Digest for sci.electronics.repair@googlegroups.com - 11 updates in 5 topics

"pfjw@aol.com" <pfjw@aol.com>: Mar 03 08:01AM -0800

Here is the Schematic: http://komkris4000.webs.com/lk150%20schematic.gif
 
It is eating the 150uF @ 75V filter caps - I replaced the OEMs first with 75V, 105F caps, they lasted about 20 hours. Then 100V, they lasted about 30 hours. The manifestation is increased HUMMmm, and the caps start to swell.
 
I am contemplating using 180V caps, but before I do so, any suggestions from the assembled multitude?
 
NOTE: All other voltages are correct and clean. When these caps are new, the amp is hum-free. As they age, a pronounced *60* cycle hum begins that appears to be volume dependent - that is, the hum goes down as volume increases.
 
The hum also remains with shorted inputs, and whichever way it is plugged in. US, nominal 120V/60hz single-phase (actual is 117V at the wallplate).
 
Thanks in advance!
 
Peter Wieck
Melrose Park, PA
"jfeng@my-deja.com" <jfeng@my-deja.com>: Mar 03 08:31AM -0800

Is it possible that you are putting them in backwards?
dansabrservices@yahoo.com: Mar 03 08:40AM -0800

> Is it possible that you are putting them in backwards?
 
Check the diode, I would guess that there is AC leaking through and perhaps the caps are the only filter of AC with no rectification at all.
 
Dan
"Mark Zacharias" <mark_zacharias@sbcglobal.net>: Mar 03 06:08AM -0600

Sometimes you have to set aside a "dog" and get on to other stuff.
 
Sanyo JA-V14:
 
Late 80's rack-type integrated amp, electronic switching and volume. Uses
flat cables to the other components. Uncommon model - not much info on them
out there.
 
Standard blown channel. Replaced the outputs and a number of drivers,
resistors, etc.
 
Let us just say that access for service was really bad. I deal with units
all the time with no bottom access but even given that, this was much worse
than usual.
 
Had to work on it opened up clamshell style at a 90 degree angle working in
between the main board and bottom chassis.
 
It also didn't help that there was a shrink-wrapped .1uF at 500V bodged in
underneath and glued to the board right over the pre-drivers I needed to
replace.
 
It was original brown gooey stuff - not yellow glue turned bad. So anyway, I
get the parts replaced but the bias won't adjust right, the voltage across
the emitter resistors was all over the place. Occasionally the relay would
come on but mostly there was a 70 volt offset.
 
Had to purchase the PDF service manual online. The manual was poor quality
and the schematic drawn in a fairly unusual way, of course.
 
I beat my head into this thing but wasn't really getting any further.
Eventually I had to set it aside for more pressing business, including that
Yamaha M-80.
 
This was in late November and December.
 
A couple days ago, business was slow and I reluctantly got back into this
thing. I could only run it maybe 30 seconds at a time to take voltage
readings across
 
this emitter-base junction, across this resistor, etc. Changed out the
differential pair and the current source transistors - no good.
 
Finally decided I didn't think there was anything preventing the early
stages from working correctly - the transistors replaced, resistors checked
OK.
 
Transistors forward biased. The circuit should be balanced but was not.
 
Now, normally there should be about 2.2 volts across the bias transistor
E-C. There was, but overall it was 70 volts above ground. Something was
leaking from the positive rail to the bias transistor.
 
You may have guessed by now. It was the glue. Just a tiny portion of it
bridged across two adjacent foil runs - not corrosive but appears like ash.
Measured 80 ohms from the bias transistor to the positive rail, in the
megohm range on the other channel.
 
Cleaned the area and cleared the "accidental resistor". Problem resolved.
 
Resoldered the IC voltage regulators and put it back together.
 
I can't retire soon enough.
 
 
Mark Z.
Cursitor Doom <curd@notformail.com>: Mar 03 03:17PM

On Thu, 03 Mar 2016 06:08:18 -0600, Mark Zacharias wrote:
[...]
 
I saw something about this on someone's YT channel a few days ago with
respect to the old late 70s CB radios where the goo becomes conductive
over time and shorts parts of the PCB out. The bloke who was explaining
it was convinced the manufacturers did it on purpose. I'm not convinced
myself, but who knows for sure?
"pfjw@aol.com" <pfjw@aol.com>: Mar 03 07:50AM -0800

On Thursday, March 3, 2016 at 10:20:44 AM UTC-5, Cursitor Doom wrote:
> over time and shorts parts of the PCB out. The bloke who was explaining
> it was convinced the manufacturers did it on purpose. I'm not convinced
> myself, but who knows for sure?
 
