Topics

Tektronix 7904 Repier


Kaveh
 

Hello everybody,
I have a Tektronix 7904. The device was probably left unused in the attic for a long time. After opening the device, I noticed that the board for the horizontal deflection was quite dirty (see picture). First I cleaned the circuit board.
the device can be switched on, the power supply seems to be OK. The vertical deflection works; the horizontal deflection does not. The two vertical channels become visible on the screen. Now when I disconnect the circuit board for horizontal extension from the power supply, two points appear for both channels that can be moved vertically but as soon as I connect them again, the screen remains dark. I have measured all voltages for the horizontal circuit board, they are all available. I replaced a defective pnp transistor. I suspect that as soon as I connect it to the CRT, there is a short in the circuit board. I checked the output transistors, they are all in order.
I measure 71 volts at + connection on the board and 117 volts at - connection. However, these measured values ​​do not match the information on the circuit diagram. According to the circuit diagram, they are at 52 volts for both.
My device was built in 1972, the manual service, which I got from the Internet, is from 1983. I have noticed that the circuit diagrams do not correspond 100% to the circuit boards installed in my device. this makes troubleshooting a bit difficult. I hope I can describe my problem here enough and hope for your tips on how I can fix the device or what the error could be.
Many Thanks


Colin Herbert
 

Have you tried checking the Service Manuals available at TekWiki?

http://w140.com/tekwiki/wiki/7904

Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Kaveh
Sent: 12 December 2020 06:08
To: TekScopes@groups.io
Subject: [TekScopes] Tektronix 7904 Repier

Hello everybody,
I have a Tektronix 7904. The device was probably left unused in the attic for a long time. After opening the device, I noticed that the board for the horizontal deflection was quite dirty (see picture). First I cleaned the circuit board.
the device can be switched on, the power supply seems to be OK. The vertical deflection works; the horizontal deflection does not. The two vertical channels become visible on the screen. Now when I disconnect the circuit board for horizontal extension from the power supply, two points appear for both channels that can be moved vertically but as soon as I connect them again, the screen remains dark. I have measured all voltages for the horizontal circuit board, they are all available. I replaced a defective pnp transistor. I suspect that as soon as I connect it to the CRT, there is a short in the circuit board. I checked the output transistors, they are all in order.
I measure 71 volts at + connection on the board and 117 volts at - connection. However, these measured values ​​do not match the information on the circuit diagram. According to the circuit diagram, they are at 52 volts for both.
My device was built in 1972, the manual service, which I got from the Internet, is from 1983. I have noticed that the circuit diagrams do not correspond 100% to the circuit boards installed in my device. this makes troubleshooting a bit difficult. I hope I can describe my problem here enough and hope for your tips on how I can fix the device or what the error could be.
Many Thanks


Chuck Harris
 

The 7904 has one tantalum capacitor on its
horizontal board that when it blows (and it will),
takes out a small 10 ohm resistor. The power supplies
will appear to be fine, but the horizontal board will
not work.

Look at C394, C396 and C390.

-Chuck Harris

Kaveh wrote:

Hello everybody,
I have a Tektronix 7904. The device was probably left unused in the attic for a long time. After opening the device, I noticed that the board for the horizontal deflection was quite dirty (see picture). First I cleaned the circuit board.
the device can be switched on, the power supply seems to be OK. The vertical deflection works; the horizontal deflection does not. The two vertical channels become visible on the screen. Now when I disconnect the circuit board for horizontal extension from the power supply, two points appear for both channels that can be moved vertically but as soon as I connect them again, the screen remains dark. I have measured all voltages for the horizontal circuit board, they are all available. I replaced a defective pnp transistor. I suspect that as soon as I connect it to the CRT, there is a short in the circuit board. I checked the output transistors, they are all in order.
I measure 71 volts at + connection on the board and 117 volts at - connection. However, these measured values ​​do not match the information on the circuit diagram. According to the circuit diagram, they are at 52 volts for both.
My device was built in 1972, the manual service, which I got from the Internet, is from 1983. I have noticed that the circuit diagrams do not correspond 100% to the circuit boards installed in my device. this makes troubleshooting a bit difficult. I hope I can describe my problem here enough and hope for your tips on how I can fix the device or what the error could be.
Many Thanks






Jim Ford
 

Thanks, Chuck. I will check those caps in my 7904.

Darn, I just had it open a couple weeks ago to check for the infamous inflammable RIFA caps. Incidentally, the only equipment I found them in was my HP 8566A/B hybrid (both sections had RIFA caps) and my HP 8350B sweep oscillator. Well, it was fun opening everything up anyway.
Except the 7904; some of the screws appear to have been tightened by automotive air wrench! Lots of WD-40, screw extraction tools, and words I won't repeat in mixed company later, I finally got them all out. One refused to budge, and I had to drill it out. Sheesh! Home Depot screws (unfortunately made in China) filled in for the ones that underwent destructive testing, and the 7904 is fortunately working fine again.

