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Thread: Trace Eliot Velocette 12R preamp heater supply

  1. #1
    Supporting Member Randall's Avatar
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    Trace Eliot Velocette 12R preamp heater supply

    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.

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    It's weird, because it WAS working fine.....

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Randall View Post
    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

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    Supporting Member Randall's Avatar
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    "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.

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    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.

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    Supporting Member Randall's Avatar
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    "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.

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    Don't forget the joker g1's Avatar
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    If that is with all secondary windings disconnected, then the PT is (most likely) bad.

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    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.

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    Supporting Member Randall's Avatar
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    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.

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    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.

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    So why it the rectifier winding drawing 8 amps?

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    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.

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

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    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.

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    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.

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    Supporting Member Randall's Avatar
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    OK, so measure in series with primary side, and secondary unloaded? As suggested in post #2?

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    Quote Originally Posted by Randall View Post
    OK, so measure in series with primary side, and secondary unloaded? As suggested in post #2?
    You may, but it won't reveal anything new. With heater circuit disconnected primary current will be low and connecting the heater circuit will increase current. There is no doubt that the PT has no winding short.
    If FS4 blows, the problem (short) must be in the following circuitry. Do you get 24VAC at the secondary with FS4 out?

    You may want to use a bulb limiter to save fuses.

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    Last edited by Helmholtz; 01-29-2020 at 08:11 PM.
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    Message for the OP: I get a sense that you're looking for a quick answer. Who wouldn't? The suggestions from this community are more pragmatic. Each member has made a suggestion that helps isolate the problem, and we expect that to be the first step in a multi step approach. It's a time tested trouble shooting method. I hope that helps you push forward. Let us know what you decide, and what your results are.
    Jcon.

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    I had another thought, and again I'm open to other interpretations. I think TR11 is a current-limit circuit to reduce inrush current. It keeps TR7 healthy during the stressful turn-on requirement of charging the caps and feeding the cold filaments. Having said that, there are a lot of opportunities for a structured troubleshoot methodology. Let us know what you come up with. BTW, is FS4 a slo-blo?
    Jcon.

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    Quote Originally Posted by log1982 View Post
    I had another thought, and again I'm open to other interpretations. I think TR11 is a current-limit circuit to reduce inrush current. It keeps TR7 healthy during the stressful turn-on requirement of charging the caps and feeding the cold filaments. Having said that, there are a lot of opportunities for a structured troubleshoot methodology.
    I think that's correct. Perhaps it would be better to start again from the beginning as a lot of what has gone on above is confusing?

    To start I'd remove FS4 and measure the transformer voltage Tx6 to Tx7 but I'm sure it's OK.
    Then Remove TR7, insert FS4 (Slo Blo) and check the voltage between ground and D7 cathode.

    This may help us see what Randall is up against.

    Click image for larger version. 

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    Last edited by Dave H; 01-30-2020 at 08:51 PM.

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    Supporting Member Randall's Avatar
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    I removed FS4 to measure TX6 and TX7, and when I powered up something went Snap! Nothing touching the chassis, everything was the same as the last time it was powered up. I informed the customer of this, and he said stop. He has already paid more in repairs then he paid for the amp, so as much as he likes it, he's done. I think it may have been the charred section of the board again, in hindsight maybe I should have dealt with that first.

    So that's all for me. Thank you guys for indulging this, and apologize if I made things confusing. I appreciate you.

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