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Recently rediscovered an old Guitar Research T64RS Valvestate head I purchased 10 years ago. I decided to take it apart and blow the dust out and clean it up. I plugged it in and discovered I have no power going to the clean or dirty channels (no leds would light up on them.). Also, to be sure I replaced the preamp tube, which wasn't the problem either. Led indicator for the power switch was on, so the fuse is good. I'm pretty certain the PT is blown. I'm a newb when it comes to transformers...I need to know what kind of transformer to search for to replace it. Here are some pics to help identify it.
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2 seemingly contradictory statementsI'm pretty certain the PT is blown. I'm a newb when it comes to transformers...
I'm not sure (yet) that your transformer is dead, did that amp work before getting stored?
Unless the mice played some Heavy Metal with it, there's no way to burn an unconnected/unpowered transformer.
Do you have some Tech friend to help you?Juan Manuel Fahey -
I highly doubt that the power transformer is bad in that you are getting the low voltage power on indicator LED to light.
The secondary winding is rectified to the + & - high voltage power rails & from there it is dropped further to the + & - 15 volt low power rails.Comment
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The amp didn't not work before I stored it. I sent an email asking the same thing I asked here to Mercury Magnetics, and I got this reply:
Hi William,
It does sound like the PT is shot.
You can test it unloaded and see if the secondary AC voltages are in spec or not. If they are, the transformers is okay and the problem is in circuit.
We really don't do anything for solid state and do not have a replacement.
We would have to get that one taken out and sent to us in order to tear it down an design an upgrade.
You would likely be looking at $125-$150
Let us know.
Thanks.
Patrick
*******************************************
Patrick Selfridge
SSN Manager
patrick@mercurymagnetics.com
Mercury Magnetics
(818) 998-7791x106
Mercury Magnetics
9167 Independence Ave.
Chatsworth, CA 91311
Also, just ordered a new dmm to check all this out. The old one was no longer functioning.Comment
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I'm going to guess that the amp is probably not worth a lot more than that, so you don't want to have a new PT custom wound
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Do what they suggested, measure it unloaded. There are 2 red and 1 black wire coming out of the PT secondary. Follow them to their connector and disconnect it.
Connect your DVM to the 2 red wires coming out of the PT. Set to AC volts and measure.Originally posted by Enzo
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Not at all. The amp brand new was $300...no chance I'm paying that much for any repair. Just want to see if I can get this thing working again. I'll check this one out and see where to go from there.
Let's say the PT is good to go. Still no current getting to the pre-amp (I've already determined pre-amp tube is still good), and no current getting to the leds on either the clean or dirty channel. Any suggestions on what to look at next?Comment
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In the picture it looks like the tube is lit up. If that is the case then the PT is working.
Look for 7815 and 7915 regulators. Measure DC voltage at each pin with your black probe connected to chassis. Post your results.Originally posted by Enzo
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I can't tell if that's the filament showing or a reflection in the glass. And it probably has a lighted ac switch, so that will light up regardless of the condition of the transformer.Originally posted by g-one View Post
First test would be a simple resistance test of the primary wiring. If you have an ohm meter pull out the plug from the wall and connect the meter leads across the ac plug and turn on the amp's power switch. Do you get a resistance reading?
If yes, then plug the amp in and test the voltage coming from the transformer's red secondary wires.
If no, then check the resistance directly across the black and white primary wires of the power transformer. Any reading there?
My guess would be that this amp runs the tube at very low voltages and really just needs a basic solid state power amp supply.Comment
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Supposing the transformer works, or you get a cheap and nice toroid such as AS-1220 - 100VA 20V Transformer - AnTek Products Corp ; what next?
What if the TDA7296 is blown?
Or some other defect?Juan Manuel FaheyComment
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Fuse is good...first thing I checked. The tube isn't glowing, that's just a reflection. The original tube looked fine, but I replaced it with a brand new tube and still nothing. I have a dmm coming in the mail later this week, so I'll be testing all of this out this weekend. I'm sure I'll be back for more questions. Just recently got back into fooling with electronics. It's amazing how much you forget over the course of a decade when you're out of practice. I really appreciate all the help this far!Comment
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"fooling with electronics"
Nice way to get hurt.
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^^^^^^^ What he said above! Brush up on your safety before anything else.
What kind of power light does the amp have? Is it an LED or is it built in to the power switch itself?Originally posted by Enzo
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Power light is in main switch. I've looked everywhere for the schematics but I haven't had any luck.Comment
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HARTKE GT60 AMP SCH Service Manual free download, schematics, eeprom, repair info for electronicsOriginally posted by Webbbilly View Post
Same dog with a different strap.
Parts ID might not match, but the schematic is basically the same.Last edited by J M Fahey; 11-12-2014, 10:44 AM.Juan Manuel FaheyComment
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