Jim Ford

------ Original Message ------
From: "Chuck Harris" <cfharris@erols.com>
To: TekScopes@groups.io
Sent: 12/12/2020 9:45:50 AM
Subject: Re: [TekScopes] Tektronix 7904 Repier

The 7904 has one tantalum capacitor on its
horizontal board that when it blows (and it will),
takes out a small 10 ohm resistor. The power supplies
will appear to be fine, but the horizontal board will
not work.

Look at C394, C396 and C390.

-Chuck Harris

Kaveh wrote:
Hello everybody,
I have a Tektronix 7904. The device was probably left unused in the attic for a long time. After opening the device, I noticed that the board for the horizontal deflection was quite dirty (see picture). First I cleaned the circuit board.
the device can be switched on, the power supply seems to be OK. The vertical deflection works; the horizontal deflection does not. The two vertical channels become visible on the screen. Now when I disconnect the circuit board for horizontal extension from the power supply, two points appear for both channels that can be moved vertically but as soon as I connect them again, the screen remains dark. I have measured all voltages for the horizontal circuit board, they are all available. I replaced a defective pnp transistor. I suspect that as soon as I connect it to the CRT, there is a short in the circuit board. I checked the output transistors, they are all in order.
I measure 71 volts at + connection on the board and 117 volts at - connection. However, these measured values ​​do not match the information on the circuit diagram. According to the circuit diagram, they are at 52 volts for both.
My device was built in 1972, the manual service, which I got from the Internet, is from 1983. I have noticed that the circuit diagrams do not correspond 100% to the circuit boards installed in my device. this makes troubleshooting a bit difficult. I hope I can describe my problem here enough and hope for your tips on how I can fix the device or what the error could be.
Many Thanks









Kaveh
 

Yes, I downloaded the service manually from there.


Kaveh
 

i'm sorry but i can't find all these capacitors on the schematic here. all the capacitors shown on my circuit diagram for the horizontal circuit have a different name and I cannot find C394, C396 and C390 anywhere. You need to have a different schematic.


Jean-Paul
 

Hello they were built over a period of years.

The schematic and parts location diagrams depend on the scope serial number.

Find it SN, search BABA, TEK WIKI, and ARTECH for that SN range of the schematic and parts layout required.

If you still cannot, please mention your scope SN and the PN of the PCB you work on, I may have a printed manual and could scan those pages if you cannot locate.


Bon chance,

Jon

Jon


Colin Herbert
 

I think you may mean BAMA (Boat Anchor Manual Archive), Tekwiki and Artek Manuals. There is also http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Tek/publikacje/ for some obscure stuff and http://hakanh.com/dl/index.htm for some hard-to-find stuff.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Jean-Paul
Sent: 13 December 2020 09:51
To: TekScopes@groups.io
Subject: Re: [TekScopes] Tektronix 7904 Repier

Hello they were built over a period of years.

The schematic and parts location diagrams depend on the scope serial number.

Find it SN, search BABA, TEK WIKI, and ARTECH for that SN range of the schematic and parts layout required.

If you still cannot, please mention your scope SN and the PN of the PCB you work on, I may have a printed manual and could scan those pages if you cannot locate.


Bon chance,

Jon

Jon


Jean-Paul
 

Kaveh Many thanks.


BRAVO on the PCB cleanup.

Beware ALL trimmer caps are hard to clean after such misue and exposure! Suggest to replace them as well as trimpots or at least use contact cleaner and iso alchol then check every one.



For documentation:
The critical missing information: For manual/schema/parts list/PCB/parts locations:

Serial of 7904 entire scope, on scope rear panel


EVERY MANUAL HAS A SERIAL NUMBER RANGE!


Bon Chance,


Jon


-PS your personal message cannot be answered at email address
as D*****t@web.de is unreachable and any messages bounce back from web.de


Jean-Paul
 

The Suspect Tantalum capacitors are in your cleaned board photo, at lower left corner by the power connector, three brown and two orange elliptic shape.

These are prone to fail sooner or later.

Jon


Kaveh
 

On Sun, Dec 13, 2020 at 01:47 PM, Jean-Paul wrote:


Serial of 7904 entire scope, on scope rear panel
I found two numbers.
1.B133854
2. PN 333-148-00
Could one of them be the serial number?


Jean-Paul
 

Hello again Kaveh,

1/ Scope SN is at top front edge of mainframe!
B = Beaverton factory B133854 = SN
I have B283019 so somewhat later.
The numbers xxx-xxx-xx are all TEK part numbers in parts list.

2/ My paper manual is 1974 and covers your SN. Most mods were in CRT and vert amp not H amp.
The BAMA and TEK wiki should have the H board schemas and parts layout and adjustments.

3/ Your posts got me curious, so I just tested my 7904 with 7B92A H plugins and 7A24, 7A26, and 7A29 Vert
Used Leo Bodnar 40 ps pulser and the TEK TG501, PG506 CAL modules.
A few scratchy controls, CRT bright and perfect, 100%

WOW!!! What a wonderful instrument, still perfect after 38 years.