This comes under the same sort of scrutiny as the foam speaker surrounds of the early 80s into the 90s, you know the ones that rotted about 45 seconds after the warranty expired and created an entirely new industry in the repair of these speakers.
 
How could so many manufacturers make the same mistake all at the same time? And most of them were the old-line Boston Sound makers - Advent, AR, EPI, KLH and more who should really have known better. My guess is that none of them considered the omnipresence of ozone around today, and so did not consider the effects of same. But it sure is coincidentally suspicious - and also the fallacy of leaping to conclusions.
 
Peter Wieck
Melrose Park, PA
"jfeng@my-deja.com" <jfeng@my-deja.com>: Mar 03 08:37AM -0800

> fallacy of leaping to conclusions.
 
> Peter Wieck
> Melrose Park, PA
The surrounds on my AR-2ax's from about 1967 are still good. They are made of rubberized cloth (probably silicone rubber). The acoustic suspension concept requires having a good seal, and foam would not work.
Jeff Liebermann <jeffl@cruzio.com>: Mar 03 07:44AM -0800

On Tue, 1 Mar 2016 11:44:44 -0800, dplatt@coop.radagast.org (Dave
Platt) wrote:
 
 
>According to the MSDS, MG's Rubber Renue is about 60-70% of a xylene
>mixture, 20-30% ethylbenzene, and 15-30% methyl salicylate ("oil of
>wintergreen"). The latter accounts for its distinctive odor.
 
I use some cleaners that add a nasal desensitizer to "control" the
odor. It's something similar "lemon fresh" or some "air freshener"
that magically eliminates odors. What it really does is temporarily
take your sense of smell out of action. Try a blast of the stuff, and
then bite into some food full of aromatic ingredients. The food will
taste like cardboard.
 
>I've found Rubber Renue to be pretty effective at removing the
>hardened varnish-like layer on rubber rollers, and restoring "grab" to
>rollers that are in reasonably decent shape.
 
The surface "varnish" is a mixture of mostly toner (powdered plastic),
clay, and phosphors. The clay is the shiny coating found on most
better papers. The phosphor give the paper the bright white color.
You can see the phosphor with a UV flashlight. It's much like coating
the rubber roller with slippery polished plastic. Too bad that most
rubber rollers are not lighter color, or you would see the surface
crud on the rollers. I've often been tempted to add some powdered
phosphors to the toner cartridge so I can see with a UV lamp where the
stuff lands.
 
>rubber may be too far gone and may not "renew" properly... I don't
>think Rubber Renue can soften up a whole roller that has hardened up
>with age.
 
Oxidation, usually caused by ozone, causes surface cracking.
<https://en.wikipedia.org/wiki/Ozone_cracking>
Early laser printers had rather high voltage corona wires which would
produce prodigious amounts of ozone and do an impressive job of
destroying rubber parts. HPII and III printers were quite good at
producing rubber rollers with surface cracking. Some of the clone
roller vendors hid the problem by pre-cracking the surface of their
rollers. Todays selenium drums use a lower voltage roller instead of
a corona wire to charge the drum, which produces no ozone. However,
the laser beam is also a source of ozone. It zaps oxygen (O2)
molecules along its path to produce ozone (O3). LED printers
eliminate the laser, so no ozone.
 
>significantly toxic... best to use it outdoors, and wear good
>chemical-resistant gloves (not rubber for tolerably obvious reasons;
>nitrile looks like a better choice).
 
Nitrile (Nitrile Butadiene Rubber) is rubber.
<http://www.aps.anl.gov/Safety_and_Training/User_Safety/gloveselection.html>
<http://www.customadvanced.com/chemical-resistance-chart.html?chemical=Xylene&rubber=NBR>
<http://www.customadvanced.com/chemical-resistance-chart.html?chemical=Acetone&rubber=NBR>
Note that Nitrile is attacked by both acetone and xylene but is
slightly better than Latex (natural rubber) for xylene. The major
benefit is that Nitrile causes fewer allergic reactions. What you
want for solvent resistance is Viton or something with a silver foil
lining, such as Norfoil (used for Hazmat service) at $10/pair:
<https://www.b4brands.com/blog/latex-vs-nitrile-vs-vinyl-gloves-which-to-choose/>
 
 
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Jeff Liebermann <jeffl@cruzio.com>: Mar 03 06:57AM -0800

On Tue, 1 Mar 2016 15:26:28 -0800, "David Farber"
 
>I checked out the delay pulse time on pin 8. It seemed normal although it
>was hard to tell because a tenth of a second goes by rather quickly.
 
