HELP!!!

Get support and show off your MetroAmp JTM 45 kit builds.

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Yeadaki
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Re: HELP!!!

Post by Yeadaki » Sat Sep 05, 2015 11:01 am

Treble tone cap was not the prob. I replaced the OT.

Now, when the standby switch is engaged there's a very loud squeal/awful noise.

OT is a Mojotone. The only difference between what I put in and what was originally there is a brown, 2 ohm wire. I presume that I simply don't connect the brown wire since I don't have a 2 ohm position on the impedance selector.

At least I've got something coming out of it now. What could be causing the loud squeal?

Thanks.
Rock on!

Yeadaki
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Re: HELP!!!

Post by Yeadaki » Sat Sep 05, 2015 11:23 am

Duh --- I got it.

I swapped the blue and brown primary's and it fired right up.

The Mojotone has the same color wires; but, they don't hook up the same.

The rich tone has returned. Indeed, it's now time to rock.
Rock on!

Yeadaki
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Re: HELP!!!

Post by Yeadaki » Sat Sep 05, 2015 3:30 pm

NEW ISSUE: V4 is hot plating.

Any suggestions?
Rock on!

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neikeel
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Re: HELP!!!

Post by neikeel » Sun Sep 06, 2015 3:45 am

This is a really good example of not giving enough info.

You told us it was built in 2009 and now it does not work despite tube changes but it sounds like you swapped the OT out in the interim but did not tell us :palm:

There is a big difference between an amp that was running fine for 6 years and stops working and an amp that stops working after you have just modded it with a different OT.

Anything else you changed to disclose? :roll:
Neil

Yeadaki
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Re: HELP!!!

Post by Yeadaki » Sun Sep 06, 2015 8:08 am

Did you read what I posted?

Let me review and summarize previous posts:

We agreed to swap the treble cap. That didn't fix it.

So I swapped (not "modified amp with") the ot.


Now, new issue:

The amp sounds fine; but, V4 is hot plating.

Bias voltage is 42.5 mV pin 8 on V5. V4 pin 8 bias voltage is 270+ mV.

Does this clear things up a bit?
Rock on!

danman
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Re: HELP!!!

Post by danman » Sun Sep 06, 2015 8:29 am

Check the negative DC bias voltage at pin 5 on each power tube socket. If you have the same - voltage on each of the two sockets, I would tend to think that the V4 power tube is bad. It could also be that pin 5 is not making good contact with the socket. If the - voltage checks good, I would try a new set of tubes to see if the red plating stops.

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neikeel
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Re: HELP!!!

Post by neikeel » Sun Sep 06, 2015 8:56 am

Yeadaki wrote:I built my JTM 45 in 2009. Today it has no output. I was playing and the sound just dropped. I've got the power light and the tubes glow.
My first inclination is to go back thru the testing points in the instructions when I first fired it up. But, I'm not sure if thats the best route.
Troubleshooting is not my strength. Can anyone point me in the right direction?
I'll start searching for similar threads.
Any help would be much appreciated.
No mention of OT swaps :whistle:
Danman is on the case :thumbsup:
Neil

Yeadaki
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Re: HELP!!!

Post by Yeadaki » Mon Sep 07, 2015 9:51 am

Maybe my post isn't visible for some darn reason. Here's what I posted

"Treble tone cap was not the prob. I replaced the OT.

Now, when the standby switch is engaged there's a very loud squeal/awful noise.

OT is a Mojotone. The only difference between what I put in and what was originally there is a brown, 2 ohm wire. I presume that I simply don't connect the brown wire since I don't have a 2 ohm position on the impedance selector.

At least I've got something coming out of it now. What could be causing the loud squeal?

Thanks."

Then I posted that I figured out the squeal (two wires from OT to valve sockets reversed).

Anywho, I suspected the bias resistor yesterday, but that's not it. Again - amp sounds fine but V4 gets orange hot in big hurry. Pin 8 reads 270+mV. Weird one for sure.
Rock on!

danman
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Re: HELP!!!

Post by danman » Mon Sep 07, 2015 11:39 am

You need to check pin 5 for the negative DC bias voltage at each socket. You should be seeing an equal value at each socket. Somewhere in the neighborhood of -45 to -50 or so. If V4 does not have proper bias voltage, it will cause the tube to red-plate. Another possibility is a bad tube in V4. What you are measuring at the 1ohm resistors on pin 8 is not the bias voltage. You are measuring the cathode current draw through a 1ohm resistor, so 1ma equals 1mv according to ohm's law.

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neikeel
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Re: HELP!!!

Post by neikeel » Tue Sep 08, 2015 5:31 am

Yeadaki wrote:Maybe my post isn't visible for some darn reason. Here's what I posted
"Treble tone cap was not the prob. I replaced the OT.
My apologies, I missed that bit futher down the thread, quite why your OT should fail?
Yeadaki wrote:Then I posted that I figured out the squeal (two wires from OT to valve sockets reversed).
Anywho, I suspected the bias resistor yesterday, but that's not it. Again - amp sounds fine but V4 gets orange hot in big hurry. Pin 8 reads 270+mV. Weird one for sure.
That tube may well now be toast.
It has cropped up before with OT primaries being oddly labelled that the centre tap and the start/end wires are mixed up.But then the amp should not really work at all! I suggest you ring them out with your meter to check which is the centre tap.
Presume your screen resistors check ok and you have correct value grid stoppers on pin 5 (4k7 or 5k6)?
Follow Danman's advice re: checking that you have a negative bias first.
Neil

Yeadaki
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Re: HELP!!!

Post by Yeadaki » Tue Sep 08, 2015 7:14 am

Thanks guys - back to work today. It may be a week before I can get back in it. I'll keep you posted.
Rock on!

Yeadaki
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Re: HELP!!!

Post by Yeadaki » Fri Sep 18, 2015 6:24 pm

Update:

When I reinstalled the board I pinched the wire from the board to pin 5 on V4.

It was pinched on one of the board mounting studs. There's not much to the tube sockets. I knew it had to be simple so I got my mirror and flashlight out and there it was.

Final summary: it was the OT that died and I induced a new fault on reassembly.

Lesson learned: don't forget the basics. Tidy wiring helps identify technician induced faults.

I very much appreciate everyone's help!
Rock on!

danman
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Re: HELP!!!

Post by danman » Fri Sep 18, 2015 9:25 pm

Glad you got it sorted out!

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