Kenwood Audio Lab Scope problem

Archived from groups: rec.audio.tech (More info?)

I've recently gotten an old Kenwood Audio Lab Scope, the 6060A model.
Appears to be about 1970 vintage. It works, but the scope's Horizontal
Position adjustment won't allow the image to be centered on the screen. The
vertical adjustment is fine.
I have the schematic & found 2 trimpots on the circuit boards, one for
vertical & one for horizontal. Adjusting the horizontal trimpot does allow
the image to be put back in the center, however, that trimpot is nearly at
the end of it's range compared to the vertical trimpot.
My question: What component type is most likely to have changed/drifted in
this circuit? It has 4 transistors (2 bi-polar & 2 FETs), a couple of caps &
assorted resistors. I'm guessing that this circuit works by adjustment of a
DC voltage, so maybe it's a cap leaking DC? Or a resistor that's changed?
Any advice appreciated.


-jim

Archived from groups: rec.audio.tech (More info?)

In article <FcydnbG4JqBJCIHcRVn-rA@comcast.com>, jim <jimbo043@bozo.net> wrote:

>My question: What component type is most likely to have changed/drifted in
>this circuit? It has 4 transistors (2 bi-polar & 2 FETs), a couple of caps &
>assorted resistors. I'm guessing that this circuit works by adjustment of a
>DC voltage, so maybe it's a cap leaking DC? Or a resistor that's changed?
>Any advice appreciated.

I'd check all of the electrolytic capacitors for leakage or high ESR.

One of the commonest failure modes in the older Tektronix solid-state
scopes (e.g. 465 and 475) is the power supply filter capacitors. They
dry out, go high-ESR, and you end up with a massive amount of ripple
on the raw (pre-regulator) DC supply rails. The regulators can't cope
with the voltage variations, AC gets onto the "regulated" rails, and
the horizontal and vertical axes of the display go bonkers.

Your scope might be suffering from a somewhat-similar problem. Check
the pre- and post-regulator voltages, for both DC correctness and for
a suitable (low) level of ripple.


--
Dave Platt <dplatt@radagast.org> AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Archived from groups: rec.audio.tech (More info?)

Ceramics might change value - but a cap causing your problem could be in the
power supply (or not).

Maybe some needed voltage is just a bit low.

Mark Z.

--
Please reply only to Group. I regret this is necessary. Viruses and spam
have rendered my regular e-mail address useless.


"jim" <jimbo043@bozo.net> wrote in message
news:EeudnbEQX-jEqoDcRVn-jQ@comcast.com...
> Thanks Mark & Dave for your comments.
> But, if it was a power supply cap problem, wouldn't that affect both
> vertical AND horizontal position circuits?
> The vertical is position is completely stable, while the horizontal still
> drifts all over the place, even after I adjusted the trimpot for that
> circuit.
> I don't see any electrolytic caps in the HOR POS circuit, just ceramic
discs
> & a few mystery parts. Do ceramics change value over time also?
> Thanks all for your time.
>
>
> -jim
>
> To fight spam, remove the *** from my address if you reply.
>
> This email was scanned by Norton AntiVirus with the
> latest update before it was sent.
> "Dave Platt" <dplatt@radagast.org> wrote in message
> news:10hq3tj6nqd46ed@corp.supernews.com...
> > In article <FcydnbG4JqBJCIHcRVn-rA@comcast.com>, jim <jimbo043@bozo.net>
> wrote:
> >
> > >My question: What component type is most likely to have changed/drifted
> in
> > >this circuit? It has 4 transistors (2 bi-polar & 2 FETs), a couple of
> caps &
> > >assorted resistors. I'm guessing that this circuit works by adjustment
of
> a
> > >DC voltage, so maybe it's a cap leaking DC? Or a resistor that's
changed?
> > >Any advice appreciated.
> >
> > I'd check all of the electrolytic capacitors for leakage or high ESR.
> >
> > One of the commonest failure modes in the older Tektronix solid-state
> > scopes (e.g. 465 and 475) is the power supply filter capacitors. They
> > dry out, go high-ESR, and you end up with a massive amount of ripple
> > on the raw (pre-regulator) DC supply rails. The regulators can't cope
> > with the voltage variations, AC gets onto the "regulated" rails, and
> > the horizontal and vertical axes of the display go bonkers.
> >
> > Your scope might be suffering from a somewhat-similar problem. Check
> > the pre- and post-regulator voltages, for both DC correctness and for
> > a suitable (low) level of ripple.
> >
> >
> > --
> > Dave Platt <dplatt@radagast.org> AE6EO
> > Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
> > I do _not_ wish to receive unsolicited commercial email, and I will
> > boycott any company which has the gall to send me such ads!
>
>