Guitar Research AC60R

[nickb]

Could some kind soul share the schematic /SM if you have it please?

Thanks!
Nick

[Enzo]

I don't know the brand, but it seems that Sam Ash is selling them, so you might contact them. Or perhaps Samson. (Same family)

[nickb]

Thanks Enzo. I was aware of the Sam Ash connection. It's fixed now. The difficulty with this POS is that they removed the identification from the power amp chip. I wonder what would happen if every customer were to ask "Does it include a service manual?"

[Enzo]

The stock answer from any company would be, "No, there are no user serviceable parts inside. Refer all repairs to qualitifed technicians."

[J M Fahey]

Just curious, which was the chip used?

[nickb]

It looked like an LM3886T but I never did find out for sure. The problem was elsewhere.

[J M Fahey]

OK, thanks.

[nurx]

Hi,
i am am electronics newby. I also have an ac60r. here it is sold under different brands. It seems to have an amplifier problem because everything, inkluding the notch fiter works fine until line out. But the speakers stays absolutely silent. All caps look well. i checked all solder spots visually, eveything looks great.
I do remeber, that first it was running normal and then, after a few minutes on, the volume turned lower and lower until it was silent.
Any suggestions? Did you find a service manual?

The only possible problem i can see are two l7915cv. They seem to run hot because they are little brownish? Could that be the problem?

[nickb]

Hi nurx,

No, I never got the sm or schematic. Sam Ash were not very helpful. I don't have it in front of me to look at as the customer has it back now so I can't be much help to you. If you are getting signals of the line out I would say the 7915's are probably OK since they likely power the preamp stages.

You can check the voltage on each 7915 pin, one will be 0V, one should be -15v and the other should be more negative that -18V.

You could check the main power rails by probing the pins of the power amp chips (black epoxy package with 11 leads on the big heatsink).

Check the speaker and wiring. If those are OK I guess at the power chip being blown, whatever it is. You'll have to do a bit of reverse engineering to make an intelligent guess if yours also has the part number rubbed off.

Edit:typos!!

[nurx]

Hi nurx,

No, I never got the sm or schematic. Sam Ash were not very helpful. I don't have it in front of me to look at as the customer has it back now so I can't be much help to you. If you are getting signals of the line out I would say the 7915's are probably OK since they likely power the preamp stages.

You can check the voltage on each 7915 pin, one will be 0V, one should be -15v and the other should be more negative that -18V.

You could check the main power rails by probing the pins of the power amp chips (black epoxy package with 11 leads on the big heatsink).

Check the speaker and wiring. If those are OK I guess at the power chip being blown, whatever it is. You'll have to do a bit of reverse engineering to make an intelligent guess if yours also has the part number rubbed off.

Edit:typos!!

Thanks allot till here! Thats exactly the help a newby needs ! :-) Ill check that 7915 tomorrow...

What do you mean with main power rails? check if the ~30V reach the power chip?
The number is rubbed off here too :-( Since the LM3886T is at about 5$ here. i will try to get one and replace it for a try. it really looks similar on the pictures i saw on the net. There is also a LM3886T*F*. Which is called to be "isolated" - does it matter whcih one choose?

nurx

[nickb]

I would expect the main power rails to be around + and - 30V.

It would be VERY unwise to put an LM3886 in without checking to see what the part actually is. That was just a wild guess on my part. You need to figure out enough of the circuit to be sure about it. Start with the power rails and the speaker out pins - so they match the data sheet? If you use the wrong part very, very bad things will happen. http://www.ti.com/lit/ds/symlink/lm3886.pdf. You should also unplug the speaker leads to prevent DC burning out your voice coil just in case things go wrong.

If there is any doubt choose the isolated part - it has no electrical connection to the heatsink.

[nurx]

I would expect the main power rails to be around + and - 30V.

It would be VERY unwise to put an LM3886 in without checking to see what the part actually is. That was just a wild guess on my part.

Your guess was right. i have the schematics! Never thought they would give it to me!
nurx

[nickb]

Could you please post the schematics? Thanks.

[nurx]

Just pass me you email by pn. I will send it to you.

[J M Fahey]

Post it here (answer > go advanced > Attachments > manage attachments > upload) so others with the same problem can repair theirs.
Thanks.