On Thu, 4 Feb 2016 05:09:48 -0600, "Mark Zacharias"
wrote:

"Trevor Wilson" wrote in message
...
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

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus



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

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus


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.

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.

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus