Sound city 120 h.t. Fuse blown

[mattiambro]

Hi,there I have in my hands a sound city 120 Mark IV that blows H.T. fuse even with no tubes. Rectifier seems to be ok, I tried it with no load gives a good Voltage.
Then I add the 200uF caps section and it was still good. Adding all the rest it blows. I did not find any shorted Cap and checked some resitors alomg tracks and seems ok.
I Know these amps are campus for being noisey but this one does a pretty consistent noise even without tubes and it increase with current Using a variac but at a certain Level it blows variac fuse that is 2Amp, main fuse of the amp is 3Amp. Anyone have suggestions? Thanks to everyone

[PRR]

> blows H.T. fuse even with no tubes

Has to be rectifier, mains caps, or wiring. Focus there.

[tubeswell]

Adding all the rest it blows. I did not find any shorted Cap and checked some resitors alomg tracks and seems ok.

With the amp off and discharged, use your R meter to measure between the HT supply nodes (at each filter cap) and the ground return. Looking for low or no resistance. (It might not be a shorted cap (or it still might be) but it could be a short between the filter cap/resistor connection point and the ground return


Or using a dimmer or variac, measure the voltages at each HT supply node (after the reservoir cap), looking for the node that is reading 0V

[Latole]

To work in a amp wich blow fuse, a lamp current limiter is often a must.

[pdf64]

A light bulb limiter with a suitable bulb wattage can be very handy in  helping to track down the root cause.

[mattiambro]

The only node where I have small resistance,(30ohm) it is the first node in series of the rectifier, and also all along rectifiers except where transformer windings are connected. Does not seems so right 

[tubeswell]

A schematic helps.  The 1st two filter cap nodes are stacked, so the chances of both caps shorting on each of these nodes is 1 in 4. Also, the 220k balancing resistors will reduce the likelihood of a fuse blowing if one of these cap segments is shorted. You infer that the fuse holds with the std-by switch off. So we can assume the reservoir cap node and the rectifier is working. (What VDC do you get there?).  The supply resistors are where the brunt of any major voltage drop will be evident. How does the 100R look? What about the 6k8? the 22k? If one of these looks burnt, lift the following cap out of the circuit and look at the idle voltages

[mattiambro]

Resistors seems fine and their values are fine . I checked wiring and seems fine, I will give anotuer shot as sono ad I can. With stand-by off I see +450V at rectifier output. With stand-by on It Is below the variac output.

[tubeswell]

...With stand-by off I see +450V at rectifier output. With stand-by on It Is below the variac output.

So, what are these voltages (at each filter cap/HT supply node)?

[Williamblake]

Also the screen grid resitors in those amps can come very close to the chassis.

[mattiambro]

Almost every node after standby has same voltages (150v input 80v at nodes) with the stand by switch on. Resistor in series of the fuse was really hot. Now I'll try rewiring that section and are what happen, I guess..  screen grid resistor does not contact chassis but thanks for the suggestion

Edit: I have disconnected the first resistor (100r) after standby switch so that only center tap of O.T. Is connected and I already have bad voltages

[tubeswell]

Almost every node after standby has same voltages (150v input 80v at nodes) with the stand by switch on. Resistor in series of the fuse was really hot. Now I'll try rewiring that section and are what happen, I guess..  screen grid resistor does not contact chassis but thanks for the suggestion

Edit: I have disconnected the first resistor (100r) after standby switch so that only center tap of O.T. Is connected and I already have bad voltages

Reconnect the 100R resistor but lift the 32uF cap

[mattiambro]

Same results

[tubeswell]

Okay so lift the screen supply cap ground node. What happens?

[mattiambro]

Same results. I see only few ohms of resistance from plates to Ground and from center tap to Ground that isn't good right? Output transformer is Hammond 1960t

[sluckey]

Same results. I see only few ohms of resistance from plates to Ground and from center tap to Ground that isn't good right? Output transformer is Hammond 1960t

Disconnect both OT plate leads from the tube sockets. Do you still have low resistance from socket pin 3 to ground?

[mattiambro]

Same results. I see only few ohms of resistance from plates to Ground and from center tap to Ground that isn't good right? Output transformer is Hammond 1960t

Disconnect both OT plate leads from the tube sockets. Do you still have low resistance from socket pin 3 to ground?

Nope

So Bad O.T. ???

[sluckey]

OK, now disconnect the OT primary center tap lead from the B+. Do you measure low resistance from the primary CT lead to ground?

[mattiambro]

Yup

[sluckey]

I would call it dead.

[mattiambro]

 

[IndustryCode783401]

I'd suggest replacing all 4 diodes in the HT power supply. You could use an ohmmeter to test these to see if one is not like the others, but if you replace one then replace all four.