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Picked up a BK 700, had a bad 83 tube, replaced with the diode mod. Lights come on, tube heater is working but no meter movement. The 6NB8 tube is lighting up as well as the two bulbs inside. What should I be checking for? It does appear the original caps are in it so those I am going to replace. All the carbon comps are actually within spec which surprised me.
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A dead meter is either the meter itself, or the circuit driving it. COnnect your volt meter across it and see if any voltage is present when it ought to be moving. Disconnect it, and measure the resistance through its coil - is it open?Education is what you're left with after you have forgotten what you have learned. -
I'd be careful with an ohm meter, checking across the movement... they run on a few mili or micro-amps don't they?Originally posted by Enzo View Post
I guess you could put a real high resistance in series with the movement's coil though... if it's open, it's open.Comment
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I thought about that, I should have said something. Most times a digital meter facing the usually not that sensitive meters in tube testers is OK. But you are right.
I ought to put an ammeter across my ohm meter and see what current it spits out.Education is what you're left with after you have forgotten what you have learned.Comment
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I did measure the resistance on the lowest setting, it is open i believe. I found a drifted 56 ohm resistor that was connected from the meter to ground. There is a 200uf 4v cap on the positive side. The schematic says it should be a 250uf 6v and doesn't show the resistor. I think I read before on how they used different meters in these.Comment
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I used to restore vintage tube testers.
In those old analog meter systems, there is usually a "shunt resistor" across the coil of the meter.... it is precision one so that the parallel resistance of the two allow a little bit of the current to flow in the meter, making it move to full scale... etc.
What I meant was to be careful if you are just holding the meter movement out, to check the coil for being blown open and with no shunt resistor, as the coil could draw quite a bit of current with the .7vdc to 2vdc from an ohm meter.Comment
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I took the meter apart and found a disconnected wire. Its resoldered and I got the meter back together, just have to test it out in circuit. That sure was a tricky job.Comment
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