A2+ beeps, works, no text display. Help please?

[TRS-Ian]

Hello Apple fans... occasionally I put down a TRS-80 and pick up and Apple and this weeks it's a ][ plus which has been sitting here for over a year waiting for me to fix it.

The good news: computer works... it beeps on power up, the power light illuminates, with no disk controller installed I can type stuff and get a beep responding to syntax errors. But I never get anything on the display.

I see the circuit design is not that different from the TRS-80 Model I and uses a 741C and a couple of 3904 transistors (3906 on a TRS-80) the composite output.

Does anyone have any pointers to get me started?

I'm hoping it's one of those "Oh yeah Ian that happens all the time on the ][ plus, just change this transistor or that capacitor and it will work again."

Thanks in advance,

Ian.

 

[david__schmidt]

I'm hoping it's one of those "Oh yeah Ian that happens all the time on the ][ plus, just change this transistor or that capacitor and it will work again."

Oh yeah, that happens all the time. The Apple II has a "liberal" interpretation of NTSC, and many modern (especially LCD) monitors have trouble interpreting the signal. What kinds of tubes have you tried pumping the video signal to?

 

[TRS-Ian]

Hi David, thanks for your reply.

Yes I'm using a known good 30 year old composite CRT monitor. It works fine for my IIe and my other II+, as well as other systems like my Coco 3 and T1K.

It's definitely a component failure on the A2+ mobo.

Cheers,

Ian.

 

[TRS-Ian]

I've had some success. That 200ohm pot between the 3904 and the video output appears to have failed internally.

I'll pick something that will fit from the electronics shop tomorrow, fingers crossed we will have a solution.

 

[TRS-Ian]

Fixed. It was the output brightness control. I also replaced it's feeder 3904 with a 2222 and have had it running for 24 hours.

No problems noted.

Cheers,

Ian.