Heathkit Oscilloscope O-12

[Pure_Brew]

Hi All,
I know this is an amp section but I'm guessing that you tube-people could help a brother out!

I got this unit recently, recapped it and fired it up. The 1V2 tube doesn't light up at all, and I'm guessing that it's related to the reason the scope has no vertical gain whatsoever (although I can change the spot position).

I tested the voltage from the transformer at the socket positions at around .588 volts, which seems to be in the operating range?

Do you think I just need another tube, or do you need more info?

Thanks in advance!

[Mooly]

Long time since I worked on valve stuff.

The 1V2 (V9) supplies the HT to the CRT... so it's not that

Measure the DC voltages around V1 and V2 particularly the Anodes of V2 and the Cathodes of V1

Edit... if voltages are missing check PSU around rail "A" fed from that 1.5k resistor around V10.

[Pure_Brew]

Wow - hey nice to hear from you Mooly, been awhile!

Hmm, well it seems that the 1V2 should be working as you say, visual inspection threw me off, and of course, I'm a noob...

I'll have to wait a bit to check voltages, as my multimeter didn't like the taste of 1200+ volts...

However, I poked around some more and found that V1 (6AB4) seems to be non-functional. That tube stands alone behind a shield, so I kind of forgot about it. Seeing that its right smack in the middle of vertical input/gain/etc., it makes sense that this could also be the culprit.

I pulled V1 right out and restarted. Well, everything else that was working before was still working.

So what do you think? Should I check the voltages on V1 from the socket/pin positions to the chassis prior to replacing the tube?

Thank You!

[multisync]

if you can move the spot up and down you can eliminate(90%) the vertical output/drivers of the display tube esp if the display moves up and down a lot( off the screen ). V1 the input vertical tube is a voltage follower/impedance converter and will have gain ~=1 or less. Most of the vertical gain come from v2a and v2b.

set you meter to 100mv AC,
set scope to the highest gain setting both selector and vernier pot,
follow the ac signal through scope,
be very careful of high voltages.
if you look at the crt tube notice the driver tube plates are connected to the crt. One tube pulls up the other pushes up. yes a push pull circuit. On the other half of the ac signal the rolls are reversed. ie pushes down, pulls down

your scope is AC coupled so your gain readings from stage to stage should be measure in ac volts. Dc is used to position and centre the beam.

I noticed the other day my old tube Heathkit scope that I bought new in 1969 is not working anymore either so I may have a look at it some time this fall and see if I can get it going again for nostalgia sake.

I count at least 5 Ac coupling capacitors. If any of them are leaking dc they might throw off the bias of the next stage and ruin the gain.

[Pure_Brew]

Thanks for the reply. I can move the spot horizontally off screen. Vertically is more limited, about an inch away from the top, or the bottom. (so about a 3 inch vertical span)

I didn't replace any of the mica caps, but I did replace all the rest. That, of course, might not matter entirely I know.

When you say set the meter to 100mv AC, what exactly should I be connecting it too?

Take Care

[Mooly]

What's the DC voltage on pin 1 of V2A and pin 6 of V2B

What's the DC voltage on pin 7 of V1 ?

[bob91343]

If you have a spot and can move it, the problem is in the low level circuits. You need to get the sweep working and the vertical signal path. Check the power supply.

[Pure_Brew]

Originally Posted by Mooly

What's the DC voltage on pin 1 of V2A and pin 6 of V2B

What's the DC voltage on pin 7 of V1 ?

I will check that, but I'll have to get back to this project, maybe next week, as there is nothing more I can do without my multimeter working correctly.


Thanks Everyone -

[Mooly]

What you could try... bearing in mind high voltages are nearby... is this. Disconnect that 0.1uf cap on the vert gain pot wiper and touch pin 2 of V2A (the end of that 10 meg) with a long metallic screwdriver and see if there is any movement or jumping of the trace. That's like "buzzing" the input of an amp.
That would help determine if the fault were in V1 or V2 stage.

On safety... that point is at essentially zero volts, but it's normal practice to only delve into working valve stuff with one hand, and keep the other hand away from the chassis or anything that's earthed. That way there's no possibility of a shock "across" you.

[Pure_Brew]

Originally Posted by Mooly

What you could try... bearing in mind high voltages are nearby... is this. Disconnect that 0.1uf cap on the vert gain pot wiper and touch pin 2 of V2A (the end of that 10 meg) with a long metallic screwdriver and see if there is any movement or jumping of the trace. That's like "buzzing" the input of an amp.
That would help determine if the fault were in V1 or V2 stage.

Just tried that and its the first time I've seen any vertical amplitude. Thanks!

The biggest difference when I turned it on, there was a rise in ripple/noise, so I imagine the cap probably smoothed that out before. Touching pin 2 of V2A causes +/- vertical jumping, alot or a little depending contact/tapping. Holding the screwdriver to the pin gives a steady-state rise in the amplitude.

So it certainly looks like the V1 stage is the issue.

Originally Posted by Mooly

On safety... that point is at essentially zero volts, but it's normal practice to only delve into working valve stuff with one hand, and keep the other hand away from the chassis or anything that's earthed. That way there's no possibility of a shock "across" you.

the other hand (behind my back) ------>