454 fireworks


Sean Turner
 

I received another patient...this time it's a 454 that is outwardly fine (actually it's in superb condition), but had a very serious HV fault. Apparently the previous user was using it and it started going snap crackle and pop. I briefly powered it on, and there is a sound reminiscent of a Tesla coil that appears to be coming from the HV cage area, starts after a second or two of the fan running.

No blown fuses, and there is HV as the crt lights up (too bright for my comfort, so I'm not going to attempt another power up. I can't see anything obvious in the HV cage, but I can't see it all without disassembly.

My question to the group is twofold: (1) have you ever seen a failure like this and (2) ideas on what to look for. The scope is as pristine inside as, very very clean and shiny. I find a hard time believing that the seemingly well sealed HV box has grime in it, so I'm thinking a catastrophic failure of some component in there.

Sean


John
 

Sean: from the symptoms, a possible scenario is that the EHT regulation has failed. I always dread going into that box!

I'd suggest you open up the back and find / remove F1437. You can then power the EHT section from an external variable PSU (current limit 2A). Power the scope and bring the PSU up from zero, whilst monitoring the -1960V.
If that can be set and the scope display is normal, this might confirm the loss of reguation. Note that if the +75V rail has gone high, this will cause the EHT to rise pro-rata, so check that first.
John


Sean Turner
 

John,

2A at what voltage? With that fuse out, I assume the on board regulator won't run, so the power supply becomes the regulator.

I'm going to look for a hardcopy manual for sure.

Sean

On Wed, Mar 10, 2021 at 07:47 AM, John wrote:


Sean: from the symptoms, a possible scenario is that the EHT regulation has
failed. I always dread going into that box!

I'd suggest you open up the back and find / remove F1437. You can then power
the EHT section from an external variable PSU (current limit 2A). Power the
scope and bring the PSU up from zero, whilst monitoring the -1960V.
If that can be set and the scope display is normal, this might confirm the
loss of reguation. Note that if the +75V rail has gone high, this will cause
the EHT to rise pro-rata, so check that first.
John


Tom Lee
 

Sean,

He said to set the current *limit* to 2A, not to force a constant 2A. As he suggested, use a variable supply set to that current limit and follow his other directions.

Tom

Sent from my iThing, so please forgive typos and brevity.

On Mar 10, 2021, at 5:31 PM, Sean Turner <sdturne@q.com> wrote:

John,

2A at what voltage? With that fuse out, I assume the on board regulator won't run, so the power supply becomes the regulator.

I'm going to look for a hardcopy manual for sure.

Sean

On Wed, Mar 10, 2021 at 07:47 AM, John wrote:


Sean: from the symptoms, a possible scenario is that the EHT regulation has
failed. I always dread going into that box!

I'd suggest you open up the back and find / remove F1437. You can then power
the EHT section from an external variable PSU (current limit 2A). Power the
scope and bring the PSU up from zero, whilst monitoring the -1960V.
If that can be set and the scope display is normal, this might confirm the
loss of reguation. Note that if the +75V rail has gone high, this will cause
the EHT to rise pro-rata, so check that first.
John




Sean Turner
 

I understand that, I apologize for not typing precisely. :o)

I'm just trying to figure out if I have a power supply that can supply the necessary voltage.

Sean

On Wed, Mar 10, 2021 at 05:34 PM, Tom Lee wrote:


Sean,

He said to set the current *limit* to 2A, not to force a constant 2A. As he
suggested, use a variable supply set to that current limit and follow his
other directions.

Tom


Tom Lee
 

He's talking about powering up the oscillator from an external supply. I don't have the schematic with me at the moment, but IIRC, it's powered off of the 12V unreg. supply. The fuse that he's presumably talking about is the one in series with that supply. Removing the fuse allows you to drive the oscillator with an external supply whose voltage can be varied. Start low and increase it while monitoring the -2kV line. But open up the schematic to double check my memory. If you understand the reasoning behind his strategy, you should be able to fill in the details.

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/10/2021 17:46, Sean Turner wrote:
I understand that, I apologize for not typing precisely. :o)

I'm just trying to figure out if I have a power supply that can supply the necessary voltage.

Sean

On Wed, Mar 10, 2021 at 05:34 PM, Tom Lee wrote:

Sean,

He said to set the current *limit* to 2A, not to force a constant 2A. As he
suggested, use a variable supply set to that current limit and follow his
other directions.

Tom


Sean Turner
 

Tom,

Yes, it makes sense. I didn't have the schematic in front of me, either. I'll go grab a PDF; have an original printed one supposedly in nice shape on the way.

Sean

On Wed, Mar 10, 2021 at 05:57 PM, Tom Lee wrote:


He's talking about powering up the oscillator from an external supply. I don't
have the schematic with me at the moment, but IIRC, it's powered off of the
12V unreg. supply. The fuse that he's presumably talking about is the one in
series with that supply. Removing the fuse allows you to drive the oscillator
with an external supply whose voltage can be varied. Start low and increase it
while monitoring the -2kV line. But open up the schematic to double check my
memory. If you understand the reasoning behind his strategy, you should be
able to fill in the details.

--Tom


Tom Lee
 

Good luck with the debug. That’s one of my favorite scopes, and well worth the effort at resurrecting.

Cheers,
Tom

Sent from my iThing, so please forgive typos and brevity.

On Mar 10, 2021, at 6:21 PM, Sean Turner <sdturne@q.com> wrote:

Tom,

Yes, it makes sense. I didn't have the schematic in front of me, either. I'll go grab a PDF; have an original printed one supposedly in nice shape on the way.

