7603 working .... then blank


tinkera123
 

Hi all,

Some background before questions:-

I picked up another 7603 Scope with 2 x 7A15A Vertical Plugins and a 7B53A Time Base ….. Beaverton B338,855 …. with Tek trolley, several basic probes, original Tektronix manuals and a few other bits and pieces …. very cheap as not many offered for sale in Australia that I know of.

So …. the common question is …. do I turn it on?? ……. or do a lot of testing first?? Usually, no …. all my old radios have been full of old electrolytics, wax, paper caps …. rat/mouse eaten …. used as a toilet etc.. But this Scope came to me as “Good operational condition” and I had photos of it working before I purchased and the guy was a retired telecommunications techo …

So yes, I gave it the once over …. power cord, correct voltage, Jumper in correct position, nice and clean, no burnt components, no burnt smells etc etc.. My intention was to turn it on …. side covers off … Safety goggles on … then check voltages at P1171 with Scope running.

I found the following issues:-

1. The display (no signal) was a bit fuzzy with some interference through it. This disappeared as the Scope warmed up.
2. The display completely disappeared for a second or two before re-appearing as normal. This decreased as the Scope warmed up.
3. Vertical Plug-in 2 Screen readout showed a “u” instead of a “V”. This slowly corrected to a “V”.

Calibrator functioned as I expected. All knobs functioned as expected. I was starting to relax …..

4. Then the Scope completely shut down ….. no Display (it transitioned from a normal looking square wave to squiggling lines to blank over 1 to 2 secs) …. Power light out …. Not sure about the Fan.

Waited for 10 mins and tried again …. The Fan operated, nothing else …. Same next morning. I will never know if these issues were already 'known' or occurred during transporting.
Currently, I am going through the circuit diagrams to work out what the possible causes could be …. making sure I can identify all the Boards, connectors, Tek terminology etc..

Questions:-
1. I would appreciate some expert opinions as to the probable cause(s)??
2. Am I correct I assuming that the voltages at P1171 are under operating conditions??

Cheers,
Ian


cmjones01
 

On Mon, Jan 29, 2018 at 02:09 am, tinkera123 wrote:
4. Then the Scope completely shut down ….. no Display (it transitioned from
a normal looking square wave to squiggling lines to blank over 1 to 2 secs)
…. Power light out …. Not sure about the Fan.
This is likely to be a failed tantalum bead capacitor, quite possibly in one of the plugins. To find it, take the plugins out one by one and try switching the scope on. If it comes out, you've found out which plugin has the problem. If it doesn't work even with all the plugins removed, the fault is in the mainframe.

To find the culprit in plugins, I just go round with my meter on continuity test range checking across all the likely-looking components. One failed short-circuit will typically make all the others on that power supply rail appear short-circuit too, so unless you have a ToneOhm or other means of isolating faults, desolder or cut one leg of any capacitor that appears shorted and check again. When I think I've found the problem, I try out the plugin with the offending capacitor removed. It may not meet its performance specs but it'll certainly tell you whether it's basically working or not.

Chris


Adrian
 

So there is a concurrent thread running on 7603 PSU failure (mine!!), which originated from the same symptoms:

All working, then a few seconds of 100Hz 'wiggling' then cold and dark, that thread might be worth a read?

PSU related so start there. Dead easy to check all the rails by removing the rear plastic cover (four screws) drop out the PSU LV Regulator PCB (four more screws) and all voltages both in and out, are easily accessible for probing on the assorted ribbon cables.

Take care 'unfolding' the board as the ribbons are stiff and in my case anyway, were snagged round stuff so it is easy to bend the SIL connector pins.  See  the SM schematic for details and yes, input voltages are on load, noticeably higher when things are not running!

Good luck,

Adrian

On 1/29/2018 10:09 AM, tinkera123 wrote:
Hi all,

Some background before questions:-

I picked up another 7603 Scope with 2 x 7A15A Vertical Plugins and a 7B53A Time Base ….. Beaverton B338,855 …. with Tek trolley, several basic probes, original Tektronix manuals and a few other bits and pieces …. very cheap as not many offered for sale in Australia that I know of.

