No vertical deflection, Ch 1 Tek 475 - UK Vintage Radio Repair and Restoration Discussion Forum

phimargin · 14th Oct 2015, 9:30 pm

Just turned on my Tek 475 (worked a few hours ago).

On channel 1: The V position control has no effect and no vertical response from the probe. The sweep/trace is good.

Is this likely to be a PSU problem?

(Channel 2 is O.K.)

Radio Wrangler · 14th Oct 2015, 10:14 pm

It is unlikely to be a power supply problem. The same power supply rails are shared between the two channels.

One possibility is a dirty switch contact in the channel selection switches. It could be some local power supply decoupling network (not shared between channels)

Otherwise, it's time to get in there with a manual and a second scope.

David

RogerEvans · 15th Oct 2015, 9:43 am

Since channel 2 is working OK you could connect the calibrator output to channel 1, display channel 2 and probe around the channel 1 amplifier to see where the signal on channel 1 disappears. If the channel 1 signal gets as far as U370, the channel switch then check the switching signals into that chip (with a multimeter, not the scope itself). If it looks like one of the Tektronix custom ICs at fault try removing, cleaning and replacing before buying another.

If you have another scope it is a lot easier!

Roger

phimargin · 15th Oct 2015, 12:17 pm

Agreed David, I only realised about the shared supplies after making the post.
"a dirty switch contact in the channel selection switches" sounds a promising idea.

Roger, good idea for further probing, definitely logical to use calibrator to Ch 1 input etc.

I'd really do with some less smudgy circuit diagrams than the scans I have.

phimargin · 15th Oct 2015, 2:37 pm

Thanks again Roger

Whilst I had vaguely wondered about using Ch 2 for troubleshooting, your specific & very practical suggestion really helped me.

The culprit was Q184, which looks like it might supply the tail current for a differential pair.

I swapped in the matching good transistor from Ch2, and Ch1 burst into life. Fortunately the transistor was a 2N3906, so not hard to replace.

I'm not sure how it could have failed though, it wouldn't be the 'whiskering' effect seen on very old types.

Regards
Stephen.

Refugee · 15th Oct 2015, 2:46 pm

Well done.
I had a fault like that in a different model of Tek scope in the time base plug in so it was well away from the incoming signal.

Radio Wrangler · 15th Oct 2015, 3:24 pm

Plastic moulded transistors have shown a few internal corrosion related slow failures,

Anyway, a success!

David

Leon Crampin · 15th Oct 2015, 3:30 pm

Plastic encapsulated small signal transistors are failing frequently now in Tek equipment - the Fairchild devices with a blob of epoxy on the top are the worst. Moisture ingress is the most likely cause, sometimes with consequent internal metal migration.

These were never claimed to be truly hermetic devices (metal can versions are available), but I think the excellent design, documentation and in some cases transistor sockets provided by Tek have proved their worth.

After all these years, there is little doubt that they made and documented about the best equipment available. Hats off to the engineers.

Leon.

Leon Crampin · 15th Oct 2015, 3:31 pm

(Simultaneous post with David).

phimargin · 16th Oct 2015, 4:15 pm

"failing frequently now in Tek equipment" - interesting, do you have a good link for more info?

The actual part is a NatSemi device.

Leon Crampin · 16th Oct 2015, 6:16 pm

No published link, but several of my Tek scopes are failing with random faults - as is to be reasonably expected. By far the majority of faults are caused by defective plastic encapsulated small signal devices. The circuit design is good, so failures can be anywhere, but 7000 series power supplies seem to have more than their fair share of small signal device failures - possibly due also to exposure to transients.

It's worth noting that when "our" Tek scopes were made, plastic encapsulated power devices such as the TO-220 had not appeared on the market. Consequently, any device of TO-5 size and above was hermetic in a Tek scope. These hardly ever fail - and the TO-66 and TO-3 devices also soldier on.

Leon.

6AL5W-Martin · 23rd Oct 2015, 1:45 pm

the 7000 series like to have bad tantals, and defective transistors are oftenly a follower of that problem. I like to replace them, a lot of work

greetings
Martin

14th Oct 2015, 9:30 pm	#1
phimargin Retired Dormant Member Join Date: Jun 2015 Location: Derbyshire, UK. Posts: 8	No vertical deflection, Ch 1 Tek 475 Just turned on my Tek 475 (worked a few hours ago). On channel 1: The V position control has no effect and no vertical response from the probe. The sweep/trace is good. Is this likely to be a PSU problem? (Channel 2 is O.K.)

