# Odd input impedances for DVDR

Discussion in 'Hobby Electronics' started by Franc Zabkar, Nov 11, 2007.

1. ### Franc ZabkarGuest

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. <shrug>

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.