So …. the common question is …. do I turn it on?? ……. or do a lot of testing first?? Usually, no …. all my old radios have been full of old electrolytics, wax, paper caps …. rat/mouse eaten …. used as a toilet etc.. But this Scope came to me as “Good operational condition” and I had photos of it working before I purchased and the guy was a retired telecommunications techo …

So yes, I gave it the once over …. power cord, correct voltage, Jumper in correct position, nice and clean, no burnt components, no burnt smells etc etc.. My intention was to turn it on …. side covers off … Safety goggles on … then check voltages at P1171 with Scope running.

I found the following issues:-

1. The display (no signal) was a bit fuzzy with some interference through it. This disappeared as the Scope warmed up.
2. The display completely disappeared for a second or two before re-appearing as normal. This decreased as the Scope warmed up.
3. Vertical Plug-in 2 Screen readout showed a “u” instead of a “V”. This slowly corrected to a “V”.

Calibrator functioned as I expected. All knobs functioned as expected. I was starting to relax …..

4. Then the Scope completely shut down ….. no Display (it transitioned from a normal looking square wave to squiggling lines to blank over 1 to 2 secs) …. Power light out …. Not sure about the Fan.

Waited for 10 mins and tried again …. The Fan operated, nothing else …. Same next morning. I will never know if these issues were already 'known' or occurred during transporting.
Currently, I am going through the circuit diagrams to work out what the possible causes could be …. making sure I can identify all the Boards, connectors, Tek terminology etc..

Questions:-
1. I would appreciate some expert opinions as to the probable cause(s)??
2. Am I correct I assuming that the voltages at P1171 are under operating conditions??

Cheers,
Ian


tinkera123
 

Hi,

Chris - I will pull those Plug-ins later today. My mind locked onto a power supply problem ... that's why I like to ask questions on a Forum .... makes me think a bit more.
I watched a few 7603 scope videos on Youtube awhile ago whilst considering which Tek scope to chase .... found them again today ... cmjones01 ... same person I presume??

Adrian - yes, I have been reading your Thread ... those connections are now on my "To Check" list.

Thanks,
Ian


cmjones01
 

On Mon, Jan 29, 2018 at 04:02 pm, tinkera123 wrote:
Chris - I will pull those Plug-ins later today. My mind locked onto a power
supply problem ... that's why I like to ask questions on a Forum .... makes me
think a bit more.
I watched a few 7603 scope videos on Youtube awhile ago whilst considering
which Tek scope to chase .... found them again today ... cmjones01 ... same
person I presume??
The very same!

Chris


tinkera123
 

Hi,

Update on my 7603 scope ......

Removed Plug-ins and turned on Scope ...... Scope still dead.

Checked voltages at Pin P1171 .....
-15vdc and -50vdc ..... okay.
+5vdc, +15vdc, +50vdc and +130vdc ... all way out of spec.

Voltages at Pin P890 (unregulated V) show that all supplies are larger than expected .... probably due to being unloaded .....

Except the +130vdc supply .... which is +97vdc and -7.8vdc (instead of +130vdc and +40vdc).

I note that the Regulation circuits rely on other supplies to power the transistors. I am assuming for the moment (away from Home) that the +50v powering the +130v Regulation section (Q850, Q852 etc) is unregulated rather than regulated +50v .... ?? And just maybe, there might be no other faults .... 🤩

Next step is to check this Supply.

Cheers,
Ian


tinkera123
 

Delving further into the 7603 Rectifier Board and the problematic +130vdc power supply .... there is a component that I am unsure of. See photo in my Album "Ian's photos 7603 scope".
What is this component?? (located in center of photo)

It measures as a 'short' in-circuit, no power.

This component and Fuse F820 are not in either of my Service Manuals, not in the Component Listing or photos of the Rectifier Board. However, it is in both my Tek 7603 scopes .... Serial nos: 388,855 and 106,586.

