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TEAC AG-790 (not A) somewhat burned

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

Reply to
William R. Walsh
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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 .............

Reply to
Wild_Bill

Hi!

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

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

Reply to
William R. Walsh

On 6/9/2010 10:59 AM William R. Walsh spake thus:

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)
Reply to
David Nebenzahl

Hi!

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

Reply to
William R. Walsh

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

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)
Reply to
David Nebenzahl

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 .............

Reply to
Wild_Bill

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Reply to
Traci Steele

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Shows a free service manual for this model.

I get lots of free manuals from this site.

You have to look closely for the D/L link, it says "click here" but NOT highlighted near the bottom of the new window that pops up once you click on the model number. Also multi part manuals are tricky to figure the procedure for downloading but it does work. Multipart manuals show as 0 1 2 3 etc. You click on one, maybe click on it a second time if you use Mozilla - you'll figure it out.

Mark Z.

Reply to
Mark Zacharias

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 mark snipped-for-privacy@labolgcbs.net

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.

Reply to
Mark Zacharias

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

mz

Reply to
Mark Zacharias

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

Reply to
William R. Walsh

Hi!

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.

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

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

Reply to
William R. Walsh

Hi!

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

Reply to
William R. Walsh

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