Shaun Merrigan's Information Pages


Category: Electronics


HP 8903B Audio Analyzer PCB Pad Replacement

Posted by Shaun Merrigan in Electronics, Test Equipment. Comments Off on HP 8903B Audio Analyzer PCB Pad Replacement

18th August

HP 8903B Audio Analyzer PCB Pad Replacement

Visual inspection of my recently acquired HP 8903B Audio Analyzer revealed that the polymer pads on the bottom side of the main PCB were seriously deteriorated:

 

 

 

Besides making an unsightly mess, this residue may form corrosive products which, over time, could damage the PCB.  Also, the pads no longer perform their bottom cover to PCB isolation function.  So all the old pads and residue had to be removed and new pads installed.  Lintless wipes, isopropyl alcohol, and nitrile gloves were the chosen tools.

 

 

 

The residue cleaned up completely, and new pads were installed. I won’t need to be concerned with this failure mode again.

 

Thanks for reading.

 

Shaun Merrigan

 


Tektronix 7854 Waveform Keyboard Rebuild

Posted by Shaun Merrigan in Electronics, Oscilloscopes, Tech. Comments Off on Tektronix 7854 Waveform Keyboard Rebuild

15th August

Tektronix 7854 Waveform Keyboard Rebuild

From the Tek Wiki:

The Tektronix 7854 Waveform Processing Oscilloscope, introduced in 1980, is a 400 MHz combined analog / digital mainframe in the 7000 series that takes two vertical and two horizontal 7000-series plug-ins. In addition to a conventional analog (real time) scope, it contains an equivalent-time sampling 10-bit digitizer and a programmable waveform processor with GPIB interface. The waveform processor can be programmed using a detachable calculator-style keyboard, using postfix operators (“reverse Polish notation” like HP calculators) that operate on entire waveforms, not just individual numbers.

Tek 7854 Mainframe #2 after cleaning

 

It is to the repair of the detachable keyboard that this post is devoted.  I purchased a keyboard for a reasonable price only to find that some keys were intermittent, and some did not work at all.

Opening up the keyboard showed that it was filled with debris:

The … Read More »


The uTracer Tube Tester and Curve Tracer

Posted by Shaun Merrigan in Electronics, Vacuum Tubes. Comments Off on The uTracer Tube Tester and Curve Tracer

14th August

The uTracer Tube Tester and Curve Tracer

I have completed building my uTracer (micro Tracer) computer based vacuum tube tester and curve tracer.  This unit, designed by Ronald Dekker described on his website:

The uTracer is much more than a simple tube tester; it is also a curve-tracer. It measures both the anode as well as the screen currents for a range of specified bias conditions, and arranges the measurements in meaningful sets of curves. From these curves parameters like the transconductance or the output resistance can then be extracted and plotted as a function of bias. Alternatively the uTracer can be used as a tester which extracts important parameters like Ia, Rp, gm and um in your favorite bias point in a matter of seconds. http://dos4ever.com/uTracer3/uTracer3_pag0.html

Ronald Dekker makes this tester available as a kit, with detailed assembly and testing instructions.  One of … Read More »


Tektronix DSA602A Digitizing Signal Analyzer

Posted by Shaun Merrigan in Electronics, Oscilloscopes. Comments Off on Tektronix DSA602A Digitizing Signal Analyzer

14th August

Tektronix DSA602A Digitizing Signal Analyzer Part 1

I recently had the opportunity to purchase one of these units, and I am thoroughly enjoying the process of  checking its operation and getting to know how to use it.  This unit is a laboratory grade, very accurate digitizing oscilloscope from about 1990.  From the Tek Wiki entry:

Inputs
3 amplifier bays, up to 12 channels (using 11A34 plug-ins)

Resolution
8 bits; averaging of N acquisitions increases by log2(N) up to 14 bits

Display resolution
50 pixels/Div

Sample rate
DSA601, max 1 GS/s; DSA602, max 2 GS/s (one channel, left plugin)

Record length
512 to 32768 samples

Sweep rate
50 ps/Div  to 100 s/Div

ENOB
7.2 bit up to 10 MHz, 6.3 bit @ 250 MHz, 5.7 bit @ 500 MHz, 4.5 bit @ 1 GHz

Screen
magnetic deflection color computer monitor, 10″ diagonal (130 mm × 168 mm), 552×704 pixels, 22×11 infrared touch-screen grid

Power
90-132 V or 180-250 V, 48-72 Hz, 465 W (DSA601A) / 585 W (DSA602A)

Features

Automated signal measurements (rise, fall, period, frequency, width, delay, phase, duty … Read More »


Tektronix Scope-Mobile Cart Repair Part 3

Posted by Shaun Merrigan in Electronics, Oscilloscopes. Comments Off on Tektronix Scope-Mobile Cart Repair Part 3

7th May

Tektronix Scope-Mobile Cart Repair Part 3

 

Part 1

Part 2 

This is the conclusion to my repair of a Tektronix 202-2 Scope-Mobile Cart.  Recall that the problem was three fractured trunnion mount bosses which rendered the cart useless (and unsafe) under load.  My proposed solution was to place the bosses into compression by putting a 6mm countersunk bolt through the trunnion mount concentric with the existing hole in the boss.  By using appropriate sized washers between the top of the boss and the frame member I ensure that the boss is placed into compression and the frame member is securely clamped.  This avoids the tensile loading which fractured the bosses initially.