0.1sec is a long time with an oscilloscope. Beg, buy, or borrow one.
It helps. However, the power good line seems to be working if you
didn't see any inordinately long delays. That leaves noise, which
you're not going to see without an oscilloscope. As I previously
suggested, use a sound card PC software oscilloscope. (No, you cannot
run the PC scope software on the machine that you're trying to test).
 
>Instant off on the power button press and then a restart on the first try. I
>wonder if the motherboard is keeping track when you get a bad shutdown and
>intentionally makes you press the power button twice.
 
I don't know about hitting the button twice, but many BIOS's will do a
rather fast boot if you reboot, but take forever to test everything if
you reboot after a failed boot. I guess(tm) the previous boot status
is stored somewhere as long as AC power is applied. It's NOT in the
power supply.
 
Incidentally, I recently fixed a Dell Optiplex 760 SFF machine that
required pressing the power on/off button twice in order to get it to
do anything. I replaced the power supply for other reasons (bulging
caps) but that wasn't the cause. After some fumbling, it turned out
to be bad contacts in the switch. However, that doesn't sound like
anything that might produce the symptoms you're seeing.
 
When in doubt, replace everything.
 
 
 
 
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Cydrome Leader <presence@MUNGEpanix.com>: Mar 02 11:13PM

> Tires are easy. Remove old idler, machine small groove in OD. Fit with O-ring of correct diameter & circumference. Replace idler. Done. Usually no need to glue, either. Flat idler get flat rings if necessary, or soft hemispherical rings. Note that perfect roundness is also achieved by this process.
 
I call BS on this.
 
Let me know the easy steps to replace the rubber on this part with an
O-ring from the hardware store.
 
http://www.smcelectronics.com/VCRIW12.JPG. Pretty common part back in the
day.
 
I find it unlikely anything MCM has in stock hasn't already turned brown
next to the rubber.
 
> Repeat: belts need only fit reliably. They do not affect rpm. They do not have to be a exact match. A nitrile toothed belt inside out will do fine.
 
> Graf, Breco, Parker (Euro, UK, US) all make 0.5mm belts in many materials, smooth, ribbed, toothed, fiber-reinforced, or not. Let your fingers do the walking. McMaster-Carr is a stocking distributor for many makers, and sell both flat stock and ready made belts of many types. The world is a big place, and much of it is at your fingertips.
 
If you have the parker EZ reference wall poster cross reference for VCR or
turntable belts be sure to share.
 
Even VCR makers started to drop all the belts towards the end. You started
to see lots of gears instead.
"pfjw@aol.com" <pfjw@aol.com>: Mar 03 05:24AM -0800

On Wednesday, March 2, 2016 at 6:13:48 PM UTC-5, Cydrome Leader wrote:
> O-ring from the hardware store.
 
> http://www.smcelectronics.com/VCRIW12.JPG. Pretty common part back in the
> day.

YIKES!!
 
That one is easy. Remove the rubber altogether and replace it. If the OEM rubber is very old, it should crumble off pretty easily. Otherwise, removal with a solvent that does not attack the white plastic should be a pretty basic technique - acetone comes to mind not knowing exactly what that white plastic might be, but likely nylon or delrin.
 
Then, replace it with a flat O-ring out of a catalog, McMaster-Carr and/or any of a dozen other sources. Don't forget to re-lubricate the bearing...
 
http://mykin.com/orings.html/?_vsrefdom=adwords&gclid=Cj0KEQiA3t-2BRCKivi-suDY24gBEiQAX1wiXL6bP7URTGduRmp-bkRG0P7Jbk0aSWqRfp7tdP2oR4EaAi8q8P8HAQ That being just one. You will have your choice of materials, some of them with far more longevity than the OEM part.
 
My lathe technique is pointed towards the Oilite Bearing type idlers very common back in the day - such as on very vintage Dual turntables and more. These tend to be quite thin, so a bit of machining will usually allow a solid new edge for the applied O-ring "spare tire".
 
My hobby activities include electronics back as far as 1919, and as recent as the 1990s (but few of those). I tend to avoid vintage moving parts as there are so many things that go wrong after 80 or more years - pot-metal decay, rust, and such - that cannot be fixed. But I have never failed to fix something worthwhile albeit over some considerable time in some cases. Right now, a bit back in the queue is an Edison Home 2-minute machine that needs the drive spring redone. I have a source for a new spring, but I will first try to salvage the existing spring if possible. If the break is at the 'hook' as is typical, I will anneal the end, cut and bend a new hook, then re-harden the end - and cross my fingers.
 
Look for solutions, not impediments.
 
Peter Wieck
Melrose Park, PA
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