JVC Power Supply Assistance

Thread Starter

samson7point1

Joined Jul 27, 2022
4
Trying to fix a JVC HM-DH30000U VCR (NTSC/US) with a bad power supply and I need some advice. This seems like something that should be simple, or at least a great learning opportunity. I'm not an expert so any level of advice is welcome, and I invite you to assume I know nothing.

From what little I know of power supply design, this one was poorly designed in that it stays hot whenever the unit is plugged in. I expect that constant heat has a tendency to cause breakdowns. There are no obvious signs of short or overheating. (I.e. no discolorations on the board or black marks, no discolored resistors etc...)

I have two of these units, one is fully working and this one will not power on. When first plugged in, the fan will sometimes spin for a second or less as though it's trying to start but gives up. I believe that the problem is isolated to the power supply because if I swap the power supply from the working VCR to this unit, this unit works normally.

The service manual can be found here: <snip>
The power supply schematic is on page 76 of that PDF.

Just probing around the power supply board one obvious problem I've found is that pins 1, 2, 3, 4, 11, and 12 on CN5302 (which goes to the digital video interface) are not showing the expected voltages. Instead 3.3v, 2.2v, and 2.5v respectively they're both showing just a few mV.

I've concentrated my troubleshooting in the area of the four voltage regulator ICs that supply power to those pins. All four of these ICs are PSQEV3's (datasheet).

Pins CN5302[1] and [2] are driven by the output of IC5302, Pins CN5302[3] and CN5302[4] are driven by the output of IC5303, and pins CN5302[11] and CN5302[12] are driven by IC5305. So one of the symptoms of the problem is that IC5302, IC5303, IC5305 are not outputting voltage.

The input voltages to those ICs look fine. IC5302 and IC5304 share an input rail and both read about 6.7v. IC5304 is outputting the expected voltage so I expect that's correct. Likewise IC5303 shares an input rail with IC5305, and they're showing about 4.9v (which makes sense considering they're supposed to have lower output than the other two).

The problem, I think, is that the "On/Off" pins for IC5302, IC5303 and IC5305 are all staying low so the regulators aren't being activated. Those three are all connected to the same signal path. IC5304 (which is working) has a different signal path for On/Off and it is working correctly. The voltage on it's ON/OFF pin is 2.4v. Since the others are the exact same chip as IC5304 (which is working normally), I temporarily bridged the ON/OFF pin from IC5304 to the ON/OFF pin on the other three to see if that would activate the other regulators, but instead it pulled IC5304's ON/OFF pin low too. Resistance between the common ON/OFF line shared by IC5302, IC5303, and IC5305 and ground is about 6.8k Ohms, so I don't *think* I have a short.

FWIW, I've tested this both with the power supply connected to the unit, and with the power supply disconnected.


Here are the steps I've taken so far:
- Checked for shorts to ground at the connectors, at Q5302, Q5303, IC5305, IC5303, IC5302, D5315, D5312
- Checked all of the electrolytic caps and replaced any that were out of spec (ESR or capacitance)
- Replaced all remaining electrolytic caps
- Replaced the power supply fuse (the old one checks out fine)
- Checked diode D5312 - checked fine with DMM
- Swapped diode D5312 and D5315 to see if the problem followed the diode (if it did, I expected to see CN5302[5] output the wrong voltage, but its output was the expected 3.3v even with the other diode in the path).
- Replaced IC5302 and IC5303

I should add that I'm not sure the problem has anything to do with those regulators.

Moderator edit: Link to service manual deleted, suspicious site, malware detected.
 

debe

Joined Sep 21, 2010
1,324
Check R5101 68k 2W resistor as it kick starts Ic5101 SMPS VCC. Also R5102 on P3 of transformer, its the VCC feed for the IC
 

Thread Starter

samson7point1

Joined Jul 27, 2022
4
Check R5101 68k 2W resistor as it kick starts Ic5101 SMPS VCC. Also R5102 on P3 of transformer, its the VCC feed for the IC
Thanks for the suggestions! R5101 and R5102 both check out (correct resistance range etc...). With the supply energized, I'm reading 11.55V on VCC for IC5101, and 168V at the drain (pin 3). The voltage on pin 1 (OCP/FB) is 311mV. I can't find a datasheet for the STR-F6555 this is the closest thing I could find. I'm guessing the OCP/FB is designed to "activate" the power regulator IC? According to that datasheet it needs at least 680mV to do that, but the fact that there is voltage on the drain leads me to believe it _is_ working. The OCP/FB pin is fed by the opto-isolator PC5101. I measured about 11.4V on both the source (pin 4) and drain (pin 3) of PC5101 so I'm pretty sure it's working properly. R5103 (10 kOhm) is showing the correct resistance. Before R5103 the voltage is 11.4 and after it's 311mV. I also checked R5104, C5104 and R5105 and all seem to check out.
 

Thread Starter

samson7point1

Joined Jul 27, 2022
4
I'm trying to understand how IC5101 (STR-F6555) is supposed to work. I found a better datasheet for a related part - I'm pretty sure it's the same exact design, just higher maximums (https://pdf.dzsc.com/F66/STR-F6626.pdf). I guess the OCP/FB is an over-current protection mechanism, so it's actually a good and normal thing it's only getting 311mV. What I'm not clear on is what the relationship between the source and drain are. I thought maybe the drain was coming from the IC, but it looks like what I'm actually measuring is output from the bridge rectifier (D5001)? Should I be seeing voltage on pin 2? What is going on in the circled section?


regulator_block_diagram.png
 

Thread Starter

samson7point1

Joined Jul 27, 2022
4
FWIW, the F6555 appears to have been faulty. Based on the behavior I believe the over-current protection was kicking in and killing off the main voltage to the rest of the supply. Replaced it and the supply is working normally now.
 
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