Thanks,
Ian


 

It is a zero ohm jumper. Sounds like it is good.

----- Original Message -----
From: "tinkera123" <iacbell@outlook.com>
To: <TekScopes@groups.io>
Sent: Friday, February 16, 2018 11:51 PM
Subject: Re: [TekScopes] 7603 working .... then blank


Delving further into the 7603 Rectifier Board and the problematic +130vdc power supply .... there is a component that I am unsure of. See photo in my Album "Ian's photos 7603 scope".
What is this component?? (located in center of photo)

It measures as a 'short' in-circuit, no power.

This component and Fuse F820 are not in either of my Service Manuals, not in the Component Listing or photos of the Rectifier Board. However, it is in both my Tek 7603 scopes .... Serial nos: 388,855 and 106,586.

Thanks,
Ian


tinkera123
 

#@&%$ .... just a piece of wire ..... :) .... thanks tmiller.

Your analysis of its performance brought back a smile to my face.

Cheers,
Ian


Roger Evans
 

Ian,

The ways in which the 7603 low voltage regulators rely on each others' power rails is quite involved!

The internal 40V supply that defines the bottom end of the voltage on C806 is supplied from regulated +50V. So if the +50V is very low then the +130V will also be low as you observe.

Can you post the voltages you measure with all the ribbon cables connected? That would help to interpret where the root cause of the problems is likely to be.

The +50V regulator uses +130V and +5V (and relies on -50V for its reference). Your measured voltage on +130V should be OK for the regulator to function but the +5V probably needs to be +1V or more for Q863 to draw current (its emitter should be around -0.7V). If that is the case then the +50V regulator should potentially be working and you can probe for the fault. Open circuit series pass transistors are quite a common failure(Q869, Q872, Q874). If CR868 is forward biassed the supply is probably current limiting and in any case it is worth measuring the voltage across R875 to see how large is the load current.

Let us know how you get on.

Regards,

Roger


tinkera123
 

Hi Roger,

Yes, I had assumed that the “+50v” power to the +130v transistors was Unregulated …. and that has proved to be incorrect.
My checking this morning shows that whilst the +50v Unreg Supply is okay, the Regulated +50v Supply is not. Therefore, the 'feed' to the +130v Supply must be the +50v Regulated.
Below are voltages with ribbons connected :-

Unregulated Supplies … P890 pins in order, measured value, correct value (vdc) ….
1 -68.4, -53; 2 -24.5, -18; 3 +13.0, +8; 4 +25.2, +18; 5 +67.1, +53; 6 +97.0, +130; 7 -7.8, +40; 8 0, 0; 9 0, 0; 10 0, 0.
…. Only +130v Unreg Supply is not working

Regulated Supplies …. P900 pins in order, measured value, correct value (vdc) …
1 0, 0; 2 -49.8, -50; 3 -14.9, -15; 4 -0.6, +5; 5 -0.7, +15; 6 +0.7, +50 7 & 8 0, 0.
…. -15v and -50v Regulated Supplies are okay.
….. +5v, +15v and +50v Regulated Supplies are not okay.

I also measured the Voltage feedback to these Supplies (Sense), P901, and got the same result.

My next steps ….
... Check the transistor connections as per Adrian’s “Screw loose?” Thread on Jan 30.
... Have a look at the transistors etc you mentioned

Cheers,
Ian


Roger Evans
 

Ian,

The measured +5V is sufficiently low that it would cause both +50V and +15V regulators to stop working (Q863 and Q919A have very small Vce) so a single failure in the +5V regulator can explain the other voltages being too low. Check the voltages on base and emitter of Q973C, Q985 and Q988 to see if there is an open circuit. If you don't find anything amiss in the +5V regulator then check base and emiter voltages on Q926, Q931, Q933 in the +15V regulator. Also measure the voltages across R989 and R935 since this will show how much load current is being drawn.

