Tek 465M Vertical Problem - Page 1
Forum section: electronics / repair

author: Citation_7
author_rank: Newbie
timestamp: May 16, 2015, 03:29:38 am
content: My old 465M has been acting up lately, just when I need it the most! The narrow horizontal sweep problem was traced to one electrolytic cap on the main (interface) board. It is C582, a 2.2uf 200V EC that was a bit bulged at the bottom and only meas. 0.04uf on the LCR meter. It caused the +95V line to sag to about +65. I was unable to get the horiz. trace to begin on the left side of the CRT. The right side looked fine. Replacing this cap fixed it!Now I have an issue on the vert. side that affects both channels. With both inputs set to gnd., the most I can lower the horiz. sweep line, on both channels, using the up/down position control, is one division below the center. Very odd. Further, the amplitude of the built-in cal sqr. wave will increase as the position knob is turned CW. The bottom will remain fixed, while the top of the sqr wave will rise, then both will rise together until they're both off the screen.I measured the V's from both the + vert to the - vert and from each to ground, while varying the position knobs. The Wht/Blu (white with blue stripe) wire, not sure if this is + or - vert, remains fixed at 32.1Vdc for all positions from FCCW to FCW of the vertical position controls. The Wht/brwn (white with brown stripe) V varies from 32.1Vdc to 5.8V from FCCW to FCW. Both V's ref'd to gnd. So one appears stuck to the +32 V rail. Output transistor short? Shorted cap?Fortunately, the service manual has some voltages listed on the schematics. Unfortunately, I don't have another scope to view the waveforms.Update: Anyone know how to remove the guards? from around the vert. output transistors? Never seen anything like that before. These trnstrs are soldered in with what appears to be a plastic disk keeping them captive inside a larger circular frame. I removed two machine screws & nuts above where the bracket makes a 90° bend, but no joy going further.Update II: I had to loosen the delay line from its anchors to the PCB in order to lift it to gain access to the vert. output xnstrs. These both tested good. Didn't find any shorted or open parts around/under them. Decided to reinstall them and power up in order to check V's. Tek's troubleshooting guide recommends shorting the bases of common-mode ckts together to null any differences between them and isolate the problem to a particular stage.In order to proceed, I had some of the schematics enlarged to make reading them easier as well as the part board locater. It's a bit tedious going from the schematic to the part list to find the reference XY coordinates, then to the diagram for every single part. But it would be a royal mess without it!Finally, I've read a few posts from guys who used to work in Tek service centers and saw many vertical board problems in 465M's come through the door. Hopefully they'll chime in here and give me a hint.

author: Grapsus
author_rank: Regular Contributor
timestamp: May 17, 2015, 03:11:14 am
content: Hi,I'm not the expert you're expecting, but here's what I'd do. On the vertical board, find the traces coming from the delay line and measure them while changing the position of the displayed channel to see if one of them is stuck like the wire that goes to the deflection plates. This might help to tell whether the problem is on the vertical CRT driver or before it.

author: Citation_7
author_rank: Newbie
timestamp: May 22, 2015, 12:09:22 am
content: Update: I discovered a defective Q4429 in the +vert amp stage. The Tek part number is 151-0434. This crosses to a 2N4261, which is quite expensive new!! Didn't find any hits for it or a substitution on this site. Any ideas?

author: Citation_7
author_rank: Newbie
timestamp: May 24, 2015, 07:39:56 pm
content: Update II:Tucker Electronics in Dallas, Tx has quite a supply of OEM Tek parts. I ordered a geniune 151-0434 from them and look fwd to seeing if the vert. problem is solved.

author: JoeO
author_rank: Frequent Contributor
timestamp: May 25, 2015, 01:41:28 pm
content: Quote from: Citation_7 on May 24, 2015, 07:39:56 pmCurious what you paid for it. Mouser has it for $22.

author: Grapsus
author_rank: Regular Contributor
timestamp: May 25, 2015, 02:13:18 pm
content: Damn, Q4429 is very close to the actual deflection plates. It's used is in a common base configuration. Is there any transistor guru here to explain why they need it ? I'm curious.With a different Q4429 sample be prepared to adjust a lot of voodoo stuff like R4443 and C4447 to get a flat frequency response.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 25, 2015, 04:03:43 pm
content: With some re-wiring, BFT92 or similar might be good enough. Possibly with a little base resistance (10-47 ohms?) or a touch of Ccb to slow it down.Tim

author: Citation_7
author_rank: Newbie
timestamp: May 25, 2015, 06:53:59 pm
content: Quote from: JoeO on May 25, 2015, 01:41:28 pm$5/ea.

author: Citation_7
author_rank: Newbie
timestamp: May 28, 2015, 03:02:51 am
content: BINGO!!We have a BINGO!My replacement xnstrs arrived today and this evening I installed a new Q4429 after testing it first. I now have full vertical position control in both channels! Both vert. output xnstrs are also hot (as they should be when ON), so I need to reinstall the heatsinks. Very, very pleased to have solved this nagging problem. It was quite the fault-hunt through pages of pdf's and a tight nest of wiring on the vert. card.Time to blow the foam off a couple of beers.........

author: Citation_7
author_rank: Newbie
timestamp: June 03, 2015, 04:17:22 am
content: Update:I disassembled the white ceramic heat-sinks used on the vert. output xnstrs, removed dried out white grease and added some new. When reassembled, these disks are not held tight, but allowed to move a bit, which is very handy because it's a trick to reinstall the xnstrs while looking at this from the side. Tek used some sort of clear, viscous liquid between the gold-plated xnstr bottoms and HS. I used a bit of white grease here.I left it on for a few hours at low intensity and these xnstrs stayed barely warm. The aluminum HS was working well, pulling heat from these ceramic disks. No worries here. Next, I'll run through the calibration procedure and tweak her up a bit.