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Odd input impedances for DVDR

I have a Tevion model TDR250HD DVDR which I purchased about a year ago. At that time I noticed some strange things about the input impedances on several video inputs. I posted my observations in sci.electronics.repair but received no suggestions. I've since received an email from someone who purchased a current model Tevion DVDR with similar measurements. Anyway here are my observations.

The resistance looking into the component inputs is 75 ohms, but the S-video chrominance and luminance resistances are 51 ohms and 38 ohms, respectively. The composite input resistance is also 38 ohms. Inside the box I see three smt resistors, 150R, 75R, and 75R, located near the sockets. It appears that each of the inputs is double terminated. For example, the chrominance input appears to be terminated with a 150 ohm resistor in parallel with an additional 75 ohm resistance elsewhere on the PCB, possibly in the video ADC. This would account for the measured value of 50 ohms. Similarly, the luminance and composite inputs would each measure 37.5 ohms. FWIW, I see no 75R termination resistors near the composite inputs. Because of the dissimilar Y/C resistances, I would think that the Tevion's S-video input would be distorting the signal.

AFAICS the only kind of arrangement that *might* make some sense is something like this:

input o--|----|---o | | 75 L | | | 150/75 _|_ _|_ = =

The DC resistance is 50/37.5 ohm, but the impedance at video frequencies would be 75 ohm.

I don't recall seeing any inductor, though.

The other person observed a resistance of 75 ohms on his composite input but mine measures 37.5 ohm. To me the latter is asking for trouble. AFAICS the input to the DVDR would be reduced to 0.67Vpp (instead of 1Vpp) assuming a 2Vpp 75 ohm source.

2Vpp 0.67Vpp source >--- 75R ---o o------|-----> decoder | 37.5R | _|_ =

All my composite recordings appear to be OK, though.

BTW, the specs page in the Tevion manual states, for both output and input, ...

Video Level: 1.00.2Vp-p 75 ohms ?????? Y output/input level: 1.0 Vp-p 75 ohms C output/input level: 0.3 Vp-p 75 ohms

There are no typos in the above specs.

Thanks for reading this far.

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar
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On low end home video equipment I have seen this. This is because in the design process, they trim with the input load rather than modify their design for the optimum. It's a cheap way to correct. If you go for the higher end products, you should not see this done.

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JANA _____

The resistance looking into the component inputs is 75 ohms, but the S-video chrominance and luminance resistances are 51 ohms and 38 ohms, respectively. The composite input resistance is also 38 ohms. Inside the box I see three smt resistors, 150R, 75R, and 75R, located near the sockets. It appears that each of the inputs is double terminated. For example, the chrominance input appears to be terminated with a 150 ohm resistor in parallel with an additional 75 ohm resistance elsewhere on the PCB, possibly in the video ADC. This would account for the measured value of 50 ohms. Similarly, the luminance and composite inputs would each measure 37.5 ohms. FWIW, I see no 75R termination resistors near the composite inputs. Because of the dissimilar Y/C resistances, I would think that the Tevion's S-video input would be distorting the signal.

AFAICS the only kind of arrangement that *might* make some sense is something like this:

input o--|----|---o | | 75 L | | | 150/75 _|_ _|_ = =

The DC resistance is 50/37.5 ohm, but the impedance at video frequencies would be 75 ohm.

I don't recall seeing any inductor, though.

The other person observed a resistance of 75 ohms on his composite input but mine measures 37.5 ohm. To me the latter is asking for trouble. AFAICS the input to the DVDR would be reduced to 0.67Vpp (instead of 1Vpp) assuming a 2Vpp 75 ohm source.

2Vpp 0.67Vpp source >--- 75R ---o o------|-----> decoder | 37.5R | _|_ =

All my composite recordings appear to be OK, though.

BTW, the specs page in the Tevion manual states, for both output and input, ...

Video Level: 1.00.2Vp-p 75 ohms ?????? Y output/input level: 1.0 Vp-p 75 ohms C output/input level: 0.3 Vp-p 75 ohms

There are no typos in the above specs.

Thanks for reading this far.

- Franc Zabkar

-- Please remove one 'i' from my address when replying by email.

Reply to
JANA

On Sun, 11 Nov 2007 22:25:48 -0500, "JANA" put finger to keyboard and composed:

What is being "corrected" ???

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar

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