TEK 475: Powersupplies repaired, but still sick.


Redguuz
 

Hi,

Thanks to the extensive help of Ozan, I managed to repair the dead power supplies (especially the +15V) in my ailing TEK 475 (vintage: December 1975), but in nice cosmetic condition (formerly IBM, not used a lot).
(the +15V supply was shorted to GND, resulting in NO -8V en -15V and a poorly performing +5V: -> 3.8V i.o 5V).

The cause appeared to be NOT a bad tantal capacitor on the power rails , as is often the case, but an intermittent short to earth at the backside of the Bottom PCB.
By pushing and pulling on this large PCB (especially at the rear side, whilst monitoring the resistance to GND of the +15V rail, I managed to (temporarily ) get no shortcircuit (i.e. 61.2 Ohms to GND, which is the normal target),
But it is extremely difficult to dismantle this board!

Results:
Scope powers up (CRT screen, Fan, Graticule lights and all filament and neon lamps).
ALL power supplies are now according to spec:

+50V = 49.996 V
+110V = 111.190
+15V = 15.0189 V
+5V = 4.998 V
-8V = - 8.014 V
-15 V = - 15.003 V

1) Scope powers up, but VERY high screen intensity , which cannot be regulated by the Brightness control (Focus works but only down to a thick (vertical line) line), see below.
2) Graticule Light, Fan (powered by +15V ) work Ok

This scope has had the (same) occasional problem of a high intensity screen (since I bought it , 5 years ago) , but gently tapping on the lid solved this normally..
However, the last time, when it did manifest itself again , tapping did not work. Next, I switched it OFF and the ON, again, and apparently "blew" the +15V supply.(by "tapping" to strongly: intermittent short to GND).

What does work:
Vertical Amps Channel 1 and Channel 2 , Vertical movement control of Ch1 and Ch2
Beamfinder
Horizontal movement control (also X-10, both fine and coarse ) but only wrt the LEFT side of the screen.
Fan, graticule, indicator lamps (neon and filament lamps).

What does NOT work:
Control of Brightness (high screen intensity) :
Focus works but NOT down to a fine line: existing problem (formerly intermittent but recently became permanent).

A-Timebase and B-timebase do NOT work anymore : New problem; maybe self inflicted . (but I did not touch the time base PCB's), at all!.

What worries me that every now and then, there are ticking noises from the CRT; I suspect the CRT to be bad, causing the high screen brightness.
Shortcut of G1 (Wehnelt) -> Kathode??

What's your opinion: (This repair gets quite complicated for a novice);

1) Try to repair or donate it to someone with more capabilities (and also a spare CRT): I know one such person.

(I had a broken HAMEG 605 : 80 MHZ scope, which I did repair: only a partly blown bridge rectifier in one power rail)
Strictly speaking, I do not need a 200 MHz scope for my Audio Hobby. The HAMEG is a nice scope as well (and more modern: mid 1980's).

2) Are these TEK 475 faults repairable using the existing CRT? (A "new" CRT is a "Total Loss" case for me: vey costly and recalibrating the scope is outside my capabilities (do not have the required equipment).
Although, a "Nice to Have", I can live without the TEK475 and use the HAMEG 605, instead.

3) A new digital RIGOL Scope is quite expensive (and I prefer the operation of Analog scopes).

4) Any thoughts??.


Tom Lee
 

Look for a loose connection to the first grid of the CRT. A floating G1 causes max brightness. That tapping used to fix it temporarily is consistent with a bad connection, perhaps with the socket. Perhaps you'll get lucky and it's that simple.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 11/8/2021 17:44, Redguuz via groups.io wrote:
Hi,

Thanks to the extensive help of Ozan, I managed to repair the dead power supplies (especially the +15V) in my ailing TEK 475 (vintage: December 1975), but in nice cosmetic condition (formerly IBM, not used a lot).
(the +15V supply was shorted to GND, resulting in NO -8V en -15V and a poorly performing +5V: -> 3.8V i.o 5V).

