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**Enzo** · 10-18-2008, 05:51 AM

Make the amp function first, THEN we can worry about the character of the result.

**pchilson** · 10-18-2008, 04:21 PM

Yes, agreed.

**pchilson** · 10-23-2008, 01:52 AM

The new power transformer arrived today. After replacing the components listed above with new I installed the transformer. The part # is the same but it looks slightly different. Maybe a different manufacturer? I hooked it up, hit the power switch and blew the fuse immeadiatly. Would hooking the red wires up in reverse do this?
For the moment it was on before the fuse went there was no loud hum as was happening before. The fuse went in probably 2 seconds.

**pchilson** · 10-23-2008, 02:17 AM

I swapped the red wires around and the same thing. Fuse goes.
How do I proceed to track this down?

Thanks

**Enzo** · 10-23-2008, 02:44 AM

The two red wires are interchangable, but the center tap is not. Are yu sure the wires are on the right posts? The red with yellow stripe is the center tap. It must connect to ground. Look on hte underside of hte board, the two main caps. There is a large copper trace that both those caps send a leg to. That is ground. isn't CP6, the center tap ground post, located between the caps?

**pchilson** · 10-23-2008, 02:53 AM

The center tap is between the large caps. The center tap and the black power wires were self explanatory in the hookup. I didn't know the reds were interchangeable, so with that said its not in the transformer hookup.

To clarify above. I replaced with new components this list:
CR21 - CR24
R96
C48, C49
CR25, CR26
R94, R95
C50, C51
Q3 - Q6
I also soldered in a socket for the MC1436Y opamp

I have changed nothing else. Is there something else that could have been in the line of attack when I shorted it that I have not replaced?

Thanks

**Gtr_tech** · 10-23-2008, 05:20 AM

Ugh.......next?

**Enzo** · 10-23-2008, 07:00 AM

Let's assume nothing. Pull the three red wires off completely and apply mains power. If the fuse does not pop, then the tranny is OK.

Since the old transformer with one side not working did not blow the fuses, right?, then either we connected the new one in wrong, or the part of the circuit the old transformer served but was not powering was not right in thefirst placce. In other words if the top half of the transformer was open. CP5 and CP6 could have been shorted together and we'd never know. So with the red wires stil off, are any of the red wire posts CP5,6,7 shorted together on the board? A quick test.

And another test would be to connect just one red wire alone and test, then connect the other red wire alone and retest. Keep the center tap connected. If one side blows fuses that is a clue. If either side blows fuses, that is also a clue, but it is a different clue.

it is also possible that you changed a bunch of parts since the old transformer was in there, and one or more of them could be either defective or installed wrong.

It is a cardinal rule of repair shops that if something on your bench suddenly develops new symptoms after you did some work, it is most likely a result of that work.

You replaced the main rectifiers CR21-24. ANy chance any are in backwards? ANy possible solder bridge underneath? It LOOKS like they all four sit poiinting the same way. Is that the case?

Look past them now. Measure resistance from TP 3,4 to ground. Either shorted there? Now measure resistance TP3 to TP4. Are they shorted together?

It is EASY to get C48,49 backwards. Note that the + side of BOTH should be towards the front of the amp. Is that the case?

Remember we are not assuming anything here. No short cuts, no "oh I know THAT couldn;t be the problem..." If we were sure what was what, we wouldn;t be here.

R96 wouldn't blow fuses.

CR25,26 - if they are not dead shorts, they won;t blow fuses.

R94,95? Hell, check them.

Q3-6, check them for shorted. Also, Q3,4 are TIP 142 and are the two on the left, is that correct? And the two on the right are TIP147. Verify them.

And of course flip the board over and look at the work, Is there a component lead we forgot to cut off? Is there a screw or nut sitting under the circuit board? etc.

There are only so many things that can blow fuses.

**pchilson** · 10-23-2008, 12:32 PM

Thanks Enzo,

I will have a go this evening and track through everything you have mentioned.
I'll keep you posted.

**pchilson** · 10-23-2008, 09:26 PM

Limited testing so far.

No long leads shorting against chassis.
All polarity specific components installed with correct polarity.

Power transformer:
Mains only - Fuse OK
Mains and Center tap only - Fuse OK
Mains, Center tap and CP7 only - Fuse blows
Mains, Center tap and CP5 only - Fuse blows

I have inspected the under side of the PCB and don't find any "bridged" traces or any apparent "burn outs". Nothing obvious on the underside of PCB.

I will do the resistance measures this evening. More later.

Thanks

**pchilson** · 10-24-2008, 03:06 AM

I have rechecked the bottom of the PCB. Replaced the TIP142s with the originals. The CP5 leg is what was knocked out before. The CP7 leg was test pointing ok. Now both leg are causing the fuse to burn. I can't find any indication of a short but there must be a bad component or somthing.

**pchilson** · 10-25-2008, 02:13 PM

Can anyone help me here?

Thanks

**Enzo** · 10-25-2008, 07:49 PM

Something is loading down the transformer enough to blow fuses. So something is either shorting a rail to ground or shorting the two rails together. Power off and unplug the amp. Verify there is no charge left in the main filters.

Measure resistance between +rail and -rail. You can do that either at the non-ground end of the two filter caps, or you can find rails on the metal tabs of the power transistors. The ones on the left are one rail and the ones on the right are the other. Are the left tabs shorted to the right ones?

Are either side tabs shorted to ground? AHA! Did you replace any of those transistors and neglect to put the insulating mica underneath one?

If neither side is shorted to ground and they are not shorted together, then either the outputs are all turning on at once, meaning a total loss of bias, an open bias string. Or we might have a backwards or shorted main rectifier. either one of those will put something on the filter caps they won;t like.

SO pull the red wires off, and measure resistance bewteen the two posts where they went. it should check as open. Also check betwen the two posts both directions with diode test on your meter. This will reveal a shorted diode.

**pchilson** · 10-25-2008, 09:00 PM

Enzo to the rescue again. I'll report back. Thanks

**pchilson** · 10-26-2008, 06:46 PM