Carvin MTS 3200 Bias runaway..... help?

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Carvin MTS 3200 Bias runaway..... help?

Postby Anticpunk on Sun Aug 14, 2011 6:14 am

Hoping some body might have had this issue with this model or a similar amp.

In standby I'm reading -57volts on the grids of all 4 6L6s, as soon as you kick it out of standby you get a fast creep up to +2volts DC. These measurements are with no power tubes in as I didn't want to have to buy another new set due to the serious redplate conditions.

I replaced every component in the bias supply except for the 20K trimpot. Diode, caps, resistors, PI coupling caps...... all of the usual culprits. With no tubes installed its obviously in the power supply or (hopefully not) the power tranny.

The bias supply pulls off of the HT tap before the rectifier diodes for B+.

Any ideas on where else to look?

Thanks in advance,

Jay
Anticpunk
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Re: Carvin MTS 3200 Bias runaway..... help?

Postby Anticpunk on Fri Aug 26, 2011 3:11 am

I replaced the bias trimmer with a chassis mounted pot for easier adjustment.

Symptoms persist. Also replaced the 1N4007s in the B+ line, disconnected the PI coupling caps from the gridstopper/tube socket line. Neither yield a change. Obviously the change is coming back thru the HT secondary somehow, but unless the 4007 in the bias supply is cooked, theres no way i should be getting positive juice with coupling caps deconn'd and no 6L6s in the sockets....

With new parts in the Bias supply and the B+ line, now in standby I get -50something but as soon as you throw the standby switch, you get a fast climb to +9Vdc settling down to around +2.5Vdc. Obviously reverse polarizing the electro cap on the bias line is bad.

If anybody is actually reading this, I'm open to suggestions........ hoping I'm over thinking this and its staring me in the face....


Thanks,

Jay
Anticpunk
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Posts: 38
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Re: Carvin MTS 3200 Bias runaway..... help?

Postby Anticpunk on Tue Sep 20, 2011 3:22 am

Since this amp is approaching 15 years old and there was some bloating in two of the filtercaps, I replaced all 6 as well.
Symptoms persist........

Also moved the CT on the b+ tap to a chassis ground instead of the PCB ground.

Unless its the power transfomer, I'm at a loss and putting it away for good........


-Jay
Anticpunk
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Posts: 38
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