Model 485 traces do not synch to triggering sources


Chuck Moore
 

Recently, upon returning from an outing, an obvious Tek product had been abandoned
on the front porch of the house. Took it inside and it had been treated well seemingly
over the years. No signs of storage with rats or mice, no grit or grime on the case etc.

Firing it up revealed a marvelous crt display. Woo-Hoo. Both channels seem to have
matching sensitivity and all the knobs and switches are in place like the day it was
unpacked from the shipping box.

Now the wart. I cannot obtain sync. No matter the source, both channels display a
running trace.

Looked at an online manual but became lost in the myriad triggering signal paths.
One thing that did catch my eye was a Tektronix IC, U780, which seemed central
to all triggering.

Before trying to wrap my head around this and going down multiple wrong rabbit
holes, are there known problems to look for in the triggering circuit?

Regards

Chuck


Mark Vincent
 

Chuck,

Put some 91% IPA on the pins of the IC and tunnel diodes near the IC. Then move the IC and diodes up and down to clean the pins and sockets. This is a common problem, dirt.

Change all the 0367 transistors! There are 11 in this scope. If you have more items with these in them, order more to replace the leaky ones. These are leaky, check as a diode from C to E, then replace them with KSP10BU. The pinout for the replacement transistor is BEC. This leakage of the transistors will cause odd problems. I have seen this enough times. There is a tantalum rated at 15V for a 13V supply on that board that will suddenly short. I pulled mine out and out in Nichicon ULD 47mfd 25V type. This tantalum is a known problem. If other switches have not been cleaned, clean them. Cleaning also goes for the other transistors and ICs.

Let us know what you find as the problem(s).

Mark


Tom Gardner
 

ISTR seeing a problem with a resistor somewhere in the U730/38/40 chain.
Check voltages and waveforms in the vicinity.

On Fri, 9 Dec 2022 at 01:44, Mark Vincent <orangeglowaudio@...> wrote:

Chuck,

Put some 91% IPA on the pins of the IC and tunnel diodes near the IC.
Then move the IC and diodes up and down to clean the pins and sockets. This
is a common problem, dirt.
Consider putting a tiny drop of Deoxit (possibly D100 but not necessarily)
on the end of a jeweller's screwdriver, and applying that directly to a
socket.

Also IPA any of the switches in the vicinity, talking the usual care with
the gold fingers.



There is a tantalum rated at 15V for a 13V supply on that board that will
suddenly short.

There are more than one :(

My notes indicate c861 c911 c965 c2101 (possibly c844 c902). Good luck
spotting them :(


Mark Vincent
 

Tom,

C861 is the one that will give trouble, 13V on a 15V tantalum. I keep 47mfd 25V ULD types in stock to replace the tantalums that are bad. Higher capacitance for others such as 100mfd can be higher in value if wanted. The decoupling off the 50V supply, I put in 15mfd in place of the ,68mfd. The original tantalum, ,68mfd, was already high in ESR. I prefer to use a 25V electrolytic to replace a 20V tantalum on a 15V supply.

Mark


Chuck Moore
 

Mark & Tom

Thank you for the navigation on this scope. I bought a 465
about ten years ago and it has been a gem despite its age.
It actually does everything I need, but a scope with 300 MHz
bandwidth is just to much to not repair.

I will post my findings and as closely as possible what solves
the problem. My big concern was the proprietary IC's might
be an issue but it sounds like the tantalum vexations struck
Tek also. Never imagined such an expensive cap would be
such a problem child.

Again thanks

Chuck


Tom Gardner
 

On Fri, 9 Dec 2022 at 15:12, Mark Vincent <orangeglowaudio@...> wrote:

Tom,

C861 is the one that will give trouble, 13V on a 15V tantalum.

Have a look at both sides of u960 on the power distribution schematic 17. C911
is also a a 15V tant on a 13V line. I've had it destroy R965. C962 is
either a 22uF 15V tant or a 0.02uF ceramic! It too is at 13V. It is on the
input to the u960 which acts as a *1 opamp.

