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On Mar 23, 2022, at 13:02, n4buq <n4buq@...> wrote:

?Readout function has been intermittent in my 7704A since I bought it last year. It will work fine for weeks and then suddenly stop. It may resume on its own after a few days but most of the time that's not the case. I've tried diagnosing the circuits when it's not working and, usually, as I'm doing that, it will suddenly start working again so I've not been able to isolate the cause (because when it's working, then I have nothing to find) - until yesterday.

It had stopped and I actually observed the moment it stopped. That seemed to be a bit of a jitter/fade event and then it stopped completely. As I was checking things, I noticed the voltage at U3445 Pin 6 was about half of what it shows on the schematic (~0.75V instead of 1.47V). I checked the supply voltage and that was within spec and ground seemed to be okay (albeit I was checking for a voltage on that pin instead of measuring for any resistance between the pin and ground which I really should have done).

Thinking Q3445 might be pulling that voltage down, I removed it but that didn't fix it. I then pulled the U3445 out just a tiny bit and pushed it back in (reseated it). At that point, U3445 Pin 6 was back up to about 1.5V and the readout was working again. For good measure, I pulled U3445 out completely and reinserted it.

I've read where certain (Texas Instrument) sockets are known problems but this one doesn't appear to be TI socket. Should I suspect something else (maybe a cold solder joint on/near that socket, etc.)? Maybe it should be replaced with a machined socket?

Hopefully that's been the problem all along but I seem to recall having reseated all the socketed components on the readout board so I'm not 100% convinced this problem won't resurface. Anyone else had problems with the readout board like this?

Thanks,
Barry - N4BUQ

On Mar 23, 2022, at 13:18, Tom Lee <tomlee@...> wrote:

?Although the TI socket’s problems are well known, all sockets can have flaky contacts.

True netal-to-metal contacts probably don’t really exist when you abut them. There’s always a thin layer of crud (oxide, eg). If the crud is atomically thin, current can still flow, thanks to tunneling (yes, it’s a real thing). But tunneling probability drops exponent with thickness, so just a “little more” crud can shut off current more or less completely. So the mystery isn’t “why are sockets and switches flaky?” The real question is “why do these often work?”

Cheers
Tom

Sent from my iThing; please forgive the typos and brevity

On Mar 23, 2022, at 13:02, n4buq <n4buq@...> wrote:

?Readout function has been intermittent in my 7704A since I bought it last year. It will work fine for weeks and then suddenly stop. It may resume on its own after a few days but most of the time that's not the case. I've tried diagnosing the circuits when it's not working and, usually, as I'm doing that, it will suddenly start working again so I've not been able to isolate the cause (because when it's working, then I have nothing to find) - until yesterday.

It had stopped and I actually observed the moment it stopped. That seemed to be a bit of a jitter/fade event and then it stopped completely. As I was checking things, I noticed the voltage at U3445 Pin 6 was about half of what it shows on the schematic (~0.75V instead of 1.47V). I checked the supply voltage and that was within spec and ground seemed to be okay (albeit I was checking for a voltage on that pin instead of measuring for any resistance between the pin and ground which I really should have done).

Thinking Q3445 might be pulling that voltage down, I removed it but that didn't fix it. I then pulled the U3445 out just a tiny bit and pushed it back in (reseated it). At that point, U3445 Pin 6 was back up to about 1.5V and the readout was working again. For good measure, I pulled U3445 out completely and reinserted it.

I've read where certain (Texas Instrument) sockets are known problems but this one doesn't appear to be TI socket. Should I suspect something else (maybe a cold solder joint on/near that socket, etc.)? Maybe it should be replaced with a machined socket?

Hopefully that's been the problem all along but I seem to recall having reseated all the socketed components on the readout board so I'm not 100% convinced this problem won't resurface. Anyone else had problems with the readout board like this?

Thanks,
Barry - N4BUQ

From: "Mark Vincent" <orangeglowaudio@...>
To: "tekscopes" <[email protected]>
Sent: Wednesday, March 23, 2022 3:21:20 PM
Subject: Re: [TekScopes] 7704A - Intermittent Readout

Barry,

You had a dirty contact on one or more pin(s)/socket/both. I will use IPA with a
brush on each IC and transistor on that board, move out, rock back and forth
enough to clean and scrape each then reseat. Unless you need or want to, there
is no need to replace any socket(s) that are not the TI problematic types.

Mark

From: "zenith5106 via groups.io" <hahi@...>
To: "tekscopes" <[email protected]>
Sent: Wednesday, March 23, 2022 3:28:52 PM
Subject: Re: [TekScopes] 7704A - Intermittent Readout

On Wed, Mar 23, 2022 at 09:01 PM, n4buq wrote:

I've read where certain (Texas Instrument) sockets are known problems but this
one doesn't appear to be TI socket

The really bad TI sockets are easily identified by the profile at the short end.
Check here
Disregard the text above "DIP sockets are ..." and the picture at the right.
The bad ones are the ones on the pictures at the bottom of the page.

/H?kan

H?kan,

Thanks for that link. I was pretty sure the ones on that board are not Texas Instrument sockets. I think HP has had their share of bad sockets and I think those might be TI sockets. Those are identified by their red color. I replaced a few of them in an HP-3456A.

