Re: stereo amp cleaning
- From: g <zekor@xxxxxxxxxxx>
- Date: Fri, 01 Jun 2007 09:28:35 -0700
On May 3, 11:12 pm, "Mr. Land" <grafton...@xxxxxxxxx> wrote:
On May 2, 6:39 pm, "Dave" <dspear9...@xxxxxxxxxxxxxxxx> wrote:
<p...@xxxxxxx> wrote in message
news:1178141401.764048.27780@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On May 1, 12:42 pm, "Dave" <dspear9...@xxxxxxxxxxxxxxxx> wrote:
a)cleaningpots and switches is only the first step. You need to
provide some (slight) amount of lubrication afterwards. 100% volatile
cleaners can cause serious friction after the skunge is removed
rebuilding additional skunge almost instantly. The idea is to exercise
the pot only as long as the cleaner is wet, then stop, thenre-
saturate to rinse out the residue. *AFTER* that, a wee spritz of
lubricating cleaner makes all go smoothly.
Ire-cleaned with a cleaner with lubricant last night.
b) switches are even 'more so' as wear can happen in any of several
ways including breaking of tabs or contact springs, and additional
friction can cause failure. So, same process. Saturate, exercise for a
few cycles whilst saturated, rinse, lubricate.
Did notre-do switches. I will though.
c) Look for broken traces on the boards, look (still) for bad caps. Do
you test the new caps *before* you install them? I have found about
0.5% of new electrolytic caps are bad or sufficiently out of spec to
be problematic. "Badness" typically manifests as intermittents... and
your delay may be related. Or, one you did not replace for whatever
reason...
I don't have an ESR meter. Is testing plain ol' capacitance any help? I'd
like to think it's a capacitor given that that's all I changed out.
d) you may believe that the power-ampsection is clean, but one thing
I have experienced that is quite peculiar but as I have seen it
twice, it cannot be all that rare... an input transistor on the power-
ampside has become sufficiently different from its other-channel mate
that similar-volume signals to it are quite different in output
volume. RARE, and the only way you will catch this is to remove the
questionable transistors and put them either on a scope or tester that
can determine their actual response.
This was an overnight change, so unless the change was mechanically induced
(definitely not ruling that out) I wouldn't be inclined to look there
justyet.
The more I think about it the less likely a pre-ampproblem seems... the
signal goes through primary amplification (a few transisitors) then through
tone controls, subsonic filter, high cut filter, balance control --> power
amp. I've got a switch to bypass the tone control, and switches to bypass
the filters, plus a switch to reverse the channels between pre-ampandamp.
None of these affect theampoutput. If I've got a bad cap in theamp
stages and am getting a DC offset at my outputs, that MAY be engaging the
speaker protection. Or, I suppose, worst case scenario I've baked one of my
giant expensive output transistors. I am assuming the speaker protection
(being two leaded) is a polyswitch. The distorted sound for a second or so
prior to speaker cut-out might match a polyswitch tripping, I don't know as
I've only ever had amps with relay protection (or none) in the past. Do you
know offhand if polyswitches tend to fail? I suppose probably as much as
any other component exposed to variable voltage and current.
I'm going to bypass the preamp section. If I can confirm that the problem
is in theamp, I could a) check DC voltages at all transistors until I find
one that's off or b) start swapping electrolytic capacitors from channel to
channel. there are only four per channel in theampsection.
e) lastly and I have seen this only once and it was A BEAR to find. A
perfectly fine looking 1/2 watt resistor in the signal path had
drifted to 3X its nominal value. How did I find it? Well, every other #
$%^&*( part checked out, so I started measuring resitance "cold"
across both channels and comparing the results component by component.
When I found a difference, I started lifting legs about that point.
And there it was.
Yes I've had several of those; they can try your patience.
Good luck with it.
Thanks, I may just need it.
Peter Wieck
Wyncote, PA
There used to be a spray on the market - can't remember who made it at
the time, might have been GC - it was called "Blue Stuff". I worked
in astereoservice department thru college, and that spray was the
best we could find forcleaningstubbornly noisy pots and switches.
When you sprayed it on it formed a mildly abrasive blue paste, which
would actually scrub the tarnish and dirt off of switch contacts. I
never saw it fail, even with the noisiest, most tarnished switches and
controls. I think it has diatomaceous earth in it.
Anyway, I think TechSpray carries it - might be worth a try
(www.techspray.com)
I just investigated this stuff, in fact I think I ordered some and
have not yet received it. Its been discontinued I think. Yes, I
ordered it two months ago, and it said they had the quanity i ordered.
Glad you brought it up. A recording studio asked me about this stuff,
and they said they depended on it. I know of no other cleaner other
than Cramolin Contaclean, that actually has something in it to
dissolve oxides.
greg
.
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