ANOTHER dead valvestate !

[Enzo]

When I face a fuse blower, I check all the output transistors for shorts between legs, also check to make sure the backsides of them are not shorting to ground at the heatsink.

I check the main rectifiers for shorts, in your case the four sides of diode bridge 1 over by the transformer.

And just to be sure, I would disconnect the power transformer wires from the circuit and leave them hanging in mid-air, then power up. Now with power going only to the transformer, does it still bow fuses?

[Valvehead]

the outputs are new--but ill re check .

i replaced the bridge rectifier with a new one tonight. Had a couple new ones---worth a shot.

with the trans sec. unhooked = no short.

[Valvehead]

no speaker is connected.

the 75w bulb glows very BRIGHT !

ill go back and check all the resistors.

measured zd1 zd2 , i have 0.9vdc at the most !

[DrGonz78]

Out of curiousity after swapping out all these parts are you testing to see what the DC you have on the output? Has it changed at all? Also, what DC voltage are you seeing at -/+ sides of your bridge rectifier?

measured zd1 zd2 , i have 0.9vdc at the most !

So what dc voltage are reading at the both ends of r101 and r102? Earlier you had "-2.2vdc at zd1 , and +2.7vdc at zd2" This latest reading is after you changed the bridge. Did you test the bridge voltages again with the new br102?

[Valvehead]

each end of r101/102 is 0v and 2.8Vdc

i also found that measuring the + end of zd1 , to ground , i get 1 ohm . i dont see where thats grounded on the scem

[DrGonz78]

each end of r101/102 is 0v and 2.8Vdc

i also found that measuring the + end of zd1 , to ground , i get 1 ohm . i dont see where thats grounded on the scem

You need voltage on both ends of r101/r102 and that is a problem. Did you check zd1/zd2 with your diode checking function on your meter. It goes to ground through PT secondary center tap. Check resistance of r101/r102 and make sure zd1/zd2 are good.

[DrGonz78]

Measure the resistance across both ends, each way, of zd2 anode and cathode. One way you should see 20 megs or so and the other way OL. If you are measuring 1ohm any way on that part it is shorted and is like just a wire connection.

[Valvehead]

all test good.


could it be C40 / 41 ?

[DrGonz78]

Test for continuity in all the rails that feed the 15v to all the amp. I think we are looking for something that is disconnected in the circuit.

[DrGonz78]

Sure test those caps too. However, you had voltage at the zeners before and now we don't. Proof check what you might have cut out of circuit...

[g1]

i also found that measuring the + end of zd1 , to ground , i get 1 ohm . i dont see where thats grounded on the scem

If you read 1 ohm from zd1 anode to ground, then it must measure 1 ohm across zd1. The cathode is grounded. So either it is shorted or the -15V line is shorted.
Or you have a meter/measurement problem. This could also explain why you don't have any DC at either end of R101 or R102.
One end of each of those resistors should have the same voltage as the collectors of the output devices.

[Valvehead]

ok---i have another meter---ill test with it

[Valvehead]

Sure test those caps too. However, you had voltage at the zeners before and now we don't. Proof check what you might have cut out of circuit...

my meter only goes so high on microfarads...how can i test the caps ?

the zeners test good out of circuit (diode setting) = open 1 way and .6v other way . correct ?

[Valvehead]

If you read 1 ohm from zd1 anode to ground, then it must measure 1 ohm across zd1. The cathode is grounded. So either it is shorted or the -15V line is shorted.
Or you have a meter/measurement problem. This could also explain why you don't have any DC at either end of R101 or R102.
One end of each of those resistors should have the same voltage as the collectors of the output devices.

ok---sorry...im getting lost here...

i just measured across zd1/ zd2 and i get 5 Meg ohms.

i have the same volts at r101/102 as the output chips tr8 tr10



i have continuity from r101/102 to tr8/tr10

and to R75/77/84/85

[olddawg]

Throwing parts at something you don't understand only compounds problems. You need to isolate the failure. Generally you isolate various system blocks. Like Enzo says. No speaker load. Isolate the power supply. Good? If good, isolate the regulator section. Check all of your rail and supply voltages. Good? If good them isolate the preamp from the power amp with the rail voltages to the power amp disconnected. Does the preamp work using the pre out or effects send? If all that is in order, then you have to trouble shoot the output stage. I come from an age when capacitance meters were not readily available and I don't trust them anyway except for very limited applications. You can always float a cap or jumper it. On that note. Are you sure you simply don't have a bad filter cap or two in the power supply?