开云体育

Hi,
I'm just getting started on a 7704A that has a few problems. There will
probably be a few more questions to follow but first off I need to know if
I can safely power it up with the top half disconnected and removed. I
don't want to cook anything!

The plugins (7A26, 7A16A, 7B85 & 7B53A) all appear to work fine in my 7603.
In the 7740A I'm not getting good vertical signal throughput, the vertical
position controls do nothing and the calibrator signals are badly
attenuated. PS looks good for voltage (will check ripple tonight). It was
full of prairie dust (Calgary). I'm hoping the worst of the problems will
be dirty board inter-connects.

Thanks!
Keith

Hello Keith,

a few friendly hints included below.

Good luck,

Magnus

* Yes, you can operate the so-called acquisition unit alias the lower part without the display unit. But you need to carefully disconnect the Pomona coax connectors J3112/J3113 which connect to the HV board. And, please leave at least one board in the acquisition unit connected, so that the switched-mode power supply has a minimum load to operate properly.

* The complete power supply including the rectifier and filter assembly or PCB/ A31 and the Power supply and inverter board assembly/A30 can be pulled out for servicing.
Please obey general safety rules and refer to the service manual, as the power supply can store lethal energy even after it has been switched off.

* My #1 hint would be to check the power supplies with the other assemblies or boards disconnected (again, min. 1 board connected), and connect them step-by-step while checking the power supples. And to do this thoroughly.
Also, check the main interconnect/plug-in sockets board, as several group members including myself have found shorted tantalum caps on this board. As you have said, the board-to-board connectors are also worth a check. And there are many in the 7704A...

* I have found Tektronix' trainings material named "7704A maintenance training handout" an irreplaceable source of information.
Dave with artekmedia.com sells an excellent copy (I am not affiliated with Dave, just a satisfied customer).

* The 7704A's switched mode power supply is a complex design and tough to troubleshoot. But, there are some very knowledgeable members in this group - so you are in the best capable hands, if you need further advice.

Keith Smith kahsmith@... [TekScopes]:

Thanks, Magnus!
The additional advice is appreciated. I bought it for a January project
(looks like a slow month for me), so I'll be taking my time to do as you
suggest.
I've already ordered the manuals from Dave ( I think his are OCR'd and
searchable) and I'll look for the other one you mentioned.

Will post back with a question or a declaration of victory :)

Keith

Hi Keith,

If, like you think, the PS is OK, then it doesn't help to take the Display unit away. [BTW in an old message you can read about a satisfactory substitute dummy load in its place.] Besides the PS unit itself you have to check that power "arrives" at all circuit boards, including the amplifier boards in the D-unit. The ribbon cable connectors there are not very heavy and may even become brittle with age. I would carefully pull them away and reconnect. The Hor Amp also makes use of +150V, supplied by the HV unit (not the main PS).
When a signal doesn't come through you can measure amplitudes at various points in the signal chain. For instance, the inputs to the amplifiers in the D-unit are easy to probe.

Albert

The bottom will power up okay with the top as long as you have minimum load. I suspect that it will power up with all four plug ins. If not, add a 1 amp or so load to the 5v line.

Jerry Massengale

The bottom will power up okay with the top OFF as long as you have minimum load. I suspect that it will power up with all four plug ins. If not, add a 1 amp or so load to the 5v line.


Jerry Massengale

Thanks guys!

I'm only getting a few minutes at a time on it these days - it's the Xmas
rush for musicians right now (thank God there *is* one).

The main reason I was thinking about powering it up w/o the Display unit
was to get better access to the main interface board while it's powered. To
be honest, I haven't even located where the TP's are on that board yet.

As Albert points out indirectly, removing the D unit also removes the
vertical amp board which is a prime suspect (or at least the connections to
it). So, I think I'll get the D unit off and clean everything I can get at,
dis and reconnect everything. Then, I'll reassemble and power up to check
for any differences. If none, I'll pull it off again and check methodically
through as best I can. Gee, I might even learn something! -- which was the
object of the exercise before I caught SAD ;)

Cheers,

Keith

Hi,
I've got the A20 main interface board out - thought I may as well have a
look for obvious ashes, etc.
One thing I've found is a rather 'bodgy' looking repair on the A22 (vert
interface) board. I'm thinking one of you might know what this repair would
most likely have addressed?
I'm referring to the area next to the 2 Pomona Coax connections.

There are some pictures here:


I'm guessing whoever did it was the same fellow who left a loose screw and
nut rolling around in there. They don't look burned, so hopefully....

And no, the board has not been washed in contact cleaner. I boosted the
contrast to open up the shadows which also brings up the finish on the
board.

Also, I haven't heard from Dave at Artek since ordering the manuals on
Thursday. I'm just hoping nothing is wrong there.