14th Oct 2015, 10:14 pm	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,870	Re: No vertical deflection, Ch 1 Tek 475 It is unlikely to be a power supply problem. The same power supply rails are shared between the two channels. One possibility is a dirty switch contact in the channel selection switches. It could be some local power supply decoupling network (not shared between channels) Otherwise, it's time to get in there with a manual and a second scope. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

15th Oct 2015, 9:43 am	#3
RogerEvans Hexode Join Date: Oct 2014 Location: Wiltshire, UK. Posts: 381	Re: No vertical deflection, Ch 1 Tek 475 Since channel 2 is working OK you could connect the calibrator output to channel 1, display channel 2 and probe around the channel 1 amplifier to see where the signal on channel 1 disappears. If the channel 1 signal gets as far as U370, the channel switch then check the switching signals into that chip (with a multimeter, not the scope itself). If it looks like one of the Tektronix custom ICs at fault try removing, cleaning and replacing before buying another. If you have another scope it is a lot easier! Roger

15th Oct 2015, 12:17 pm	#4
phimargin Retired Dormant Member Join Date: Jun 2015 Location: Derbyshire, UK. Posts: 8	Re: No vertical deflection, Ch 1 Tek 475 Agreed David, I only realised about the shared supplies after making the post. "a dirty switch contact in the channel selection switches" sounds a promising idea. Roger, good idea for further probing, definitely logical to use calibrator to Ch 1 input etc. I'd really do with some less smudgy circuit diagrams than the scans I have.

15th Oct 2015, 2:37 pm	#5
phimargin Retired Dormant Member Join Date: Jun 2015 Location: Derbyshire, UK. Posts: 8	Re: No vertical deflection, Ch 1 Tek 475 Thanks again Roger Whilst I had vaguely wondered about using Ch 2 for troubleshooting, your specific & very practical suggestion really helped me. The culprit was Q184, which looks like it might supply the tail current for a differential pair. I swapped in the matching good transistor from Ch2, and Ch1 burst into life. Fortunately the transistor was a 2N3906, so not hard to replace. I'm not sure how it could have failed though, it wouldn't be the 'whiskering' effect seen on very old types. Regards Stephen.

15th Oct 2015, 2:46 pm	#6
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: No vertical deflection, Ch 1 Tek 475 Well done. I had a fault like that in a different model of Tek scope in the time base plug in so it was well away from the incoming signal.

15th Oct 2015, 3:24 pm	#7
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,870	Re: No vertical deflection, Ch 1 Tek 475 Plastic moulded transistors have shown a few internal corrosion related slow failures, Anyway, a success! David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

15th Oct 2015, 3:30 pm	#8
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: No vertical deflection, Ch 1 Tek 475 Plastic encapsulated small signal transistors are failing frequently now in Tek equipment - the Fairchild devices with a blob of epoxy on the top are the worst. Moisture ingress is the most likely cause, sometimes with consequent internal metal migration. These were never claimed to be truly hermetic devices (metal can versions are available), but I think the excellent design, documentation and in some cases transistor sockets provided by Tek have proved their worth. After all these years, there is little doubt that they made and documented about the best equipment available. Hats off to the engineers. Leon.

15th Oct 2015, 3:31 pm	#9
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: No vertical deflection, Ch 1 Tek 475 (Simultaneous post with David).

16th Oct 2015, 4:15 pm	#10
phimargin Retired Dormant Member Join Date: Jun 2015 Location: Derbyshire, UK. Posts: 8	Re: No vertical deflection, Ch 1 Tek 475 "failing frequently now in Tek equipment" - interesting, do you have a good link for more info? The actual part is a NatSemi device.

16th Oct 2015, 6:16 pm	#11
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: No vertical deflection, Ch 1 Tek 475 No published link, but several of my Tek scopes are failing with random faults - as is to be reasonably expected. By far the majority of faults are caused by defective plastic encapsulated small signal devices. The circuit design is good, so failures can be anywhere, but 7000 series power supplies seem to have more than their fair share of small signal device failures - possibly due also to exposure to transients. It's worth noting that when "our" Tek scopes were made, plastic encapsulated power devices such as the TO-220 had not appeared on the market. Consequently, any device of TO-5 size and above was hermetic in a Tek scope. These hardly ever fail - and the TO-66 and TO-3 devices also soldier on. Leon.

23rd Oct 2015, 1:45 pm	#12
6AL5W-Martin Hexode Join Date: Nov 2006 Location: Biedenkopf, [Hessen], Germany. Posts: 425	Re: No vertical deflection, Ch 1 Tek 475 the 7000 series like to have bad tantals, and defective transistors are oftenly a follower of that problem. I like to replace them, a lot of work greetings Martin