Author Topic: High pictched noise coming from terminal I got from eBay - no screen  (Read 4367 times)

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Offline alank2

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Re: High pictched noise coming from terminal I got from eBay - no screen
« Reply #25 on: June 26, 2017, 10:17:19 pm »
It still does a multi frequency beep on the piezo so I think the Z80 is coming up, but beyond that I'm not sure.
 

Offline james_s

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Re: High pictched noise coming from terminal I got from eBay - no screen
« Reply #26 on: June 27, 2017, 10:19:31 pm »
You could try connecting the RS232 line through a null modem cable to a PC and see if you can type on the terminal and receive it on the PC. Hard to know what baud rate it is but I'd try 9600 and 19200 first.

Did you check carefully for cracked traces or loose connectors?
 

Offline alank2

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Re: High pictched noise coming from terminal I got from eBay - no screen
« Reply #27 on: June 27, 2017, 10:39:07 pm »
Hi james_s,

The RS232 works (last time I checked).  I could send keys and receive them.  I could even see the keys affect the output video signal before it stopped working sadly.

I'm still baffled as to how I killed it.  I might have touched a couple leads of a transistor that amplifies it allowing a higher voltage to backfeed into it perhaps, but even then it had some resistance to go through and an output collector AND IC (SN74S09N) to get to the main IC.  Perhaps I should try to probe the pins on it a little more closely to see if I can see any signs of life.  It is a tqfp style chip with tiny leads and thin near each other tracks so it is difficult to probe without shorting pins together, but at this point I'm not sure that there is much risk.

Thanks!!!

Alan
 

Offline alank2

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Re: High pictched noise coming from terminal I got from eBay - no screen
« Reply #28 on: June 28, 2017, 03:24:40 pm »
So james_s, generally speaking, how compatible are 14" monochrome tubes with each other?  If I keep this one for parts and get another 14" terminal like a dec model or something, will this one be a replacement tube for another one?
 

Offline alank2

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Re: High pictched noise coming from terminal I got from eBay - no screen
« Reply #29 on: July 04, 2017, 02:58:14 am »
I got a DEC vt420 off of eBay and it is working great.  One question - which will prolong the CRT more - turning down the brightness or turning down the contrast?
 

Offline davegsm82

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Re: High pictched noise coming from terminal I got from eBay - no screen
« Reply #30 on: July 04, 2017, 10:58:11 pm »
Overall reduction in brightness will prolong tube life, don't cycle it where possible, if you're going to use it on and off over the space of an hour for example, then just leave it on.

Reducing brightness will also prevent phosphor burn, where you will see ghosting of previous data/characters etc when the device is off. No way round it and nothing you can do to rectify it, just use it only when needed.

What are your plans for the terminal? I have one in the loft, I love it's 70's/80's design, it was given to me by a good (now passed on :( ) friend, so one day I'd like to get it working. I'd love to have it hooked up to some sort of standalone chat system, using either internet or SMS/GPRS data links. Maybe one day....
 

Offline alank2

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Re: High pictched noise coming from terminal I got from eBay - no screen
« Reply #31 on: July 05, 2017, 02:19:23 am »
Between brightness and contrast, you think it is more important to limit brightness then?

I'm making sort of a retrocomputing wall in my office and plan to have this connected to the PDP-8 clone I have for now.  I've had it connected to a raspberry pi with tty enabled and that was pretty cool too.  I'd like to get one of those Altair 8800 clone's down the road.  I've also made one of Grant Searle's multicomp CP/M projects it would be ideal for.
 

Offline TVguy

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Re: High pictched noise coming from terminal I got from eBay - no screen
« Reply #32 on: July 05, 2017, 10:33:33 am »
...Blue/yellow are on a tiny small cable going to a 3pos connector on the neck PCB.  The third wire in this connector is separate (white) and goes to a pcb post....
... The other two pins of Q702 are 0.54V and the center one goes into Q703.  Its center pin is 0V resulting in 0V to the socket terminal 2.

You have no video output supply voltage. Collector of Q703 should be around 70 V to 90 V DC.
No cathode bias means the CRT will be switched hard on with maximum beam current.
My guess is that the white lead from P702 should connect to the post shown in the image below.
Measure the DC voltage on the vacant post and if it is a suitably identified DC value; move the white lead from its current location to the vacant post.

 

Offline alank2

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Re: High pictched noise coming from terminal I got from eBay - no screen
« Reply #33 on: July 05, 2017, 11:58:21 am »
TVguy I considered this possibility but really recall it being connected to the other post.  I should have taken a picture.  I'll get this back out of the box and try it and see what it shows.  I wish I hadn't killed the video signal output from the IC!!!
 


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