Thanks, Guys

Keith

Hi Keith,

Not sure what exactly you mean. The filter R77/C77 is normally mounted "in the air" as shown. There seems to be a repair near Q60. There are also components at the rear side of A22. Maybe R60 (to emitter Q60) there has been replaced? Or the transistor(s)?
Did you check that the center pins of the Pomona connectors are not bent away?

Albert

=========

One thing I've found is a rather 'bodgy' looking repair on the A22 (vert
interface) board. I'm thinking one of you might know what this repair would
most likely have addressed?
I'm referring to the area next to the 2 Pomona Coax connections.

There are some pictures here:


---
Keith

On 12/08/2014 07:09 PM, Keith Smith kahsmith@... [TekScopes] wrote:

Hi,
I've got the A20 main interface board out - thought I may as well have a
look for obvious ashes, etc.
One thing I've found is a rather 'bodgy' looking repair on the A22 (vert
interface) board. I'm thinking one of you might know what this repair would
most likely have addressed?
I'm referring to the area next to the 2 Pomona Coax connections.

There are some pictures here:


I'm guessing whoever did it was the same fellow who left a loose screw and
nut rolling around in there. They don't look burned, so hopefully....

And no, the board has not been washed in contact cleaner. I boosted the
contrast to open up the shadows which also brings up the finish on the
board.

Also, I haven't heard from Dave at Artek since ordering the manuals on
Thursday. I'm just hoping nothing is wrong there.

Thanks, Guys

Keith

Saw a message this morning on the HP Group. Dave is still in the process of the Florida move and doesn't have all his computers set up as of yet.

Mark

Thanks, Mark! I didn't know about that.
I'll make out in the meantime with plain scanned manuals. Jeez, the
original book would be really nice.

Albert,
Thanks for that. It was really just the soldering job that had me
wondering. -was borrowing Dave Jones term 'bodge' for the flying connection
--could have been an early revision?
Anyway, It seems I'm in the soup now, and I'd guess a few of you could see
the possibility coming, as did I (but hope springs eternal).

I vacuumed the boards, getting the worst of the dust off and
dis/re-connected all of the inter-connects. I also lifted and rocked all of
the socketed IC's ai found. I had the main interface board out as you
know, but I didn't remove the 3 smaller boards on the plugin side nor the
logic board. Also, I didn't pull the socketed transistors. I wish I had. I
didn't do anything on the PS except disconnect J3112/13 as that was the one
thing I had some confidence in --the first voltage checks I did there were
all just inside 'in spec', although as I mentioned previously, I didn't
check ripple..

I put it all back together (had taken lots of photos and marked all the
Pomonas). Now she's in a coma. Switch lights are dim, trigger lights on
both time bases are flashing as are several of the illuminated switches on
the 7A53A, all in sync with the (new) ticking of the PS. All voltages at
P32G on the regulator board are very low and rise/fall as the PS tries to
get started(?).

After that, I tried the plugs in the 7603 again and all seems to be well
there. <sigh of relief>

OK, says I. The education begins. I googled around a bit. Found the Tek
Toubleshooting Scopes pdf, also an interesting
old 7704A thread at

which contained this little nugget by Barry 777:

"One bit of probably useful info on the 7704A: Since they have hundreds of
transistors and IC's with sockets - and somewhat marginal soldering
sometimes on all the boards, my first course of action is usually to pull
all boards and reflow every single solder joint - especially at the plug-in
sockets which suffer more strain. Also, I push-and-pull every transistor
and IC several times to clean off those wires/terminals (even though most
appear to be gold-plated and theoretically corrosion-proof). It's actually
better to wick up the old solder first, otherwise you'll just contaminate
the new solder - but that is a lot of work! Whenever possible, I reflow
BOTH sides of the board since it appears to be a 3-layer board.

"Also, take special care to ensure that you're getting good contact on
those tiny plugs and jacks that connect the upper half of the unit to the
bottom half. My 7704A "worked" when I got it, but had intermittents in
almost every part of the scope - vertical, horizontal and readout functions
mainly. After performing my easy magic on all the boards, most of the
intermittents disappeared - but then I only had half the usual horizontal
gain (but the readout information did cover the full width). After pulling
it apart AGAIN and reflowing some more stuff, I finally traced it to those
tiny plugs and jacks near the enclosed high voltage section (attached to
the large black plug with the handle). Finally, I have a fully functioning
scope. Now it's time to to clean the controls and switches on the 8
plug-ins I bought for it."

I'd be interested to read any thoughts you may have... is he over the top?
(Murphy seems to have his own room at my place, too.) I'm pretty decent at
soldering, have good irons and a rework station with vacuum.



Now, I haven't seen any signs of heat damage anywhere. I guess I should
open up the PS first and look for that in there, too.

