Monday, November 2, 2015

Sansui G-7500 Highlight

Sansui G-7500 Receiver 

We have another G-series Sansui in for a repair I thought Id highlight again, this time i'm featuring the G-7500 model.

This unit came in with catastrophic damages! A fire had erupted on the F-2980 assembly under the bias network and blown the right output bank. Again, the culprit is the F-2980 area, we have many many Sansui G series come in with major damage to this board. The topography is similar in most of the 7500-8000 series with minor output and filter differences. Also different in the 7500 is the output bank assemblage. The 7500 uses again TO-3P type packages but with typical packages emitter resistors, the heatsink is also more similar to the G7700 on up units, utilizing a solid block heatsink fitted to the chassis as shown later in this write up. 

Pulled TO-3P Output Bank F-2981/2

F-2980 Damage
First noted was significant repair of damaged PCB tracing from a prior technician to the other channel. Thats why its important to be familiar with aged units, certain soldering and heat precautions need to be taught and developed. Also was the repair and debri removal of the burnt PCB section. Etching new tracing and flushing the board with 99.99<% iso alcohol helps remove old flux and stains to give a cleaner working surface, its also necessary when adding new eyelets to PCB.

Damaged PCB

Cleaned PCB

The damage to the final, and associated BJ-transistor devices was significant, I noted both NPN/PNP TO-3P outputs confirm what I suspect of being shorted along with the R70/71 .33 5W emitter resistors, again these are mounted on the actual F-2980 footprint unlike some of the other G-series which located them to the output device PCB assemblies 2981/2.  The damage then extended to  TR19 which is a readily available 992fbu BJ-transistor, actually blew the silicon package apart! Next we tracked down to the stabilizing resistors and bias array which had all the 1/4W resistors actually blown and burnt apart, along with a 1/2W 10ohm. As is typically the 1st stage FET differential and 2nd stage pairs where intact and fine. This particular fault took out the 100ohm bias potentiometer as well, which is important to notate, this shows significant draw. 

Damaged F-2980 (Notate Burns!)

New coupling lyrics were replaced along with 1/2W MOX and carbon film resistors and new 5W wire-wounds, the bias potentiometer was replaced with a 500 ohm single turn precision pot along with updating 2473(4148) general type diodes.

Repaired F-2980

As a pre-cursor on the F-2980 assemblies I replace the lyrics with high temp long life FC/FR or high temp PW's along with removing and cleaning the caustic glue from the board.  Watch the outputs and monitor with a 1K sine and re-biased/offset adjustments were correct.

I also wanted to note the different lamping used in the 7500, this models uses just two 8v globular incandescents behind the meters for the entire display, about a thousands time easier to replace in this model. 

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