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Trying to bring back a AA501
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Eric
So I am finally working up the nerve to put an AA501 on the bench and run it through its checks. From a preliminary check in It looks like the voltage section is working well and tracking with my meter that is freshly in cal. However, it seems the distortion side needs some help. I have not tried to do a calibration on it yet as I have been a little nervous to open it up. With a shorting bar on the inputs it says 0.0079% distortion. With the outputs floating it is about 0.0303% with a signal applied from a wein bridge at about 1Khz 6.5V Pk I get a distortion figure of 100.3% distortion. On a scope that oscillator is a clean sign wave. On volts a 6.5V Pk voltage reads 4.65 VRMS. Math says it should be 4.63 so that could be with in the margin of error. A fluke 8840A reads 6.54~ off the peak detector of the oscillator. Any gotcha’s I should know about a AA501 before I start running through the service manual. I have had to repair the front connector it was very loose to the case. But that has been repaired.
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Jean-Paul
Hi there, AA501was design by Bruce Hoffer and Richard Cabot, who later started Audio Precision
Complex disassemble, take photos and notes, get the full service manual. Most frequent issues are dead tantalum capacitors . Bon Chance Jon
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Hello,
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These units use a handful of reed relays for input range level switching (A14 board on my DA4084 units). I encountered one of these relays which was intermittent and it caused the auto range function to continuously hunt and never settle down. This resulted in garbage distortion measurements. Even when selecting the range manually, the distortion readings were erratic. I located the flakey one by tapping each range relay with a stick and watching the result on the display. Replacing the offending relay fixed the problem. Shaun Merrigan
-----Original Message-----
From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Jean-Paul Sent: Friday, December 27, 2019 16:49 To: TekScopes@groups.io Subject: Re: [TekScopes] Trying to bring back a AA501 Hi there, AA501was design by Bruce Hoffer and Richard Cabot, who later started Audio Precision Complex disassemble, take photos and notes, get the full service manual. Most frequent issues are dead tantalum capacitors . Bon Chance Jon
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Ke-Fong Lin
Hi Eric,
I have about the same distortion readings for my (working) AA501, that is around 0.029% open and around ~0.009% shorted. The level meter part seems to be working, have you tried various voltages to check all the voltage ranges? In particular, trying each range manually, then auto-ranging? Then try the distortion meter using manual range and auto-ranging? Best regards,
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Eric
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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Eric
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.
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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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Eric
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?
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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.
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It appears to me that the MM74C175N has a VCC (max. operating) of 15V and the 74HC175N VCC operating range is -.5V - 7.0V. Otherwise they appear to be similar. Both operate at TTL levels. Quad Flip Flops with common clock and clear to all 4. As long as your operating VCC was compatible, it looks like the 74HC175N could sub in.
-- Michael Lynch Dardanelle, AR
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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.
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Arden
-----Original Message-----
From: Eric Sent: Saturday, January 04, 2020 6:03 PM To: TekScopes@groups.io 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.
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Both operate at TTL levels ....Sorry, Michael, but you are wrong. The input true and false thresholds are different for CMOS and TTL. Only a 74HCT (notice "HCT" for TTL interface compatibility) part may be used with TTL parts only because its input thresholds are offset to meet TTL output levels. Arden -----Original Message----- From: Mlynch001 Sent: Saturday, January 04, 2020 6:33 PM To: TekScopes@groups.io Subject: Re: [TekScopes] Trying to bring back a AA501 It appears to me that the MM74C175N has a VCC (max. operating) of 15V and the 74HC175N VCC operating range is -.5V - 7.0V. Otherwise they appear to be similar. Both operate at TTL levels. Quad Flip Flops with common clock and clear to all 4. As long as your operating VCC was compatible, it looks like the 74HC175N could sub in. -- Michael Lynch Dardanelle, AR
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Meh, I said "Hz" when I meant MHz. Oh, also, high speed logic designs require much more rigorous power and ground rail decoupling because the faster the logic the noisier the environment. But still you may be OK with the 74HC part.
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Arden
-----Original Message-----
From: KB6NAX Sent: Saturday, January 04, 2020 6:32 PM To: TekScopes@groups.io Subject: Re: [TekScopes] 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. Arden -----Original Message----- From: Eric Sent: Saturday, January 04, 2020 6:03 PM To: TekScopes@groups.io 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.
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OK, learned something. LSTTL and TTL are different animals? SN74HC175N Will drive 10 outputs of LSTTL. Just trying to learn how logic works, I will likely encounter a similar issue in the future.
