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In Reply to: Band posted by steven oda on July 26, 2002 at 10:42:10:
the bandend is soldered to the same lug with the xfrmr output - the rest of the caps are soldered correctly - or at least, that's my working assumption, because a) it worked until the new diode went in, and b) they were installed the same way originally (i.e., polarity matches schem. and previous installation)could the higher bias voltage have damaged some other component downstream, causing the signal to break?
Follow Ups:
Todd,It's possible. Though you probably would not get a negative voltage reading (zero volume, no inputs, no power tubes, and speakers connected) at pin 5, if one of the resistors downstream of the the diode (between the diode and pin 5, of the EL34) was burnt.
Red lead to pin 5 and black lead to ground. Set meter to read 200 VDC range. If you can't get the proper voltage---buy a new diode. Re-insert it and try again. Remember, the voltage should read NEGATIVE.
To test a diode, it should let voltage through in one direction. But, not the other. Ohm (DC) setting on the DVM can quickly test this. But, this is a crude measurement and can't tell you the degree of potential thermal damage. Better to buy another diode.
Measure the voltage on pin 3, without the EL34 tubes inserted. Should be around 500 VDC (positive). Set the DVM to 1000 volts VDC range.
Red lead to pin 3. Black lead to ground.
If pin 5 reads -48 volts VDC and pin 3 reads around 500 volts VDC---and you still can't get any sound from the speakers (after you've turned the amp off and inserted the EL34's)---check for lit plates on the tubes.
(The reason I suggest not putting the EL34 into the amp, while testing for voltages---is to prevent any further damage to the tubes. Your reading maybe a tad higher, since there will NOT be the load of the tubes to drop the readings).
Also, check for 6.3 volts VAC across pins 2 and 7. Maybe, you have a bad connection to the heater pins. If okay, plug in the EL34's.
If the heaters are fine. Anode voltage is fine. Bias voltage is in proper range. And still no noise from the speakers.
Be sure all tubes light up. Check the heaters, again. 12A(_)7 should measure 12.6 volts across pins 4 and 5. 6SN7 types measure 6.3 volts across pins 7 & 8. 6FQ7 measure 6.3 volts across pins 4 & 5.If for some reason the current draw was too much in the power stage---a fuse could have blown. Probably need to hunt around for any blown out fuses. These can be hidden on a circuit board (on the top or bottom of the board).
It could also be a bad coupling cap(s) from the phase inverter or gain stage.
Check for continuity with the amp off and DVM set to Ohm's (DC) in the Meg ohm range. Could be a bad solder. Bad cap. Bad resistor in the RC circuit of the gain stage or PI. OR a bad tube in these sections.
One other option you don't even want to think about is a blown OPT. There are ways to test the continuity (Ohm meter, again with amp off). But, this is crude and can really determine only breaks in continuity or shorts.
Give the TA board the results of some of these tests & maybe we can help narrow down the problem area.
Good hunting.
Todd,ONe other thing, many amps have a screen grid resistor which acts as kind of a fuse---to prevent excess current flow, within the tube (and potential thermal runaway).
In Marshall amps a 250-1000 ohm, 5 watt resistor is tied to pin 4 of the EL34's. Check this resistor for its measured value. And any signs of the "burnt toast" effect.
as this gives me plenty to do, and I can avoid the usual tour-guide stuff ("no, really, I'd love to see Stone Mountain for the 150th time ...")thanks for all the help, and I will post on this once I get it fixed (or come to another major deadzone) ...
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