Regards,

Roger


tinkera123
 

Hi Roger et al,

I am unsure as to how to identify an ‘open circuit’ around a transistor versus a transistor that is turned off. For example, voltages around U973A and C are as follows ... A e -1.26 b -0.56 c -0.67 and C e -0.11, b -0.84, c -0.65

I think that A is turned on (although c is very low) ..... as b is approx 0.6v more positive than e. However, this is not the case for C. So, is C ‘turned off’ completely or ‘open circuit’???

Q985 appears to be ‘on’ e -0.66 b -0.11 c -0.65
Q988 e -0.55 b -0.66 c +13.32 ??

Both R985 and R935 have no voltage drop across them.

I think that I need to probe around one of the ‘working’ supplies and learn how they are supposed to operate. I will tackle the -50v supply as the Manual has a good explanation as to how it works. Then I might be able to more effectively use your assistance.

Thanks,
Ian


Roger Evans
 

Ian,

Sorry to be so slow in replying, I have been away and looking at schematic diagrams on the phone is no fun!

I quite agree with your comments about difficulty in distinguishing an open circuit transistor from one which is biassed to cut off. With the power off you can do diode checks on b-e and b-c junctions but it is not very convenient unless the transistors are socketed.

Your measurement of -0.84V on the base of U973C is very puzzling and may indicate where the fault lies. U973D and U973E form a differential amplifier with a constant current source in the common emitter supply. Whichever of the pair has the more positive base voltage should take the majority of the current. When the regulator is working properly the collector of U973E (base of U973C) should sit at +5V plus the forward base-emiiter voltage drops of U973C, Q985 ansd Q988 so about +7V. The voltage divider of R970, R971 sets the base of U973E around 0V, the base of U973D is grounded via R975 and the gain of the differential amplifier stabilises the output of the regulator at +5V.

You measure -0.7V at the output of the +5V regulator so R970/R971 will set the base of U973E around -5V while the base of U973D is 0V minus the voltage drop of its base current across R971. The base current shouldn't be more than tens of uA (the emitter current is about 0.5mA) so the base of U973D should be only fractions of a Volt negative. In this state U973D should be drawing most of the common emitter current and the collector of U973E should be at a much higher voltage than you measure. Probing U973D and U973E might confirm this or we need to look elsewhere.

If the measurements show that U973 is suspect you can replace with a CA3046 or get the Tektronix replacement - is it socketed?

Regards,

Roger

PS There are some professional engineers around this group who can be much more precise and more authoritative than me.


Dave Hills
 

I just read over this thread this morning, and perhaps I can offer some insight.

First, the +5v supply is not necessary for the +50v supply to "turn on" and supply
an output. It is necessary for the +50v regulator to precisely control it's output, but
it should be able to supply something greater than 40 volts without the +5v. This is
due to the presence of CR861. It ensures that Q863 remains turned on, "stealing"
nearly all the emitter current from Q860, keeping it turned OFF until the base of
Q860 reaches 0v. This will be the case until the +50v feedback reaches about +49.5v,
turning off CR861 and driving the base of Q860 to 0v.

I would start looking at the voltages on Q860 and Q863 first, to determine if this
error amplifier is functioning. With +50v at +0.7v and +130v at +97v as you reported,
you should see approximately:
Q863 e= -13.0v, b= -12.4v, c= -0.6v, or Vbe= +0.6v, Q863=ON
Q860 e= -13.0v, b= -12.9v, c= +2.4v, or Vbe= +0.1v, Q860=OFF

If these voltages look reasonable, I would next look at Q876, Q869, Q872, and
Q874 for problems there.

Oh, and always check any carbon comp resistors for having drifted high and out of spec.

Dave


tinkera123
 

Roger, Dave, et al,

Slow response is no problem to me. I am appreciative of anyone spending their time to help. This process for me is just as much about learning the circuitry as it is about getting the Scope running properly. So, occasionally, I take time out to review the theory and/or just re-think the problem. This issue has had me reviewing my understanding of transistor circuitry and troubleshooting processes. I have been re-checking all transistors in-circuit power off, in-circuit power on, and out-of-circuit where possible.