C2101 is a 20V tant on a 15V line, and a "nearby" voltage is marked as
13.1V. Not sure what the actual operating voltage is, but looking at the
schematic it won't be vastly different. See the calibrator schematic 16.

As I indicated, the other two are only "possibly" an issue.


Tom Gardner
 

On Fri, 9 Dec 2022 at 15:55, Chuck Moore via groups.io <wd4hxg=
icloud.com@groups.io> wrote:

Mark & Tom

Thank you for the navigation on this scope. I bought a 465
about ten years ago and it has been a gem despite its age.
It actually does everything I need, but a scope with 300 MHz
bandwidth is just to much to not repair.
The 485 is the scope on my bench. I've had it stably triggering on a >1GHz
signal, the limit being the greatly reduced signal amplitude rather than
the trigger circuit.

The 485 has two rare features:

- the cal out has a risetime of <1ns, i.e. suitable for testing the
scope's risetime
- it has a real 50ohm input attenuator, i.e. not a crappy 50ohm resistor
slapped across a 20pF//1Mohm input. It also has a separate traditional
1Mohm attenuator.

Plus I like the blue trace :)


Tom Lee
 

Tek learned that the voltage ratings on tants need to be chosen more conservatively. Eventually they settled on using tants rated for twice the expected nominal operating voltage. That cut down the failure rate a lot. Better late than never.

When replacing a failed tant, use the 2x rule of thumb to guide your choice of replacement.

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 12/9/2022 07:55, Chuck Moore via groups.io wrote:
Mark & Tom

Thank you for the navigation on this scope. I bought a 465
about ten years ago and it has been a gem despite its age.
It actually does everything I need, but a scope with 300 MHz
bandwidth is just to much to not repair.

I will post my findings and as closely as possible what solves
the problem. My big concern was the proprietary IC's might
be an issue but it sounds like the tantalum vexations struck
Tek also. Never imagined such an expensive cap would be
such a problem child.

Again thanks

Chuck




si_emi_01
 

Good advice for Tantalums and Aluminum Electrolytics as well.

One of the things I like best about my 485 is the clean, bright, well focused trace it produces. My 2465BDV is the best it can be but is no match for the thin, bright beam on my 485.

One other thing that I found decades ago with my 485 that had a trigger issue was Potentiometer R755 (Trigger Sensitivity) associated with Tunnel Diode CR751 was dirty. As with servicing any equipment with calibration pots, I always mark the position of all Pots on a Board I am working on. Moving the wiper through the travel cleared whatever debris was there and I never have had a problem since.

By the way, triggering and displaying a trace out to 670 MHz with the 50 Ohm Input Attenuator was no issue for my 485. Very good Analog Oscilloscope.

Ross

-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Tom Lee
Sent: Friday, December 09, 2022 10:28 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Model 485 traces do not synch to triggering sources

Tek learned that the voltage ratings on tants need to be chosen more conservatively. Eventually they settled on using tants rated for twice the expected nominal operating voltage. That cut down the failure rate a lot. Better late than never.

When replacing a failed tant, use the 2x rule of thumb to guide your choice of replacement.

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 12/9/2022 07:55, Chuck Moore via groups.io wrote:
Mark & Tom

Thank you for the navigation on this scope. I bought a 465 about ten
years ago and it has been a gem despite its age.
It actually does everything I need, but a scope with 300 MHz bandwidth
is just to much to not repair.

I will post my findings and as closely as possible what solves the
problem. My big concern was the proprietary IC's might be an issue but
it sounds like the tantalum vexations struck Tek also. Never imagined
such an expensive cap would be such a problem child.

Again thanks

Chuck





Mark Vincent
 

Chuck,

If your S/N is below B15590, modify the circuit in the front end that drives the Z input relay. This change is to protect the IC from spikes from the relay turning on or off.

There is a 100mfd condenser in each attenuator as a decoupling. The condenser is axial tantalum and is run almost at its rated voltage. I put in a 16V type to replace the 10V type that sees 9V. A ULD or UHE type would work.

Mine has the same option as Tom G.'s. That is why I like mine even more.

Mark