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Perhaps spoke too soon but we are close.

The +15v bridge rectifier is running at scorching hot and the -8 and +5v are not far behind.

this was the initial finding when I had first fixed the scope way back when...


The trace disappears after maybe fifteen minutes and only half appears with the beam-finder there-after.

I'm off to bed, and shall have another look in the morning, but we definitely still have a detail to resolve.

WOOHOO!!!!

Oh my gosh!


IT LIVES!!!!!!



I was in such a hurry earlier that didn't even touch any controls, however turning the storage off, resolves the problem.


Now I need to properly reassemble the A5 board set-up, and all the front panel and controls.



Not sure if this is because it is all half dismantled still, but the 'vertical position' control doesn't raise the wave above the median line


I know it is not yet 100% resolved, but I still lack the words to convey my gratitude.


What a mission! What a challenge!

Have learned invaluable information along the way. Tomorrow, I shall find the correct components, and hopefully the vertical postioning will be resolved.


It is difficult to know who to thank the most....



James

James,

If you are in storage mode, David T. is right. If not, see if your d-c restorer diodes, CR1444 and CR1445, are bad along with R1445. The diodes can be replaced with 1N4937 and the resistor replaced with a Vishay VR37 series. I think by your description, David T. called it right.

Mark

Quick check reveals all the LV supplies are still in order

Very quickly as have to go out for the evening.

Changed the components, turned the scope on and appeared a nice clean trace.

After 10 seconds or so the trace faded and the CRT started to fill with intense bright green light.

After reducing the 'intensity' to min, it remained with two separate blurry areas, like unfocused spots on the screen.



No time now to test anything but shall welcome any opinions.


Thanks


James

On Sun, Mar 27, 2022 at 12:59 PM, James55 wrote:

1N4148's are not a problem, but for the 3v Zener, only have the choice
between a 3.3v and a 2.5v until the shops open tomorrow.

3.3V would be fine.
Ozan

On Sun, Mar 27, 2022 at 05:19 PM, Mark Vincent wrote:

James,

A 1N4148 will work to replace the 1N4610.

Mark

1N4148's are not a problem, but for the 3v Zener, only have the choice between a 3.3v and a 2.5v until the shops open tomorrow.

Would be great just to do a test, just in case I have missed something?

On Sun, Mar 27, 2022 at 09:38 PM, Raymond Domp Frank wrote:

without that shorted, +5V and +65V wouldn't be shorted together and CR948
and
CR949 wouldn't have blown.

... and neither would VR948.

Correction: Unless CR948 shorted out and then, depending on position of S948 and R948...

Raymond

On Sun, Mar 27, 2022 at 09:35 PM, Raymond Domp Frank wrote:

without that shorted, +5V and +65V wouldn't be shorted together and CR948 and
CR949 wouldn't have blown.

... and neither would VR948.

Raymond

On Sun, Mar 27, 2022 at 08:49 PM, James55 wrote:

You were spot on, and thankfully I had managed to locate exactly what you had
suggested. In fact, the VR 948 zener is also bad.

It would have to, to make sense of your findings. Don't forget to check R948 (trigger holdoff pot) for a short across, as Mark suggests; without that shorted, +5V and +65V wouldn't be shorted together and CR948 and CR949 wouldn't have blown.

Raymond

Mark

I have just seen your post.

You were spot on, and thankfully I had managed to locate exactly what you had suggested. In fact, the VR 948 zener is also bad.

Thank you


James

And with the A5 reconnected to the scope, all the LV rails are once again fine.

So, I have obviosly learnt something during this long thread...

Hat off to you all.

Now just need to find suitable diodes.


James

James,

A 1N4148 will work to replace the 1N4610.

Mark

So, after re-checking all the caps for shorts, and then removing the chips to no avail, I moved on to testing every diode and may have found the problem.

CR948 (Tek 152-0333-00) and CR949 (1N4152) were (and still are) shorted out.

With these two lifted from the circuit we no longer have the low 4Ω reading between the +65 and +5v rails, so fingers crossed it is only these two.

For sure I don't have a 152-0333-00 (1N4610) here, although I do have various diodes here, if anyone has a suggestion as to a temporary replacement, in order to test the scope?