Roger, I understand your logic re operation of the circuitry of the +5v Supply. My probing of U973, Q985 and Q988 indicate that they are Good. U973 is soldered in.

I then measured the Voltage drop across all Current Limiting resistors. Not surprisingly, most were zero (probably as everything is turned off), except, R875 1ohm in +50v Supply, Vdrop of 0.22v ….. could this indicate a max current before the Supply turns off?????

So …. I was puzzling over the +50v Supply, in particular the purpose of CR861, when I read Dave’s very timely explanation. Thank you Dave.

I have checked all the transistors in this circuit and they appear Good. Q863, Q860, Q869 and Q872 can be tested out-of-circuit, so I am confident of those.

As to Q874 ….. I need to re-test this transistor with Q872 and Q876 removed from the circuit, as I was getting a momentary ‘short’ when I reversed the bias across b to c (Diode tester shows some capacitor charging(??), that is, an mV value constantly increasing).

I will try to test voltages around the +50v Supply tomorrow before my grand-daughter arrives and helps grandpa by ‘colouring in’ my circuit diagrams …. 😊

Cheers,
Ian


Dave Hills
 

R875 1ohm in +50v Supply, Vdrop of 0.22v ….. could this indicate a max current before the Supply turns off?????

Absolutely! The current limit threshold looks to be around 200mA or 200mV across R875. Check CR875 for a short.

Dave


Roger Evans
 

Ian,

Having convinced myself that there had to be a fault in U973 if U973E was conducting and U973D was biassed off, can you satisfy my curiosity by saying what you measured on base, collector and emitter of U973D and E? I have a 7603 myself and a 7613 with almost identical power supply so I am preparing myself for the future!

Many thanks,

Roger


tinkera123
 

Hi Roger,

Let me see if I can lay this info out in a meaningful fashion. And please, let me know if my logic is astray ....:)

U973D .... npn
Pin no:, Transistor pin, Voltage (all measurements in-circuit)
9, b, -0.02
10, e, -0.71
11, c, -0.55
Conclusion ..... no shorts, turned off.

Diode Tester measurements, mV, with a 1.0mA test current
Forward bias b to e and b to c 789mV
Reverse bias e to b 21257mV and c to b 1906mV
Conclusion ..... Good

U973E .... npn
Pin no:, Transistor pin, Voltage (all measurements in-circuit)
12, b, -5.03
13, e, -0.71
14, c, -0.84
Conclusion ..... no shorts, turned off.

Diode Tester measurements, mV, with a 1.0mA test current
Forward bias b to e and b to c 794mV
Reverse bias e to b 2160mV and c to b 2233mV
Conclusion ..... Good

Cheers,
Ian


Roger Evans
 

Ian,

Thanks for taking the time to do this, I really appreciate it.

U973D looks OK for a transistor which is turned ON (I think this might just be a typo). Its base is 0.69V more positive than its emitter and with zero collector load its Vce is small (0.16V with collector more positive)

U973E should be turned OFF, its base is 4.3V negative with respect to its emitter. It has a 180k collector load resistor so with a high impedance voltmeter its collector voltage should read something similar to the +50V supply ie +0.7V. In fact its collector voltage is more negative than its emitter which is distinctly suspicious.

To be absolutely honest, when trying to work out what was happening in the +5V regulator I had temporarily forgotten that +50V on R977 was in fact only +0.7V. I am sure the first thing to do is to find the cause of the overcurrent on +50V. You can isolate the various boards to narrow down the culprit, there is a chain of ribbon cables from regulator board to Zaxis to horizontal amplifier to vertical amplifier, also a ribbon cable from regulator to calibrator board. The +50V sense return from the main board is decoupled by electrolytic capacitor C9 so you may want to check that separately with a multimeter since if you pull the cables to the main board you will lose the sense return on all supplies. If after fixing the +50V you still have a problem with +5V then it might be worth rechcking U973.

Thanks for putting up with me thinking and learning in public.

Roger