This unit came in this last week for a forum member here. He’s up in my area from New Jersey for a wedding so I was hoping to get this back before he goes homes. Great meeting you again Raymond in person!
This unit came in for a simple clean up and spot check, reporting and LED’s. Upon inspection I noted several issues with the Scott 350R. This unit would not bias correctly or output on the left channel. along with massive arching occurring.
Like many of the low wattage Pioneers, the Scott 350R uses a buckled power switch in conjunction with he speaker selector. This particular switch uses two sets of internal contacts within a plastic body. I have gone in detail before the clean up procedure which is still junk. The switches self destruct because of the gapping at the contacts and the plastic case, the contacts WILL ARCH and destroys the plastic, gapping the contacts yet further.
The only true fix is to bypass the original switch. For cosmetics sakes we use a 7A DPDT toggle switch thats mounted on the rear of the chassis with a new X1Y2 Safety surge capacitor installed.
The particular board had several bloated electrolytic within the power supply due to the proximity of the current limiting resistors, these x3 were replaced with 220/50V high temp Panasonic FC capacitors.
It was noted that the current limiting protection circuit was faulty within the driver stage, this was due to the main output Q42 leaking under full load, as this cascades it shunts its excessive current through R234 a 150 1/2W resistor that would then short out. As a bonus to Raymond we decided to upgrade the whole driver.
Pulled Components ...yuck city
The original single 5 leg differential transistor a 2SK type was replaced with 1% matched KSA992 transistors, along with the bias control to a TO-126 type 3503 package vs the original TO-92L for better dissipation. The pre-drivers and drivers were upgraded to modern low noise BJT’s as well. New Fairchild TO-3LP outputs installed as well. Electrolytics within the driver stage were upgraded to a low impedance high temp (105C) form of Nichicon PW’s and all associated 1MFD input/outputs to a high grade Panasonic ECW polypropylene film capacitors.
All new LED’s were custom tailored and installed with all new LED mounting tabs and epoxied in. All inputs cleaned and she’s ready to go home!