Panasonic SA-HT70 not working! :-(

Check for +28v on pin 14 of IC502. If missing, safety resistor FP549 is open. Failing that, you've probably fried IC502. I've got the relevant part of the schematic as a pdf if you need it. Contact me off group with a valid e-mail if you do.

Arfa

I'm sorry, but I'm a bit of a novice....

Should I be test>> Hi all,

The reference is ground ie any piece of metalwork on the unit such as the antenna socket outer, which will definitely be ground. However, I must say that if your level of expertise with this equipment is actually such that you are not aware that ground is the standard reference point unless otherwise stated, then it is unlikely that you are going to be able to troubleshoot the unit to the point of being able to fix it. Also, you should be aware of the safety angles of working on a piece of live equipment. Whilst the 'HT70 is better than some for safety in that it uses a linear rather than switch mode supply, there is still exposed line voltage in places around the power supply, so do take care ...

Arfa

Right click on one of my posts' headers, then left click on properties, where you will find my address.

Arfa

First you should locate FP549, and check the voltage ( with respect to ground ) on either side of it. If there is +28v on one side, and nothing on the other, then it is open circuit, and will need to be replaced. Officially, it is a 500mA fuse, but is actually shown on the schematic as a fusible resistor. It is quite common for such wire-ended circuit protectors or 'fuses' to actually be special low value resistors which are designed to go rapidly open circuit once their nominal power rating is exceeded. I would expect this one to be around 1 ohm at 0.5watt - but a fusible type, not a standard resistor. If you can obtain an official wire-ended 500mA circuit protector, then fit that. If not, the fusible resistor. If you're lucky, there will be no other damage. But make the checks first, before proceeding to replacing it.

BTW, it is common convention to bottom post on newsgroups ( without wishing to start another furious debate on the subject |:-\ ) as it makes the thread a lot easier to follow

Arfa

Arfa:

I rechecked the voltages on the unit.

With the unit in standby mode, pin 14 on IC502 reads about +2.0 V. FP549 also reads about +2.0 V on either side.. The voltage on both of these gradually drops the longer the unit has been plugged in. Also, If I unplug the unit, I get the same readings. I suppose charge is stored in the capacitors somewhere. If I press the power button, the voltage reading (for both IC502 and both ends of FP549) jumps to about +12.0 V, then the unit goes into standby mode after 1 second, and the readings go back to about +2.0 V (dropping gradually). Any suggestions? Thanks

By the way, I can't figure out the right place to right-click. I'm not seeing an email address.

OK. Well that basically tells us that the problem is not the protector, or probably its feed either, so it will be necessary to start checking the output voltages of IC502, which is where it gets complicated, because as it only stays on a second or two, before dropping back to standby, you don't have long to evaluate what is appearing on the chip's outputs. As it is also possible that the input voltages never have time to reach their normal running level, it takes a degree of experience to determine if output pins that are reading low in that second or two, are *heading* for the right level ...

For an e-mail direct to me, use snipped-for-privacy@ntlworld.com

Arfa

Ok. What do you recommend as my next step? Should I try and measure the voltages on all 14 pins and post the results?

Arfa Daily wrote:

You should have the schematic that covers this part of the machine now, and all of the voltages are printed at the pins of IC502. However, as I said, it's probably going to be difficult for you to interpret what you're seeing in the brief time it remains out of standby, but you can give it a try, and I'll see if anything leaps out at me.

Arfa