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Hello!

First than anything else, thanks to everybody who has suggested ways to
deal with this repair.

But saga continues... What seemed to be a simple trouble (power supply
ripple) is still unresolved. Summing up, my 545B shows a distorted signal,
obviously LINE frequency related, as shown in this oscillogram:



Problem has been traced to the +350VDC supply, which has, under load, a
large ripple (before the main relay actuates, ripple is lots lower). This
oscillogram (sorry for the bad focusing of my picture) shows the AC voltage
just after the rectifying diodes of the 350VDC supply (so it is NOT the
regulated voltage):



(LINE frequency in Spain is 50Hz)

Please, note that it shows 500mV but I used a 10x probe, so each division
is 5V. So ripple is high, about 18Vpp. As after this stage comes the series
regulator (6080), driven by the voltage comparator (6AU6), I don't know if
this ripple can be considered 'normal'... I have done similar checkings to
other supplies in the 545B and none shows as much ripple under load. I have
replaced rectifiers but trouble persists. I have also checked most of the
6AU6 surrounding components and all are fine. Both tubes have been swapped
with good ones but problem remains. Could the transformer be bad and unable
to supply the necessary current under load?. Could the +350VDC supply have
a larger than usual load in the circuit, causing that drop?

BTW, I feel like following somebody else in this repair; first, I have
found that the wire coming from the power transformer for that supply was
evidently resoldered... and, in fact, it was a cold soldering job!. Then I
have found that the main fuse, along its holder, is just missing (I will
replace it ASAP). Also, some diodes and resistors in the power supply
section have been replaced.

So I am afraid somebody tried to repair the scope and found that the power
transformer was bad and discarded the scope... hope I am not right!

Please, let me know what else could I check or any other info you can provide.

Thanks!

JOSE
----------------------------------------------------------------------
73 EB5AGV / EC5AAU - JOSE V. GAVILA
La Canyada - Valencia (SPAIN)

EB5AGV Vintage Radio Site:

European Boatanchors List:

Looks like a problem with the power supply filter capacitor.



--- In TekScopes@y..., "JOSE V. GAVILA (EB5AGV/EC5AAU)" <eb5agv@c...>
wrote:

Hello!

First than anything else, thanks to everybody who has suggested

ways to

deal with this repair.

But saga continues... What seemed to be a simple trouble (power

supply

ripple) is still unresolved. Summing up, my 545B shows a distorted

signal,

obviously LINE frequency related, as shown in this oscillogram:



Problem has been traced to the +350VDC supply, which has, under

load, a

large ripple (before the main relay actuates, ripple is lots

lower). This

oscillogram (sorry for the bad focusing of my picture) shows the AC

voltage

just after the rectifying diodes of the 350VDC supply (so it is NOT

the

regulated voltage):



(LINE frequency in Spain is 50Hz)

Please, note that it shows 500mV but I used a 10x probe, so each

division

is 5V. So ripple is high, about 18Vpp. As after this stage comes

the series

regulator (6080), driven by the voltage comparator (6AU6), I don't

know if

this ripple can be considered 'normal'... I have done similar

checkings to

other supplies in the 545B and none shows as much ripple under

load. I have

replaced rectifiers but trouble persists. I have also checked most

of the

6AU6 surrounding components and all are fine. Both tubes have been

swapped

with good ones but problem remains. Could the transformer be bad

and unable

to supply the necessary current under load?. Could the +350VDC

supply have

a larger than usual load in the circuit, causing that drop?

BTW, I feel like following somebody else in this repair; first, I

have

found that the wire coming from the power transformer for that

supply was

evidently resoldered... and, in fact, it was a cold soldering job!.

Then I

have found that the main fuse, along its holder, is just missing (I

will

replace it ASAP). Also, some diodes and resistors in the power

supply

section have been replaced.

So I am afraid somebody tried to repair the scope and found that

the power

transformer was bad and discarded the scope... hope I am not right!

Please, let me know what else could I check or any other info you

can provide.

Thanks!

JOSE
--------------------------------------------------------------------

--

73 EB5AGV / EC5AAU - JOSE V. GAVILA
La Canyada - Valencia (SPAIN)

EB5AGV Vintage Radio Site:

European Boatanchors List:

Snips:

Looks like a problem with the power supply filter capacitor.

Problem has been traced to the +350VDC supply, which has, under

load, a

large ripple (before the main relay actuates, ripple is lots

lower). This

oscillogram (sorry for the bad focusing of my picture) shows the AC

voltage

just after the rectifying diodes of the 350VDC supply (so it is NOT

the

regulated voltage):



(LINE frequency in Spain is 50Hz)

I'm looking at the schematic of the 545A, and I suspect that the 545B has
the same 350V supply. The (silicon) bridge rectifier feeds the 125uF
reservoir capacitor through two ten-ohm 1W resistors in parallel. It also
has an 82k resistor in parallel.

So check that the 10-ohm resistors are present and measure correctly, and
the same with the 82k.

Are you working from a schematic, BTW, or diagnosing blind?

Craig

Hello!

Thanks to both of you... but my message was from September, I think, and
the oscilloscope was fixed back then ;-)

It was a capacitor, as you suggest.

Merry Christmas!

JOSE
----------------------------------------------------------------------
73 EB5AGV / EC5AAU - JOSE V. GAVILA
La Canyada - Valencia (SPAIN)

EB5AGV Vintage Radio Site:

European Boatanchors List:

Craig Sawyers [mailto:c.sawyers@...],
answering Jose Gavila:

<snip>

I'm looking at the schematic of the 545A, and I suspect that
the 545B has
the same 350V supply. The (silicon) bridge rectifier feeds the 125uF
reservoir capacitor through two ten-ohm 1W resistors in
parallel. It also
has an 82k resistor in parallel.

So check that the 10-ohm resistors are present and measure
correctly, and
the same with the 82k.

I don't think that trouble in those resistors could
account for a ripple. IIRC the purpose of the 82K
is to prevent some cap (maybe this one) from getting
reverse voltage during warmup. The 10's are there
mostly for troubleshooting. You measure across them
to calculate current.

Even if you don't have a manual or a cap tester, you can
find out if the cap is ok. Measure the current
as I mentioned above. The formula for current
in a cap is I = C * dV/dT . Estimate
dV/dT from your scope trace. Plug in I and
dV/dT. Does C come out near the printed value?

To get in deeper, you really need a manual.
It tells what I should read, and it might even
say how much ripple is acceptable. In any case,
how's the +350 line? If it's smooth over the
specified range of line voltage, you're ok.

Regards,
Dave Wise

Aargh, remind me to read the whole thread next time.

Dave Wise

Even if you don't have a manual or a cap tester, you can
find out if the cap is ok. Measure the current
as I mentioned above. The formula for current
in a cap is I = C * dV/dT . Estimate
dV/dT from your scope trace. Plug in I and
dV/dT. Does C come out near the printed value?

Yeah - that was my next line of advice. I'd run it the other way though;
assume the cap is good, and then calculate the current from the ripple. The
6080's are good for 100mA per section - so does the ripple indicate
significantly *more* than 200mA? If it does, then the capacitor is much
less than its correct value, and is probably shot.

Craig