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Old 16th November 2012, 12:46 AM   #1
mike567 is offline mike567  United States
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Default Mighty Midget Problems

I am having problems with mighty midget. The amp has been working but all of a sudden blew an output transformer in the right channel.
I purchased new output transformers (edcor gxe10-8-8k)

I installed the new transformers but the right channel starts heating up in about 30 seconds even without tubes. slight scratching noises in right speaker. small burn near r16 in board (arc over??) . checked filter capacitors, most resistors in the vicinity,

Can any body give some guidance where to start next?

test equipment is limited to a DMM. and digital cap checker

thanks in advance
mike
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Last edited by mike567; 16th November 2012 at 12:56 AM. Reason: schematic link
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Old 17th November 2012, 07:25 AM   #2
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Please, measure what are U plate and U cathode of output tubes in both channels.
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Old 17th November 2012, 01:56 PM   #3
mike567 is offline mike567  United States
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Quote:
Originally Posted by azazello View Post
Please, measure what are U plate and U cathode of output tubes in both channels.
with no tubes installed (transformer gets hot quickly)

cathode voltage is 0 on good channel and 0 on bad channel
plate voltage is 174 on good channel and 10 on bad channel

cathode capacitors and resistors are good. First things I checked.

Probably in the power supply?
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Old 17th November 2012, 02:32 PM   #4
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Cathode voltage 0 on the good channel is wrong, because U cathode resistor 200 ohm must be more 10-12 volts.
Check, please, for short inside on the socket of output tube in bad channel.
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Old 17th November 2012, 02:58 PM   #5
mike567 is offline mike567  United States
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the voltages were checked with NO tubes installed
cathode resistor goes from pin 9 to ground with no other connections .

I have verified that cathode resistor and bypass capacitor on both tubes were good.
I did not install the tubes because the transformer on the bad channel heats up very quickly. I can disconnect the transformers and check again if you wish.

I am very glad to have your help.

Last edited by mike567; 17th November 2012 at 02:59 PM. Reason: more info.
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Old 17th November 2012, 03:01 PM   #6
mike567 is offline mike567  United States
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The sockets show no sign of shorting. They are new ceramic sockets
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Old 17th November 2012, 09:48 PM   #7
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Please: 1.Check resistance between socket cathode pin and ground, must be 200 ohm. Check for short between socket pins. /But first, switch off an amplifier/..... 2.Check voltage on cathode resistor 200 ohm with tube /tester must be in DC mode/ for short time.... 3. Disconnect C11-0.22 uF and see what will happen..... 4. Disconnect R17 360 kohm..... 5.Change for a short time output tubes, from left to right channel..... 6. Resistance between first grid /pin 8/ and ground must be 221 kohm / amplifier must be switched off/.
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Old 17th November 2012, 10:02 PM   #8
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With only 10 volts on the plate of the bad channel with no tubes in, it has to be shorted somewhere around there. Unplug the amp, discharge the B+ capacitors, and measure resistance, at the tube sockets of both plates.

The 174 on the good channel is lower than what is supposed to be there because the bad channel is shorted, from the op transformer to pin 11 of the 6T10.

You should have had the fuse blow first.

Last edited by DeathRex; 17th November 2012 at 10:06 PM.
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Old 17th November 2012, 10:34 PM   #9
mike567 is offline mike567  United States
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I will check all of the above tomorrow. I have 6 spare tubes which I have tried. the problem does not follow the tube. I hesitate to power the amp up with the transformers
attatched because the bad channel will heat up the transformer in about 30 seconds.

the amp is not built with the recommended power transformer. the one I used has 500 vac ct. the rectifier was modified from bridge to full wave with center tap to ground 330 ohm power resistor removed from the circuit. the amp worked for about 2 months before I heard a small chirp then silence in the bad channel.
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Old 18th November 2012, 05:47 PM   #10
mike567 is offline mike567  United States
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Problem was found too be some tracking across the PCB near the 360k resistor ans ground.

I wish to thank azazello and deathrex for your help.
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