TEK 475A VOLTS/DIV compensation problem

[fabio.deangelis]

Hello EVVBloggers,
I have recently restored an old TEK 475A oscilloscope bought on eBay for a few Euro. The main problem was a faulty high voltage multiplier and a bad electrolytic capacitor on the +15 volt rail. Another issue was the “blocked” fan cooler that I have attempted to restore by replacing a faulty U1690 with four transistors and grease the motor. The fix apparently worked but unfortunately the fan motor did not start at power on all times. I had to make an agonizing decision, replacing the sophisticated Tek fan motor assembly with a small computer fan, attached to the +15 volt with a resistor ☹
The scope is now working very well in all functions but a weird issue in the VOLT/DIV compensation. I am unable to compromise the compensation among the 0.1, 0.2 and 0.5 Volt positions. Once I compensate the 0.1 position (C32, C33 if I remember correctly), then I got a bad image (under compensated) on 0.2 and 0.5 positions. If I re-adjust C32,C33 for a flat square image on those positions then I got a bed image (over compensated) on 0.1 volt position. The problem is present on both channels, even if it is more evident on CH1. I believe I have successfully performed the vertical calibration steps present before the VOLT/DIV compensation adjustment. I did a test by replacing the “compensator unit” (C32 C3) taken from another 475 , but the result is unchanged. Note that I have thoroughly cleaned the attenuator switches, which appear to work well.
Any clue what could be the issue here ?

[andrewtaylor]

please check the switch contacts of the rotray switches V/div.

Cleaning with Isopropyl might remove  dirt that leads to contact problems which influensce the compensation boxes..

[fabio.deangelis]

Thanks for your suggestion, that was my first corrective action I disassembled the rotary attenuators and thoroughly cleaned each contact with isopropyl alcohol and paper strips as advice here on this forum. Thus, the prime suspected are still the attenuator switches ? Any other areas ?

[BlownUpCapacitor]

The attenutator networks are socketed right? In individual x10, x2, x5 modules, the sockets may have excess oxides on them, and you'll need to clean them.

If that doesn't solve the issue, swap out all the attenuator units, x100, x10, x5, x2 with the ones in channel 2. I assume channel 2 has no issues.

If the issue persists, probe cathode of CR104. Observe the waveform while you input a square wave into channel one. See if both observed waveforms match with the test scope and scope under test.

If it doesn't, the issue is not in the attenuator portion but rather the amplifier portion.

[David Hess]

I have had a few attenuator modules where the connection between the pin and the substrate was bad.  I fixed them by using 2% silver solder to solder the pin where it contacts the hybrid substrate trace.

[fabio.deangelis]

Hello thanks for your replies, I have found a compromise between vertical amplifier adjustment and the incriminated CH1 compensator. The results is not perfect but  acceptable. I agree the problem is in the switch assembly, as the problem is only on those specific VOLT/DIV positions. Thanks again for your time !