Simpson Model 455 Oscilloscope Repair/Restoration

There is a very high chance that somewhere a capacitor has packed up. I would check all caps within the assigned X amplification area as well as the vacuum tube responsible for it. In fact all caps would actually need testing. It could equally be tarnished tube sockets or bad (weak) tubes. Would also look at measuring resistance values but especially anything that is 1M or over. High “MegaOhms” value resistors (including pots) in old gear tend to sift in value more often than other (lower value resistors).

All of the above is by assuming that you have the schematic diagram/service manual at your disposal.

Regarding the glitchy Vertical, chances are that there are dirty/tarnished Switches &/or Pots & VRs as it “inevitably” happens to the equipment of that age. Good luck.

PS: I just remembered that MCL has a very nice Video on YouTube showing a Tube based Scope restauration that might be of some inspiration to you (even though) is a different make/model than yours but…still. Here it is:

https://www.youtube.com/watch?v=GDZS1yEVqko

@ovi4

I was able to obtain the a scan of the original manual, which does contain the schematic. Thanks for the advice, I’ll go through and start working my way through the caps and resistors. As for the Tubes I have both the originally included tubes and the replacements I picked up (New old stock). I was able to check them in my old Heathkit TC-2 Tube tester.  

However you bring up a good point, that I completely overlooked, possible issue with the tube sockets themselves.

Thanks, now it’s back to the workbench. 😎

@ovi4 

I’ll check that video out.

I would also add, can you move the collapsed spot all around the CRT with the position controls? If not that points you to the deflection circuitry. If you can move it around, you would be looking into horizontal amp/timebase. A good cleaning with contact cleaner always helps intermittent connection issues in controls.

It’s Alive !  😎 

Some more troubleshooting revealed a 12kΩ (1 Watt) resistor went “open”, it feeds the Pentode Grid # 2 on the pair of 6AW8A tubes in the Horizontal amplifier. The 2 Pentode grids (#2) are supposed to measure approximately +170 vdc , after replacing the resistor the measured voltage is +169.8 vdc. All waveforms are no longer collapsed and the horizontal axis settings and horizontal position controls are all functional.  

This then prompted me to check the resistor for the pair of 12BY7A tubes in the Vertical section. It is a 3.9KΩ (1 Watt, 5%), and was measuring out of spec at 4.89kΩ. As it appeared to be working it’s way towards becoming open, I replaced it as a precaution. This also helped bring the Grid voltages for the pair of 12BY7A tubes in line with the schematic. The Schematic calls for +240vdc , I measure now +235vdc.

At this point I was able to go through the adjustment section of the service/instruction manual.

– Adjusted the VDC Balancer (Variable adjustment for the Volts/CM)
– Adjusted the HDC Balancer (Variable Horizontal Sync Input)
– Adjusted the Astigmatism and calibrated the Focus control

Other Maintenance tasks performed:

– Cleaned the Horizontal and Vertical Pots with contact cleaner, this improved their operation, and almost got rid of all the “glitchiness”. May need a little more cleaning to fully resolve.

– Replaced a few of the Transistors in both the Horizontal and Vertical areas that seemed sketchy.

– Going to go through and see if any other resistors may be going out-of-spec

– Also may need to “physically” rotate the CRT about 1 ~ 2 degrees, as the waveforms displayed are not level. The have a slight angle which rises towards the “right” side of the screen.

All that being said, I am excited with this progress so far. Thanks all for your insight and thoughts!

Just an update, going through the scope, starting with the “Test Signal Generator” and the “Horizontal board”. I have found quite a few of the resistors have drifted out-of-spec (High). I’ve replaced them, and things are going very well.  The Trace is now stable, was able to clean up the .5v (1kHZ) test signal generator. Had some issues with the 2SC717 Transistors, very odd that some of them test as normal NPN BJT but other with same marking test as N-JFET. The schematic shows them to be standard transistors and not FETs, not sure what was done previously to the scope. I was able to source replacement 2SC717 and they are working as expected. 

 Next Steps:

– Will go through the “Vertical Board” and check the resistors there as well.

– Have to “Rotate” the CRT about 1 ~ 2 degrees clockwise, as all the traces are not appearing level.  Using a Flat DC input and the line start low on the left and goes up to the right at a small angle less than a few degrees. Does anyone have any advice for safely rotating the tube so as not to damage it. 

– Waiting for the Outer Case to come back from “Powder Coating”

– Have figure out a way to “Re-label” some of the markings on the front of the scope for the Volts/cm dial and input, they have been worn away. Thinking maybe a clear sticker with the text printed on it in black. Not sure what my other options are, I can only image the original markings and text were “Silk Screened”. 

Thanks again all, for joining me on this adventure.. 

Generally, there is some form of a clamp near the neck (connector) of the CRT. Just loosen it up and make your adjustments. When done snug the clamp back down enough to keep it from moving. Additionally, you can use a dry erase marker on the CRT to mark the line so you can see it while powered off if you wish to do so.

Houston, we have a problem…. The Horizontal amplifier and Horizontal sweep are not working. The Vertical amplifier works perfectly and correctly displays the amplitude of the waveform being measured. However, the Horizontal is compressed into a single line, regardless of time base or Horizontal sync, also the Horizontal position potentiometer is unable to move the displayed traced either left or right.

