On Tue, Feb 2, 2021 at 05:22 AM, Ozan wrote:

Interleaved

Maybe I’ll check those 2 diodes and the 300ohm resistor, but I’m sure
they’ll probably be ok.

The transistors and 300 ohm are inside the chip, you can see schematic of the
chip at:
https://w140.com/tekwiki/images/4/49/Tek_155-0124-00.pdf

That would certainly explain why I couldn’t find them. 😂

If you want to confirm the currents are OK another way is to measure voltage
drop across 47-ohm resistors R780 and R770. Schematic shows 100mV delta-V is
expected on these resistors.

Ozan

Will do that and report back. Thank you.

Maybe I’ll check those 2 diodes and the 300ohm resistor, but I’m sure
they’ll probably be ok.

The transistors and 300 ohm are inside the chip, you can see schematic of the chip at:
https://w140.com/tekwiki/images/4/49/Tek_155-0124-00.pdf

If you want to confirm the currents are OK another way is to measure voltage drop across 47-ohm resistors R780 and R770. Schematic shows 100mV delta-V is expected on these resistors.

Ozan

Ozan,

I guess the chip must have drifted over time then...
I measured R751 and it hasn’t drifted at all. It’s value is 24.29k, which is right on the money.
Maybe I’ll check those 2 diodes and the 300ohm resistor, but I’m sure they’ll probably be ok.

After all that tiny non linearity isn’t that much of a real problem. It’s very small.
I’ll report back after locating and checking the 3 components.

Hi Stephen,
These voltages look OK except pin 5 is a little low. The imbalance you are seeing in this stage could be making up for the offset of discrete amp following U760 so unless you see a waveform shift when you turn on horizontal x10 mode no need to touch R749. Step 3 of page 5-15 explains R749 adjustment but this is not the reason for nonlinearity.

At this point I have no other suggestions, may be the IC became slightly nonlinear over time.

Only thing that bothers me is pin 5 voltage is low. Pin 5 has two diodes and a 300-ohm resistor in series to pin 4 (-5V). I expected the current through R751 to flow into pin 5 :
I_pin5=(29.955-(-3.3835))/24.3k= 1.37mA.
This should generate 2*Vbe+300*I_pin5-V_pin4=-3.188V at pin 5
Service manual marks even higher voltage at -3V but you are measuring -3.38V.

If the bias currents to diff pairs inside U760 are low, they would become nonlinear at two ends of the swing because they would be current starved.

Either chip drifter over time (nothing to do), or 24.3k resistor became higher over time (unlikely), or there is a leakage path at pin5 (unlikely without physical evidence).

Ozan

Ozan,

I measured voltage at pins 1-2-7-8

Here are the results:

1. 4.1202
8. 4.1172
2. 5.966
7. 5.986
And finally
5. -3.3835
R751. Side A: 29.955, Side B: -3.3835

There is a slight imbalance between the 2 sides of the differential signal.
Could that be corrected by touching R749 (MAG REGIS) maybe?

On Sat, Jan 30, 2021 at 12:11 PM, Stephen wrote:

On Sat, Jan 30, 2021 at 09:03 AM, Ozan wrote:

Hi Stephen,
Voltages at pins 7 and 8 are important, they are the other side of the
differential signals 1 and 2. We want to make sure there is no imbalance.
--
Ozan

Ok, I’ll probe them tomorrow and will report back.

While measuring pins 7 & 8 could you measure voltages at both sides of R751 (at the terminals)? One side should measure same voltage as pin 5 and other side should be +30V.

The voltage at pin 5 is a little low according the the schematic note.

Ozan

On Sat, Jan 30, 2021 at 09:03 AM, Ozan wrote:

Hi Stephen,
Voltages at pins 7 and 8 are important, they are the other side of the
differential signals 1 and 2. We want to make sure there is no imbalance.
--
Ozan

Ok, I’ll probe them tomorrow and will report back.

Thanks Ozan.

Hi Stephen,
Voltages at pins 7 and 8 are important, they are the other side of the differential signals 1 and 2. We want to make sure there is no imbalance.
--
Ozan

Ozan,

Here are the voltages I found today:

1. +4.1370
2. +5.967
3. -1.0252
4. -5.086
5. -3.39
6. -1.09
7. N/A
8. N/A
9. -3.17
10. -0.3455
11. +0.0565
12. -5.84
13. -2.94
14. -0.2709
15. +4.85
16. +0.0002

I didn’t bother to check pins 7 and 8. Will do if important.

Sometimes it is good to be at where you started :)

Ozan

How do you mean?

What I meant is for a moment we thought things got worse and the we thought horizontal amp was toasted. It is good news there wasn't any damage and we are back to looking at the original problem, not something worse like a bad IC.

By the way, I myself fried few components during debug sessions, so it is not unusual.

Ozan



Ozan

On Fri, Jan 29, 2021 at 09:17 AM, Ozan wrote:

Sometimes it is good to be at where you started :)

Ozan

How do you mean?

Sometimes it is good to be at where you started :)

Ozan

On Wed, Jan 27, 2021 at 08:58 PM, Ozan wrote:

After changing the zener let us know new voltages at the critical pins
(earlier list).

Ozan

Back in business.
Changing that WR749 brought the scope back to life.
I’ll take the new measurements soon.
All I can say at this point is the it still has the same non linear issue.

After changing the zener let us know new voltages at the critical pins (earlier list).

Ozan

Ozan,

One side is at 8.5xxVDC and the other is a pt 8.6xxVDC...
It’s definitely shorted.

I’m guessing next is to replace it and go from there...
Will do and report back.

Thanks Ozan,

Will do and report back.
I hope it’ll fix things and that the chip isn’t bad. That would suck...

Hi Stephen,
You are right. My schematic scan is so fuzzy I can't tell the last number but part list is much cleaner and it is actually VR749.

VR749 is the suspected zener diode. Before removing it I would quickly (quickly because 8.6V instead of 5V is not good for U760) measure voltages on both sides of VR749 to confirm it is bad. For a good zener you should see ~ 3.6V difference between two sides. In your measurement it looks like zener went short.
Ozan

On Mon, Jan 25, 2021 at 09:34 AM, Ozan wrote:

In summary, I really meant VR745.

...

This is the one controlling the +5V supply of the chip.

Ozan

Hi Ozan,

Are you sure it’s not VR749 instead?
I don’t see any VR745 anywhere on the parts list...
VR749 is a 3.6V-0.4W zener d’iodé.

I assume you were taking about VR764, right?

In my schematic VR764 is the one connected to the "beam find" function. It looks a little off but still in a reasonable range.

VR745 is the zener dropping 8.6V before it becomes +5V of the chip. If the anode of VR745 (pin 15) is really 8.53V as you measured it needs to be changed.

In summary, I really meant VR745.

I will also check VR745 like you suggested.

This is the one controlling the +5V supply of the chip.

Ozan

Ozan,

Yes it is Dangerously off. I think I’m gonna replace VR764, just to be on the safe side before I power it up again, and recheck the voltages on both sides of it.
I assume you were taking about VR764, right?

I will also check VR745 like you suggested.