List the Tek Factory Assembly Errors You Have Found


Jim Adney
 

On the calibrator ground thread, someone mentioned finding a miswired ground resistor, original from the factory. I thought it would be interesting to let people list the Tektronix factory assembly errors they have found. I'll start this with my list of two.

I was running thru a collection of AM502s that had various problems. Most of what I was doing was replacing burned out incandescent lamps with LEDs and replacing the dropping resistor. To get reliable service, i also had to Deoxit the front panel switches. While going over one unit I noticed a ground connection had not been soldered. The wire lead was there and the ground lug was there, but there was no solder. IIRC, this was part of the input AC/GND/DC selection circuit. Easily fixed. BTW, I have parts from one AM502 that I couldn't save if anyone needs spares.

We picked up a surplus 475 or 485 that didn't work.at all. Opening it up, it looked clean, but there was clearly some problem with the power supply. When I looked over the board from the solder side, it looked perfect. The top side looked perfect, too, but a small glint of silver sticking out from under a cap caught my eye. This was a radial lead electrolytic, plugged into the board and soldered on the other side. I figured there was a scrap piece of wire caught under it and tried to pull it out, but it wouldn't budge, so I unsoldered the cap from the board. I discovered that this cap had long leads that had been pushed into the holes in the PC board, but one of the leads had caught on the board and didn't pass thru. Instead, most of it, a couple inches, curled up under the cap. It must have been okay for a few years, but eventually wore thru something and shorted, probably to the capacitor case. After straightening the leads and reinstalling the cap, the scope worked just fine.


Dave Wise
 

On my 1S1’s heater regulator board, the pass transistor was mounted with the emitter pin touching the heat sink, which shorted collector and emitter and ran the tubes hot.

Dave Wise


 

I had a very similar experience with a large ceramic capacitor (about the size of a quarter) in the power supply of a 475 or 475A I was working on. I traced the bad supply rail to what appeared to be a shorted cap. After verifying that I had a suitable replacement in stock I unsoldered the suspect cap. Once it was out I decided to check it, and it tested fine, so I figured that the short must be somewhere else and soldered it back in place. When I checked for the short again I couldn't find it, and when I powered the instrument up the supply rail was now working.

I figure that one of the legs had been bent down and was shorting to another pad on the PCB. When I tested the cap I straightened the leads so I would have enough lead to clip onto, and when I reinstalled the cap with the leads straightened I must have cleared the short.

I'm not sure that this was precisely a factor assembly error, but it didn't appear that the cap had ever been reworked, so I assume it was this way from the factory. I suspect that the lead was bent to almost touching, and that a later service or repair attempt bent the lead enough to get it make contact.

-- Jeff Dutky


Wayne
 

I can't even remember the model of the scope, but it was one of the old black fluted knob models, from around the time of the 512 or shortly after. It was about 1967 or so and I was repairing test equipment for a technical school. This scope worked fine until it warmed up for about 45-60 minutes, then the triggering became more and more unstable until it would not trigger at all. 4 or 5 people (including me) had worked on it, and it became a "hanger queen" sitting under the bench. One night I determined I was going to fix it. Finally found the problem was slightly low filament voltage on some of the tubes in the triggering circuits. At any rate, the problem was probably a factory defect. Grounds were provided via small square bus bar that ran through the triggering section. One section of bus bar was placed so close to the filament terminal on a tube socket that when it got warm enough, expansion caused a not-quite-a-short circuit between the filament connection and ground, dropping the filament voltage. As the heating proceeded, the connection got better and better, and triggering eventually quit. Bent the bus bar a bit and the problem was solved. I suppose someone could have been working on it before and bent it after production, but the scope was really pristine and showed no hints of any repair work.


Jared Cabot
 

While rebuilding my 2235 Scope, I found one wire for the Variable Holdoff pot wasn't soldered, wire was in the hole in the lug, but someone forgot to poke it with the hot end of a soldering iron.

I can only assume that no-one really used that function so it was never noticed. A simple fix anyway.


Holger Lübben
 

Hi!

The ALT-Mode didn't work correctly on my first 7854. Initially it wasn't complicated to find the error - a diode installed the wrong way. But I couldn't beleave that this mistake wasn't found at the factory tests and that the owners in the first 30 years didn't complain about that error. I was totally biased by "Hey - its a Tektronix, that can't be possible"...
I even took out he board and inspected it in detail. ;-)
A picture is in the photo section

Holger


Morris Odell
 

I've had a few experiences like this, here are just a few that come to mind:

1. I picked up a P6045 active FET probe in non working but otherwise as-new condition. It even had an "Instrument Checked" tag from the local Tek distributor. The reason it didn't work was that the tiny 100 ohm resistor in series with the probe tip had never been installed! I installed a small SMD part and it hasn't missed a beat.

2. On one of my non-working 7000 series plugins one of the screws that held a PCB down was missing. It happened to be the one that acted as the chassis return for the PCB so once it was replaced all was well.

3. Recently I've been helping a friend restore a late model 549 and we have found several mistakes and differences between the manual and reality. One interesting one was a couple of resistors that had been added in series with the relay contacts, probably to prevent them welding when the power supply caps charged up. The manual schematic showed them as an addition. These had the effect of increasing the ESR of a filter cap to the point where the power supply ripple was excessive and that in turn affected triggering.

Morris


Dave Wise
 

Not a one-time assembly error but a consistent problem with the manufacturing process.

Type 1A7, jumper wires between the Input board and Main board.
Manufacturing tinned the wires before crimping the contacts.

That was a blunder; due to cold flow, it’s not possible to maintain gas-tight contact with solder.
The jumpers on Input board pins C, D, E, and F carry input tube current and any shift in resistance is magnified as noise and drift.

Fix: Solder the crimps, at both ends.

HTH,
Dave Wise

From: David Wise
Sent: Friday, April 09, 2021 3:29 PM
To: TekScopes@groups.io
Subject: RE: [TekScopes] List the Tek Factory Assembly Errors You Have Found

On my 1S1’s heater regulator board, the pass transistor was mounted with the emitter pin touching the heat sink, which shorted collector and emitter and ran the tubes hot.

Dave Wise


Dave Wise
 

Looks like a design error.

Type W, above S/N 7000
HF Common-mode rejection trim cap.

In early W, the HF CMRR trimmer is C113, from input tube V113 plate to ground. This cap balances against fixed C213 on the other input tube plate.
At S/N 7000 the tubes were replaced by a dual JFET. The trimmer was moved to the second stage as C155, from R158 to ground, but C213 didn’t become C255, it was simply left out.
As a result, every late W has C155 at minimum C, and may or may not have optimum balance.

Fix: Add a fixed cap – say, 4.7pF - off R154.

HTH,
Dave Wise