Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Packerswin14 on April 10, 2012, 12:56:52 pm
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I've got a 1975 Silverface Master Volume Twin Reverb in. Someone previously made modifications to it which resulted in a variety of small fires. Both channels worked at about half volume when I got it, albeit with some noise, but no reverb or vibrato. I've replaced the burned and melted components, all electrolytics, got the reverb repaired. Checked all the jacks and grounds. Tested the tube, and also swapped it twice. V5 is oscillating. Speed and intensity pots have been cleaned, and both test ok. Then I started really looking at the schematic and layout and discovered just how modified this was-- it looked okay to me, based on other twins, but once I looked at the Fender layout drawing, I realized that whoever changed it left out some components and made some pretty drastic changes from the original design, including forgetting to put some components in. Once I put these parts back in, still no vibrato, to my surprise.
I think I've got it back to the original design now, which has gotten rid of the hum, crackle and pop. But still no vibrato. New footswitch, plus I've been testing it with the jack shorted all along. New roach, just to be sure. Almost every component has been replaced, as you can see-- several of them were charred, and I replaced most of the rest while I was at it. I'm a bit mystified. I've followed every wire and component, checked every connection with the continuity tester, lifted the boards and checked with a mirror and light to see if there was a huge blob of stray solder shorting components in the middle of the sandwich or underneath. Either I've missed something, or someone wiser than me has seen this before. Anyone have an idea?
Here are voltage readings with the vibrato off and on, of V5:
1: 430/153 to 372
2: -13.5/0 to -2
3: 0/ 1.5 to 2.5
6: 382/ 246 to 380
7: -13.5/ 0 to -13
8: 0/ -20
And resistance to ground with the vibrato on and off from pins of V5:
2: Infinity/994K
3: 2.7K/ Infinity
7: 101K/995K
8: Infinity/Infinity
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And resistance to ground with the vibrato on and off from pins of V5:
2: Infinity/994K
3: 2.7K/ Infinity
7: 101K/995K
8: Infinity/Infinity
You can't take resistance readings with power on.
V5-2 should be 1M/3M
V5-3 should be 2.7K/2.7K
V5-7 should be 1M/3M
V5-8 should be 100K/100K
Looks like your pin 8 'may be' missing a 100K cathode resistor.
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Thanks, Sluckey. Here are the correct resistance readings:
2- 1M/1.7M
3- 2.7k/2.7K
7- 1M/1.7M
8- 99K/99K
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Here's a pic
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Couple more pics
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And finally,