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I have a Tek 485 that I have had and used lightly for 20+ years with no issues.? That is until last night.? I hadn't used it in couple of months. After turning it on I went to adjust the Intensity and it went totally bright and stuck there.? The Intensity flickers when the control is moved, but when left in any position, it's always stable on high.? A second symptom simultaneously appeared.? The retrace of the beam is not blanked.? So, both issues point to a problem with intensity modulation of the beam.?


I thought I had the service manual for this, but I am unable to find it.? I am sure the first thing I need to do is get a copy of this.? What is the best source for this?? I am a retired electronic engineer, so I have no problem digging into the scope.? But I only have one, so I hope I don't need another to troubleshoot this one.


Any suggestions on what areas of the circuitry to look at would be appreciated.

As an EE, you may have been taught the term like I did in school 'it's broke'. The class and instructor laughed. A bit of New Year's humor.

Likely you have at least one d-c restorer diode that is open or shorted. Replace ALL of them with 1N4937. In the schematic on page <13> these are to the right of the 'focus' control and have CR16xx. The schematic is found by looking up 'tektronix 485' online then looking for the w140 page. There will be pdfs of schematics and information, . You can find a paper manual online. I prefer a paper manual over a digital one. I did replace the two 22meg bleeder resistors in the d-c restorer circuit with Vishay VR37 series. The original 1/4w carbons will be well high in resistance. The symptom of these being too high is a bright trace when turned on then dimming after 15-20 minutes without touching the intensity knob. I did replace R1745 with a 1/2W in the Z circuit. C1748, in mine, are 47mfd 25V Nichicon ULD types. UHE will work fine also.

There is a 13V supply with a 15V tantalum used. Replace this with a 25V type. I used the ULD in mine. Also, in the front end, there is a 100mfd that should be changed to a higher voltage. I do suggest making the mod for the higher s/n types to protect the input IC from damage when switching from 1meg to 50 ohm. This is shown in the schematic. I did this to my earlier s/n model.

Check all of the 0367 transistors! These are known to be bad from C to E checking as a diode. The early ones are a bit larger in body size with round leads that will be tarnished. Mid types will be normal size with square leads that may or may not be bad. The last ones made with a blue tinted face will likely be good. Only in the 7D15 will the number be 0402 for these. They are the same transistor. I used KSP10BU as replacements. Other models and plugins have the 0367s in them. If you find ones that are bad, replace all in the piece. I have seen this number cause problems such as triggering not being right.

I did replace some resistors with higher wattage and mounted them a bit off the board to allow for better air flow and to make any probing easier. The tantalums are also suspect. Seeing 15V on a 25V tantalum will be alright. If the tantalum is rated at 20V, raise the voltage. Using good quality electrolytics, low ESR, high temp and long life types, with a higher capacitance works. If a tantalum is working fine and the voltage of it is high enough above the voltage across it, it is not necessary to change it.

I chose to restuff the two main filters in the power supply with Nichicon LGR types of higher capacitance. I wanted new in mine to make sure age would not be a problem.

Mark

One thing to look out for is bad IC sockets. The low profile ones used in this and other models use contacts that make contact with the sides of the pins. Little contact. The best replacements are ones by Augat or MilMax machine types that use round contacts that are firm. Second choice are the better ones that have a dual wipe.

Mark,

Thanks for the reply. I also posted this on the TekScopes2 group and have many replies there. Here is a link to that thread:

/g/TekScopes2/message/17100

With all the suggestions I have my work cut out for me. It may take a while, but I will report the results.

Greg

Greg,

I also take each knob off to oil the set screw and shafts. This makes it smoother to move and prevents freezing due to electrolysis. I will put a bit of oil on a flat blade screwdriver then put inside on shafts of the time/div. This is for ones where the metal shafts go deeper inside. I also oil the fans.

Mark