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

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" wrote in message
...
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

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

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)

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

On 6/9/2010 1:13 PM William R. Walsh spake thus:

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. ;-)

No dig, no snarkiness. I'm always amazed when Franc seems to pull free
manuals for the most obscure pieces of equipment out of the ether,
seemingly. And no shame in using them.


--
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)

There are several approaches for replacing thru-hole components in areas
where damage has occurred to the PCB (lifted/missing solder pads, etc).
Often, the long length of new replacement components can be utilized by
poking the leads thru the (component side of board) original holes and
laying the component leads along the stripped/cleaned/tinned copper trace
(solder side of board), and soldered in place.
This method is usually as secure as the original mounting.

If there were severely charred damage at the original thru-holes, a Dremel
or similar small tool could be used to drill thru-holes in undamaged areas,
and make suitable adjustments.

The variety of existing mounting conditions is nearly infinite, in that
there could always be other factors involved in different types of board
assemblies.. nearby components, hardware, heatsinks, obstructions etc.

I'm sure there have probably been millions of repairs made, where
replacement component leads have simply been soldered to associated circuit
points, but the primary factor should always be safety.

Years ago, it was a fairly common practice/shortcut (but somewhat sloppy in
some cases) to clip the leads of a failed axial lead component above the
board surface, and then form small eyelets on the leads of the replacement
component, then solder the eyelets to the stubs of the old component.
This method could result in significant time reduction involved to complete
a repair, where a lot of disassembly would be required to access the solder
side of a board.

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


"William R. Walsh" wrote in message
...
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.

William R. Walsh wrote:
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

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"William R. Walsh" wrote in message
...
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


Actually, just downloaded it, looks good and it's in a single 1 meg file. I
will try to email it to you.

If necessary email me at

and reverse the domain to read "sbcglobal" and notice the underscore _
between the first and last name.

I can reply to the email and attach the manual.

Mark Z.

"Mark Zacharias" wrote in message
...
"William R. Walsh" wrote in message
...
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


Actually, just downloaded it, looks good and it's in a single 1 meg file.
I will try to email it to you.

If necessary email me at

and reverse the domain to read "sbcglobal" and notice the underscore _
between the first and last name.

I can reply to the email and attach the manual.

Mark Z.


Yeah - I guess it bounced, so email me and I can send it to you.

mz

Hi!

I'll send you an e-mail. The address from which these messages come is
no longer valid (although I did think messages would still go there,
the biggest problem is that I forgot the password a few years ago).

William

Hi!

Often, the long length of new replacement components can be
utilized by poking the leads thru the (component side of board)
original holes and laying the component leads along the
stripped/cleaned/tinned copper trace (solder side of board), and
soldered in place.

That is exactly what I'm planning to do. I was concerned about
components coming loose due to thermal cycling. That concern actually
comes from being told that simply soldering broken circuit boards
together again won't work, jumper wires must be used. I've never
experimented, because I want to do only safe and effective repairs.

This method is usually as secure as the original mounting.

That's exactly what I wanted to be sure of.

If there were severely charred damage at the original thru-holes,
a Dremel or similar small tool could be used to drill thru-holes
in undamaged areas, and make suitable adjustments.

The holes are damaged, but they're not so badly damaged that I cannot
use them. They looked better after I scrubbed the board.

As it is, the previous owners don't want the set back. They haven't
been really upfront about what went wrong, either. I'm guessing that
someone got a little too carried away with the volume control and
pushed things just a little too far. There are speaker fuses in this
receiver, something I do not believe the previous owners were aware
of. Neither the speaker or power fuses are blown.

It does seem like this receiver has more of a power supply than its
amplification section would ever be able to use. The power transformer
alone looks like it should be at home in something with a much larger
amplifier, and by itself, weighs ~16 pounds.

William

Hi!

Yeah - I guess it bounced, so email me and I can send it to you.

Okay, I popped over to the eServiceinfo site and found the manual
after a quick search. Guess I should have checked into that first...

Looking over it now...I'm due to be near a RadioShack store tonight,
so I may drop in and see if they have suitable replacement parts. (I
know they're not necessarily the cheapest/best, but they'll do.)

William