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From: William R. Walsh on 9 Jun 2010 09:40
Hello all...

A TEAC AG-790 stereo receiver has appeared on my bench. I'm not sure
what happened to it. Resistors at positions 7R41, 7R47 (and a
seemingly unmarked neighbor--can see an outline for it, but no number)
and 7R48 have all been so hot that they've badly damaged the
underlying circuit board. Date codes put the time of manufacture
sometime around 2005.

The resistors themselves still have their blue body color. The color
bands are gone and I don't know what the replacements ought to be. No
other parts are visibly distressed or blown, including the final
transistors (which also seem to check out electrically).

Just for grins, I plugged it in and tried it out. Amazingly, it does
play but only on the right channel for loudspeakers and headphones
alike. I'd have expected it to shut down, blow up or go into
protection.

The circuit board has been burned badly enough that I think just
replacing the damaged parts will be difficult. What can be done in
this case for a reliable repair?

William
From: Wild_Bill on 9 Jun 2010 13:42
If the leads of the new/uncut replacement resistors aren't long enough to
bridge across the burned area, you may need to implement a small custom
circuit board to hold the resistor(s).
I believe trimming away the scorched/burned material is a good practice.

Because of the failure mode, be absolutely sure to use a known good quality,
flameproof circuit board material. One of the common PCB specs is 94V (I
think) related to flame resistance.

I've made small boards and installed them as riser boards where space
permitted, attached securely to the main board so the component was properly
supported.

--
Cheers,
WB
..............


"William R. Walsh" <wm_walsh(a)hotmail.com> wrote in message
news:8ce364d3-ae59-4781-842e-296161bf97f8(a)u7g2000yqm.googlegroups.com...
> Hello all...
>
> A TEAC AG-790 stereo receiver has appeared on my bench. I'm not sure
> what happened to it. Resistors at positions 7R41, 7R47 (and a
> seemingly unmarked neighbor--can see an outline for it, but no number)
> and 7R48 have all been so hot that they've badly damaged the
> underlying circuit board. Date codes put the time of manufacture
> sometime around 2005.
>
> The resistors themselves still have their blue body color. The color
> bands are gone and I don't know what the replacements ought to be. No
> other parts are visibly distressed or blown, including the final
> transistors (which also seem to check out electrically).
>
> Just for grins, I plugged it in and tried it out. Amazingly, it does
> play but only on the right channel for loudspeakers and headphones
> alike. I'd have expected it to shut down, blow up or go into
> protection.
>
> The circuit board has been burned badly enough that I think just
> replacing the damaged parts will be difficult. What can be done in
> this case for a reliable repair?
>
> William

From: William R. Walsh on 9 Jun 2010 13:59
Hi!

> If the leads of the new/uncut replacement resistors aren't long
> enough to bridge across the burned area, you may need to
> implement a small custom circuit board to hold the resistor(s).

So...

If it were possible (meaning that the leads will reach), does that
mean that scraping off the solder mask from the other side of the
board and installing the new resistors there would be acceptable?

I can do that, but I am concerned about longevity...solder joints not
cracking, that sort of thing.

It's too bad this couldn't have been something easy, like a broken
board. I've done that a few times now with excellent and durable
results.

> I believe trimming away the scorched/burned material is a good
> practice.

I cleaned off most of the burn damage on the solder side by scrubbing
with simple soap and water. What was there was mostly soot.

> One of the common PCB specs is 94V (I think) related to flame
> resistance.

I've seen that written (as "94V-0") on circuit boards and looked it
up. As best I remember, that was a fire resistance indication.

I've never tried to make a PCB or even a project board before. I have
some idea of what's involved in both...

TEAC America does make available a service manual for this receiver
and it should have the resistor values I need. While it's not too
unreasonably priced ($14+S&H), their online ordering is incredibly
hokey. They really do expect you to respond to a "price quote" e-mail
and submit all of your details--including credit card number!--as a
reply.

I'm not so worried about fraud, being as I've had to do that before
(with VMware back in the day)...but in this day and age of easily
implemented online stores? Heh...what year is it at TEAC?

William
From: David Nebenzahl on 9 Jun 2010 14:09
On 6/9/2010 10:59 AM William R. Walsh spake thus:

> TEAC America does make available a service manual for this receiver
> and it should have the resistor values I need. While it's not too
> unreasonably priced ($14+S&H), their online ordering is incredibly
> hokey. They really do expect you to respond to a "price quote" e-mail
> and submit all of your details--including credit card number!--as a
> reply.

Maybe you should just wait until someone like Franc locates a free
online manual.


--
The fashion in killing has an insouciant, flirty style this spring,
with the flaunting of well-defined muscle, wrapped in flags.

- Comment from an article on Antiwar.com (http://antiwar.com)
From: William R. Walsh on 9 Jun 2010 16:13
Hi!

> Maybe you should just wait until someone like Franc locates a
> free online manual.

I'm not sure if that's a "dig" implying laziness or a suggestion, but
I'm taking the high road on this one. ;-)

I did do some searching for a downloable service manual but I never
found one that was free. I _could_ call TEAC America parts and
service, but I'm not often near a phone and it's not my preferred way
of doing things.

William
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