Strange squealing in amp when powered up.

[Matt J]

Mark,

That is an awful thing to find! I looked over your post, applied the B+, and checked the middle and edges of the board from a powered eyelet. Thankfully no AC or DC was measured.

Thanks for the tip!
- Matt J.

[Structo]

Finding a bad cap from the get go is not good.

If I follow what you're saying, it is the first 20uF cap on the main filter board that is testing bad?

Dang, those caps are expensive too.

I swear I read about more Sprague caps being bad then most other brands.

If you get no reading on it, that means it's open, and not shorted, so I don't understand why that would make it blow a fuse.

I would look over the way you have the filter board wired and the polarity of everything on it.

Carefully double and triple check everything.

[Matt J]

You got it right Tom, it's the first 20uf cap on the board. I tested it with different meters, all came back with no reading. The other caps all read ok. Could a voltage spike resulting from the tubes not being in when first powered up have caused a faulty part? I've ordered the parts to build a new power supply board, buying more of those Sprague caps was no fun on my wallet either!

I'll let everyone know how it goes when the parts arrive and I start building the board. I'm still trying to figure out what to do with the Red/Yellow wire from the PT. I don't want to strip it back and heat sink it just yet without being 100% of what I'm doing.

- Matt J.

[martin manning]

Do you mean "no reading on you meter when it's set to read capacitance? What's the DC resistance of that bad cap? It would have to be shorted to explain the behavior you see.

If it's the HT center tap, that red-yellow has to be grounded or you won't have any B+... don't cut it!

[CHIP]

That red/yellow wire is your center tap. It must be grounded!
Looks like Martin beat me to the punchline.

[rogb]

Do you mean "no reading on you meter when it's set to read capacitance? What's the DC resistance of that bad cap? It would have to be shorted to explain the behavior you see.

If it's the HT center tap, that red-yellow has to be grounded or you won't have any B+... don't cut it!

May as well fuse it too, while you're at it. The more "bulletproofing" the better when it comes to expensive iron and components. Try a 500mA fast blo

[Matt J]

Here's an update:

I did not get a chance to measure any DC resistance on the filter capacitor before I removed the power supply board. As I was dissembling it I did notice that the bad capacitor's lead looked weak where the lead enters into the capacitor itself. Soon enough as I took it out the lead broke right off. I still tried measuring it's capacitance, nothing came up again on the multimeter. I also double checked all the capacitors and resistors on the circuit board and connecting to the tube sockets. Three came up with values that were noticeably off: the 4.7k negative feedback resistor, the 220k OD entrance resistor, and the 180k resistor connecting to V2-b. I made sure to change these to the correct value.

Well, while I'm waiting for the parts to build a new power supply board to arrive I started looking over the Power Transformer. I wanted to make sure the secondaries were putting out the correct amount of voltages, especially after the PT had to deal with the faulty filter capacitor (and whatever problems that might have caused). I grounded both the Red/Yellow and Green/Yellow wires, hooked up the secondaries and bias tap. Connected it to the limiter, turned the mains on and the bulb did not dim. I turned the limiter off, there was the sound of electrical noise, then the fuse blew.

Does this sound like a PT that has something wrong with it?

- Matt J.

[Bob-I]

I grounded both the Red/Yellow and Green/Yellow wires, hooked up the secondaries and bias tap. Connected it to the limiter, turned the mains on and the bulb did not dim. I turned the limiter off, there was the sound of electrical noise, then the fuse blew.

Does this sound like a PT that has something wrong with it?

- Matt J.

I'd start by disconnecting the secondaries, all of them. Insulate them with wire nuts, and check the voltages one set at a time. Make sure to use the limiter until you're sure everything is good, DON'T disconnect the limiter until you bulb dims as it should and you have voltages that are close to correct.

Be VERY careful when messing with the AC on the secondaries, these voltages are far more harmful than the DC after the rectifiers.

[martin manning]

I'd start by disconnecting the secondaries, all of them. Insulate them with wire nuts, and check the voltages one set at a time.

...and with the power off, check the DC resistance between each winding and all the others, and to the core. It should be infinite.

[Matt J]

Hey everyone, here's the latest update:

I changed over the Power Transformer to a Hammond 290FX I had purchased earlier for a possible future build. There's no CT so I reinstalled the two 120ohm resistors from the heaters to the ground.

I rebuilt the power supply filters to the design similar to the Ceriatone Overtone Special Ford Mustang with two 220uf radial caps in a totem pole configuration. I replaced all the dropping string resistors too to 3 watt metal oxide.

With the amp connected to the limiter and no tubes in, throwing on the mains results in the bulb dimming and the pilot light slowing lighting up as the capacitors charge. Switching from standby to operate does not cause any problems.

I let the amp run for a while to slowly build up the capacitors. After a while I turned the limiter off to check the voltages, this resulted in a loud popping sound coming from inside the amp. I immediately turned the amp off and started checking over things. I could not get a reading on the two 270k resistors of the totem pole stack in the power supply. Thinking these may have blown or been faulty, I replaced them.

I reconnected the limiter and went through the start up procedures again, everything seemed fine. I tried turning it on with the limiter off. Throwing the mains on resulted in the loud popping sound again even with the stand by still on. I shut the amp down and left it at that. Once again I am getting no reading on the two 270k resistors.

I'm attaching some photos I just took of the new board. Does anything look amiss? Here are where the wires from the board run, from left to right:
Yellow wire - from bias supply board
1st black wire - from choke to 20uf filter cap
Orange wire - from 220uf caps to standby switch
2nd black wire - from choke
Red Wire - Output Transformer CT
Yellow wire - from junction of black choke wire and OT CT to standby switch
Brown wire - Screen supply wire from 20uf cap to screen resistor on tube socket.

Also, you'll notice a lot of the PT wires are running around. That's because I just haven't trimmed them back yet. I wanted to get everything in working order first.

Thoughts anyone?

- Matt J.

[Matt J]

The fuse never blew during any of this by the way. I'm using a 5 amp slow blow as the schematic calls for.

[ToneMerc]

I rebuilt the power supply filters to the design similar to the Ceriatone Overtone Special Ford Mustang with two 220uf radial caps in a totem pole configuration.

What exactly is "to the design similar to the Ceriatone"?

You changed the PT and the power board and you still have basically the same issue which makes me believe your power supply isn't wired/configured correctly.

I'm attaching some photos I just took of the new board. Does anything look amiss?

I would rather see an exact wiring diagram or schematic verses the pictures because your board is laid out differently. Therefore lacking a familar frame of reference and since I can't see under the board, it's really hard to make out if anything is a miss or not, barring the terribly obvious.

For example, those two black wires at the top left that appear to be terminated to the chassis ground, how is each connected at the board?

TM

[markr14850]

Yellow wire - from bias supply board

That wire is from the top of the totem pole? What does it have to do with your bias?

[ToneMerc]

Yellow wire - from bias supply board

That wire if from the top of the totem pole? What does it have to do with your bias?

My first thought too, but as I reread his old posts I concluded that he really means Bias/HV board or I hope so.

TM

[Matt J]

Yep, it's from the bias/HV board.

I'm drawing up something right now that shows where everything on the board goes.