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Old 11th February 2012, 02:42 PM   #11
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TP11 (vt) shows .016 volts. TP7 (fout) has a fixed high frequency signal and does not vary with the tuner settings.
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Old 11th February 2012, 03:52 PM   #12
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The problem seems to be control circuit of the front end sections ( the PLL loop - IC701 - the prescaler, IC702 - the pll synthesizer ) and that's just a little bit out of my league. You could check those IC's ( and IC703 ) with a voltmeter to see if the voltages on the pins correspond to those in the schematic, maybe you can find the damaged one.
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Old 11th February 2012, 03:53 PM   #13
nattawa is offline nattawa  Canada
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Quote:
Originally Posted by A Sound Mind View Post
TP11 (vt) shows .016 volts. TP7 (fout) has a fixed high frequency signal and does not vary with the tuner settings.
The problem probably lies somewhere outside the rf front end. The tp11 should show a changing dc voltage when you tune the frequency on the control panel which in turn causes the frequency of the local osc signal at tp7 to change. Try applying a variable dc voltage at tp11 with a lab dc supply through a 1kohm resistor and see if you can tune in some stations, so as to rule out the frontend.

The fact that the rf frontend sends the local osc signal out indicates a digital PLL tuning control is employed. This system takes the local osc frequency (at tp7), counts it down and compares against an internal reference frequency, the error is then translated into the tuning voltage (tp11) to tune the local osc frequency. The same tuning voltage also tunes the rf input and amplifier circuit. Such system is usually part of a u-con. So what you want to check, after ruling out the rf frontend, is the circuit around the u-con, for example the pwm filter that generates the tuning voltage, its power supply, and so on.
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Old 11th February 2012, 04:33 PM   #14
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Varying the voltage at TP11 did in fact tune in stations. So it appears that we can rule out the front end.
Now I'm having trouble understanding the next step. What is a U-con? Where is the PWM filer? It isn't identified on the schematic. There are a large amount of components in these areas, including several ICs, so I'd need some help in zeroing in the areas to check.
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Old 11th February 2012, 05:29 PM   #15
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Here's some possibly useful info:
IC710/TD6104P
1-5.54V
2-5.49V
3-4.56V
4-0V

IC702/TC9147AP
4-4.74V
5, 22, 23, 24, 27, 28, 32, 33, 35, 36 - 0V
29-.015V
30-5.6V
34-.33V
40-5.05V
41, 42 - 4.95V

Now this one looks off:
IC 703/TD6301PA
1-0v
28-12.61v
Per the schematic, Pin 28 should read 5.5v.
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Old 11th February 2012, 05:39 PM   #16
nattawa is offline nattawa  Canada
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u-con = micro controller. I don't have the schematic and can't discuss in details. consumer electronics usually use cheap controllers that would not offer sufficient resolution at the DAC ports for tuning voltage, they often get away with an i/o port programmed as pwm working with an external power supply paired with an R-C LPF to obtain the resolution needed. Again without a schematic I can't tell there is such circuit in your receiver. Can you post the controller part of the schematic?
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Old 11th February 2012, 05:52 PM   #17
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Here's the controller portion of the schematic.
Let me know if you need more sections.
Attached Images
File Type: jpg HK690i_Controller.jpg (850.5 KB, 28 views)
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Old 11th February 2012, 06:39 PM   #18
nattawa is offline nattawa  Canada
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Look at the LPF block in the schematic, above the u-con IC702. If possible, disconnect the drain lead of Q702(or simply remove Q702), then measure and make sure the 30.0V supply is good. If 30.0V is confirmed good, try to replace C709 with a new one, preferably a film cap as opposed to an e-cap. watch the voltage rating of 50V or higher.
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Old 11th February 2012, 10:20 PM   #19
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I pulled the FET and did not read 30v, or any voltage at all for that matter.
+44 volts should be feeding the bridge rectifier (D71) on the tuner board, which is then converted to 30 volts at Q75/Q76. This voltage is not present.
The voltage source on the main board is good, so the loss is somewhere in between. Again, the schematic is difficult to follow, so it may take a little while to find where the loss is occuring.
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Old 11th February 2012, 11:01 PM   #20
nattawa is offline nattawa  Canada
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That's right. The 30.0V takes supply from the power amp positive rail (44.0V), through R82 as current protection, through in-rush limiting circuit Q75 an Q76, then stabilized by D75. all you need to do is an ohm measurement between the 30.0V line and GND making sure they don't short to each other, then checking up R82 Q75, Q76, D75(30v zener), and the resistor across b-e of Q76, as well as C74(the cap parallel to D75).

Easy fix.

Last edited by nattawa; 11th February 2012 at 11:09 PM.
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