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745 PA Mystery [message #29326] Thu, 29 August 2024 20:27 Go to next message
rainy_daze
Messages: 2
Registered: January 2016
Location: Chesterfield, MO
Junior Member
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.
Re: 745 PA Mystery [message #29328 is a reply to message #29326] Fri, 30 August 2024 05:36 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4786
Registered: June 2004
Location: NY
Senior Member
Hello.
First a question, why did you replace both of the fuses where either of then blown / open?

Not to be mean here, but if they both tested good and you replaced them anyway then I question weather you know enough about electronics to repair this amp on your own and not harm it more.

Anyway here's what I can offer with the info you have provided.

The schematic for this amp can be found in this site's Technical section .

It's listed under model K200a.

It's the first schematic that comes up when you open that section.

Your amp being the PA model has boards pc101, pc201 and pc401.

The red wires bring the power supply 24 vdc into the main preamp board  ( 101 ) and the reverb board pc401 from the pc201 board on the rear wall of the amp.

This 201 board is the voltage regulator for the 24 vdc and the audio driver that feeds the driver / phase splitter transformer mounted to the right of that board.

If you have no 24 vdc or if the red wire off of that 201 board going to the transformer has no dc on it then the amp will not have any output to the speakers.

I sence that your amps issue is on that 201 board, or that driver / PI transformer is open.

When I get home I will look up how that transformer should test out resistance wise so you can test it.

Just for added safety to the amp I would replace that 5 amp fuse with a 3 amp so that any further misshaps create less semiconductor damage.
Re: 745 PA Mystery [message #29329 is a reply to message #29328] Fri, 30 August 2024 09:05 Go to previous messageGo to next message
rainy_daze
Messages: 2
Registered: January 2016
Location: Chesterfield, MO
Junior Member
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.
Re: 745 PA Mystery [message #29330 is a reply to message #29326] Fri, 30 August 2024 13:16 Go to previous message
stevem is currently offline  stevem
Messages: 4786
Registered: June 2004
Location: NY
Senior Member
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]

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