Panasonic AG7500 professional S/VHS recorder repair - no capstan spin. - Page 1
Forum section: electronics / repair

author: Yansi
author_rank: Super Contributor
timestamp: November 04, 2020, 12:50:02 am
content: Hello,I just got a Panasonic AG7500 video cassette recorder. Pretty impressive looking professional machine. Unfortunately, it has a lot of problems. Except from some mechanical issues such as the cassette lift mechanism is stiff (motor can't load a cassette), we have some electrical one:Capstan does not spin.If I manually load a cassette and power the machine up, it seems to accept the cassette and leaves it put. Then if I press the Play button, it pulls the tape out, spins the cylinder. Spins for a few seconds, then unloads the tape back and stops. Capstan does not spin.Fortunatelly, full service manual was available:https://elektrotanya.com/panasonic_ag-7500pan.pdf/download.htmlI have tried to troubleshoot myself, but come to a dead end.First, I have located the schematics part, that spins the capstan. In the scanned manual above, the relevant part is pages 97 through 99. The power stage is page 99 top right corner.I have checked power supply voltage, is about 26V at standstill - which I find rather high, especially if the schematic calls for 20V. Huh. Maybe sign of a possible other problem? Have not checked where the 20V should come from yet.Then I have tried to look for the datasheet of the BLDC controlled AN640G, found nothing, but a Russian clone KP1043XA7 (KR1043HA7). But does not matter, I have figured out the functionality of it anyway. Pin 4 is the input control voltage, pin 7 is the reverse rotation select. No other control inputs to that IC as far as I can see. (Pin 5 is just a reference voltage, divided from a 5V supply rail).So, I have measured voltage at pin 4 at idle (still) condition and found 1.2 V. Schematic calls for about 3.8, which is probably a voltage when running. So I have dug further, as what provides the control voltage. It goes through an amplifier opamp IC11, which works correctly (input voltage at pin 5 is 0.812 V). Digging further, the voltage comes from IC6 (AN6357). Digging even further, I have found that the capstan microprocessor, IC1 on page 97 of the manual, never enables the capstan. It is a pin 33 (marked as CAP.ON). This voltage line always sits at voltage very close if not exactly 0V (measure just 16 mV there). The voltage does not change even if I press the PLAY button.So I thought there may be a short circuit, due to the very very low voltage there. Unfortunately it is not the case. There is no short to ground. The CAP.ON (capstan enable line) is the green one marked in the schematic parts I have uploaded below.Unfortunately, I am out of ideas what tail to chase next. When should the controller enable the capstan? What is the condition for enabling the spin? Any ideas, or experience repairing VCR machines?I may try manually overriding the CAP.ON line and force it spin, after pressing the play button. Unfortunately, I think that would not be enough, as there are many other control lines for other circuitry of the capstan, that do seem to not have appropriate levels. So the issue is likely not in a faulty MCU (I also pray and hope), but elsewhere. But what control signals should I chase then?I think I should check the PSU. The 26V is suspicious.Many thanks for any hints.

author: Yansi
author_rank: Super Contributor
timestamp: November 04, 2020, 01:41:28 am
content: So, examining the inputs to IC1 (the capstan microprocessor), the only inputs that can influence its internal states are these:The serial port. (ugh. Not fun).pin 13 cylinder reference (not sure what it does)pin 14 head switch (not sure what it does)pin 15 capstan lock (however, this should be ignored when spinning up, shouldn't it?)pin 16 forced lock off (not sure what it does either)pin 18 frame lock (not sure what it does)pin 19 HD audio on (unlikely to be a cause of not spinning capstan).Do, these are the only inputs to the microprocessor. Others are output pins, so this group of pins should be decided upon, wheather to enable the capstan.//EDIT: Power supply 20V rail okay, it is an unregulated one straight from the filtercap.