Author: Provo Electronics

We are not fixing audio today.  I upgrade my old friend Dewault 920  . This hard working  guy  was in service with me from 2010 .  Got good reputation and perfect resume .   As you know this tool works for electrical jobs and small projects and use not powerfull NI-MH battery  7,2 Volt.  After 5- 9  month  of work you have to buy new battery. In this case we will transfer  it to lithium 7.4 volt battery . We use two 18650 lithium elements in series. Gentle soldering requaired. Create new charge port and drop new batteries in old plastic shell .   DO NOT USE OLD CHARGER !!!!     It will damage new Lithium battery.

In my case i used simple audio RCA plug as charging port. Charger we will use 8.4 volt  1-2 Amp for lithium batteries only.   8.4 volt only with protection and current control . Will do update how my upgrade process  goes.

Case 432200. 12/27/2024. Sansui AU-719. Increasing volume leads to cut left channel in half.
Nice amplifier. Build to last. Problem acting as bad volume pot.
After detailed diagnostic found contacts oxidation on speaker protection relay. Relay replacement solved this problem.

Case   345560  .   12/23/2024.  Poineer SX-880 . No sound. Unit in protection mode. Protection relay not clicking  after turn on .

After troubleshooting  found  –  only one channel causing this issue.  ( DC offset voltage test on protection relay) .  After we know what chanel is bad  we focus on this specific channel. Found bad 2SA899 driver transistor( you can use 2sa939 , 2sa914) and bad soldering   joints on board.  After changing transistor , rebuild joints – still no sound. Investigating voltages on this transistor  found resistor 330 ohm connected to base 2sa899 changed value to 10000 ohms. Same story in working channel  – from 330   to 1500 ohms. Changing resistors in  both channels solved the problem.  Our guess – resistor change value problem developed by bad soldering  joints on main board and   hard working transistors in line after  46 years . Or they getting older like people do   or something else  on chemical level .
Nice receiver , easy to work on it ,   enormous power .  All critical and major components designed on  one board.