Hi Tom,
Thanks for thinking for me! I hadn't given this even a moment's
thought! Obviously, my electronic alertness is a bit off after having been out
of it for around 20 years...
You're right about the 1N5406, it's a
"standard recovery" diode. The circuit runs at around 50KHz. I don't know the
characteristics of these diodes at this frequency. The SMPS doesn't see it as
a shortcut though...
It's one of 2 diodes in a half-bridge, supplying 5V to
... the light bulbs only, through a CLC-filter.
The load on the
transformer may be another matter. I'll try and find a way to access some
measuring points while the scope is switched on or at least I'll watch for
effects that may be caused by this.
Thanks again for
this!
Raymond
--- In TekScopes@..., "Tom
Jobe" wrote:
>
> Hi Raymond,
> A
question about your diode substitution.
> What frequency does that diode
run at in the 7704A?
> And is the 1N5406 suitable for that
frequency?
> The data sheet for 1N5406 makes me think it is only good
for low frequencies.
> I don't have a 7704A manual handy to answer my
own question.
> tom jobe...
>
>
>
> -----
Original Message -----
> From: raymonddompfrank
> To: TekScopes@...
> Sent: Tuesday, May 22, 2012 12:47 PM
> Subject: [TekScopes] Re:
7704 power supply
>
>
>
>
>
> Hello
guys,
> I have been able to repair my 7704A! It turned out that one
bridge rectifier diode (CR3182) on the inverter/rectifier board was shorted,
causing a periodic short-circuit in transformer T3101. It was the transformer
making the noise. I used a 1N5406 from a broken Apple PowerMac G5 as a
replacement.
> I can't understand why it broke down the moment when I
first switched the PS on without the load of the display unit (D7704).
>
> Thank you all very much for your help, hints and encouraging
words.
> This forum has certainly been good to me with my first
posting!
>
> Next, I'll try and resolve the brightness/focus
issue.
>
> Best regards,
>
> Raymond
>
> --- In TekScopes@...,
"raymonddompfrank" wrote:
> >
> > I am
a newbie Tek collector, having used my own 465 until some 20 years ago. I am
reasonably knowledgeable in electronics.
> >
> > The other
day, I purchased a 7704 after only giving it a very quick test. I agree that
wasn't very clever. Unfortunately I noticed some brightness and timebase
issues and I decided to have a look inside the instrument. That wasn't very
clever either...
> > I checked the +54.2 +- 0.25V without removing
the power supply. It turned out OK (54.4). I decided to check some other
voltages on the top regulator board and decided to remove the display unit
(D7704) to reach in. I realized that many switching power supplies don't like
very light loads but decided it was easier and the location of the description
of how to remove the D7704 in the maintenance manual was such that I concluded
it was safe. Also, I haven't found any warning re. this procedure.
>
> Anyway, since I removed the D7704 and switched the PS on, there's a
hissing sound from it and from what I measured, all preregulated voltages are
much lower than normal: +54V now is 30V, -54V is -30V, 7V is 2V etc. Also, the
front lights remain off (not surprising).
> > I'm anxious leaving the
PS on for more than a few seconds.
> >
> > My Question: Did
I indeed blow up the PS and if so, is there a component that usually blows up
and is the repair doable or even easy? There's no nasty burning smell or
anything.
> > Or is this just a coincidence, and have I just
witnessed another electrolytic capacitor dying?
> >
> >
Thanks for any ideas!
> >
> > Raymond
>
>
>
