Re: tuner repair problems

In article <6K1Sf.85$Zf3.19@clgrps12>,
Dave <dspear99ca@xxxxxxxxxxxxxxxx> wrote:

I made a conscious effort when replacing the caps to look up the rating on
the schematic, locate the new part, locate the old part, remove the old one,
hold it up next to the new one, read the info on each, then put the new one
in and chuck the old one into a can on my bench. When I finished I had used
up all 44 caps.

You would't be the person (by a lot!) to either misinterpret a decimal
point in a component value, or to encounter a schematic which has a
misplaced decimal due to an error in printing or production.

That's one reason I tend to favor the European notation - the actual
multiplier is used where the decimal point would be placed, and
preference is given to using notations with the digits to the left of
the decimal. In this notation, the part value you actually used would
be a 220n (220 nanoFarad, or .22 uF) while I'm suggesting the use of a
22u, or at least something no smaller than 4u7 or so.

So I try to match the impedence if I can, right?

Not match, really, but make-a-reasonable-relationship-with.

To a first approximation, what you'll be looking at is something
like a series voltage divider. The impedance of the coupling cap and
the preamp's input impedance are in series, and the preamp only gets
to "see" the portion of the voltage which gets past the cap.

You want to use a cap which is large enough, so that its series
impedance at the lowest frequency you're interested in (20 Hz is the
conventional number) is significantly less than the input impedance of
the preamp.

If we assume a tuner input impedance of 4.7k (a fairly conventional
value), and 20 Hz, we'd find that a cap whose impedance equals this at
20 Hz is

C = 1 / (2*pi*20*4700)

which works out to about 1.7 uF. Using a cap of this value would
result in a rolloff of around 6 dB at 20 Hz.

It's possible that the caps you pulled out were actually 1.5 uF, and
that the tuner's designed to have some rolloff in the deep bass.

I personally wouldn't use anything smaller than a 4.7 uF in this
application and would probably go larger.

Is there a
non-expensive-obscure-tool way to measure impedence of the preamp inputs and
tuner outputs and size the output coupling caps accordingly?

The first thing I'd do is look at the preamp manual. The input
impedance of the line-level inputs is often specified.

The second thing I'd do would be look at the schematic, or trace the
circuitry from the input jack towards the switching circuitry. The
usual input would have a resistor from input to ground to set the
input impedance.

If the preamp is of the "direct coupled" design, you could simply
measure the resistance at the "tuner" input with an ohmmeter (with the
power safely off, of course) to "see" the value of this resistor...
it'll probably measure with reasonable accuracy. If there's an
additional DC-blocking cap in the preamp prior to the input resistor,
this trick won't work.

You can probably rely on the rule-of-thumb that old-style
glassfet/firebottle (vacuum tube) audio components usually had input
impedances in the 100k-ohm range, while transistorized equipment is most
commonly in the 4k7 - 10k range. If you use 4k7 as a conservative
assumption you'll probably be OK.

Any ideas on the on-again, off-again digital display? You know it'll never
give me a problem while the test leads are hooked up.

I tend to look for cracked/fatigued/"cold" solder joints in cases like
this, especially on pins where any component having significant mass
is soldered to the board.

Simply re-heating all of the solder connections associated with the
display (ideally, using a bit of solder-wick and liquid flux to
remove the existing solder, then re-wetting the joint with
high-quality solder) may very well fix the problem.

Gently wiggling any cable connectors might also find the problem.

--
Dave Platt <dplatt@xxxxxxxxxxxx> 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!
.

Relevant Pages

  • Re: tuner repair problems
    ... the preamp's input impedance are in series, ... to "see" the portion of the voltage which gets past the cap. ... Simply re-heating all of the solder connections associated with the ...
    (rec.audio.tech)
  • Re: How to measure a dynamic mics output impedance?
    ... it might sound better wtih a preamp that has a 2500 ohm input ... impedance, or a 1200 ohm impedance, or you may even like what loading ... have input transformers and others don't. ... Transformerless mics connected to transformerless preamps tend ...
    (rec.audio.pro)
  • Re: Amp - loss of power?
    ... >> Ian Iveson wrote: ... >>> another possibility may be shorting input cap C12. ... >>> your preamp much good. ... > 50VDC on its output if C12 has been replaced with a resistor. ...
    (rec.audio.tubes)
  • Re: tuner repair problems
    ... The first thing I'd do is look at the preamp manual. ... impedance of the line-level inputs is often specified. ... The center post of my tuner input has a 560k resistor to ground. ... additional DC-blocking cap in the preamp prior to the input resistor, ...
    (rec.audio.tech)
  • Re: Alembic F-2b
    ... a QSC PLX2402 power amp. ... The F-2B is pretty much a Fender Showman preamp without the power ... output impedance to be around 40k to 60k. ... Unity gain just means 1-to-1, in case you haven't heard that before. ...
    (alt.guitar.amps)