Topics

TDS744

Vintage Test
 

Hi All,

I've just laid my hands on one of these for a decent price, knowing it didn't work. The owner said it worked one day and jus did tick, tick etc the next - a familiar story! When I stripped it, I found a load of duff diodes/transistors and two-way devices in the standby circuit, which I replaced. I powered it up - still the same, except that the front-panel LEDs all flashed too now. On a whim, I disconnected everything from the processor board, and lo and behold -it powered up and worked properly into a VGA monitor. Now, to the problem; the CRT driver assembly is clearly faulty, but there's no schematic for it anywhere. I have the manual for TDS540, which has a lot of similarities, but is monochrome and not colour, so the CRT driver assembly is way different. Do any of you happen to have any experience with these 'scopes?

Cheers,
--
you can never have enough oscilloscopes, DMMs, valve testers or soldering irons . . .

 

There's 4 or 5 TO-39 transistors on the CRT neck board. One of them is prone to shorting and causing that problem. Later revisions had a heatsink on it to keep it cooler.

Sometimes it will be discolored from thermal stress giving it away visually.

The TDS544A schematic with the early revision color CRT driver board is on Hakan's page; it may be of some help...

There's also a capacitor that often leaks and causes the same problem. Facing the front of the board, it is located on the left side/edge somewhat towards the rear - there's three in a row, and it's the largest of the three. 100uF 50V I believe. the leakage gets so bad that it begins conducting under the cap and starts burning the PCB due to carbonizing (sp?)

Jay

dnmeeks
 

I've had a couple of TDS7x4 scopes with issues on that board. Below are some notes that I wrote up once for the group. Also I have a schematic that I got somewhere for a TDS544 HV board, and I was able to use that to troubleshoot the '754. As I remember, the ref designators were the same, but the board layout was slightly different. I put that schematic in a new folder called "TDS544_HV".
I also think I have a HV board from a TDS744 (that is bad) for parts: let me know if you want it. I think this board was DOA in a scope that was badly busted up and I didn't try to repair it - i was able to get a working board so I just replaced it.


Here are my old notes:

TDS754C arrived with dim screen and also the screen was too large in both dimensions. It was wrapping slightly around the edges of the tube.
I could adjust the brightness using the trimmers on the left side of the high voltage power board (these trimmers are available with the outside cover removed, just in front of the fan).
But even with the brightness back up, the screen still overlapped. The brightness adjustment didn't affect the screen overlap either.

There is a 75V and -75V rail on the high voltage board that is derived from the same flyback as the high voltage. +75V is available at J60, a wire leaving the PCB, just in back of the CRT socket.
On my board, the 75V rail measured 67.9V, or about 10% low.
Turning the HV adjustment (R58, on the right side of the board near the edge) had no effect at all.

I have another HV board from a TDS744, and ohming around U45 (the PWM generator that controls the HV) didn't help - it looked the same as the old board.

However, U45 was indeed bad. Replacing that IC (MC34060P) brought the 75V rail up to 76V (and I adjusted it down to 75V). The screen is much brighter (I turned the brightness back down), and the screen size is normal.

Here is a summary of the controls on the HV board, for future reference (note that your board may be different so no guarantees here):

HV adjust: R58
Horiz Size: R172
Horiz Position:
Vertical Size: R270
+21V adjust: R186
Vertical Position: R402 (available next to the fan)
Contrast: R404 (next to the fan)
Brightness: R403 (next to fan)
Contrast: R404 (next to fan)
Trace rotation: R401 (next to fan)

Hopefully that will help someone in the future.

Siggi
 

On Fri, Jul 6, 2018 at 18:37 Vintage Test via Groups.Io <mel.purcell=
icloud.com@groups.io> wrote:

I've just laid my hands on one of these for a decent price, knowing it
didn't work. The owner said it worked one day and jus did tick, tick etc
the next - a familiar story! When I stripped it, I found a load of duff
diodes/transistors and two-way devices in the standby circuit, which I
replaced. I powered it up - still the same, except that the front-panel
LEDs all flashed too now. On a whim, I disconnected everything from the
processor board, and lo and behold -it powered up and worked properly into
a VGA monitor. Now, to the problem; the CRT driver assembly is clearly
faulty, but there's no schematic for it anywhere. I have the manual for
TDS540, which has a lot of similarities, but is monochrome and not colour,
so the CRT driver assembly is way different. Do any of you happen to have
any experience with these 'scopes?

