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Please help with TDS340A


James
 

HI all,
I am out of my depth, no question but I'm going to ask and see what you guys think.
I have a TDS 340A which has behaved well but now has some kind of switching artifact that makes it impossible to use. There is a pulse at 31.5kHz which can be seen clearly with a coiled wire attached to a probe and which seems to come from the front of the screen. This pulse is getting through either channel into everything I am trying to measure, I cannot tell how. If the probe is grounded the signal is clean as you would expect, but every circuit I attach a probe to now shows this wretched pulse. Is this a result of some foolery on my part, a scope about to go on the blink, or something quite different? What can I do to try to fix it. I am no EE, just a basic hobbyist and unhappy with the idea of serious surface mount work - I will likely screw up worse than it is! I do have an old scope that functions in a manner of speaking which I could use to probe if anyone thinks I can achieve anything useful, or am I better off getting another functioning scope, or finding (where in UK) a person willing to try to fix it?
Any and all help gratefully received!
Many thanks
James


Harvey White
 

31.5 Khz sounds like something generated by the display.  You could disconnect the display power, use an external display, and see if it goes away.  Possibly a bad filter capacitor in the display itself.

Harvey

On 9/9/2020 2:37 PM, James wrote:
HI all,
I am out of my depth, no question but I'm going to ask and see what you guys think.
I have a TDS 340A which has behaved well but now has some kind of switching artifact that makes it impossible to use. There is a pulse at 31.5kHz which can be seen clearly with a coiled wire attached to a probe and which seems to come from the front of the screen. This pulse is getting through either channel into everything I am trying to measure, I cannot tell how. If the probe is grounded the signal is clean as you would expect, but every circuit I attach a probe to now shows this wretched pulse. Is this a result of some foolery on my part, a scope about to go on the blink, or something quite different? What can I do to try to fix it. I am no EE, just a basic hobbyist and unhappy with the idea of serious surface mount work - I will likely screw up worse than it is! I do have an old scope that functions in a manner of speaking which I could use to probe if anyone thinks I can achieve anything useful, or am I better off getting another functioning scope, or finding (where in UK) a person willing to try to fix it?
Any and all help gratefully received!
Many thanks
James



James
 

Thanks Harvey. So is there a simple way to connect an external display, and is it just the HT to the tube that I disconnect on the scope? Please excuse these very basic questions!


Harvey White
 

I had to look up this scope.

IF you have option 14, there's a VGA connector on the back.  Plug a standard VGA monitor in there.  Otherwise, you'd have to tap off the signals and feed them to an external monitor if you can figure out where the signals are supposed to be.

I have a TDS540A, which has the VGA connection as a standard item.

For the display, what you have is essentially a small computer running a VGA display, with the scope electronics hooked to that computer.  So the electronics capture and digitize a waveform, that's read by the computer, the processor then formats the display, and presents you the signal.

What I *think* is happening is that there's likely a bad electrolytic in the power supply for the display.  (or on the display board itself).  If you can completely unplug the monitor, then the interference should go away.  However, without an external monitor to see what the output is, you can't tell.

So with another scope, I'd start looking for noise on the power supply outputs.  Since it's VGA, (and a CRT), you've got a lot of energy at 30.8 Khz (their spec), and the horizontal drive is generally used to generate the high voltage just as in a standard (old style) TV.

For testing, I'd just unplug the monitor completely (including power), and look for interference on the power supply outputs. Then I'd plug the monitor back in, and see what happens in those same leads.

IF you have option 14, it's a lot easier.

Harvey

On 9/12/2020 2:38 PM, James wrote:
Thanks Harvey. So is there a simple way to connect an external display, and is it just the HT to the tube that I disconnect on the scope? Please excuse these very basic questions!



James
 

Thanks again Harvey.
I took the scope apart and pulled out the display module. Praise be, it's through hole components! It also appears that the HV section and horizontal deflection parts are all on this module. Following on I have a couple of further dumb questions:
The CRT has a white connector at the end, I presume it works like a valve base and I just gently pull it away from the CRT pins? Any advice/tips on doing this?
The service manual is full of obsolete part numbers for the electrolytics, with no datasheets easily obtainable. The caps are a mixture of 35-160V types from Nichicon and similar and all look fine but as yet unmeasured. It's probably easier to just pull them out and replace them, there's only a handful. Do I need particular spec capacitors or am I likely to be ok if replacing 105C +50-20% types with others that meet this spec (and obviously the same or higher working voltage)?
The parts list distinguishes CAP;FXD;ALUM from CAP;FXD;ELECTLT. Are these all aluminium electrolytics or is there some other difference I am unaware of? They all look like electrolytics!
Many thanks as ever in advance.


James
 

...Also what do you recommend to glue the electrolytics to each other and/or the board. They currently are mostly stuck; do I use hot glue or silicone and if so which grade is suitable?


 

Personally I'd use neutral cure silicone. Hot glue gun glue is a pain to remove when you need to.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of James
Sent: 21 September 2020 12:07
To: TekScopes@groups.io
Subject: Re: [TekScopes] Please help with TDS340A

...Also what do you recommend to glue the electrolytics to each other and/or the board. They currently are mostly stuck; do I use hot glue or silicone and if so which grade is suitable?


