Sansui BA-3000 blowing mains fuse following restoration

[BradWil]

getting 1.2mV DC offset, regardless of VR01 setting.
BTW working on the LEFT driver amp only at this point, as it is all original, apart from a recap and trimmers.
All on DBT

 

[BradWil]

if both drivers are failing to get BIAS and DC, should I be looking elsewhere?
Left driver is original apart from a recap and right driver has some transistors changed as perviously listed.

 

[richardpp]

Driver board supply voltage test points on F-2512. Nominal 65Vdc regulated, 68Vdc unregulated, allow for dbt.

*** Be extremely careful the probe tip doesn't short blue and red connections ***



Back in the morning ACST

 

[BradWil]

On DBT, here are my measurements

 

[Hyperion]

I'd say the 2 @ 57.5V are just DBT affected = Ok, 66.5V = Ok and the 0.4V clearly needs looking at, faulty (open) series pass transistor?

 

[richardpp]

@Hyperion, good call!

Here are test points for the series pass transistors on F-2508.

 

[richardpp]

Preliminary, check fuse F02 on F-2511 and the connector block TM01 also on F2511. Expect to see ~90Vdc at TM01.

 

[BradWil]

measurements to GND and still on DBT

F-2508 regulated PS

TR05 emitter 66.5VDC
TR10collector -65.7VDC

F-2511 protection

F02 closed

Connector block TM01
Black 0VDC
Red 69.2VDC
Blue -69.3VDC

 

[richardpp]

That all looks good.

I didn't specify, but around 90V at TM01 unloaded, so around 70V if loaded looks good.

Suggest checking for voltage either side of R16 10Ω on F-2508. Not sure if you have access to R16, you may need to remove the board to check it. Otherwise check for a broken trace on F-2512.

 

[BradWil]

will do and report back

Here are the voltages on the left and right F-2507 driver connectors to F-2512. These are measured to GND and with driver boards removed and both outputs (F-2510) connected. Pins without values were all zero VDC. All on DBT.

 

[BradWil]

That all looks good.

I didn't specify, but around 90V at TM01 unloaded, so around 70V if loaded looks good.

Suggest checking for voltage either side of R16 10Ω on F-2508. Not sure if you have access to R16, you may need to remove the board to check it. Otherwise check for a broken trace on F-2512.

View attachment 3831726

F-2508 R16 measures 10.2Ω
Voltage to GND on the TR10 Collector side of R16=-66.2VDC
Voltage to GND on the rail side of R16=3.2mVDC
DBT on.
Drivers removed.

 

[BradWil]

With F-2508 removed and both F-2507 drivers removed, left to right pin voltages on F-2512 as follows:

0,0,0,0,0,0,0,0, +81.2VDC, -81.2VDC - so looks like the rails are reaching the regulated PS.

 

[BradWil]

there are no broken traces on any of the boards

 

[BradWil]

F-2508

 

[richardpp]

F-2508 R16 measures 10.2Ω
Voltage to GND on the TR10 Collector side of R16=-66.2VDC
Voltage to GND on the rail side of R16=3.2mVDC
DBT on.
Drivers removed.

The problem is on F-2508, there should be approx 66V on both sides of R16. I’ll need to check the schematic …..

 

[BradWil]

Here are the semis repalced on F-2508. I can begin to remove and test each transistor ...

 

[richardpp]

my guess is that there is a poor solder joint on R16, even if your meter shows continuity. Remove and resolder.
But do check resistor values around R16.

Just saw your last post. No need to remove transistors, the regulator appears to be working fine.

 

[BradWil]

These resistors on F-2508 removed, measured and replaced with 0.5W 1% MF.
R16 10Ω
R19 22KΩ
R27 47KΩ
R32 56KΩ
R22 5.6KΩ

All measure ok but I replaced them to be certain

Still no BIAS on either amp

 

[richardpp]

Ok I see this is a little more complicated than I thought.

I'll lay out what I'm seeing so far, someone else might be able to help.

You can't bias the amplifier because the negative 65V regulated voltage supply is not reaching the connections on F-2512 where the driver boards plug in.






The regulator board F-2508 has ~80V on the input.

The negative side of the regulator has -65V on the output of the series pass transistor but after R16 this falls to within a few millivolts.

The positive side of the regulator has +65V on its output which goes directly to the driver boards.

Since the positive side of the regulator tracks the negative side, if after R16 the voltage is effectively 0V, why doesn't the positive side also read 0V?

I would be interested to see if -35V is missing as well. Can you see a connector for the meter boards that might let you test the regulated 35V supply?

 

[Hyperion]

Back on post #44 your picture of the track side of the connection PCB shows solder connections x4 which seem to have been sucked dry of solder?

Are my eyes deceiving me?