The cause appeared to be NOT a bad tantal capacitor on the power rails , as is often the case, but an intermittent short to earth at the backside of the Bottom PCB.
By pushing and pulling on this large PCB (especially at the rear side, whilst monitoring the resistance to GND of the +15V rail, I managed to (temporarily ) get no shortcircuit (i.e. 61.2 Ohms to GND, which is the normal target),
But it is extremely difficult to dismantle this board!

Results:
Scope powers up (CRT screen, Fan, Graticule lights and all filament and neon lamps).
ALL power supplies are now according to spec:

+50V = 49.996 V
+110V = 111.190
+15V = 15.0189 V
+5V = 4.998 V
-8V = - 8.014 V
-15 V = - 15.003 V

1) Scope powers up, but VERY high screen intensity , which cannot be regulated by the Brightness control (Focus works but only down to a thick (vertical line) line), see below.
2) Graticule Light, Fan (powered by +15V ) work Ok

This scope has had the (same) occasional problem of a high intensity screen (since I bought it , 5 years ago) , but gently tapping on the lid solved this normally..
However, the last time, when it did manifest itself again , tapping did not work. Next, I switched it OFF and the ON, again, and apparently "blew" the +15V supply.(by "tapping" to strongly: intermittent short to GND).

What does work:
Vertical Amps Channel 1 and Channel 2 , Vertical movement control of Ch1 and Ch2
Beamfinder
Horizontal movement control (also X-10, both fine and coarse ) but only wrt the LEFT side of the screen.
Fan, graticule, indicator lamps (neon and filament lamps).

What does NOT work:
Control of Brightness (high screen intensity) :
Focus works but NOT down to a fine line: existing problem (formerly intermittent but recently became permanent).

A-Timebase and B-timebase do NOT work anymore : New problem; maybe self inflicted . (but I did not touch the time base PCB's), at all!.

What worries me that every now and then, there are ticking noises from the CRT; I suspect the CRT to be bad, causing the high screen brightness.
Shortcut of G1 (Wehnelt) -> Kathode??

What's your opinion: (This repair gets quite complicated for a novice);

1) Try to repair or donate it to someone with more capabilities (and also a spare CRT): I know one such person.

(I had a broken HAMEG 605 : 80 MHZ scope, which I did repair: only a partly blown bridge rectifier in one power rail)
Strictly speaking, I do not need a 200 MHz scope for my Audio Hobby. The HAMEG is a nice scope as well (and more modern: mid 1980's).

2) Are these TEK 475 faults repairable using the existing CRT? (A "new" CRT is a "Total Loss" case for me: vey costly and recalibrating the scope is outside my capabilities (do not have the required equipment).
Although, a "Nice to Have", I can live without the TEK475 and use the HAMEG 605, instead.

3) A new digital RIGOL Scope is quite expensive (and I prefer the operation of Analog scopes).

4) Any thoughts??.








Mlynch001
 

Could be a problem with the DC restorer circuit. I have had a couple of VR1374 82v Zener diodes that failed and caused uncontrollable brightness. Also check the solder joints on the CRT socket for failed joints.

--
Michael Lynch
Dardanelle, AR


Ozan
 

Hi Martin,
Congratulations for the +15V fix. It was a good puzzle.

On Mon, Nov 8, 2021 at 05:44 PM, Redguuz wrote:
....
1) Scope powers up, but VERY high screen intensity , which cannot be regulated
by the Brightness control (Focus works but only down to a thick (vertical
line) line), see below.
...
This scope has had the (same) occasional problem of a high intensity screen
(since I bought it , 5 years ago) , but gently tapping on the lid solved this
normally..
....
What worries me that every now and then, there are ticking noises from the
CRT; I suspect the CRT to be bad, causing the high screen brightness.
Shortcut of G1 (Wehnelt) -> Kathode??
As Tom suggested it could be a loose connection. Are the ticking noises happening when you shake the scope or are they more like high voltage discharge noises? If you have a high voltage probe it is worth looking at the cathode voltage (-2450V). If you don't have a HV probe at least base fo Q1306 should be ~ 0.8V or so if HV loop is regulating assuming R1303A and R1303B are good and you are measuring ~ 50.6V at the anode of CR1304. This check is not as good as using an HV probe but still checks most of the HV regulation loop.

