began work today
So today I began working on this 7070. It was a little dusty from on the interior so I used my magical shop van attachments to clean up the case and the interior. I didn't use any electronics cleaner or deoxit yet I wanted to verify the problems the unit was having as described earlier(powers on, no audio though left or right channel, tunes appropriately to frequencies and indicates when an fm stereo signal has been reached).
Tested unit, tuning seems functional as in it will dial in and fm stereo will light up so I am going to ignore that board for now. We have power, the unit powers on and supplies voltage to the lights, however the protection relay doesn't click on so I assume there is some dirty DC voltage on the main supply board.
F-2625
The main power board relay did not click on so I suspect there are issues with caps and resistors on the main board. Specifically, the fuse able resistors on the main board have mostly turned white to indicate rupture. also Capacitors C07, and C08 look swollen with the wrapping split on c08 and I suspect may have caused the fuse resistors to blow in the first place. There is also a broken diode (10D) on the main board which shows resistance in both directions. I need to find a replacement part number for this diode. I know it is a common diode and I need to search for the replacement but I was hoping someone will come along and give me the part number. Im pretty sure radio shack still carries them.
F-2624
On inspection there were mostly blown fuse able resistors which I have ordered replacements for.
My first steps to check the driver circuit adjustment checks. Each channel on A and B read at around 4.00 V DC that was unchanged with adjustment to VR-01 and VR-02. The Bias current read a little high on both channels but where adjusted back to 30mA through VR04, VR04. I have also note the ZD01 and ZD02- (24v 1/2 W ZD's) had some odd black corrosion along the leads and needed replacing, those have been ordered.
F-2626
The power supply gave some crazy readings (Unpowered, aux from top four,
1. 1.49 2k and rising.
2. 8.9 20K and rising.
3. 10 20k and rising.
4. 12 20K and rising.)
I have ordered replacement resistors for the fuseable resistors for the board. The zener diode had some odd corrosion so I ordered a replacement for that as well (13v 1/2W ZD's).
There are no obvious burns to the circuit boards and I believe the passive replacements will suffice to bring this unit back into service. Once I have the components in the mail I will replace the caps, resistors, and diodes.
I am going to post some pictures to show you the work I am describing.