James

James,

On page <7>, there is part of the A5 board in the centre where the 5 and 65V are tied through parts. Check the 948 and 949 parts. Maybe the A Trig. Holdoff pot is shorted from each end, diodes shorted or something wired wrong. I could be wrong about any of these being bad. I would check here first to see if something is bad/wrong then if all are good, move on to other areas. If someone else knows otherwise, jump in. This was my guess where to start.

Mark

Hi all,

Thanks for the suggestions.

Been a busy few days so haven't worked on the scope. Also didn't manage to get the prints I had wanted, but today is a free day, and this will be great practice in problem solving.

Just doing a quick resistance test across the LV rails (with the A5 board connected) reveals that between +65 and +5v there is only 4Ω, whereas with the A5 removed, the measurement is over 4.5kΩ, so for sure that needs to be looked into.
And in case anyone was wondering, the LV rails all return to their correct voltages with the A5 board removed again.



@ Roger,

If you want a quick check on which power rail is causing the problem you can completely and individually isolate +15V, +5V and -8V from the rest of the A5 board by lifting one side of the decoupling inductors L582, L584 and
L585, square 12C on the A5 component layout diagram (they are not shown on schematic <5> but are shown on the power distribution schematic). C582 and C584 are immediate suspects, but you may have already changed
them.

Unfortunately lifting none of the inductors resolved the short and yes the capacitors have recently been changed with 35v tantalums.



Wish me luck...


James

I like to overengineer, and I've got them, anyway.? I was more concerned about the 2 1/2 digit types.

Harvey

On Fri, Mar 25, 2022 at 02:30 AM, James55 wrote:

Measurements were taken as each disconnected part was reconnected and all was
going fine.

Until, after replacing the A5 Trigger Generator and Sweep Logic board, the
voltages on the +65v, +15 and +5v rails all went to almost zero.
The +140v is on 72 volts, whilst the minus 15 is around -3v and the minus 8
rail being around -7.4v

I've watched this thread every now and then and I've seen a lot of back and forth. Please ignore me if this has been addressed:

If I understand you correctly, with the board out, power supply voltages are OK, inserting the board kills them. I suggest disconnecting the board again and with your resistance meter look for shorts on each power supply connection going into that board.
As said, a single voltage supply short, like the +65V (the reference in this 'scope), can bring down all others.

Raymond

If you want a quick check on which power rail is causing the problem you can completely and individually isolate +15V, +5V and -8V from the rest of the A5 board by lifting one side of the decoupling inductors L582, L584 and L585, square 12C on the A5 component layout diagram (they are not shown on schematic <5> but are shown on the power distribution schematic). C582 and C584 are immediate suspects, but you may have already changed them.

Best of luck,

Roger

With a sensitive enough voltmeter you can measure the voltage drop on the
PCB traces and figure out where the current is going.
3 1/2 digits won't be enough.



On Fri, Mar 25, 2022 at 12:28 AM David Templeton <vexorg.calibra@...>
wrote:

You don’t need more than 3.5 digit for fault finding and to get it working

David

On 25 Mar 2022, at 01:56, Harvey White <madyn@...> wrote:

?I'd like more than 4 digits, but you take what you can get. You'd

generally want to replace the "failing" rail with power from the supply
(other supplies not needed), current limited to about 100 ma. Somewhere
around here we've talked about voltage drops across parts when looking for
shorted capacitors.

Harvey

On 3/24/2022 9:30 PM, James55 wrote:
Situation update...

Measurements were taken as each disconnected part was reconnected and

all was going fine.

Until, after replacing the A5 Trigger Generator and Sweep Logic board,

the voltages on the +65v, +15 and +5v rails all went to almost zero.

The +140v is on 72 volts, whilst the minus 15 is around -3v and the

minus 8 rail being around -7.4v

A quick check seems to show that none of the tantalums are short, and

nothing is obviously burnt or damaged, so tomorrow I shall repeat what
worked for the A6 main interface board.

That is, I'll get prints of the component layout and of the schematic,

then I'll colour the components of the failed rails (in a separate color
for each rail), whilst working through the schematic, taking and recording
the readings of each component in turn.

@ Harvey

There is a 15v 2A current limiting PSU here and the DMM has four digits

The questions will come as I look closer.