Dynaco ST 70 bad PT or not?

[Randall]

This kit stereo amp came in for blowing fuses. The guy says he changed the filter can and the preamp board and that's when it started. The filter can isn't pretty, but it doesn't look at fault. First I pulled all the tubes, bright light bulb. Then I started disconnecting windings one at a time until all that was connected was the secondary center tap, and I still got a bright light bulb. Once I disconnected the center tap, it stopped. The funny thing is, if I put a neon on pins 4 and 6 of the rectifier socket, and apply 10v to the primary, the neon comes on. It doesn't act exactly like I'm used to it flashing, it is more of a partial glow. I don't see it come on at all with other winding combinations. Also, I see 30 ohms between CT and pin 4, and 21 ohms between CT and pin 6 of the rectifier. I assume I'd see some reading since it is center tapped, but is this too low?

Am I seeing a false positive here? I'd sure hate to tell this dude his hi-fi PT is toast.

http://diyaudioprojects.com/Schemati...-Schematic.htm

[Helmholtz]

PT might have a short between the HV winding and core/ground. Measure DCR between CT and chassis.

[Randall]

I must have made an error, because now when I power it up with everything disconnected, I still get the bright bulb. So, I'd say she's toast.

Interestingly, I read open from CT to chassis. In fact the only reading I get from CT other then the above mentioned pins 4 and 6, is 4 ohms to the bias supply winding.

[Helmholtz]

To make sure, you can do an additional test: With ALL secondary wires disconnected and on the bulb limiter, measure primary and secondary voltages and see if voltage ratios are correct or not.

[Randall]

With all of the secondary wires disconnected I get 9.5 vac on primary , and 47 vac on the secondary.

[Helmholtz]

With all of the secondary wires disconnected I get 9.5 vac on primary , and 47 vac on the secondary.

Are you sure you disconnected all secondary wires including the heater windings (total of 12 wires)? What is your bulb wattage?

[Chuck H]

Well the voltages seem strangely low, but the ratio seems about right.?.

[Chuck H]

Are you sure you disconnected all secondary wires including the heater windings (total of 12 wires)? What is your bulb wattage?

So we're including CT's for the audio tube filament circuits then.?. I wondered if it should matter myself, but didn't know.

[Randall]

All secondary wires are either disconnected or wired to open socket pins. Filament winding is not disconnected, but there are no tubes installed, so why would we need to? Same with the rectifier, the B+ wire off pin 8 is disconnected, so that is the same as those four wires not connected, correct? I know a socket can short, but all the tests to ground or other pins show open. The three CTs are disconnected, as is the bias supply. The rest are terminated at open socket pins, that all measure open to ground.

My limiter bulb is 100 watt.

[Chuck H]

All secondary wires are either disconnected or wired to open socket pins. Filament winding is not disconnected, but there are no tubes installed, so why would we need to? Same with the rectifier, the B+ wire off pin 8 is disconnected, so that is the same as those four wires not connected, correct? I know a socket can short, but all the tests to ground or other pins show open. The three CTs are disconnected, as is the bias supply. The rest are terminated at open socket pins, that all measure open to ground.

My limiter bulb is 100 watt.

In the schematic there are capacitors to ground from the center taps of both power and signal tube filament windings. These are AC circuits to begin with and there is also no guarantee that the capacitors are not faulty. Why would we not remove every lead from circuits just to be sure?

[Randall]

Because the filament CTs are disconnected, as I mentioned. The caps test good, but they are not a factor at this point, because they are disconnected.

[Enzo]

Keep in mind that arcs can happen and your meter will not detect them. Your meter uses maybe a volt to test circuits. Something that only arcs or leaks at say 300v won't show up.

If the transformer sounds like it is dead, we are at the what is there to lose stage. We can decide not to remove transformer wires to empty tube sockets, but those sockets could hide the problem. If the transformer is to be replaced anyway, pull the wires off the socket circuits because we have to do that to replace the transformer. And that leaves ALL wires from the transformer up in the air. If anything changes, then a socket wire has some sort of issue. If nothing changes it is just one more nail in the transformer coffin.

[Helmholtz]

If the transformer sounds like it is dead, we are at the what is there to lose stage. We can decide not to remove transformer wires to empty tube sockets, but those sockets could hide the problem. If the transformer is to be replaced anyway, pull the wires off the socket circuits because we have to do that to replace the transformer. And that leaves ALL wires from the transformer up in the air. If anything changes, then a socket wire has some sort of issue. If nothing changes it is just one more nail in the transformer coffin.

Exactly that were my motives. Couldn't have phrased it better myself.

[Helmholtz]

Well the voltages seem strangely low, but the ratio seems about right.?.

Well, thinking about it, that's actually no surprise on the bulb limiter (representing a relatively high series resistance). An internal or external short means a load to the winding concerned. As all windings are well coupled, the loading will be "seen" by all windings. Consequently the (reflected) impedances of all windings will be reduced proportionally but voltages ratios may stay more or less unchanged - when supplied by something like a current source and only few turns are shorted.

Seems that verifying voltage ratios on a bulb limiter is no meaningful test, sorry.

[g1]

I had thought he was still testing with the 10VAC on the primary and so TX must be ok. My bad.