Trace Eliot Velocette 12R preamp heater supply

[Randall]

I am in the weeds with this one. Neither preamp tube lights. Heater supply fuse FS4 blows instantly with no tubes in at all. Lookong around for shorts, I find TR1 with 2.5 ohms across E to C in circuit, so I pull it and it measures the same. Fired it up with a fresh fuse and no tubes, and it still blows.

I don't really understand what is going on here with all these transistors.

Another thing I notice is charring under the board next to TR7 and another trace that goes to the Hi/Lo power switch. BD647 comes up as a NPN silicon darlington, designed to be used in a pair. I don't see what it's function is here.

I could sure use some help with sorting this one out.

http://schems.com/bmampscom/trace_el...locette12r.pdf.

[nosaj]

I am in the weeds with this one. Neither preamp tube lights. Heater supply fuse FS4 blows instantly with no tubes in at all. Lookong around for shorts, I find TR1 with 2.5 ohms across E to C in circuit, so I pull it and it measures the same. Fired it up with a fresh fuse and no tubes, and it still blows.

I don't really understand what is going on here with all these transistors.

Another thing I notice is charring under the board next to TR7 and another trace that goes to the Hi/Lo power switch. BD647 comes up as a NPN silicon darlington, designed to be used in a pair. I don't see what it's function is here.

I could sure use some help with sorting this one out.

http://schems.com/bmampscom/trace_el...locette12r.pdf.

Simplify, disconnect the transformer from the circuit an power it up while monitor draw. Could be a shorted winding.

nosaj

[Randall]

"Simplify, disconnect the transformer from the circuit an power it up while monitor draw. Could be a shorted winding."

Good idea. It measures 8 amps ! That can't be good.

Also I pulled TR7, and it reads 9.2ohms B to C, and makes long beeps in either direction on my diode setting.

[The Dude]

That's a good first step. That circuit is a way over-complicated regulator, IMO. Seems a bit of overkill. If the transformer is ok, check bridge rectifier diodes D6-9 also. Since all of those transistors "interact", it's probably a good idea to check them all and the zener. The tube filaments will give you false readings as you troubleshoot the supply, so I'd remove those tubes until you get the supply working. Then check supply to ground to make sure you've removed any shorts as you go. Those op amps run on a single supply instead of the traditional +&-15V. One or both of those could also be shorted.

[Randall]

"If the transformer is ok,"

What is the concensus on that winding drawing 8 amps unloaded? Well it does for a few seconds, then the mains fuse blows.

[g1]

If that is with all secondary windings disconnected, then the PT is (most likely) bad.

[g1]

I should add that this is assuming no other short in the primary circuit which is not shown in schematic.
A power switch shorting to chassis or something like that is also possible.

[Randall]

This was with only the DC rectifier winding disconnected, measuring current across the unloaded secondary. The other HT voltages are good, and the power tubes running on AC filaments light up when the rectifier winding is connected to circuit, and the FS4 is blown. So it seems the mains fuse only blows when my current meter replaces the fuse. A little research provides these amps to be prone to PT failure. There used to be a few replacement vendors, but so far they all look to be out of stock. Except for Merc Mag, which wants $130 for one. And indeed I have fixed various issues with this particular amp maybe 5 or 6 times in as many years. They run hot as hell, and this is a sub-tropic environment.

[Helmholtz]

The other HT voltages are good, and the power tubes running on AC filaments light up when the rectifier winding is connected to circuit, and the FS4 is blown.

So no reason to suspect the PT. If a transformer has any winding short all voltages will be down.

[Randall]

So why it the rectifier winding drawing 8 amps?

[log1982]

Hi all.
To answer OP's question about the TR7, it's the pass transistor in a linear regulator circuit. The circuit takes in a wide range of input voltages and outputs a fixed voltage, 25.4 according to my calculations.
It looks overly complicated but these parts are exactly what you would find in a linear regulator chip.
When you said you pulled TR1, did you try to operate it with TR1 not in the circuit? It can't work that way. Your output will be unregulated, and high. If you think TR1 and TR7 are bad, (your measurements seem to indicate that) are you replacing them?
The interesting thing is that the circuit has short-circuit protection (TR11). It's either not working, or the problem is up stream.
The good thing about discrete circuits is that it's normally pretty easy to isolate and localize problems. If it were me, I'd take out TR7 to isolate the regulated load to see what it does.
And please make sure TR7 and TR1 are good.
Jcon.

[Helmholtz]

It's not the winding. How did you measure current? I guess you connected the "load" via your ammeter?

[Helmholtz]

measuring current across the unloaded secondary.

No good idea. This way your meter is shorting the secondary and you measure short circuit current. Amperemeters have very low internal resistance.

[g1]

As above, any current measurement must be in series with load. A meter set for current measurement is basically a piece of wire between the 2 probes (short circuit).
I had thought you were measuring that 8amps on the primary side, with meter in series with winding.

[Randall]

OK, so measure in series with primary side, and secondary unloaded? As suggested in post #2?