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