Consumer electronics "war stories"

On Thu, 4 Feb 2016 05:09:48 -0600, "Mark Zacharias"
<mark_zacharias@sbcglobal.net> wrote:

"Trevor Wilson" <trevor@SPAMBLOCKrageaudio.com.au> wrote in message
news:dhf4nvF7fagU1@mid.individual.net...
On 3/02/2016 11:31 PM, Mark Zacharias wrote:
Any more recent successs stories to brag about?

C'mon, don't we all enjoy patting ourselves on the back, really?


Mark Z.

**I recall the first time (1980-ish) I discovered those fusible
resistors that go high after a few years. With no obvious signs of
distress. Now I just head straight for the buggers.

Then there's those low value (</=47 Ohms), 1/4W cracked carbon
resistors that go O/C when subjected to ca. 60+ Volts with no signs of
burning (Marantz 1200b, 240, 250M, 500 models). Over the years, I
learned to suspect any resistor over the value of 100k, if the circuit
is displaying some kind of mysterious fault that cannot be explained
by a semiconductor failure or cap leakage.

--
Trevor Wilson
www.rageaudio.com.au

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A brand new "war story".

As I am nearing the end of my career, I wonder if any one unit will be
the high water mark so far as feeling the satisfaction of fixing a
really tough one.

This may be it.

A Yamaha M-80 power amplifier. I've worked on a few in the past -
difficult but doable.

I've always thought that "M-80" was a humorously ironic model number for
an amplifier so flammable.

Initial inspection:

Burned resistors on both channels, a vented 1000uF 100v cap (one of a
pair) on the main board, lots of brown glue, some green corrosion
visible on component leads.

Strangely - only one output transistor was bad. I knew this was going to
be a tough one but I figured I could do it - just give the customer a
pretty high estimate.

Replaced those larger caps, lots of bad drivers, pre-drivers, signal
transistors, several burned and corroded resistors, one bias transistor.

Replaced the one bad output and it's mate. I figured the same current
ripped through both, so I wanted at least do that.

I knew the speaker relays would need service, so I took them out of
order and did that job.

Bringing up on a variac, the fires were out, bias adjusted OK, but no
sound.
Another bad resistor.


Replaced this, but now there was a -86 volt(!) offset. Couple more bad
resistors.

Each time a component replaced it was necessary to monitor bias when
bringing it up.

Bring it up again, no offset but one channel oscillates. Fine. Trace
down and replace the bias transistor on that channel that was breaking
down. Replace it and: one channel low in gain, approximately 1/2 the
other channel.

Replace a bad 3.9K 1/2 watt resistor in the feedback loop. That was
easy. NOPE.

Now BOTH channels oscillate like crazy. Apparently a larger 3.9K 2W
resistor was corroded and got nudged while replacing the other. Replaced
that. No more oscillation. NOW:

Still no change on the gain problem.

Bad 430 ohm resistor hiding UNDER a power resistor, and not even visible
until the other was removed.

Unit now repaired and functioning properly.

This thing took approximately five whole days worth of bench time.

I'm going to spend a very generous amount of time patting myself on the
back for this one.


**All for an amp worth, what(?), $300.00? It can be a bugger of a
business. How much can you charge your client? $200.00?

On the flip side, I did a couple of really ancient (ca. 1972) Accuphase
amps recently. The client was willing to spend around $700.00 on each. He
bought them for a song and they typically sell for a couple of Grand on
eBay.


--
Trevor Wilson
www.rageaudio.com.au

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After an initial assessment, I gave the customer an approximately 300.00
estimate.

Midway through the process I advised him it would be more.

Billed out at 400.00.

He hasn't picked it up yet, but it's part of a set with the preamp, tuner,
cd, and cassette so I'm not too worried.

Yeah - these days you take what you can get.

I was pretty confident I could fix it, but one problem kept hiding behind
the last one, and oscillation problems kinda turn my knees to jelly.


Mark Z.
Click to expand...

Mark,

These were the worst! I've seen 20 or more transistors blown on these
amps. I admire your tenacity. The Accuphases were so much easier to
repair.

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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?
 
On Thursday, March 3, 2016 at 10:20:44 AM UTC-5, Cursitor Doom wrote:
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?
Click to expand...

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
 
On Thursday, March 3, 2016 at 7:50:40 AM UTC-8, pf...@aol.com wrote:
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
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.
Click to expand...
 
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.
 
On Thu, 3 Mar 2016 15:17:37 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

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?
Click to expand...

It was rumored that the glue was made by Sony and that they sold it to
other manufacturers but didn't use it themselves until many years
later. T

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On 04/03/2016 02:17, Cursitor Doom wrote:
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?
Click to expand...

I think it might be used much more recently too. I have a DVB-T set top
box that used to work fine but started locking up and generally
crashing. The PCB had some brown glue that got dabbed in various places
including around a RAM chip and onto some 0402 components. THe glue had
started to discolour and it looked like it was corriding the components.
I picked off the glue and cleaned it a bit with solvent and I think I
replaced one decoupling capacitor and now it works fine, no crashing.
https://www.flickr.com/photos/37778900@N06/25484493195/
 
On 3/3/2016 4:41 PM, Chuck wrote:
On Thu, 3 Mar 2016 15:17:37 -0000 (UTC), Cursitor Doom
curd@notformail.com> wrote:

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?

It was rumored that the glue was made by Sony and that they sold it to
other manufacturers but didn't use it themselves until many years
later. T
Click to expand...

I don't know who made amn sold it, but Sony products also had the
glue (resistor) problem.

Mikek
 
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.
Click to expand...

The Advents were acoustic suspension. When the surround was good the piston
motion of the woofer was damped , apparently enough to do the job.

mz
 
On Thu, 03 Mar 2016 16:41:53 -0600, Chuck wrote:

It was rumored that the glue was made by Sony and that they sold it to
other manufacturers but didn't use it themselves until many years
later. T
Click to expand...

Sony, yes it was them; I remember now. The goo was suspicious as it was
commonly used on high impedance areas of the boards of multiple
manufacturers where there was no ostensible need for it.
 
pf...@aol.com wrote:
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.
Click to expand...

** You can add Bose and JBL to that list.

First time I saw crumbling surrounds was on a pair of JBL LE8Ts sold in 1970 - lasted less than 5 years. The local ( Australian) JBL agents replaced the cones at no cost when the owner complained bitterly about it.

Interestingly, the use of foam surrounds was taken up by nearly all makers in the USA but few elsewhere. In the Europe and the UK, rubber roll surrounds were the norm: eg KEF, Celestion, Philips, Seas and Wharfedale woofers.

Neoprene rubber has a indefinite life and many such woofers from the 70s are still working today.


..... Phil
 
Circa 1987

Harman Kardon HK-400XM:

Three-head cassette deck.

Intermittent failure to record on one channel. Playback fine.

Customer sent to HK twice for factory service.

Twice HK replaced the record / play head assembly. Problem persisted.

On the bench, at failure mode, discovered full peak-to-peak bias at the
output of the affected channel, swamping out the audio.

Defective (intermittent coil) in bias trap. IIRC a small module.

That customer still looks me up 30 years later when he has a problem.

Couple years later, same problem on a Kenwood KX-1030. Open coil.

Over 30 years - never seen this since.


Mark Z.
 
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