Tektronix DM465B with DM44 -- Focus pot and DM44 LEDs. - Page 1
Forum section: electronics / repair

author: JohnTinkers
author_rank: Contributor
timestamp: June 18, 2025, 04:18:33 pm
content: Hi --Recently acquired a Tek 465B with DM44, with expectations to find a shambles. VERY pleasantly surprised to get a unit with Tek calibration stickers INTACT, no damage, scope working beautifully but no display in the DM44.Found that the +5 voltage supplied to the DM44 was missing, traced it to four tantalum capacitor suicides in the DM44 supply, and the 5V regulator had committed seppuku as well. Recapped the supply and promptly got magic smoke from that supply -- my fault. Fixed that and the display was on but I needed to replace two chips -- the A/D converter pair. Did that, and the unit worksbeautifullyand was amazingly well-calibrated!Sorry for the novel. Two issues remain...On the DM44, the ms and us LEDs illuminate but are barely visible. Digits are fine. First thought was that the LEDs were degraded, and needed replacement, but measure first.There is a mere 1.56 volts at the LEDs, with 4.0V on the other wide of a 470 ohm pullup resistor (R3325) on the common anode side of both LEDs.5.0V from the repaired power supply comes in at 4.986.The +5 test point shows 4.913.The focus pot, hidden under the high-voltage shield, works. But it is EXTREMELY STIFF. I can leave the unit off for a day or two and pop the shield, but I don't know whether that pot is sealed/I assume it's under the shield because it handles high voltage.My questions, and thanks for the help...Thoughts on reducing the size of R3325 in the DM44, or should I replace the LEDs, or both? Board voltages seem fine, and the troubleshooting flowchart in the service manual does not mention the LEDs.Can the focus pot be cleaned/ lubricated, or is it time to hunt one down? I'm reticent to just pop open a HV shield and go exploring, especially since I don't know the discharge time for the HV caps.Thanks for any help you can offer!

author: med6753
author_rank: Super Contributor
timestamp: June 18, 2025, 06:37:44 pm
content: The focus pot on 400 series scopes always seems to go stiff but it can be fixed. Very carefully get a little deoxit into the pot and slowly turn it back and forth and it should free up.

author: JohnTinkers
author_rank: Contributor
timestamp: June 18, 2025, 10:38:58 pm
content: Thanks for the tip! I will try that. I assume you are referring to Deoxit F5 and not F100...

author: JohnTinkers
author_rank: Contributor
timestamp: June 19, 2025, 05:37:43 pm
content: Quote from: med6753 on June 18, 2025, 06:37:44 pmHaving a bit of a problem finding a place into which I can insert the Deoxit. I shot some into a tiny dropper bottle so I wouldn’t soak the board in Deoxit but every drop that I tried to get in the one sorta-hole I found on the underside seemed to go on the board and it’s still stiff. Looking for a replacement pot, although not urgently because it still works even though stiff.

author: med6753
author_rank: Super Contributor
timestamp: June 19, 2025, 11:41:42 pm
content: Yes, deoxit F5. Temporarily tilt that capacitor up and set the scope upright on it's rear legs. Get the deoxit on the pot shaft and it will soak down internally.

author: JohnTinkers
author_rank: Contributor
timestamp: June 23, 2025, 04:11:42 pm
content: Quote from: med6753 on June 19, 2025, 11:41:42 pmSquirted F5 into a dropper bottle and got it in there and -- IT WORKED! Focus much smoother now. Thanks for the help!!