It tests ok at 4816nF with 1.4ohms ESR. I need to work out a way of getting everything connected together outside the case so I can do measurements. I?m hoping it?s something fairly easy to spot as high voltage even at low current is daunting. Cheers A

You?re right with pin 8, tracing the circuit I can see why someone recommended I check Q101 and Q102 which look OK in my tester. The contrast control itself tests OK resistance wise, it goes from pretty much 0ohms to 610M. Yes, it works with the external control. It was the first thing I checked last night. Cheers A

Morning! It is, 610k or 0.6M :) Cheers A

On Fri, Nov 24, 2017 at 8:41 AM, Brent Hilpert via cctalk <cctalk at classiccmp.org> wrote: Indeed... I am working from the Model 4P service manual which I downloaded from archive.org I think. I can't find a schematic for the control panel, but the parts list says the 2 pots are 500 Ohms (contrast) and 500k brightness. One of diagrams at the start of the manual suggests the former has 3 wires (wired as a potentiometer), the latter 2 (wired as a variable resistor). with the ends to +12V out and ground, the wiper to DC Control (pin 8) The brightess pot is wired between pins 3 and 4 (ground) and forms part of the potential divider for the CRT first anode (g2 across the Pond :-)). Note that the brightness control might have quite a high voltage on it. I am assuming the fault is correctly described, a lack of contrast and not brightness. Can you get a bright raster if you turn the brightness control up? OK... Firstly, measure the voltage on pin 8 of the monitor PCB. Can you get it to swing from 0 to around 12V? Scope the video output from the CPU board (on pin 2 of the monitor PCB). Is it a good TTL level signal? Measure the voltage on pin A of the CRT base PCB. Expect around 60V here. Check that C101 is not shorted. You could try the machine with it removed to see what happens. Have you checked the TR103 on the CRT base PCB? -tony

Evening! Yes, it tops out at 11.82V. Yes. Given that it?s TTL and a 3-wire connection (composite plus H&V sync) I should be able to feed that into a PET monitor shouldn?t I? Wish I hadn?t given away my ?big? TRS80 Model 4 :/ 60.3V It tests as 2 diodes which is correct for a bipolar transistor I believe. Cheers Tony, A

Yep, it runs from 0-11.82V. Indeed. My brain was saying ?video? but my fingers typed ?composite? :) I?ll make up an extension cable I think, got more PETs than I know what to do with. I used to have multiple Model 3s AND a Model 2 but they had to go when I moved to a smaller house back in 2010. Perhaps embarrassingly I can?t remember where mine came from but given I?ve had it since the early noughties that?s not surprising. It may even have been from a car boot sale. Nothing happens, in that the picture is the same as it is with both those components fitted. Cheers A

On 2017-Nov-26, at 7:42 AM, Tony Duell via cctalk wrote: With a scope the objective would be to look at the levels when pixels are 'on' in the video signal. For white/on pixels, you need conductivity through TR102.E-C and TR103.E-C, to allow electrons to flow from GND, through those transistors, into the cathode of the CRT, and be emitted from the cathode towards the screen. For a white pixel then: - the video signal (and to a lesser level, the base of TR102) must be going positive, to turn TR102 on. - TR103 collector should be going 'lower' during the pixel period, the black level is approx. 60 V, the white level should drop well below that. You probably won't see much voltage variation at CRT PCB point H / TR102.C / TR103.E as it's just current-switching there with no collector pull-up. You could try connecting CRT PCB point H briefly to GND through a 150-300 ohm R, or short TR102.C-E (not 103). That should bring up a white screen. If it doesn't it would suggest the problem is around TR103. If it does the problem is likely around TR102. If your multimeter has an hfe mode, you could try pulling TR102 & 103 and see if they show gain. The monitor board schematic on page pdf.173 of https://amaus.net/static/S100/tandy/systems/model%204/Tandy%20Model%204P%20… presents DC voltage levels to be expected at the transistors and other points. That's for the green screen monitor, looks to be the same as the white-screen (pdf.172) except for some minor component value diffs.

Replying to myself here but to my surprise the replacement 2SC2228 arrived today and the machine is running nicely now, albeit with some screen wobble. I also tested the replacement in my transistor tester and it was correctly identified as a BJT NPN so I should?ve questioned it at the beginning but hey. Live and learn. It?s currently running the built-in RAM test which is a lot more than it?s done for the last 15 years :) Cheers folks!