Tektronix 475: No Horiz Sweep

[MrBungle]

The blanking uses current summing which is difficult to debug with a voltage measurement device. I would pull each of the relevant coaxial connectors off in turn and see if that results in a trace. Then follow that back. If it doesn’t do anything swap out the summing transistor. That does goes bad occasionally. I think it’s on the bottom left of the interface board.

I had a duff driver circuit on the vertical channel switch once which caused this. Towards the rear of the vertical board I wiggled all the transistors and it worked.

I’ve got a sick 465 here with a similar problem at the moment

[Anthony]

Mr. B
Thank you. I did eventually realised that a summing transistor would not reveal much with voltage measurements. I also, as you suggest, already tried disconnecting all other inputs, Q1332 and the other 2 coaxs - made no difference. Nor does lifting CR1337.

Of course I am only able to look at one side of the board without removal, maybe there are horrors on the other side?

I wonder about breadbording Q1338 and 1344 - seems a bit desperate!

[MrBungle]

I wouldn't worry about the other side of the board. I've had one out. It's not pretty and is difficult to get out. All the debugging and repairs can be done from the top (thank goodness!)

I'd start here with a large print out of the schematic and board layout from the service manual and start recording voltages starting with supply voltages to the blanking circuit subsection. I reckon there's a duff semiconductor or cap in there somewhere!

At least it's totally broken. What's terribly more frustrating with these is when they partially work, like the common problem with the 475 where the unblanking is too slow at high sweep speeds and unblanks after the trace has left the right of the screen. Grr.

[Anthony]

Mr. b
Yes that sort of fault must be terribly frustrating. Perhaps i`ll have it to look forward to!
Best wishes
Anthony

[MrBungle]

Maybe. It’s worth it for the feeling you get when the things work though

Out of interest a couple of 475’s have popped up on eBay today for a reasonable fee if you need parts.

I actually grabbed a broken 475A with DM44 this afternoon so will join you in the bug hunting no doubt as well.

[Anthony]

Mr. b
Thank you for the tip.
Best
Anthony

[RogerEvans]

Anthony,

I was going to reply to your post on Tekscopes but since there is a bit more background information here it may be more appropriate to offer my thoughts here.

The thread here began with a description of a sweep problem including 'the Horiz. Trace control only moves the spot(s) on the right hand side of the screen and not the full screen' and ' will not trigger in Norm/Ext or Auto/Ext' and 'no A+gate'. Normally the Z unblanking should come on permanently in X-Y mode but there are plenty of potential faults that could cause unblanking not to be generated at all, hence the lack of a spot in XY mode until you press the beam finder. With the asumption of a fault in the sweep circuitry causing the absence of the unblanking pulse, there is no evidence of any problem in the Z amplifier, the emitter of Q1344 is also a low impedance point because of Q1352 base-emitter junction and CR1352.

In most cases that I have come across you can override the lack of unblanking by adjusting the Grid Bias pot on the Z axis board R1375 on diagram <12>. This should enable you to see how much of the sweep circuitry is working. If you use the slowest sweep you can see where it starts and ends and whether the speed changes during the sweep.

The manual shows Q688 and Q698 as being the sweep bistable multivibrator and also Q698 directly generates the unblanking pulse to J694. If Q688 is conducting and Q698 is not conducting then the beam will be blanked. I would certainly check Q698 for open circuit but there are many other possibilities.

Regards,

Roger

[Skywave]

Originally Posted by MrBungle

I wouldn't worry about the other side of the board. I've had one out. It's not pretty and is difficult to get out..

Unfortunately, 'difficult to get (the board / part) out' is a common feature with Tek. kit: the 475 being a 'good' example.

Al.