This was the most challenging project I ever undertook as I never thought this guy would come back to life.
This scope was donated to me by a friend of mine. It was in a house which was being demolished and we had to break open a door to get in to the room to grab this fella. The owner did it just for me, as she knows I am after old scopes 🙂 So it was indeed a real passion to restore this, so that I could tell her that all the effort she put in – just to get this scope to me – was not in vain. After almost 4 months, this is how it is now, in my lab 🙂
Since its easy to combine all the pictures in to a video , I am posting two videos here, covering the entire restoration project.
Video #1 – Tektronix 547 Restoration – Part 1 of 2
Video #2 – Tektronix 547 Restoration – Part 2 of 2
Video #3 – Here is a Quick video of the completed Unit – Tektronix 547 Overview
Video #4- Tektronix 547 Internal Safari (on a Live scope)
Some of the detailed technical aspects of the restoration are not covered in the video, so I am going to explain it here. This is my third 500 Series scope that I’m restoring, so a quick advice on the steps that worked so well for me.
- Clean, Inspect
- Test Tubes, Transistors
- Recap – All Electrolytic and Mic/Paper based on need/condition/type.
- Test Heater Power Supply, Bring up Heater.
- Test DC Power Rails, Bring up and Test DC Power Rails – aka -150, +100, +225, +350.
- Troubleshoot – Fix , until the scope is back to life
Steps 1-5 is covered in detail in the video. I am covering only the troubleshooting part here in detail. Here are some important hints/tips.
Here is a reference on Time Delay Tube and Heater warm-up.
Video #5 – Time Relay Tube/Heater Warmup in Tektronix 500 Series Scopes –
Video # 6 – Time Relay Tube/Heater Warmup in Tektronix 500 Series Scopes -In action
For further reference.
Tekwiki – http://w140.com/tekwiki/wiki/547
Most important – Serial No/Schematic. There are differences in schematic between different serial numbers. Ensure that you use the right schematic. Also some components are added only after a specific serial number. For e.g. you may not find a diode in a chassis which is present in the schematic, because it was added from a later serial#. Check the electrical part list in the manual for details.
- Component Numbering – Makes it easy to track parts. This is very useful when we need to check Electrical Parts List to identify the components. As an example, for recapping, we need to scan through the parts list to identify all electrolytic and paper caps. There is a very handy map to locate where each part is. For example V113 or C108 or R134 will be in B-Sweep Generator (the swinging door with tubes in the chassis). Here is a grab from service manual.
2. Power Rail Resistance – Again from the service manual. Very important to check rail resistance before you power up DC voltages.
3. HV supply – The HV supply to Anode is at +8150 and Cathode is at -1850. Total Anode to cathode potential is 10KV. As always heater is elevated close to cathode potential. The HV Test Point is close to Cathode Voltage. This same rail is also used as the feedback reference to the HV Oscillator.
There are three sections of HV supply.
A) V822 – for Grid – Modulated with UNBLANKING Pulse from Sweep
B) V832/842/852 – HV for Anode/Post acceleration – +8150V. HV Rectifier plus voltage doubler. Note the HV Caps there.
C) The Cathode -HV and HV Feedback – this is with V862.
How to test HV transformer Failure/Overloading
Here is the path of the HV Feedback. Its simple, the screen grid voltage to V800 will keep increasing to compensate for the transformer loss or other failure which is bringing HV out to become low, remember -1850V is the HV out here.
V862 rectifies the HV -1850V for the Cathode. The resistor network further reduces it and picks a sample -160V as the reference input to V814B – 12AU7. This is the output sample, – 160V. The HV Oscillator is regulated by this sample voltage.
If the output is going down due to a HV transformer issue, you will see the plate voltage of V814B go down from -2. However, remember, the output voltage or output sample voltage can go down even if V862 or any components in the cathode HV circuit is bad. That is the issue I had, a bad V862.
Second point to test is the screen grid of V800, Pin 8. Steady state reference is +90. If the HV transformer or feedback loop is bad, you will see this increasing, to +120V or more.
Here is an example after running the scope for 10 minutes with a failed V862.
4.Deflection plates – Remember, do not short circuit the Deflection plate connectors aka Vertical or H Amp outs to ground. Here is a close up of the CRT deflection plate connectors. If the H AMP and V AMP are faulty or not delivering correct output, we will not get a trace, as they can pull the beam off the screen. That is the reason why I disconnect all deflection plates to confirm if the basic CRT functions are working.
