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Hi, I have a TV-7 tester that I acquired 4 years ago, and today it shows a line adjustment problem. The meter needle can only reach 30, even I turn the line adj. control to the maximum level.
Given that the 5Y3 tube was replaced 2 years ago (when it wore out) and the 83 rectifier tube has been in its position for roughly 10 years, I suspect that the 83 rectifier tube may wear out as well; but unfortunately, I do not have another tester that allows me to test these tubes. Also, I am rather reluctant to send my tester to the technicians, as I will have to wait for weeks or even months to get my tester back.
My question is: could a weak / bad 83 rectifier tube cause the line adjustment problem? If so, I may buy a brand-new 83 rectifier tube to replace the old one, and see what will happen first.
Best regards,
Follow Ups:
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: 07/07/12
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; its 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
see images at: www.pbase.com/jamato8
Photo 2
Photo 1
I had the same symptoms several years ago and the problem was the meter.
Make sure you check your mains voltage first. I was helping a friend diagnose his TT-1 tester over the phone recently and come to find out his house mains had spiked to 128vac and caused his tester to not calibrate or line adjust.
The schematic looks like the tranny wants to see 93v after the line adjust.
Many thanks for the TV-7 maintenance document.
I am currently reading that document, and trying to figure out where the problem lies (although I am not a technician). It seems that I also need a multimeter to check whether the resistors (R124, R123, R125 ) go bad.
I have worked on the TV7's and there is one resistor in particular that can go out. I don't remember the number from memory but have replaced this same resistor on a couple of testers.
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
see images at: www.pbase.com/jamato8
IIRC:The line adjustment is a simple rheostat If you lift off the top plate by uscrewing the manty black screws on the perimeter you canlift the top plat off and access the circuit. Rheostat was a relatively low ohmage IIRC.
Stu
Don't think it is as easy as replacing a tube.
It looks like there are some diodes and other parts in that particular circuit. Look at Page 9 of the attached article.
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