These are straight TTL ICs.? The logic 1 level is roughly 4 volts or so.? All that a logic 1 needs to do is back bias an (equivalent) input diode.? Look at DTL logic (old!) to find an equivalent circuit, and then start replacing the input diodes with (typically) a multiple emitter transistor for a nand gate.
The 16 ma for a chip is also not too out of line, in my experience.
What I'd be looking for would be either slow switching times at the chip outputs or significantly lower 1 output voltages, or higher than normal 0 output levels.
Should two TTL outputs be shorted (that are not open collector), bad output levels are to be expected even from good chips.
Note:? the output high driver of a TTL chip is effectively an emitter follower with the collector and base resistor going to VCC.? There's a limit to what the transistor can pull up, so the 1 output is rated at about 400 ua source, and the standard TTL output (a saturating transistor to ground) is typically 16 ma. Since the output sinking transistor is driven (more or less) by the full VCC through an effective resistor, it gets plenty of base drive.
Harvey
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On 12/4/2021 9:40 AM, Zentronics42@... wrote:
Shaun,
Just tried to order the Tauntek but the boards are sold out. I was informed that there will be a delay in getting new boards so. But no worries I am on the waiting list. I will take a closer look at the drum switch. I would imagine it is a cam switch or something in the IC's that the cam is switching. I tested U330 but totally unloaded the single IC is drawing about 16 ma which seems a touch high to me. And when I wire a gate to get a logic level flip the (high) output is only 4Vdc on some of the gates and this is with the IC out of the unit on a breadboard. So it cant get to 5V totally unloaded. VCC is 5.2 to match the TG 5V rail. This is why I am thinking the IC is marginal. Though I could also be asking to much from an IC of this vintage.
Zen
-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Shaun M
Sent: Saturday, December 4, 2021 9:23 AM
To: [email protected]
Subject: Re: [TekScopes] Sick TG501 Main 1Mhz ref not running.
Zen,
I had a similar problem (double time marks) on a TG501 unit several months ago. It turned out that one finger of the drum switch assembly was not opening when it should have due to mechanical damage. I found the problem using the switch matrix in the SM which allowed me to figure out the proper position of each switch finger. The matrix is located on the ¡°Display and Switch Details¡± page of the schematic section in the SM.
On the subject of vintage IC testing: The Tauntek tester lets you see pin voltages and overall current for a given chip as well as the usual logic testing.
Shaun M