VIVE TEKTRONIX!

Jon


Chuck Harris
 

In the spirit of give a man a fish, and you feed him for
a day. Teach a man to fish, and you feed him for life.

Here is how to fish:

The horizontal board has several power supplies that
enter the board, notably +15V, -15V, -15V DCPL, +5V,
and +5V DCPL.

Each of these points is filtered by a tantalum capacitor,
and a small RF choke, the -15V DCPL also has 10 ohm 1/4W
resistor in series with the choke.

These are colloquially known as "L" type low pass filters.
Their purpose is to remove RF noise coming from the power
supply leads so that it doesn't reach the circuits on the
horizontal board.

You don't need a schematic to find them, they will shine
right out at you if you look at the horizontal board.

You can recognize them by their epoxy blob style tantalum
capacitor, that is decorated with colorful bands of paint.

You can recognize the small RF chokes as they are built
like 1/4 W resistors wrapped in fine wire.... Sometimes
they are 1/4 watt resistors wrapped in fine wire...

Typically, the shorted tantalum capacitor will cause the
choke that comes before it to turn into a charcoal crisp.

Sometimes the shorted tantalum capacitor will remove itself
from the board like a tiny suicide bomber. You will only
see a pair of 1/10th inch spaced short wires sticking
upright from the board where it used to be.

You may replace these shorted capacitors with high quality
electrolytic capacitors of the same, or higher voltage, and
a minimum of 2x the capacitance, to a maximum of about 5x
the capacitance.

-Chuck Harris

Kaveh wrote:

i'm sorry but i can't find all these capacitors on the schematic here. all the capacitors shown on my circuit diagram for the horizontal circuit have a different name and I cannot find C394, C396 and C390 anywhere. You need to have a different schematic.






Jean-Paul
 

Chuck perfect response as always.

I have to check my trusty 7094 for tant timebombs....

jon


Kaveh
 

On Sun, Dec 13, 2020 at 03:12 PM, Jean-Paul wrote:


WOW!!! What a wonderful instrument, still perfect after 38 years.
Yes you are right. this is really a great device. therefore I really want to fix it. The tantalum capacitors are okay, I checked them all. I will still replace them with high quality electrolytic capacitors. As I saw in the circuit diagram of the converter, tantalum capacitors are also built in. I hope they won't cause any problems anytime soon. I could address that, but the device has to work first. Can your service Manuel send me the circuit diagram for the horizontal circuit as a PDF? that would be good.


Jean-Paul
 

All info and manuals here

http://bama.edebris.com/download/tek/7904/

Jon


Jean-Paul
 

With the terrible pcb conditions of the Horizontal I would check every PCB and also see if any interconnect connections and connectors are corroded.

The TEK black harmonica that mate to board 0.025" sq gold pins are especially delicate.

Jon


Kaveh
 

On Sun, Dec 13, 2020 at 03:13 PM, Chuck Harris wrote:


In the spirit of give a man a fish, and you feed him for
a day. Teach a man to fish, and you feed him for life.
nice quote but who is the fish here and who is the fisherman?


Chuck Harris
 

The fish is diagnosing tantalum problems (without a
schematic). I am hoping that with a little advice,
you will learn how to recognize tantalum capacitors,
find failed capacitors on your own... So, with any
luck, you will be the fisherman.

On tantalums: It bears repeating that tantalum capacitors
are probably the most reliable form of electrolytic
capacitor ever made. That said, they do have a weakness,
though.

In the old days, when tektronix was designing the 7000
series scopes, the manufacturers of tantalum capacitors
advised that the capacitors did not need to be spec'd
at a higher than working voltage. They believed that
using a 5V tantalum capacitor on 5V was a perfectly
safe thing to do. They suggested that you use the nearest
rated voltage capacitor that had a rating above the
operating voltage. So, for 5V, 6V caps were used. For
15V, 16 or 20V capacitors were used.

In many respects they were right. Typically a tantalum
capacitor specified so closely to its operating voltage
will last a very long time. 5-30 years in some cases.

However, from our point of view, and perhaps NASA's, that
is not as good as we might like. We like older equipment,
and the capacitors specified as they once did will often
die in spectacular ways.

NASA found that the answer was to specify such capacitors
at 2 to 2.5 times the operating voltage. When this is
done, the capacitors have essentially an unlimited lifetime.

There is no need to replace good tantalum capacitors. There
are only a few, typically those connected with low impedance
to a power supply, that are problematic.

Those in timing circuits, and those that are voltage rated
as I mentioned above are mostly beyond reproach.

-Chuck Harris

Kaveh wrote:

On Sun, Dec 13, 2020 at 03:13 PM, Chuck Harris wrote:


In the spirit of give a man a fish, and you feed him for
a day. Teach a man to fish, and you feed him for life.
nice quote but who is the fish here and who is the fisherman?






Don Bitters
 

B133854 is the serial number. The other number is a part number.

Don Bitters