front panel to the back plane and the cpu board.  Unfortunately I didn't have any way to changing the CPU board but I'm thinking perhaps that may be the problem.

Any chance anyone knows in which positions the switches should be?

As it stands now the oscilloscope turns on as soon as I switch on the power and won't turn off until I press the power switch.  The ON/STBY button does nothing.

[tautech]

Is this switch a soft key type(rubber membrane), maybe the conductive layer is non conductive or the tracks dirty?

[abeasley]

tautech, I pulled off the front cover and cleaned the contacts and  tested them, everything looks good.  Sometimes when it's been off for a while I have to press the ON/STBY button for it to boot but after that it could care less what I do with that button.  I've gone through all the trouble shooting steps and it says replace the A01 Backplane, A06 Front Panel, or A07 Auxiliary Power boards and I've replaced all three, granted they were from an old 420 and could possibly be bad as well.

I guess I'm going to move on to the next project and hope someone smarter than me runs into a similar issue and comes up with a fix!