 

The holes in the trunnion mount were countersunk to fit the flat head socket bolts:

 

 

 

 

The cart was reassembled with the repaired trunnion mounts:

Felt lining was added to the plug in storage compartment:

 

Finally, the cart … Read More »


Tektronix 556 Dual Beam Oscilloscope Testing: Vertical Bandwidth

Posted by Shaun Merrigan in Electronics, Oscilloscopes, Tektronix 556. Comments Off on Tektronix 556 Dual Beam Oscilloscope Testing: Vertical Bandwidth

27th January

Tektronix 556 Dual Beam Oscilloscope Testing: Vertical Bandwidth

I have been doing some basic testing of my Tektronix 556 Dual Beam Oscilloscope, and I have documented some parts of this process.  I have also captured images of the traces, which I will share here.

Test Setup: Tektronix 556, 1A2 plug-in, Timebase A, Tektronix SG 503 Generator, and Tektronix TG 501 Time Mark Generator

I found the -3db vertical bandwidth of the 1a2/556 to be 65 Mhz, which is to be expected for the 50 Mhz rated 1A2.

 

TM 500 Module with Plug ins

50 kHz Reference Signal 6v peak to peak

5 MHz Sine Wave 6v peak to peak

20 MHz Sine Wave 6v peak to peak

40 MHz Sine Wave 4.8v peak to peak

50 MHz Sine Wave 4.3v peak to peak

65 MHz Sine Wave 4.2v peak to peak

I will continue this testing by investigating the absolute maximum trigger … Read More »


Tektronix 556 Fan Motor Repair Part 2

Posted by Shaun Merrigan in Electronics, Oscilloscopes, Tektronix 556. Comments Off on Tektronix 556 Fan Motor Repair Part 2

22nd January

Tektronix 556 Fan Motor Repair Part 2

In this continuation of Tektronix 556 Fan Motor Repair Part 1, I have removed the motor and mount assembly and disassembled it.  Now I need to figure out the problem and rebuild the motor.

Tektronix 556 Fan Motor Overview, mounted in place

Tektronix 556 Fan Motor removed from mounts.  The Tektronix part number is 147-0029-00.

 

Tektronix 556 Fan Motor removed from mounts.

Tektronix 556 Fan Motor removed from mounts.

The next few images show the disassembled parts of the motor:

Rotor, end bells and bushing assembly

End bell inside, retainer, spring washer felt and bushing.

End bell and bushing assembly

Rotor and spacers

At this point I found several issues by visual inspection:

The original rear bushing had been replaced, probably due to excessive internal wear.
The replacement itself had excessive internal wear and it did not fit in the retainer properly due to its shape.
The replacement … Read More »


Tektronix 556 Fan Motor Repair Part 1

Posted by Shaun Merrigan in Electronics, Oscilloscopes, Tektronix 556, Vacuum Tubes. Comments Off on Tektronix 556 Fan Motor Repair Part 1

19th January

Tektronix 556 Fan Motor Repair Part 1

I was recently fortunate enough to acquire a Tektronix 556 Dual Beam Oscilloscope with many accessories and in good condition.  Unfortunately, the cooling fan on the scope was very loud and more concerning, it emitted an alarming metal on metal contact sound.  After verifying basic operation of the oscilloscope, I decided to repair the cooling fan.  This process is documented in the images and commentary that follow.

Part 1: Removing the Noisy Fan Motor

556 Cooling Module with filter and mesh screen removed.  The fan is connected to the chassis wiring at the ceramic terminal which is circled:

Fan to chassis wiring connection terminal.

 

Inside of cooling module and bottom side of motor (fan blade removed):

Back side of motor (fan blade removed).  This is a 4 pole 1725 rpm (loaded) motor.  It moves a very large volume of air.

An … Read More »


Tektronix Scope-Mobile Cart Repair Part 2

Posted by Shaun Merrigan in Electronics, Oscilloscopes, Tech. Comments Off on Tektronix Scope-Mobile Cart Repair Part 2

7th January

Tektronix Scope-Mobile Cart Repair Part 2

This post continues my repair of the Tektronix Scope-Mobile Cart and  trunnion mounts. Here is Part 1, which details the problem and my proposed solution.

The original design used 5/16″ button head bolts which screwed into the bosses and attached the aluminum box frame member to the polymer trunnion mount:

It is obvious that overtightening these bolts put the bosses into tension which caused them to fail.  The gap between the member and the boss “spring loads” the system and puts additional tensile stress on the boss.  While an aluminum casting could easily handle these stresses, the polymer obviously could not.

What I did was to glue the fractured bosses into place using a low surface energy epoxy and use them as a drilling guide (see Part 1).  Then I drilled completely through the trunnion mounts with a drill press, using … Read More »


Tektronix Scope-Mobile Cart Repair Part 1

Posted by Shaun Merrigan in Electronics, Oscilloscopes. Comments Off on Tektronix Scope-Mobile Cart Repair Part 1

6th January

Tektronix Scope-Mobile Cart Repair Part 1

I was lucky enough to obtain a Tektronix 202-2 Scope-Mobile Cart.

 

Upon examining it, I found that the scope tray was loose and the tilt mechanism did not function properly.  The cause for this was broken trunnion mounts on both sides of the cart as follows:

 

 

Three of the four bosses were fractured with the fracture surface indicating tensile overload.  I suspect that the bolts were overtightened and the bosses fractured. Overtightening loaded the bosses in tension by trying to compress the box frame member.  Unfortunately this type of polymer (some type of hard thermoplastic) is not good in tension.

Other mobile carts I have seen use cast aluminum for these trunnion mounts, which is a better choice for strength and durability.  The cart I have for my 556, a 205-2 (triple wide), uses cast aluminum trunnion mounts.

 

 

 

I … Read More »