Tony Fleming
 

Harvey is the BEST!!! I need him to download his brain and email me a copy
of it! Thanks Harvey for helping everyone here!!!

On Mon, Sep 21, 2020 at 6:05 AM James <@Jazid> wrote:

Thanks again Harvey.
I took the scope apart and pulled out the display module. Praise be, it's
through hole components! It also appears that the HV section and horizontal
deflection parts are all on this module. Following on I have a couple of
further dumb questions:
The CRT has a white connector at the end, I presume it works like a valve
base and I just gently pull it away from the CRT pins? Any advice/tips on
doing this?
The service manual is full of obsolete part numbers for the electrolytics,
with no datasheets easily obtainable. The caps are a mixture of 35-160V
types from Nichicon and similar and all look fine but as yet unmeasured.
It's probably easier to just pull them out and replace them, there's only a
handful. Do I need particular spec capacitors or am I likely to be ok if
replacing 105C +50-20% types with others that meet this spec (and obviously
the same or higher working voltage)?
The parts list distinguishes CAP;FXD;ALUM from CAP;FXD;ELECTLT. Are these
all aluminium electrolytics or is there some other difference I am unaware
of? They all look like electrolytics!
Many thanks as ever in advance.






Harvey White
 

Comments Interleaved:

On 9/21/2020 7:04 AM, James wrote:
Thanks again Harvey.
I took the scope apart and pulled out the display module. Praise be, it's through hole components! It also appears that the HV section and horizontal deflection parts are all on this module.
Pretty much have to be.  The monitor is pretty much a standard VGA monitor (I'd think) and needs the usual assortment of HV section, Horizontal and Vertical drivers (for the yoke), and a video driver.


Following on I have a couple of further dumb questions:
The CRT has a white connector at the end, I presume it works like a valve base and I just gently pull it away from the CRT pins?
That would be right.  Make sure that you are pulling only on the socket, and not the pin holder/base of the tube as well.  I'd be tempted to try to very gently rock it back and forth in case the pins are somehow stuck.  It should be keyed, either by a keyway in the plastic base or, if like a 7 or 9 pin valve, pin spacing
Any advice/tips on doing this?
The service manual is full of obsolete part numbers for the electrolytics, with no datasheets easily obtainable. The caps are a mixture of 35-160V types from Nichicon and similar and all look fine but as yet unmeasured. It's probably easier to just pull them out and replace them, there's only a handful.
I'd  do a little probing of the power inputs with another scope (I think you said you had one) to look for that 30 KHz waveform. That might tell you which to replace.  Alternatively, you can (assuming it's a 15 volt line), tack a 100 to 1000 uf 25 volt capacitor across that at the input to the monitor and see what that does or doesn't do.

Another possibility is that if there's a shield around the HV (that's HT for you) transformer, then see if the screws are tight.



Do I need particular spec capacitors or am I likely to be ok if replacing 105C +50-20% types with others that meet this spec (and obviously the same or higher working voltage)?
The parts list distinguishes CAP;FXD;ALUM from CAP;FXD;ELECTLT. Are these all aluminium electrolytics or is there some other difference I am unaware of? They all look like electrolytics!
I suspect that they're all standard aluminum electrolytics, since this is mostly consumer level off-the-shelf electronics in the display.  I'd go for 105 degree C capacitors (typically you'd be getting 85 or 105, 105 lasts longer at higher temperatures, and even at lower ones), working voltage equal to or up to 50% more (that's arbitrary, but a slightly larger working voltage is better).  Do not use a 15 volt or 16 volt rated capacitor on a 15 volt line, just not enough headroom.

and for the actual value, most electrolytics are about -20% + 100% in tolerance, so putting a 150 uf capacitor in place of a 100 uf capacitor should be fine.  If you think it's part of a signal chain, as in a filter for a particular frequency, or something doing timing, then go for the same value.


Many thanks as ever in advance.
Not a problem.  Check out some of the other repair threads some time to see some general "what do I put in its place?" advice. Not sure if anyone ever collected this and put it somewhere.

Harvey





James
 

Hi and thanks to all, especially Harvey. I am not much further forward. I haven't gone for replacing the caps as advised by Harvey, but have managed instead to probe the power supply into the scope, and to upload a pic of what the interference looks like taken at the screen. This pulse is lessened but still present all the way around the scope. My old Trio scope shows almost nothing at all when the same probe is held against its CRT. The PSU output is clean and the trigger pulse is about 5V at the same frequency of 31.5ish kHz.

Is this info helpful to you who know so much more than I? I am out of my depth so moving very cautiously here!


James
 

Oh, the trigger pulse is a flat topped rectangular wave, looks clean both without and with the display section attached.


James
 

Hi all,
Another call for help to improve my scope. Does anyone know what might be causing this problem? I have unsoldered all the alu caps and checked, a couple were marginal and I replaced since it seemed sensible to do so, but none were way off spec. Issue remains.
Do I just junk the scope and if so what models are good replacements for tube audio work at the hobbyist end of the market? Thanks as ever!