If cathode voltage is good and nothing is loose usually the issue happens to be DC restorer as Michael suggested.

Ozan


Roger Evans
 

You should be able to (temporarliy) reduce the screen brightness by adjusting R1375 (Grid Bias), this should not be affected by a failure in the Z axis amplifier. This would save any damage to the phosphor and might restore good focus if there are no other high voltage or CRT problems. You can reset the Grid Bias later without affecting the calibration of the scope.

Regards,

Roger


 

On Tue, Nov 9, 2021 at 03:39 PM, Roger Evans wrote:


You should be able to (temporarliy) reduce the screen brightness by adjusting R1375 (Grid Bias)
Unfortunately, this is rarely the case. The negative voltage that is used for grid control and grid bias (adjustment) is generated by the very circuit that possibly is malfunctioning. In such a case, grid and cathode are at (almost) the same potential, instead of the grid being (several 10's of volts) more negative than the cathode at all times.

Raymond


Roger Evans
 

I did look at the 475 grid supply before posting and at least superficially the grid bias circuitry is separate from the z-axis amplifier but exactly how the two outputs are combined to set the voltage at the 'bottom' of the voltage multiplier I would not claim to understand.

Roger


 

On Tue, Nov 9, 2021 at 05:50 PM, Roger Evans wrote:


I did look at the 475 grid supply before posting and at least superficially
the grid bias circuitry is separate from the z-axis amplifier but exactly how
the two outputs are combined to set the voltage at the 'bottom' of the voltage
multiplier I would not claim to understand.
Hi Roger,
The following may help you on your way. It's a rough and brief description of what is supposed to happen:
A voltage pump, mainly consisting of the components below and slightly to the right of TP1364, and fed by the AC signal from T1320 (via R1326), generates a voltage of a few hundred volts. The Z amplifier modulates that voltage, e.g. to cause periodic beam (un)blanking. The grid bias adjustment makes sure that the grid is just negative enough with regard to the cathode so that the beam is blanked when it's supposed to be off. The Z-axis signal unblanks the beam by making the voltage difference from grid to cathode less negative at the right time and for the right duration to show the beam/trace. By setting the grid bias just negative enough (to allow full blanking), the full less-negative range is available for visible intensity.
If the voltage pump isn't working, all this more and less negative voltage doesn't exist, cathode and grid will be at (almost) the same voltage (-2450V) all the time and beam blanking won't occur, resulting in full brightness only.

Raymond


 

On Tue, Nov 9, 2021 at 05:50 PM, Roger Evans wrote:


exactly how the two outputs are combined to set the voltage at the 'bottom' of
the voltage multiplier I would not claim to understand.
I forgot to go mention:
a. The voltage at the "bottom" of the HV multiplier is generated by T1320 and is *constant*.
b. What I wrote in my previous email functions without and is completely independent of the HV multiplier. The HV multiplier receives an AC voltage from T1320, just like the voltage going to R3126. Together with the other (= gnd) side of the multiplier input (and output), a voltage cascade inside U1321 is fed and its output is the source of the PDA-voltage.

Raymond


Redguuz
 

Thank you Raymond,

I read on another forum that the absence of brightness regulation could spell another disaster: a faulty winding on T1320 (feeding R1326).

Does this happen often? (The HV transformer is quite buried in the innards of the scope).


Mark Vincent
 

It sounds like your d-c restorer diodes are bad (leaky). Use 1N4937 diodes to replace them. Also replace the 22meg 1/4W carbon with a Vishay RT37 style. These are 1/2W 1%. The original carbons are usually well out of tolerance.

