2270 transformer testing

[mistermojo]

Hey All,
I began recapping my 2270 this week and ran into some trouble. This is my first time doing this. First I did the power supply, reassembled and everything was working great. Next, I replaced the power switch and tested, again, no problem. Then I did three caps on each P750 board, but didn't test anything before moving to re-lamping.

When I was putting in the LEDs for the indicator lights I couldn't remember if there was polarity to worry about, so I soldered one in and hit the power button to test it. Nothing happened. I found a blown main fuse. I did not have the light "bar" secured and it *may* have shorted to the chassis, but I'm not sure. Seems like the fuse was already popped, since I didn't have a flash or pop sound when I turned it on. I replaced the fuse and tried running it again and the fuse blew immediately.

Since then, I've un-soldered all the secondary wires from their terminals. Power on with no secondaries attached and the fuse did not blow. Attached the two green wires to the panel lights, powered on, lights lit and fuse still good. Attached the yellow wires, powered up, still good. Attached the black wire, also still good. Attached orange back to bridge rectifier and when I power it up, the fuse blows.

Next, I removed the bridge rectifier and tested across the leads. In one direction I get no reading, in the other I get 5.x from negative to each of the AC poles, and ~8.1 to the positive. Does that sound right? I'm thinking it is still ok, but not sure.

If the rectifier isn't bad, then I'm guessing the transformer could have shorted. How do I test it? Maybe I should check the voltage coming out of the orange wires? What are the resistance values of the secondary windings? I can't find specs anywhere.

What else could it be? I've triple-checked all the caps that I replaced and they are correctly mounted. I did tweak one of the output transistor legs when re-assembling. Maybe I damaged it? How would I test that?

I'm a newbie here. I know how to solder very well, but don't know much about electronics, so this troubleshooting is a learning experience for me. All help greatly appreciated.

 

[petehall347]

post pics showing topside and underside of the boards you have worked on

 

[mistermojo]

post pics showing topside and underside of the boards you have worked on

I'll do that when I get home later. Thanks. You must be wondering if I put a cap in backwards or bridged some solder? Pretty sure I did not, but I'll check and post.

 

[petehall347]

yes i think its something you did wrong .

 

[mistermojo]

 

[mistermojo]

The three blue caps are the ones I replaced. 759, 760, and 761. Both channels shown here.

 

[mistermojo]

I put an ohmmeter across the terminals of the output transistors and got different readings on each of them. Is that telling?

 

[Steven Tate]

Did you put a round mica insulator between the square bias transistor and heat sink when you reassembled? Good mica or silicone insulators under outputs?

 

[mistermojo]

that's a good question. I think so, but I just found one now when I was taking it apart, so maybe not? I'll re-assemble and make sure I did. Need to get back to work. Would that cause a short?

 

[Steven Tate]

I think the metal on the transistor would short to chassis. Whether that would blow fuses, I’m not sure. Somebody here probably does know.

 

[petehall347]

that is what i was thinking after studying the pics that look ok from i can make out . use your meter to check ohms from transistor case to heat-sink to see if shorted . they need to be isolated

 

[mistermojo]

Thanks Pete and Steven. I'll get it back together as soon as I get home and report back.

 

[mistermojo]

Got everything back together, including bridge rectifier and it still blows fuse. Insulators are in place. I'm back to thinking it could be the transformer.
What should I try next?

 

[Steven Tate]

You could disconnect power to the power amps (carefully protect wires once disconnected), and see if it will power without blowing the fuse. Since you worked on the amp boards and lamp board right before it started blowing fuses, you need to figure out which one is shorting out. Are you using a Dim Bulb Tester when you power up? You need to be doing that for sure. If you remove amp power and bulb stays bright when powered up, the problem is elsewhere. If it dims, the problem is in one or both of the amps. I’m also suspicious of the power switch if you went into it.

 

[mistermojo]

That's the red and the white wires coming from the filter caps, right?

 

[rBuckner]

Those are the positive and negative dc supply wires for the power amps. One each will go to the power rectifier and two each will go out, one set per amp module. There is a thread in the DIY subforum here that explains using the diode test function of a meter to do a basic health check of transistors. I recommend testing all outputs per that thread.

 

[mistermojo]

I just realized a mistake. It appears that the output transistors are two different types. I was treating them as if they were all the same. I'm looking at the schematic, but the numbers I see there are different than the numbers on the transistors. I'm going to test them and see if I can figure out which is which. On the schematic they are H001 to H004, right? H001 and H003 are PNP, while H002 and H004 are NPN?

 

[mistermojo]

I tested all the output transistors and the NPN in the right channel appears to be bad. I'm going to build a dim bulb tester while I'm waiting for parts. Did I likely short something else out too? Should I replace all four?

 

[rBuckner]

Check all transistors and diodes before powering up since indeed more may be cooked by the blown output. I agree with Steven's excellent suggestion to build and use a DBT for the power up. Two of the outputs are NPN and two are PNP. Well loved replacements are the OnSemi MJ21193 and 4.

 

[mistermojo]

Got my OnSemis today. Built my DBT this morning. Hopefully I'll be back in the sound tonight!