5. Switches and Sockets – I cannot stress enough the trouble these switches cause if not cleaned properly. All rotary switches need proper cleaning. Even on instruments that are in a better condition, if you find weird symptoms, try to clean the switches first. The same applies to the sockets as well. I did not find many problems with vacuum tube sockets, but transistor sockets are a pain. Clean them as much as you can and ensure the transistor is seated firmly to the contacts in the socket.
6. Horizontal Amp Troubleshooting – I usually inject an external signal and trace it down the Amp to find the problem. This is when the entire Horizontal is down. If you have a sweep which is known and good, then it’s not required. However I prefer this method to avoid problems in the sweep section. Remember, since its all DC Coupled, a faulty sweep output can cause confuse/mess things up.
Here is a simple signal flow diagram for H-Amp. This is shown here to to help with the flow and understand where the signal becomes differential from single ended.
Basic oscilloscope probing through the path is all that is needed to spot the culprit. Sweep input could be a triangle/saw tooth wave with modified duty cycle from a simple function generator. The service manual is really comprehensive, you can check DC levels as well at each stage to ensure its all biased properly.
Important: While measuring DC voltage levels, ensure the scope is set to the required settings as prescribed in the manual against each section. The front panel settings for each section of schematic is different.
7. Time base – Tunnel Diodes. These are harder to identify as failed. Also, the signal is differential and tiny coupling transformers to transfer the pulse.
Here is the Sweep circuit from Timebase B. Here is how some of the components look in the chassis.
8. Sweep – The whole signal is really tiny, riding on a 102V DC level. Differential input scope can help troubleshoot this. Never plus the scope across both the rails. Here is the detailed pic.
Watch the orange boxes. They are at DC level.
9. Testing a Tunnel can be tricky. If you have a curve tracer, it is easy. But even if you don’t have one, you can add a simple series resistor – to limit current – and use a sweep wave from a function generator across it. Next, measure the voltage drop across the resistor on an oscilloscope. Here is an example. 1st wave is the input. 2nd wave is the voltage across the resistor. You can see the notch in the waveform, which is the current drop or valley.
Here is the tunnel diode in a breadboard with series resistor.
Or ideally, the curve tracer 🙂
On this project, I was bitten by a bad tunnel diode in the B-Trigger. Since it is used trigger the sweep, the sweep will not lock, rather will only freerun. This is the D45 in timebase B which is the main time base.
After may days of online research, found a closest Russian military tunnel diode with matching (closely) characteristics – 10mA – which was used as replacement. Part # 1I308G. Ebayed from Russia, took ~4 weeks to arrive.
Reference of common available TD at http://w140.com/tekwiki/wiki/Russian_tunnel_diodes
First challenge is to install limbs to this fella. Its very temperature sensitive, do keep it cool, use pliers or other heat dissipating methods while soldering.
Here it is installed inside the ceramic strip. The second diode you see next to it is also a tunnel diode, 4.5mA one.
And the outcome, locked and delayed sweep.
10. Lead inductance/Capacitance – This is what service manual says.
For me the delayed trigger was intermittent. It worked only after 2uS on Timebase B. C424 is installed deep in to the ceramic strip… (something like) as shown in the 2nd Row – (this is not C424, but C424 is installed same way).
It had dust on its surface which was causing the issue. The reason I suspected that is because the issue is not present on HF, and totally random.
Here is the capacitor as I pulled out of the scope.
I didn’t even replace it. Just cleaned it and all started working. The lesson here is, respect component lead lengths, dust, stray inductance/capacitance and resistance. The gold Tunnel diode you see in the picture is the Delay Pickoff tunnel.
That’s all I can think of as notes, the rest of it is all covered in the video. 🙂
I have no commercial affiliation with any of the products/organizations/individuals mentioned in this blog.
The information provided here is for educational purpose only.
You are free to distribute this as long as it stays original with all information as is here and it is free and you don’t scavenge any tubes from any scopes or this blog.
NO electrons were harmed during the repair/filming of this instrument restoration. All free electrons found inside the unit were rehabilitated to the nearest vacuum tube.
NO EXTRA SCREWS OR PARTS WERE OFFICIALLY FOUND AFTER REASSEMBLY. UNOFFICIAL EXTRA ITEMS WERE DISPOSED OFF SECRETLY AND DECLARED AS “EXCESSIVE ASSEMBLY” DURING MANUFACTURE.
Please do report any errors or stupidity.