Thanks,
Barry - N4BUQ

----- Original Message -----

From: "zenith5106 via groups.io" <hahi@...>
To: "tekscopes" <[email protected]>
Sent: Wednesday, March 23, 2022 3:28:52 PM
Subject: Re: [TekScopes] 7704A - Intermittent Readout
On Wed, Mar 23, 2022 at 09:01 PM, n4buq wrote:

I've read where certain (Texas Instrument) sockets are known problems but this
one doesn't appear to be TI socket

The really bad TI sockets are easily identified by the profile at the short end.
Check here
Disregard the text above "DIP sockets are ..." and the picture at the right.
The bad ones are the ones on the pictures at the bottom of the page.

/H?kan

From: "Harvey White" <madyn@...>
To: "tekscopes" <[email protected]>
Sent: Wednesday, March 23, 2022 9:01:31 PM
Subject: Re: [TekScopes] 7704A - Intermittent Readout

If I remember properly, one thing that distinguished the Texas
Instruments sockets was that the IC pin was gripped by the small edges,
not by the larger flat surfaces.

Harvey


On 3/23/2022 9:47 PM, n4buq wrote:

H?kan,

Thanks for that link. I was pretty sure the ones on that board are not Texas
Instrument sockets. I think HP has had their share of bad sockets and I think
those might be TI sockets. Those are identified by their red color. I
replaced a few of them in an HP-3456A.

Thanks,
Barry - N4BUQ

----- Original Message -----

From: "zenith5106 via groups.io" <hahi@...>
To: "tekscopes" <[email protected]>
Sent: Wednesday, March 23, 2022 3:28:52 PM
Subject: Re: [TekScopes] 7704A - Intermittent Readout
On Wed, Mar 23, 2022 at 09:01 PM, n4buq wrote:

I've read where certain (Texas Instrument) sockets are known problems but this
one doesn't appear to be TI socket

The really bad TI sockets are easily identified by the profile at the short end.
Check here
Disregard the text above "DIP sockets are ..." and the picture at the right.
The bad ones are the ones on the pictures at the bottom of the page.

/H?kan

I didn't know that. Was that done to lessen the stress on the pin's bending point?

If I recall correctly, the sockets in the HP3456A gripped on the flat sides.

Thanks,
Barry - N4BUQ

----- Original Message -----

From: "Harvey White" <madyn@...>
To: "tekscopes" <[email protected]>
Sent: Wednesday, March 23, 2022 9:01:31 PM
Subject: Re: [TekScopes] 7704A - Intermittent Readout
If I remember properly, one thing that distinguished the Texas
Instruments sockets was that the IC pin was gripped by the small edges,
not by the larger flat surfaces.

Harvey


On 3/23/2022 9:47 PM, n4buq wrote:

H?kan,

Thanks for that link. I was pretty sure the ones on that board are not Texas
Instrument sockets. I think HP has had their share of bad sockets and I think
those might be TI sockets. Those are identified by their red color. I
replaced a few of them in an HP-3456A.

Thanks,
Barry - N4BUQ

----- Original Message -----

From: "zenith5106 via groups.io" <hahi@...>
To: "tekscopes" <[email protected]>
Sent: Wednesday, March 23, 2022 3:28:52 PM
Subject: Re: [TekScopes] 7704A - Intermittent Readout
On Wed, Mar 23, 2022 at 09:01 PM, n4buq wrote:

I've read where certain (Texas Instrument) sockets are known problems but this
one doesn't appear to be TI socket

The really bad TI sockets are easily identified by the profile at the short end.
Check here
Disregard the text above "DIP sockets are ..." and the picture at the right.
The bad ones are the ones on the pictures at the bottom of the page.

/H?kan

If I remember properly, one thing that distinguished the Texas Instruments sockets was that the IC pin was gripped by the small edges, not by the larger flat surfaces.

Harvey


On 3/23/2022 9:47 PM, n4buq wrote:

H?kan,

Thanks for that link.? I was pretty sure the ones on that board are not Texas Instrument sockets.? I think HP has had their share of bad sockets and I think those might be TI sockets. Those are identified by their red color.? I replaced a few of them in an HP-3456A.

Thanks,
Barry - N4BUQ

----- Original Message -----

From: "zenith5106 via groups.io" <hahi@...>
To: "tekscopes" <[email protected]>
Sent: Wednesday, March 23, 2022 3:28:52 PM
Subject: Re: [TekScopes] 7704A - Intermittent Readout
On Wed, Mar 23, 2022 at 09:01 PM, n4buq wrote:

I've read where certain (Texas Instrument) sockets are known problems but this
one doesn't appear to be TI socket

The really bad TI sockets are easily identified by the profile at the short end.
Check here
Disregard the text above "DIP sockets are ..." and the picture at the right.
The bad ones are the ones on the pictures at the bottom of the page.

/H?kan

.

On 24 Mar 2022, at 03:25, Mark Vincent <orangeglowaudio@...> wrote:

?Barry,

The scraping is only enough for contact to be made by the pin and socket contact. Unlikely you will see any scratch marks. Do not scratch the gold. The oxidation is more on the socket than the gold pin. If you do have those crummy TI sockets, get ones in stock to replace them when you need it. Mill Max would be a good choice. I would like to get ones in stock, 8, 14 and 16 pin, for when I need to replace them. What make and model(s) numbers are best to use, I do not know. Here is where paying a bit more per socket would be worth it. There are some cheap ones out there that are only somewhat better than the old TI types. I would want a dual wipe type to make contact on both sides of the IC pins.

The feel of the old TI types makes the moving the IC pins feel like a scratch or scraping. The side of the pins moving against the IC socket contacts are transferring the feel to the IC body when they are moved. The holes are usually barely bigger than the IC pins and the contact can be seen if you get it in the right position in a light source.

Mark