Mark


 

On Tue, Nov 9, 2021 at 11:08 PM, Redguuz wrote:


I read on another forum that the absence of brightness regulation could spell
another disaster: a faulty winding on T1320 (feeding R1326).
I consider that highly unlikely (to say the least). The HV (-2450V) seems to be there and you've got very high brightness, so PDA (from U1321) seems to be there as well. Both need T1320 and the feed to R1326 is somewhere in between on T1320.
I think the negative bias voltage is gone (that's the gist of my earlier mails today), caused by what Mark Vincent mentions or you have a leaking/shorted DC restorer cap (C1371, C1372) or even C1326. By all means replace R1278 (22MOhm) while you're in there.

Raymond


Redguuz
 

Thanks All,

Really appreciate all your suggestions.

@ Raymond:
I also concluded that the HV system (oscillator/transformer T1320) does work, but IMHO it does NOT exclude the possibility that there could be an intermittent loose connection from a specific secondary HV tap (the one connected to R1326).
This is also mentioned in several threads
(there was also the suggestion to convert to a HV transformer of a 465: this one uses a better potting material than the transformer used in the 475.)


Maybe I should firstly acquire a EHT probe to better diagnose the problem.

I will firstly get some spares and report back on the results of the various suggestions.

Thanks! Martin


 

Hi Redguuz,
Of course, all kinds of problems are possible, of which I currently consider a defective winding in T1320 the least likely. A bad R1326, C1326 or so are much more likely.
Keeping the KISS-method and Occam's razor in mind, Mark's and my recommendation for replacing a few components seems most appropriate. These are the culprits in about 80% of symptoms like yours (they "often" fail after a few decades), and replacement is cheap and easy. Evn if they're not the problem, they might well be soon. Also, danger of (more) damage to the 'scope (and you) would be much smaller.

Raymond


Redguuz
 

PS Raymond,

I gather that in your last mail "R1278 (22MOhm)" should read " R1378(22MOhm)" .


Redguuz
 

Raymond,
Very much appreciate your good advice especially the Safety Warnings!!.


 

Yes, I meant R1378, not R1278. These carbon comp. R's usually drift high and I would expect the negative voltage becoming too high, resulting in blanking, not full intensity but by all means, replace it while you're in.

Raymond


 

On Wed, Nov 10, 2021 at 02:24 PM, Raymond Domp Frank wrote:


and I would expect the negative voltage becoming too high
That should have read "and I would expect the negative voltage possibly becoming higher, in turn resulting in a tendency to *reduce* brightness, not increase to full intensity.

Raymond


EB4APL
 

I never have repaired a 475, but I have fixed several scopes (Tek and other brands) with the brightness at maximum symptom, and always the culprits were one or more diodes in the DC restorer that were leaking. Since you need a specialized test setup, the best approach is to replace all of them once you are there.

Ignacio

El 10/11/2021 a las 3:36, Raymond Domp Frank escribió:
On Tue, Nov 9, 2021 at 11:08 PM, Redguuz wrote:

I read on another forum that the absence of brightness regulation could spell
another disaster: a faulty winding on T1320 (feeding R1326).
I consider that highly unlikely (to say the least). The HV (-2450V) seems to be there and you've got very high brightness, so PDA (from U1321) seems to be there as well. Both need T1320 and the feed to R1326 is somewhere in between on T1320.
I think the negative bias voltage is gone (that's the gist of my earlier mails today), caused by what Mark Vincent mentions or you have a leaking/shorted DC restorer cap (C1371, C1372) or even C1326. By all means replace R1278 (22MOhm) while you're in there.

Raymond

--
El software de antivirus Avast ha analizado este correo electrónico en busca de virus.
https://www.avast.com/antivirus


 

On Wed, Nov 10, 2021 at 03:59 PM, EB4APL wrote:


I never have repaired a 475, but I have fixed several scopes (Tek and other
brands) with the brightness at maximum symptom, and always the culprits were
one or more diodes in the DC restorer that were leaking. Since you need a
specialized test setup, the best approach is to replace all of them once you
are there.
Yeah, many of us consider replacing the DC restorer diodes a no-brainer in a 'scope with these symptoms.

Raymond