Nakamichi 700 II PCB Power Supply problem.

[internauta20]

Hi to everyone...

A few days ago I took on the task of rescue my old Nakamichi 700 II (1973-79) and the first thing I come across is a lack of 12 Volt output from the power supply PCB, the voltage It is not stable and varies randomly to such a degree that in the multiple measurements I made I registered 0. 4.5, 9.7,12.5, 13.2 and even 14.6 Vol. All of the above once you have tested the output of the corresponding winding in the transformer and checking that it is correct . In fact, the outputs that feed the vumeter bulbs are correct and they turn on when current is applied to the equipment.
I am not a scholar on the subject, I checked the 8 main components of the 12vol stage, the rectifier package, the two transistors, the capacitors and the two resistors but still I cannot find the origin of the problem perhaps due to my inexperience in the subject of checks on components.
I am very grateful for the help of a good Samaritan who enlightened me about it and guided me to the right path to solve this little problem.

Please see attached images and sorry to google for the bad translation

 

[dhnash]

If you have confirmed the presence and stability of the transformer secondary voltage which is used to power the level (VU) meter lamps, you should check the DC voltages shown on the +12VDC power supply schematic to determine where the instability in voltage first appears starting with the +17.8VDC that should be present across capacitor C405. If this voltage is not present and stable, C405 may be defective (most likely) or the bridge rectifier may be defective (less likely). If the +17.8VDC is present and stable, you should then check the voltage across capacitor C407 which should be +13.2VDC. If this voltage is not present and stable, C407 may be defective. All voltages should be measured in respect to the connection labeled GND in the schematic.

 

[perryinva]

You might just want to pony up the $12 and replace all the almost 50 year old capacitors in the whole PS first, and then see if the instability is still there!

 

[internauta20]

Greetings to everyone in the forum...

Thank you very much for your answers and suggestions, I appreciate it very much.

For work reasons I had to leave home, this week I am going to buy a full recap and I will be reporting the results. Once the problem is solved, I will be doing the corresponding tests, there is still a long way to go for the great sound to return to this beauty and I hope to have all of you in the future.