I have a Tektronix 465 that won´t produce a full width trace in the .5s-10ms; .5ms-1ms; 5us-1us ranges. All other ranges are fine. The time per division seems fine, but the beam starts its trace before it gets back to the left side of the screen. This only happens when the scope is "free running" in Auto mode. If the scope is triggered (Int, Ext, or Line) the trace begins at the far left of the screen. Originally there was no trace for the above ranges. I found R1151(287k ohms)open circuit and when replaced the trace was restored but with the problems indicated above. I'm beginning to think it might be the sweep control IC, U870, Tek part No. 151-0049-01. I am prepared to get a complete Trigger Generator and Sweep PCB if necessary. I think the part number is 670-2234-02 or 670-2234-04Can anyone help me either source the parts or with the fault?

: I have a Tektronix 465 that won´t produce a full width trace in the .5s-10ms; .5ms-1ms; 5us-1us ranges.

Sure sounds like a switch not making contact: is there one that's only closed on these three ranges?

Hi Alan,

Thanks for your suggestion. I've been right through the switch contacts for these ranges and the only common switch is one which goes open circuit for those ranges. By following through what's connected in when the switch is open circuit I found the open circuit resistor. I've checked most of the components around the same area but can't find anything that seem out of order. Any other ideas? I'm realy stuck on this one.:
: : I have a Tektronix 465 that won´t produce a full width trace in the .5s-10ms; .5ms-1ms; 5us-1us ranges.

: Sure sounds like a switch not making contact: is there one that's only closed on these three ranges?

All I have is a 466 manual to look at, and the circuit seems a bit different. R1151 here goes to the trigger holdoff circuit, which seems a likely place to look. Resistors don't usually open up by themselves.

You might post to the Tektronix forum, or to sci.electronics.repair where there are some experienced Tek techs on hand. I doubt if any of them would think to look here. If you've done this much troubleshooting already, and know pretty much what you're looking for, the people on the other forums will be happy to help out. The Tek forum is:

http://www.reprise.com/host/tektronix/forum/default.asp

Though I think there are more people available on the sci.electronics.repair group, who have worked with these models.

Hello again Alan,

Yes R1151 goes to the trigger holdoff circuit. I've been a tech for many years and have found resistors to go O/C for no apparent reason. I think the bonding between the metal film and lead must become fragile with age and break away. The restor looked perfect - no signs of burning and no signs of excessive current around that part of the circuit. The fault appears to have fixed itself for now, but I think I may have had a dry solder joint as the last thing I did was lift one end of C837 (150pf). I think I'll have a close look at the board tomorrow and touch up any suspect joints. Thanks for your assistance and interest. It's the first time I've used a discussion forum. I did post on the Tek forum but havn't had any feedback yet. There was one message posted by someone with exactly the same problem as mine so I go to the Tek forum and steer him towards the holdoff circuit.

: All I have is a 466 manual to look at, and the circuit seems a bit different. R1151 here goes to the trigger holdoff circuit, which seems a likely place to look. Resistors don't usually open up by themselves.

: You might post to the Tektronix forum, or to sci.electronics.repair where there are some experienced Tek techs on hand. I doubt if any of them would think to look here. If you've done this much troubleshooting already, and know pretty much what you're looking for, the people on the other forums will be happy to help out. The Tek forum is:

: http://www.reprise.com/host/tektronix/forum/default.asp

: Though I think there are more people available on the sci.electronics.repair group, who have worked with these models.

Hi,
Dumb me: I noticed later that you had already posted to the Tek forum. That one takes so long to load, that I wait a day or two before I check for new messages.

Guess I've been lucky: I've never had a film resistor open, but I have had two change value, one in the 5V reference regulator of a Tek 7504. As I remember, when I unsoldered it, the lead fell off. The other was a meter shunt in an AVO tube tester, a high-quality part and not all that old. It changed from something like 5 ohms (I forget exactly) to 4 ohms, quite suddenly. Or maybe it was from 4 to 5. At any rate it caught me by surprise when the 100 mA meter scale didn't read quite right.

I wonder, since an open resistor shouldn't have affected anything else in the circuit, if in checking the associated switch contact, you got a bit of dirt in it, so it no longer closes? It doesn't take much. Or maybe one of the nearby contacts? I've had nothing but trouble trying to clean those things with slips of paper pulled between the closed contacts.

Cheers, Alan