Subject: Zenith VCRs  "glitch" playing some prerecorded tapes.   8/95

     There are tracking problems with some recent models of Zenith brand 
VCRs  when playing some pre-recorded (rental) tapes. The particular 
models I worked on were VR-2410 and VR2411. The customer complained of a 
periodic loss of picture when playing some rental tapes (movies)... the 
VCR playback picture would break up repeatedly, throughout the film, and 
the "auto-track" indicator on the machine would flash. The VCR would 
play its' own recorded tapes just fine as well as most prerecorded tapes 
from the rental outlet. The "bad" rental tapes would play fine on any 
other VCR! So is it the machine or the tape? Actually, it's both! I ran 
some tests on one of those VCRs and sent all the information I had on 
the problem to Zenith several months ago.
     A recent service bulletin from Zenith indicates that the "bad" 
rental tapes were probably recorded in a VCR that had an "index search" 
function, which lays down a modified CTL (control track) on the tape 
that the Zenith, and possibly other brands (Goldstar, for one) will not 
play properly. It's the combination of the tapes and that particular VCR 
that creates the problem. Zeniths' solution is to modify the Control 
Track circuitry in the VCR (add a transistor and resistor) to invert the 
CTL pulse. 
     Although it was written for service techs, I will quote the Zenith 
Bulletin verbatim:
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SUBJECT:            Unable to Auto-Track Some Rental 
                    or Pre-recorded Video Tapes                   

Auto tracking problems with some rental or pre-recorded video tapes has 
occurred in 1992 "J2" and 1993 "L" Line VCRs. The symptom is a breakup 
of the video with audio slur that occurs every 8 to 10 seconds. Placing 
the VCR in the manual tracking mode by depressing the adjust key in most 
cases does not cure this situation. The cause of the problem is the VASS 
(VHS Address Search System) signal that is encoded onto the rising edge 
of the CTL pulse. This VASS signal is causing the servo circuit to not 
be able to access the CTL signal correctly. 

The problem can be corrected by the installation of a transistor on the 
foil side of the PCB between the servo IC-201 and the system IC-202. 
This transistor will invert the CTL pulse so that the VASS signal is on 
the falling edge of the CTL pulse where it will not upset the servo 
circuit. 

Install transistor (Part# 921-2134 or 921-2161) between pin 56 of servo 
IC-201 and pin 17 of IC-202 the system IC. Open jumper wire W2C4, since 
it is located under the chip, cut the foil near pin 56 of IC-201. Solder 
the base connection of the transistor to the pin 56 side of IC-201 and 
the emitter terminal to the nearest ground foil. Connect the collector 
to the pin 17 side of IC-202 and to a 10K ohm resistor. Connect the 
opposite side of the 10K ohm resistor to the +5 volt supply located at 
jumper W2H5.


Note: This modification is not to be installed in models VRL-2210, 
VRL-4110 or VRL4210. Carefully install the transistor and resistor as 
close to the PCB as possible maintaining proper lead dress and safety. 

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