My intuition is that I've jiggled a transistor somewhere, and that I should
pull them all, clean and re-seat. If I do that, should I wipe them with
something? I have some De-oxit Gold and some MG silicone based contact
spray. I've stayed dry, so far.

Thanks again!
Keith

Hi Keith,

It's so easy to make a mistake with all those connectors. You wouldn't be the first person reseating a plug the wrong way around. Double check that all pins 1 (arrow) are at the right place. Maybe you can see which side was which in your photos, one side is full plastic and leads/pins are visible at the other side.
I would first try to get the 'scope more or less working. If you start now to resolder or whatever, you will never know whether you introduced new problems or made things better.
Also leave the PS alone. It's in tick mode now while it was not previously. So the tick mode is most likely caused by a fault elsewhere. Again this makes me think of a wiring fault.

Hi Keith,

It seems I hit the send button before I was ready.
You pulled J3112/3113 at the PS side. Are you sure these are in place again (and no bent inner conductor)?
In one of my 7704A a 20 kHz cable had a shortcut in the APD connector. "Nice" fault!

Albert

Points well taken, Albert.
I'm confident about the J3112/13 but I'll double check anyway.
Will do, on the earlier half of your post. I'll be a bit slower the rest of
this week --that was my day off.
Also it's time to clean the bench, move some gear and make room to work on
this behemoth!
If you saw it all, you might think (excepting my lack of brain power) that
I was related to Jim Williams ;)

Keith

On Tue, Dec 9, 2014 at 1:39 PM, aodiversen@... [TekScopes] <
TekScopes@...> wrote:

Hi Keith,

It seems I hit the send button before I was ready.
You pulled J3112/3113 at the PS side. Are you sure these are in place
again (and no bent inner conductor)?
In one of my 7704A a 20 kHz cable had a shortcut in the APD connector.
"Nice" fault!

Albert

[Non-text portions of this message have been removed]

--
Keith Smith - KeithSmith.ca_Freelance Guitarist & Location Recording Service
<>


[Non-text portions of this message have been removed]

Well, I'm getting some time in on this today. During the week I made a
spreadsheet to help me get a grip on all of the inter-connects for at least
the +50, -50, +15, -15, and +5v supplies.
That allowed me to work through it all fairly quickly and hopefully I
haven't missed too much in the process. I still have Magnus' "Do it
thoroughly." ringing in my ears.


I checked for continuity with the power supply removed entirely, starting
from the P20G connector pins to all of the other boards and found no
problems.

This may be contentious: I've acquired a couple of Huntron Trackers (1000 &
2000) that were thrown in with a couple of scopes some time ago. I've never
seen them mentioned here and would be interested to read any opinions on
their efficacy.
I used the Huntron 2000 for the continuity tests. I liked the indication of
whether the circuit being tested appears to be totally resistive or if some
other function is going on. I'm certainly not suggesting I possess any
expertise in its use.
All supply paths tested looked purely resistive, as expected.

Assuming that the PS is in tick mode due to an overcurrent problem, I
started looking for shorts.

I checked the resistance to ground (Pin 3 of P20G) of all the interconnect
points and added those values to the spreadsheet. Measurements were made
with a Fluke 87V.
The low R values of the +/-15 and +5 v supplies quickly indicated a
problem, I think, and were repetitive throughout the system ,but I followed
through for the sake of completeness.
Later, I checked R values from P20G-3 to the supplies on pins 1, 2 and 4,
while I disconnected the A10, A33, A34 boards and the entire Display unit.
The highest value I could get was about 400 Ohms (suspiciously close to the
in-circuit resistance of C2085).

I find the symmetry of the R values on both 15v supplies a bit curious.
Could be a clue?

I'm guessing the next thing is likely the C2081/83/85/87 caps, which all
look to be tantalum. I don't have an ESR meter, but I do have a Schmitt
trigger built from Alan Wolke's (W2AEW) video, so I could pull them and try
to get a rough ESR value on a scope. It might be quicker to just replace
them though. I can probably get them tomorrow at my local supply house. Any
suggestions on suitable replacements would be appreciated.

For what it's worth, 85 & 87 have nice fat ellipses on the Huntron, while
81& 83 look pencil thin even at 60 Hz.
Whatever is going on, I'm feeling it's probably on the A20 board or its
close cousins. Make sense?

Thanks again!
Keith

Hi,

Please give the measured resistance from all the low voltage power outputs to ground and what plugins were present if any. This a ballpark value but the +5v should be less than 100 ohms the +-15v should be 100 to 200 ohms or so, the +- 50v should be higher than the 15v lines. The +5Vlight line will be less than 10 ohms depending on what is connected.

Where do you get 400ohms for C2085?