Sean

On Wed, Mar 10, 2021 at 05:57 PM, Tom Lee wrote:


He's talking about powering up the oscillator from an external supply. I don't
have the schematic with me at the moment, but IIRC, it's powered off of the
12V unreg. supply. The fuse that he's presumably talking about is the one in
series with that supply. Removing the fuse allows you to drive the oscillator
with an external supply whose voltage can be varied. Start low and increase it
while monitoring the -2kV line. But open up the schematic to double check my
memory. If you understand the reasoning behind his strategy, you should be
able to fill in the details.

--Tom




Sean Turner
 

Thanks! I'm hoping it's a simple fix. The scope looks practically new inside and out! I think I may use it in the radio shack since it's nice and small compared to my other laboratory Tek scopes. :o) The case design is a model of repairability too...just two things to loosen to remove both halves.

Sean

On Wed, Mar 10, 2021 at 06:27 PM, Tom Lee wrote:


Good luck with the debug. That’s one of my favorite scopes, and well worth
the effort at resurrecting.

Cheers,
Tom

Sent from my iThing, so please forgive typos and brevity.


Paul Amaranth
 

Absolutely worth it. I had a 454 that I restored and calilbrated
and I love it. I also have a 454A in the queue that I'll get to
sometime. I really like the larger screen on that.

Paul

On Wed, Mar 10, 2021 at 06:34:49PM -0800, Sean Turner wrote:
Thanks! I'm hoping it's a simple fix. The scope looks practically new inside and out! I think I may use it in the radio shack since it's nice and small compared to my other laboratory Tek scopes. :o) The case design is a model of repairability too...just two things to loosen to remove both halves.

Sean

On Wed, Mar 10, 2021 at 06:27 PM, Tom Lee wrote:


Good luck with the debug. That’s one of my favorite scopes, and well worth
the effort at resurrecting.

Cheers,
Tom

Sent from my iThing, so please forgive typos and brevity.






!DSPAM:604981d3135882394211157!
--
Paul Amaranth, GCIH | Manchester MI, USA
Aurora Group of Michigan, LLC | Security, Systems & Software
paul@AuroraGrp.Com | Unix/Linux - We don't do windows


Sean Turner
 

On the flip side, compared to my 519, it's a huge viewing area!

Sean

On Wed, Mar 10, 2021 at 06:51 PM, Paul Amaranth wrote:


Absolutely worth it. I had a 454 that I restored and calilbrated
and I love it. I also have a 454A in the queue that I'll get to
sometime. I really like the larger screen on that.

Paul


Sean Turner
 

I did some quick power ons with the covers off (and one hand behind my back and the other on the power switch!) Looks like the arcing is getting started in the area of C1457, and then when it really gets going, R1460, R1461, and the anode lead all are enveloped by arc, and it looks to also be reaching C1460. And as mentioned before, it sounds like a Tesla coil! Would be super cool if it wasn't an oscilloscope.

Sean


Sean Turner
 

John,

I had the opportunity to remove F1437 and power the HV supply from a regulated bench supply. My findings were that sparking begins around C1457 at about 10.5 V (at which point it is drawing approximately 1.2 amps).

Sean

On Wed, Mar 10, 2021 at 07:47 AM, John wrote:


Sean: from the symptoms, a possible scenario is that the EHT regulation has
failed. I always dread going into that box!

I'd suggest you open up the back and find / remove F1437. You can then power
the EHT section from an external variable PSU (current limit 2A). Power the
scope and bring the PSU up from zero, whilst monitoring the -1960V.
If that can be set and the scope display is normal, this might confirm the
loss of reguation. Note that if the +75V rail has gone high, this will cause
the EHT to rise pro-rata, so check that first.
John


Sean Turner
 

Good call on checking the +75 supply too...it's running at +100!

Sean


John
 

Let's hope it's not Q1197 gone s/c as it's a pig to replace (mechanically). With the EHT unpowered at least you can trouble-shoot the PSU. NB the -12V is the reference for both +75 and +12V rails so needs to be correct / set first. Once you have the power-supply working fingers crossed the EHT section comes good.
John


Sean Turner
 

John,

The -12V rail is spot on, and so is the +12. I did do a quick and dirty diode test on Q1197, which showed two good diode drops, but that was in circuit and at low. I suppose it could be breaking down at operating voltage? I agree, getting at it will be...interesting...if the fault with +75V regulation is narrowed down to that.

Sean

On Mon, Mar 15, 2021 at 12:57 AM, John wrote:


Let's hope it's not Q1197 gone s/c as it's a pig to replace (mechanically).
With the EHT unpowered at least you can trouble-shoot the PSU. NB the -12V is
the reference for both +75 and +12V rails so needs to be correct / set first.
Once you have the power-supply working fingers crossed the EHT section comes
good.
John


John
 

Sean: If Q1197 isn't s/c it's 99% likely the drive circuitry, which at least is accessible!
John


Sean Turner
 

OK, I have the scope on the bench again for a closer look at all the components in the +75 supply. Obviously I haven't torn it apart to test out of circuit, but now that I tested it again, Q1197 seems to have an emitter-collector short. I don't *think* this would result from how it is connected. Am I missing something?

Looking at the mechanical bits, it looks to me like the power transformer has to be undone to get to nuts that hold the metal part that the pass transistors heat sink to.

Sean

On Mon, Mar 15, 2021 at 03:43 PM, John wrote:


Sean: If Q1197 isn't s/c it's 99% likely the drive circuitry, which at least
is accessible!
John