745 PA Mystery [message #29326] |
Thu, 29 August 2024 20:27  |
rainy_daze
Messages: 2 Registered: January 2016 Location: Chesterfield, MO
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Junior Member |
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I recently picked up a black 745 PA. It's an early "Frankenstein" style head from 1966, with reverb on one channel. It powers up but has no output.
Here is what I've done so far:
- replaced both fuses
- cleaned all pots and jacks
- checked for any loose wires or components
- replaced the original RCA 36892 output transistors. I have four MJ15022G on one side and a pair of 2N3055 on the other side behind the power amp board.
- replaced all capacitors on the preamp, reverb, and power amp boards
After all that, there is no change. I am still not getting any sound from the amp besides a slight buzz, which doesn't get any louder with the volume knob on either channel. I have tried multiple speaker cables and guitar cables. I am using a Kustom 2x15" cabinet from another amp for testing.
I checked the big resistors and they all tested fine. I noticed none of the resistors are warm when the amp is on, except for the one connected to the pilot lamp. The filter caps measured about 40v. I measured the ohms on the reverb tank and got 172 ohms on the input, and no reading on the output. I assume the tank is bad. Does anyone know what it should read? Does anyone have a schematic for this model? Any tips or advice to get this amp working again would be greatly appreciated.
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Re: 745 PA Mystery [message #29329 is a reply to message #29328] |
Fri, 30 August 2024 09:05   |
rainy_daze
Messages: 2 Registered: January 2016 Location: Chesterfield, MO
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Junior Member |
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Hi, thanks for the reply.
I changed the fuse because someone put 7A in there.
Found the schematics! I didn't realize they would be under K200A.
I measured the red wire coming out of the transformer and into PC201 and found that it did not give me any voltage reading.
Let me know what my next step is. Thanks again.
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Re: 745 PA Mystery [message #29330 is a reply to message #29326] |
Fri, 30 August 2024 13:16  |
stevem
Messages: 4786 Registered: June 2004 Location: NY
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Senior Member |
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The positive 34 volts that half the output section runs on is fed into Q6 on the 201 regulator board and ends up being regulated down to 24 to 27 volts and that’s what powers the driver / PI transformer and the other two boards .
The schematic has voltages on it you need to check for after proving out that all the transistors on that board are good and that R207 a 5 watt 25 ohm resistor is not open and that the ground connection for the board is good.
The gray wires in the amp carry the audio signal.
The input side of the driver transformer off of the 201 board should test out at about 9.7ohms when it’s unsoldered.
On the output side of the transformer the tan and green wires should check at 5.9 ohms and the brown orange wires about 6.1 ohms.
[Updated on: Fri, 30 August 2024 13:17] Report message to a moderator
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