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10th Mar 2007, 8:09 pm | #1 |
Retired Dormant Member
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Thorn 1500 - no line sync
Long time since I saw one and never had this fault on one before; usually only mains droppers and repairing the tuner latch bar.
I have changed the 30FL2 osc valve, but still no line sync: line hold control does nothing. Last edited by Mike Phelan; 2nd Dec 2007 at 5:47 pm. Reason: Caps and punctuation needed |
10th Mar 2007, 9:25 pm | #2 |
Dekatron
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Re: thorn 1500 no line sync
Can you tell if the frame lock is OK? If you rock the frame hold about slightly, you should be able to tell. Anyway, the line hold/horizontal hold control is R65.
This tells us that the line oscillator is working, but on the wrong frequency. Right...well... C44, W3 and W4 (swap them both) and R53 and R60. From here, it goes to TR10. Also worth a look is C51. Hope this helps! Cheers, Steve P
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10th Mar 2007, 9:26 pm | #3 |
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Re: thorn 1500 no line sync
Check R44 47k res. The most common fault on any set I can recall!
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10th Mar 2007, 11:49 pm | #4 |
Pentode
Join Date: Jan 2006
Location: Stockton On Tees, Cleveland
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Re: thorn 1500 no line sync
Hi Simon,
Almost certain I did this model with a similar fault about 2 years ago. The fault was in the field timebase; new PCL805 and checked the pentode cathode resistor (300 ohm) I think it was. If that doesn't cure it, check C70, C73 and C75. the cross couplers and C81 the linearity feedback cap. Also look for discoloured resistors. Regards. Mike. Last edited by Mike Phelan; 2nd Dec 2007 at 5:50 pm. Reason: Caps and punctuation needed |
10th Mar 2007, 11:57 pm | #5 |
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Re: thorn 1500 no line sync
Frame hold seems to be ok.
I changed R44 47k - had risen to 58k, changed w3 and 4 with 1N4148 equiv as one of them was o/c. Checked r53 r60 both ok. I also changed vt10, and every now and then the line hold can be made to lock for a split second, but the control doesn't appear to do anything except when it decides to lock by itself. Then the control has temporary control for a split second. C51 had only risen by half a mfd. Any suggestions for w3/4 and vt10? Last edited by Mike Phelan; 2nd Dec 2007 at 5:53 pm. Reason: Caps and punctuation needed |
11th Mar 2007, 12:12 am | #6 |
Pentode
Join Date: Jan 2006
Location: Stockton On Tees, Cleveland
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Re: thorn 1500 no line sync
Hi Simon.
If you switch the set off then on again do you get temporary line hold ? Regards. Mike. |
11th Mar 2007, 12:44 am | #7 |
Dekatron
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Re: thorn 1500 no line sync
What are the voltages on VT10. Should be 44v, 1v and a DC voltage from the W3-W4 junction. It also gets a feedback line from the LOPT, via C45, R51 and C49.
Is the line hold control itself OK? If it is, then the DC on the cathode of W4 should change. If it does, so should the DC on R54. There's resistors and a cap across W3-W4; are these OK? The other end of R54 goes to the base of VT10 as a controlling bias. The output goes from the collector to V1b. Check C50 as well. Cheers, Steve P
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11th Mar 2007, 11:37 am | #8 |
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Location: Warnham, West Sussex. 10 miles south of DORKING.
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Re: thorn 1500 no line sync
Have you changed the actual line hold control? These are just glorified presets with a knob, and used to give loads of problems. It's probably partly O/C.
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11th Mar 2007, 2:14 pm | #9 |
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Re: thorn 1500 no line sync
Someone has beaten me to it and changed the hor hold control with a newer closed type.
A thought of mine was that the knob shaft was spinning inside the control, but this wasn't the case as it tracks from one end to another perfectly ok. Last edited by Mike Phelan; 2nd Dec 2007 at 5:55 pm. Reason: Caps and punctuation needed |
12th Mar 2007, 8:11 am | #10 |
Dekatron
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Re: thorn 1500 no line sync
Hello again Simon,
It may appear to track but have you measured its resisitance? It may be mechanically ok but intermittently o/c. This is a very odd fault for a 1500. I believe from memory its 390K. A 470k will work ok at least for a test. Regards John. Last edited by Mike Phelan; 2nd Dec 2007 at 5:56 pm. Reason: Typo |
14th Mar 2007, 4:49 am | #11 |
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Re: thorn 1500 no line sync
Tracking problems do exist with this model.
Try a 380K 10W wirewound preset. This should help in this case. Good luck |