Guitar Research AC60R

[nurx]

I fear it is not a broken amp. I checked the speakers with another amp and it was silent. All 3 of them! I do not understand how this could happen. I am too afraid to connect a speaker to the amp to test it. It might get blown too doesnt it?
Any suggestions? New speakers are quite expensive and i cannot be sure that there isnt a defect on the amp. Should i check the speaker-out for dc voltage?

[nickb]

The LM388T is probably blown.That often causes the output to be connected to one of the power rails and burns out any DC coupled speaker. The SP3 tweeter should have survived as it's AC coupled.
(1) Remove the LM3886 - cut the leads and carefully desolder one at a time is easiest.
(2) Check the fuse is the correct rating
(3) Check the power rails +B and -B, switch off and allow to caps to discharge - bleed the charge with a resistor and check with a voltmeter
(4) Replace the LM3886, power up without the speakers. Check for DC across the output terminals. If close to zero you are good to connect up a speaker and test.

[nickb]

BTW the schematic has at least one error: the bridge rectifier connections are wrong.

[J M Fahey]

Disconnect the speakers one by one and measure their terminal to terminal DC resistance.
We expect around 7 ohms on the 8" and 14 ohms on the Tweeter.

*If* they are open , similar ones are inexpensive.

[nurx]

You are a great help to me! I can literally feel my brain growing :-)

Does AC coupled mean that there is a cap to the speaker that filters low frequencys and dc?

I will be back here when i do my reparation try. Actually tomorrow the parts for a "tremulus lune" will arrive and that will be on pole position :-)

Well and thanks to everybody trying to understand my bad english. Guess thats not allways easy :-)

[nurx]

I just did that and saw that the tweeter is a 8Ω 0.5Watts but i think that doesnt matter?
The 8" are 8Ω too. i desoldered them and checked them with my multimeter on DC Resistance. The tweeter is at 0Ω. One 8" has ~46kΩ and the other one 0Ω. Means they are all broken doesnt it?

[nickb]

As the wise Mr Fahey said, expect ~7 ohm DC on the 8 ohm speakers and ~14 on the tweeter. You need a use a low ohm range if you have one. Now the tweeter is marked as 8 ohms, you say. The schematics state 16 ohms so I guess they made a change. It's of little consequence.

It sounds like one 8in speaker is blown (the high resistance one) and the others are OK.

Yes, the capacitor blocks low frequencies and what is DC but the lowest frequency of all!

[nurx]

Sorry, i missunderstood my multimeter. it doesent say 0Ω. Its says overload on the tweeter and one 8".

[nurx]

I think about replacing the speakers with two 8" 8 Ω Fullrange Speakers without the additional tweeter.
VIS BG 20-8 - VISATON Breitbandlautsprecher, 20cm bei reichelt elektronik

Do you think this will cause problems? I dont know what upper-end of the frequenzy range of the original 8" speakers is...

[nickb]

I meat to add, the existing tweeter may be of the piezo type. They don't have a voice coil and so you can't test with with a DC ohmmeter as they look like on open circuit. You have to use a real signal You could also try a small battery - you should hear something as to connect and disconnect it.

[nurx]

Well, i soldered in a lm3886tf (isolated) and had to replace all three speakers. It is working well again. I hope the lm3886 will work long because i read that the isolated one needs a larger cooling area...
Thanks for ll support

[cefetanto]

Hello, the post parade this awhile but if someone can help me, I have a blown amp, this transformer with the open primary, unca specification is T78-38S, which also has in its schematic. Could tell me what the output voltage (secondary) and its power?
Thanks.

[J M Fahey]

The LM3886 loaded with 4 ohms is happy with +/_ 28V DC rails, so you will need a 20+20VAC transformer, power/VA rating around 80 to 120 VA.
Some catalogs show transformer physical size, so it should be another guide to get a similar one.
Weight is also a guide.

[Emmettjett]

Does anyone have the schematic to Guitar Research (red single stack) Model T64RS? Sam Ash sold them however they were no help in getting info on this item.

[eman]

Thanks for the thread and schematic. I have a friends and the transformer primary is open. So the tranny is not spec'd on the schematic. I do see 7815 and 7915 regulator after which gives me a hint voltage wise and I assume from both looking at it and the fact it is a 60 watt amp that its 6-8 amps current. I'm sure I can't find the exact transformer but something close should do. Any help appreciated. The part number comes up empty A-78-38U. SS 031006 is also stamped on it. This is a 120V version.

Eman