[TechAssist] Re: RCA CTC177BE tuning problem
- From: "Dave Huston" <DaveHuston@xxxxxxxxxxxx>
- To: <techassist@xxxxxxxxxxxxx>
- Date: Tue, 9 Jan 1996 15:56:49 -0500
Thanks Gonzalo. I gave the customer an estimate of aprox. $160. If he
approves it I'll try your suggestions.
----- Original Message -----
From: "Gonzalo Arrivas" <gna2000@xxxxxxx>
To: <techassist@xxxxxxxxxxxxx>
Sent: Wednesday, January 09, 2002 3:19 PM
Subject: [TechAssist] Re: RCA CTC177BE tuning problem
>
> Dave,
>
> Here's a tuner troubleshooting you can follow:
>
> TROUBLESHOOTING THE TUNER
> IN THE CTC 175-187 CHASSIS
>
> Step 1 Check the voltage at U7401-10 (5VDC)
> This will verify B+ and establish a good ground.
>
> Step 2 Monitor the voltage at Q7401-C Tuner Control
> If the voltage is changing when you change channels this would
> indicate U7401 is working.
>
> Step 3 Check the voltage at U7501-4 Quad Op Amp
> This is the 33 volt used for channel tuning.
>
> Step 4 Monitor the voltage at Q7402-c Band Switch Transistor
> You should have a negative DC voltage for band 1 (-11 to -14 VDC)
> You should have a positive DC voltage for band 2 (+11 VDC)
> This will switch positive or negative as you change bands.
>
> Step 5 Monitor the voltage at U7401-9 Mixer/OSC
> This is the band switch used at U7301-7. 3 VDC activates the VHF OSC at
> pin 11. When UHF is selected, U7301-7 should go to 0 VDC. The UHF OSC at
> U7301-14 should be running. When OSC isn't running the voltage is about
> 10 to 11 VDC and when the OSC is running the voltage will measure about
> 4.5 to 5 VCD. Another good thing to know is that if U7301-1 is low
> (less than 5 VDC) the chip is probably bad and the tuner alignment won't
> respond because the RF AGC will remain high.
>
> Step 6 The voltage at U7401-9 is also used for RF Band Switch
> Q7403 and Q7404 collectors are connected together and the state of the
> common
> collectors determines the VHF/UHF RF switching. When the voltage at
> U7401-9
> is high (7 VDC), Q7403 turns on. Its collector is 0 VDC and the VHF RF
amp
> Q7102 is turned on by lowering the source voltage to about 4.5 to 5 VDC.
> When U7401-9 is low (0 VDC), Q7404 turns on. The collector is high (11
> VDC).
> Q7102-S will go high turning the UHF RF amp off. At the same time
> Q7101-D (UHF RF amp) goes high and the UHF RF amp is turned on. An
> overview
> would be when a VHF band is selected, U7401-9 is 7 VDC. You should have 3
> VDC
> at U7301-7. The VHF Mixer/OSC will be running and the voltage at U7301-11
> will
> be about 4.5 to 5 VDC. Q7403 will be in saturation and the collector is
at
> 0 VDC. When U7401-9 is 0 VDC, U7301-7 is 0 VDC and the UHF Mixer/OSC is
> selected. U7401-14 will now measure about 4.5 to 5 VDC. Q7404 will turn
> on,
> collector is high and the UHF RF amp is selected.
>
> Step 7 Q7401 off U7401-14 is the Tuning PLL Transistor
> Its collector is used to control the 33 VDC for tuning. This voltage is
> used
> at U7301-3 as a tuning reference for the QUAD OP AMP. The digital
> information
> stored in the eeprom comes into U7501 pins 5,10 and 12. This information
> is
> converted to an analog value at pins 7,8 and 14 and used to tune the
> varactor
> diodes to band pass the selected channel frequency. You can use a DVM to
> measure
> the voltage at U7501 pin 3. This is the tuning reference. Selecting
> channel
> 2, you should have about 1.35 VDC on pin 3 and about 1 VDC at pins 7,8 and
> 14.
> Selecting channel 13, the voltage at pin 3 is about 2.01 VDC. Pins 7,8
and
> 14
> about 6 VDC. Now you should go to the varactor diodes and measure
> approximately
> the same voltage at their cathodes. (Keep in mind that these voltages
will
> change
> as you chage channels). When in the air mode, you may use the chart at
the
> bottom
> page 49 in the CTC 175/176/177 training manual for reference. When in the
> cable
> mode, use the chart on page 51.
>
> VARACTOR DIODES are designed such that their capacitance is dependent on
> the voltage
> across the diode. As the tuning voltage varies across the diode, a
> varactor acts
> like a variable capacitor causing the tuned circuit to vary its frequency.
> Different
> channels are tuned in this manner. (Note: Varactor diodes work with
> reverse bias).
>
> Thanks to Bob Rose for providing the above information...
>
> >From: "Dave Huston" <DaveHuston@xxxxxxxxxxxx>
> >Reply-To: techassist@xxxxxxxxxxxxx
> >To: <techassist@xxxxxxxxxxxxx>
> >Subject: [TechAssist] RCA CTC177BE tuning problem
> >Date: Tue, 9 Jan 1996 07:47:11 -0500
> >
> >
> >This one tunes channels 2-13 fine. UHF channels won't change. I get the
> >same
> >channel no matter what the OSD says. Did standard RCA tuner resolder to
no
> >avail. Has anyone seen this before?
> >Dave
> >
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>
>
>
> G.N.Arrivas, CET, Pittsburg. CA
> Tel/Fax: 925-432-2057
> Email: gna2000@xxxxxxx
>
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