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In Reply to: RE: TV-7 tube tester line adjustment problem posted by Richard.Sun on July 03, 2012 at 10:00:19
Hi all, I borrowed a Fluke 15B digital meter and a Heathkit IT-17 tube tester from my friend, and did some tests.(1) 5Y3 and 83 rectifiers are all tested good, so no problem with the tubes.
(2) The resistors are tested good (R124 = 245k Ω, R123 = 1k Ω, and R125 = 1k Ω), but there is a small blue resistor (= 2k Ω) bringing my attention (please see photo1, red circle), as based on the maintenance document (page10, Figure 6), this 2k Ω resistor should not be in line voltage test circuit (by the way, my tester is TV-7/U, not the later models: TV-7B/U or TV-7D/U).
(3) C103 is being replaced by a VISHAY 100uF / 25V capacitor (please see photo2), and the test reading I got for this VISHAY capacitor is: capacity = 2.5 — 10 nF (far less than 100uF), resistance = 258.4 Ω.
Does this suggest that the problem lies with the filter capacitor (C103)? Also, I am concerning about that small blue 2k Ω resistor in line test circuit — should I remove it?
Many thanks,
Edits: 07/07/12Follow Ups:
Richard,
Someone added that 2K resistor to calibrate that circuit, later models had a pot IIRC.
C103 is not a filter cap, it's simply a cap used to damp the meter movement. You can replace it, it could be the cause but I doubt it. If you are measuring the cap value in circuit your measurement may be off. If in doubt get a new cap, check it before you put it in, and be sure to observe polarity.
BTW, TV-7s use almost no filter capacitance at all - the unit works on pulsating Dc. There is just a 2700 pf cap as a "filter"! The unit also uses a copper-oxide rectifier in the meter circuit.
Check the rheostat itself, it should measure about 200 Ohms from one end of the winding to the other.
If the rheostat and the cap are good then my suggestion is to send it to Kara Chaffee and let her fix it up and calibrate it. TV-7s are quite complex, and she's an expert!!
Thanks Jim; I did check the rheostat (R126), and when turning the line adj. control, my measurement ranges from 0.012 ohms (minimum) to 211 ohms (maximum). So I think it works fine.You are right about my measurement of the capacitor value; it’s inaccurate if measuring it in test circuit. I need to get a soldering gun / iron to remove this capacitor from the circuit & test it again.
Edits: 07/07/12 07/07/12
and she has an excellent reputation. Sometimes it is just good to get the tester tuned up anyways.
The Elaborate DAC=Johnny's Balloon Meshplate 27 Balanced Preamp=Balanced Six Pacs=Newform Research 645
iRiver H140/MB Balanced dac/amp/Ultrasone Ed. 9/JH13 Pro IEM's
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Photo 2
Photo 1
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