I did look for a schematic before I recapped the CRT driver in my TDS784D,
but at the time there wasn’t anything around that I could find. Since then,
Håkan has made this <
http://www.hakanh.com/dl/docs/hardtofind/TDS544A_A30-32(Disp).pdf>
available for download. Maybe it’ll help? There may be other relevant docs
on his site, he keeps adding stuff over time.

People also convert those to LCD screens. There’s the NewScope kit, but you
can also just drive an LCD from the VGA output.

Vintage Test
 

Guys,

Thanks very much for all of your help and the link to Hakan’s info - I’ll certainly have a look! I managed to do a deal with a US seller for a complete, working board(s), but will repair mine in case others might need it in the future. While looking for a replacement, I noticed that the later versions have a different HT trans former, so will look into this and get back to you all. Once again, many thanks.

--
you can never have enough oscilloscopes, DMMs, valve testers or soldering irons . . .

 

While looking for a replacement, I noticed that the later versions
have a different HT transformer,
Also the aluminum cover over the P/S is slightly different to fit the newer transformer.
/Håkan

Vintage Test
 

Hi All,

I am totally stumped with this scope now! Many thanks to Håkan for the schematics, but I can’t figure out why neither of the two CRT drivers work - both take so much current (>1.5A) that the LV PSU 25V rail is pulled down so hard that the PSU goes into protection. I’ve checked all of the semiconductors and electrolytics, and am baffled. The scope is fine on an external VGA monitor. Do any of you have any suggestions please?

Cheers, Mel

--
you can never have enough oscilloscopes, DMMs, valve testers or soldering irons . . .

 

I am totally stumped with this scope now! Many thanks to Håkan for the schematics, but I can’t figure out why neither of the two CRT drivers work - both take so much current (>1.5A) that the LV PSU 25V rail is pulled down so hard that the PSU goes into protection. I’ve checked all of the semiconductors and electrolytics, and am baffled. The scope is fine on an external VGA monitor. Do any of you have any suggestions please?
Double-check the TO-39 transistors on the neck board. There's one that usually shorts out, look for discoloration of the can. Later revisions added a heat sink to the device.

dnmeeks
 

I've had a couple of TDS7x4 scopes with issues on that board. Below are some notes that I wrote up once for the group. Also I have a schematic that I got somewhere for a TDS544 HV board, and I was able to use that to troubleshoot the '754. As I remember, the ref designators were the same, but the board layout was slightly different. I am not sure how to attach a PDF... maybe you can't?
I also think I have a HV board from a TDS744 (that is bad) for parts: let me know if you want it.
Here are my old notes:

TDS754C arrived with dim screen and also the screen was too large in both dimensions. It was wrapping slightly around the edges of the tube.
I could adjust the brightness using the trimmers on the left side of the high voltage power board (these trimmers are available with the outside cover removed, just in front of the fan).
But even with the brightness back up, the screen still overlapped. The brightness adjustment didn't affect the screen overlap either.

There is a 75V and -75V rail on the high voltage board that is derived from the same flyback as the high voltage. +75V is available at J60, a wire leaving the PCB, just in back of the CRT socket.
On my board, the 75V rail measured 67.9V, or about 10% low.
Turning the HV adjustment (R58, on the right side of the board near the edge) had no effect at all.

I have another HV board from a TDS744, and ohming around U45 (the PWM generator that controls the HV) didn't help - it looked the same as the old board.

However, U45 was indeed bad. Replacing that IC (MC34060P) brought the 75V rail up to 76V (and I adjusted it down to 75V). The screen is much brighter (I turned the brightness back down), and the screen size is normal.

Here is a summary of the controls on the HV board, for future reference (note that your board may be different so no guarantees here):

HV adjust: R58
Horiz Size: R172
Horiz Position:
Vertical Size: R270
+21V adjust: R186
Vertical Position: R402 (available next to the fan)
Contrast: R404 (next to the fan)
Brightness: R403 (next to fan)
Contrast: R404 (next to fan)
Trace rotation: R401 (next to fan)

Hopefully that will help someone in the future.

nonIonizing EMF
 

On Sun, Sep 8, 2019 at 08:05 PM, dnmeeks wrote:


I am not sure how to attach a PDF... maybe you can't?
There is a Files section where files can be uploaded. Seems like a file can be attached though the group rules/settings might not allow for. File uploads are allowed.