Going through the schematic, Horizontal amplifier section uses two 6AW8A (Triode / Pentode) Tubes. I suspect the issue is either in this section of the circuit, or the feed controlling the Tubes. According to the schematic here are the normal voltages for each tube, and what I am currently measuring:

Tube A
Pin 1 Triode Cathode +16v , Measuring -.78v
Pin 2 Triode Grid –> 56Ω –> Transistor 2SC458 Collector +15v, Measuring -2.7v
Pin 3 Triode Plate +100v, Measuring +106.7v
Pin 4 & Pin 5 are the Heater, they check ok.
Pin 6 Pentode Cathode / Grid # 3 –> 820kΩ –> -20v, Measuring -21v at the 820kΩ (Feed Side) and +11.2v at Pin 6 (other side of 820kΩ)
Pin 7 Pentode Grid # 1 –>56Ω–>20kΩ(variable)–>Pin 1 Triode Cathode, measuring -.072v
Pin 8 Pentode Grid # 2 +170v, measuring +228.8v
Pin 9 Pentode Plate +130v (this goes to the CRT), measuring +228.8v

Tube B
Pin 1 Triode Cathode +16v , Measuring +1.23v
Pin 2 Triode Grid –> 56Ω –> Ground, Measuring -.019v
Pin 3 Triode Plate +100v, Measuring +106.3v
Pin 4 & Pin 5 are the Heater, they check ok.
Pin 6 Pentode Cathode / Grid # 3 –> 820kΩ –> -20v, Measuring -21v at the 820kΩ (Feed Side) and +11.2v at Pin 6 (other side of 820kΩ)
Pin 7 Pentode Grid # 1 –>56Ω–>20kΩ(variable) H-Position Control–> Should be +15v ~ -15v, measuring +5.9v ~ -6.2v
Pin 8 Pentode Grid # 2 +170v, measuring +228.8v
Pin 9 Pentode Plate +130v (this goes to the CRT), measuring +228.8v
 

Things I have checked:

– Resistors in Horizontal Amplifier portion of circuit, replaced any out-of-spec, all resistors are in spec to within 1%
– Checked the 5 transistors in the Horizontal section ( 3 – 2SC458, and 2 – 2SC717). Also have new replacements, and tried changing them, no effect.
– Have 2 “New” Old stock 6AW8A tubes, tried changing them, no effect.  Tested both the original tubes and new tubes with my Heathkit Tube tester and they seem to test ok.
– Removed the Tubes from circuit and rechecked the measurements, no change. 

This makes me think it’s as if the Horizontal amplifier is not being engaged.  The 2 10KΩ power resistors for the Horizontal amplifier are cold, under normal operation they should be warm, similar to the 2 power resistors for the Vertical Amplifier tubes.

At a loss for now… Looking for some guidance or ideas. 

Here are the scans for the Schematics:

Simpson Model 455 Oscilloscope Horizontal & Sawtooth Generator Amplifier Schematic:

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Simpson Model 455 Power Schematic:

[img] [/img]

Simpson Model 455 Oscilloscope Vertical Amplifier Schematic:

[img] [/img]

So it was working fine, now horizontal is dead? Pins 8 and 9 on both horizontal tubes makes me suspicious. Another silly question, have you checked the deflection switch S301? External deflection would cause issues.

@radtekman 

Checked S301 (Deflection Switch), tests ok. 

Pins 8 & 9 on the the Horizontal tubes is definitely a concern, it seems as if that section is not being triggered. Since the Voltage is high on pin 9 of both tubes, it would seem they are not operating because there should be a load / draw to bring that voltage down to +130v.

Wondering if there is an issue somewhere with possibly Q404 and Q405

Unfortunately other than schematics, I can’t locate a service manual for the scope. Trying to determine which points I can check for a waveform coming in, and what it should be / look like at various points in the Horizontal area.

Some progress…

I was able to restore the Horizontal Position control, and can now move the trace (waveform) left–to–right with no issues. Resistor R309 (820Ω) which is tied to Pin6 (Pentode Cathode / Grid # 3 ) on both Tubes went very high out-of-spec. Replaced it with a new 820Ω (2 Watt) and Horizontal Position control is restored.

However the Horizontal trace (waveform) is still compressed into a small area (about 1 cm) and will not expand to fill the screen. I feel the Sweep timing is working, as I am able to adjust the time base and and see the changes to the waveform. 

Some updated measurements now with the Horizontal Position control restored:

Tube A
Pin 1 Triode Cathode +16v , Measuring -.78v
Pin 2 Triode Grid –> 56Ω –> Transistor 2SC458 Collector +15v, Measuring -2.7v
Pin 3 Triode Plate +100v, Measuring +103.1v
Pin 8 Pentode Grid # 2 +170v, measuring +160.8v
Pin 9 Pentode Plate +130v (this goes to the CRT), measuring +115.5v

Tube B
Pin 1 Triode Cathode +16v , Measuring +1.23v
Pin 2 Triode Grid –> 56Ω –> Ground, Measuring -.019v
Pin 3 Triode Plate +100v, Measuring +103.1v
Pin 8 Pentode Grid # 2 +170v, measuring +160.8v
Pin 9 Pentode Plate +130v (this goes to the CRT), measuring +115.5v

Not sure what I may be missing here, feel like I am overlooking something in the Horizontal amplifier.

I feel like lack of drive would be in the pentode section of that circuit. Was R309 actually reading 820k or was that a typo? Additionally you can try an external deflection and see if it reacts different. I don’t understand why the external input is only ran to one side of the CRT, but it might help figure out what is going on. You have gotten this far, so close to getting it straight!