Re: Trying to bring back a AA501


Eric, the 74C157 and 74HC157 are the same logic functions but are members of two distinctly different semiconductor processes. The 74C157 belongs to the metal gate family, the first of the CMOS families. It's maximum toggle speed is in the neighborhood of 5 - 10 Hz and it's operating voltage ranges from 3 - 15V. The 74HC157 belongs to the later silicon gate high speed CMOS family. It toggles at up to around 35MHz. It’s operating voltage maxes at 6 volts. So the rub is knowing what the rail voltage is in the 501. If it's 5 volts the "HC" part may work OK, but it's not guaranteed if the logic in the 501 is susceptible to timing issues with much faster through gate transmission times.


-----Original Message-----
From: Eric
Sent: Saturday, January 04, 2020 6:03 PM
Subject: Re: [TekScopes] Trying to bring back a AA501

I have found a bad IC in the 501 it is the MM74C175N from On
semiconductor. It also seems this part is obsolete. Would I be safe in
subbing a 74HC175N from Texas Instruments. I am not sure if the
difference in C and HC logic will matter?

On 12/31/2019 9:33 PM, Eric via Groups.Io wrote:
OK a little more digging and something I can dig in to at least not sure if this is the only issue but it is a start. I was looking around U1410 on the main board and ran through the truth table. It turns out Pin15 on U1410 has a constant output of about 5VDc. The others go to 14.5V when they should be high. Also when pin 15 should be high the distortion levels look in the ball park when Pin 15 should be low the distortion goes to +100% distortion and the function output from the aa501 is a sign wave as opposed to the distortion elements. The frequency it can’t get past is 959 Hz which is when Pin 15 should go low.

On 12/31/2019 9:35 AM, Eric via Groups.Io wrote:
Well it looks like the AA501 is not as bad off as I though. I got a service manual still need to go over it in detail however it looks like the fault is isolated to “higher frequency”, anything ~1Khz it looks like I am getting numbers that are at least in the realm of sanity. Anything over that and I get 100%+ distortion. I am starting to suspect ½ and opamp has gone out.

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