Thanks! -- Michael Lynch Dardanelle, AR
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KeepIt SimpleStupid
My GOTO place has it: http://www.utsource.net/sch/MM74C175N
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On Saturday, January 4, 2020, 10:31:47 PM EST, Mlynch001 <@mlynch001> wrote:
OK, learned something. LSTTL and TTL are different animals? SN74HC175N Will drive 10 outputs of LSTTL. Just trying to learn how logic works, I will likely encounter a similar issue in the future. Thanks! -- Michael Lynch Dardanelle, AR
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Eric
I did find out that the AA501 does need the VCC of 15V so the HC series logic is out for sure. I am going to order some of the direct replacement parts as there is a supply house that has then mouser and digikey were out. Texas Instrument cross reference says a CD40175BE is a comparable part as well. I have also confirmed the fault as with this IC out of circuit the 501 says 100+% distortion on all frequencies also one of the outputs is open when i check it with my small curve tracer. On this one pin 14 is dead.
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On 1/5/2020 1:08 AM, KeepIt SimpleStupid via Groups.Io wrote:
My GOTO place has it: http://www.utsource.net/sch/MM74C175N
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Watch out! The CD and 74 pinouts are different. You don't what to hack up your circuit board.
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Also, be advised, when troubleshooting IC's in general having the schematic of the device gives understanding of how inputs and outputs work. For example a tri-state output could be "off" and not "open." A CMOS input is high impedance so you don't know if the input stage is blown unless a clamp diode is shorted. The ultimate is to try the truth table, a lot of work on complex parts. Parts substitution can't be beat for getting results. I always buy two or more when in that situation. eBay to the rescue - Item #372418190226 Arden
-----Original Message-----
From: Eric Sent: Sunday, January 05, 2020 6:51 AM To: TekScopes@groups.io Subject: Re: [TekScopes] Trying to bring back a AA501 I did find out that the AA501 does need the VCC of 15V so the HC series logic is out for sure. I am going to order some of the direct replacement parts as there is a supply house that has then mouser and digikey were out. Texas Instrument cross reference says a CD40175BE is a comparable part as well. I have also confirmed the fault as with this IC out of circuit the 501 says 100+% distortion on all frequencies also one of the outputs is open when i check it with my small curve tracer. On this one pin 14 is dead. On 1/5/2020 1:08 AM, KeepIt SimpleStupid via Groups.Io wrote: My GOTO place has it: http://www.utsource.net/sch/MM74C175N
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David DiGiacomo
On Sun, Jan 5, 2020 at 1:31 PM KB6NAX <gumbear@...> wrote:
That is not correct. CD40175 and 74C175 have the same pinout.
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Eric
This one did fail the truth table check when it was in the device. VCC is about 15V which means a HIGH is 15V. Output 3 has a 5V output and does not move from there. When it should be high it is at 5V when it should be low it is at 5V. In the 501 the truth table is frequency dependent on the input. As you roll through the frequency the outputs should do their thing. 1 2 and 4 seem to function as expected.
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On 1/5/2020 3:30 PM, KB6NAX wrote:
Watch out! The CD and 74 pinouts are different. You don't what to hack up your circuit board.
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Chuck Harris
I am having trouble imagining how a 15V CMOS device could spontaneously
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generate 5V on one of its outputs.... I can see ~0V, or ~15V, or even 7.5V, but 5V is an odd value for a device that has no resistors in it. Are you sure that it isn't being bullied around by whatever it is driving? old style CMOS has very little drive capability. -Chuck Harris Eric wrote:
This one did fail the truth table check when it was in the device. VCC is about 15V
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Is CR1412 good? I.e. not shorted?
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On 1/5/2020 11:15 PM, Eric wrote:
This one did fail the truth table check when it was in the device. VCC is about 15V which means a HIGH is 15V. Output 3 has a 5V output and does not move from there. When it should be high it is at 5V when it should be low it is at 5V. In the 501 the truth table is frequency dependent on the input. As you roll through the frequency the outputs should do their thing. 1 2 and 4 seem to function as expected.
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Dave Wise
That happened here a few weeks ago. Fluke 8000A DMM. After a transient, it would only display even digits. Turned out the 7447 display driver developed an internal short on its 1's input. The MOS A/D chip was fine. Whew!
Dave Wise ________________________________________ From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Chuck Harris via Groups.Io <cfharris=erols.com@groups.io> Sent: Sunday, January 05, 2020 10:36 PM To: TekScopes@groups.io Subject: Re: [TekScopes] Trying to bring back a AA501 I am having trouble imagining how a 15V CMOS device could spontaneously generate 5V on one of its outputs.... I can see ~0V, or ~15V, or even 7.5V, but 5V is an odd value for a device that has no resistors in it. Are you sure that it isn't being bullied around by whatever it is driving? old style CMOS has very little drive capability. -Chuck Harris
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