Jerry Massengale

HI Jerry,
It's all in the spreadsheet but it pretty much boils down to this:
P20G3 Ground
P20G6, -50v, 13.2k
P20G1, -15v, 124Ω
P20G2, +5v, 86Ω
P20G4, +15v, 124Ω
P20G5, +50, 1.7k
These were pretty consistent though all the boards, except the +50 was a
lot higher on the 41 & 42 boards (P41i3 and P42i3) at 18.8k.

400Ω was on the +15v, I think. I just quickly tried that expecting a
greater change with the boards disconnected, so I didn't write it down.
I can't probably duplicate that right now because I have the A24 trigger
board removed to get at those caps.
The in-circuit Resistance across C2085 was 350Ω -probably doesn't mean
much(?).
I forgot to mention in the above post that no plugins were installed,
because I wanted to get at the slot pins for continuity testing.

On Sun, Dec 14, 2014 at 9:20 PM, jerry massengale j_massengale@...
[TekScopes] <TekScopes@...> wrote:

Hi,

Please give the measured resistance from all the low voltage power outputs
to ground and what plugins were present if any. This a ballpark value but
the +5v should be less than 100 ohms the +-15v should be 100 to 200 ohms or
so, the +- 50v should be higher than the 15v lines. The +5Vlight line will
be less than 10 ohms depending on what is connected.

Where do you get 400ohms for C2085?

Jerry Massengale

-----Original Message-----
From: Keith Smith kahsmith@... [TekScopes] <
TekScopes@...>
To: TekScopes <TekScopes@...>
Sent: Sun, Dec 14, 2014 7:34 pm
Subject: Re: [TekScopes] 7704A -just a quickie

Well, I'm getting some time in on this today. During the week I made a
spreadsheet to help me get a grip on all of the inter-connects for at least
the +50, -50, +15, -15, and +5v supplies.
That allowed me to work through it all fairly quickly and hopefully I
haven't missed too much in the process. I still have Magnus' "Do it
thoroughly." ringing in my ears.



I checked for continuity with the power supply removed entirely, starting
from the P20G connector pins to all of the other boards and found no
problems.

This may be contentious: I've acquired a couple of Huntron Trackers (1000 &
2000) that were thrown in with a couple of scopes some time ago. I've never
seen them mentioned here and would be interested to read any opinions on
their efficacy.
I used the Huntron 2000 for the continuity tests. I liked the indication of
whether the circuit being tested appears to be totally resistive or if some
other function is going on. I'm certainly not suggesting I possess any
expertise in its use.
All supply paths tested looked purely resistive, as expected.

Assuming that the PS is in tick mode due to an overcurrent problem, I
started looking for shorts.

I checked the resistance to ground (Pin 3 of P20G) of all the interconnect
points and added those values to the spreadsheet. Measurements were made
with a Fluke 87V.
The low R values of the +/-15 and +5 v supplies quickly indicated a
problem, I think, and were repetitive throughout the system ,but I followed
through for the sake of completeness.
Later, I checked R values from P20G-3 to the supplies on pins 1, 2 and 4,
while I disconnected the A10, A33, A34 boards and the entire Display unit.
The highest value I could get was about 400 Ohms (suspiciously close to the
in-circuit resistance of C2085).

I find the symmetry of the R values on both 15v supplies a bit curious.
Could be a clue?

I'm guessing the next thing is likely the C2081/83/85/87 caps, which all
look to be tantalum. I don't have an ESR meter, but I do have a Schmitt
trigger built from Alan Wolke's (W2AEW) video, so I could pull them and try
to get a rough ESR value on a scope. It might be quicker to just replace
them though. I can probably get them tomorrow at my local supply house. Any
suggestions on suitable replacements would be appreciated.

For what it's worth, 85 & 87 have nice fat ellipses on the Huntron, while
81& 83 look pencil thin even at 60 Hz.
Whatever is going on, I'm feeling it's probably on the A20 board or its
close cousins. Make sense?

Thanks again!
Keith

[Non-text portions of this message have been removed]

[Non-text portions of this message have been removed]

--
Keith Smith - KeithSmith.ca_Freelance Guitarist & Location Recording Service
<>


[Non-text portions of this message have been removed]

Hi,

p20G 7 to 9 should be a very small resistance but not zero. This the lamp circuit. The readings you gave look okay. Add 2 ohms from the +5v line to ground and see if the 7704a will power up.

Jerry Massengale

On 12/15/2014 03:48 AM, jerry massengale j_massengale@... [TekScopes] wrote:

Add 2 ohms from the +5v line to ground and see if the 7704a will power up.

2 Ohms? 2.5 amps? Is that high a current what you meant? I guess it's just 12.5 Watts,
so OK...

I believe 2 amps has been suggested when 5v is the only extra min load. My extender uses 4 ohms but that is with 20s on the 15s and 500s on the 50s.

Jerry Massengale