Date   

Re: TEK 7704A pre-death behaviour a clue to the fault?

Simon Owen
 

Update: I just measured the +50V supply as being within tolerance but with
1.8 full volts of ripple.

Further evidence for the case against an electrolytic capacitor?

On Thu., 8 Oct. 2020, 10:26 Simon Owen via groups.io, <Sowenbd=
gmail.com@groups.io> wrote:

Thanks a million, great advice both of you. I shall proceed as you suggest
and report back!

On Thu., 8 Oct. 2020, 10:21 Raymond Domp Frank, <hewpatek@gmail.com>
wrote:

I concur with Eric that most likely, it's a matter of one or more bad
caps.
With the 7704A, the display unit may simply be separated from the base
unit, at the "Main Interconnect". You just have to loosen the screws
holding down the rails linking the top and bottom sections, to the left
and
right of the mainframe.
After a quick look at the schematics, I can't find a reason why running
the base unit powered on with the display unit disconnected, could do any
harm.
Checking the base unit under power on its own not only may allow to
narrow
down the problem area but measuring resistances across the power
connections of the display unit may indicate the problem area(s).

Raymond










Re: TEK 7704A pre-death behaviour a clue to the fault?

Simon Owen
 

Thanks a million, great advice both of you. I shall proceed as you suggest
and report back!

On Thu., 8 Oct. 2020, 10:21 Raymond Domp Frank, <hewpatek@gmail.com> wrote:

I concur with Eric that most likely, it's a matter of one or more bad caps.
With the 7704A, the display unit may simply be separated from the base
unit, at the "Main Interconnect". You just have to loosen the screws
holding down the rails linking the top and bottom sections, to the left and
right of the mainframe.
After a quick look at the schematics, I can't find a reason why running
the base unit powered on with the display unit disconnected, could do any
harm.
Checking the base unit under power on its own not only may allow to narrow
down the problem area but measuring resistances across the power
connections of the display unit may indicate the problem area(s).

Raymond






Re: TEK 7704A pre-death behaviour a clue to the fault?

 

On Thu, Oct 8, 2020 at 01:13 AM, Simon Owen wrote:


What you're saying makes sense to me though. Are you thinking it could be
one or both of the huge beercan caps?
No, the beercan caps sit straight across the mains input, before the oscillator causing the ticking, so no tick mode. They fail not often but if they do, they usually cause the main fuse to blow and, with some bad luck, damage the input rectifier,

Raymond


Re: TEK 7704A pre-death behaviour a clue to the fault?

 

I concur with Eric that most likely, it's a matter of one or more bad caps.
With the 7704A, the display unit may simply be separated from the base unit, at the "Main Interconnect". You just have to loosen the screws holding down the rails linking the top and bottom sections, to the left and right of the mainframe.
After a quick look at the schematics, I can't find a reason why running the base unit powered on with the display unit disconnected, could do any harm.
Checking the base unit under power on its own not only may allow to narrow down the problem area but measuring resistances across the power connections of the display unit may indicate the problem area(s).

Raymond


Re: TEK 7704A pre-death behaviour a clue to the fault?

Simon Owen
 

Thanks Eric
I forgot to mention I've replaced all the tantalum and electrolytic caps on
the rect/filter board.

Now I recall a tantalum cap on the board on top of the CRT burned out
shortly after its first resurrection so I did a wholesale replacement of
tantalum caps throughout.

What you're saying makes sense to me though. Are you thinking it could be
one or both of the huge beercan caps?

Thanks,
Simon

On Thu., 8 Oct. 2020, 09:56 Eric, <ericsp@gmail.com> wrote:

First thing I would suspect is a leaky electrolytic or a tant that is
shorted. I am leaning more to the electrolytic though. Seems like a "warm
up" with a cap charging in time. Sounds like one of the rails is coming up
slowly

Eric

On Wed, Oct 7, 2020, 6:50 PM Simon Owen <Sowenbd@gmail.com> wrote:

Hi all
I am embarking on the likely arduous journey to resurrect my 7704A from
tick mode.

It was actually in tick mode when I bought it so I got a good deal (I
hope!). I got it home and pulled some plugins out and gave them a good
shove home and presto, it hummed to life.

I got a couple of months of tinkering with it but alas back in tick mode.

I have put together a test load and I'll be measuring voltages and ripple
shortly.

It just occurred to me that before its most recent death there was
potentially a warning sign: the scope would tick a few times before
coming
to life. Then I remember it would tick for longer and longer, but I found
that if I pushed the beam finder button it would 'snap out of it' and get
down to business. This clearly didn't last.

Does anyone have any wisdom on this symptom so perhaps I will have an
idea
of the likely problem? When it was running during its 'pre-death' period
I
didn't notice anything else peculiar.

Assuming I can rule out the power supply as the cause of the trouble, I
wouldn't know where to begin!

Thanks a million
Simon Owen










Re: OT CRT data wanted

 

Out of curiosity, can you describe the 54DGP11 tube? Or put a picture in the photos directory?

Regards, Tom

On 10/7/2020 6:27 PM, Mark Vincent wrote:
I have two crts that I would like the data on and schematic of unit(s) they
were in. The numbers are Dumont/Fairchild F7770-0P31 and ETC (Electron Tube
Corp.) 54DGP11. Thanks.

Mark



--
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus


Re: TEK 7704A pre-death behaviour a clue to the fault?

Eric
 

First thing I would suspect is a leaky electrolytic or a tant that is
shorted. I am leaning more to the electrolytic though. Seems like a "warm
up" with a cap charging in time. Sounds like one of the rails is coming up
slowly

Eric

On Wed, Oct 7, 2020, 6:50 PM Simon Owen <Sowenbd@gmail.com> wrote:

Hi all
I am embarking on the likely arduous journey to resurrect my 7704A from
tick mode.

It was actually in tick mode when I bought it so I got a good deal (I
hope!). I got it home and pulled some plugins out and gave them a good
shove home and presto, it hummed to life.

I got a couple of months of tinkering with it but alas back in tick mode.

I have put together a test load and I'll be measuring voltages and ripple
shortly.

It just occurred to me that before its most recent death there was
potentially a warning sign: the scope would tick a few times before coming
to life. Then I remember it would tick for longer and longer, but I found
that if I pushed the beam finder button it would 'snap out of it' and get
down to business. This clearly didn't last.

Does anyone have any wisdom on this symptom so perhaps I will have an idea
of the likely problem? When it was running during its 'pre-death' period I
didn't notice anything else peculiar.

Assuming I can rule out the power supply as the cause of the trouble, I
wouldn't know where to begin!

Thanks a million
Simon Owen






TEK 7704A pre-death behaviour a clue to the fault?

Simon Owen
 

Hi all
I am embarking on the likely arduous journey to resurrect my 7704A from
tick mode.

It was actually in tick mode when I bought it so I got a good deal (I
hope!). I got it home and pulled some plugins out and gave them a good
shove home and presto, it hummed to life.

I got a couple of months of tinkering with it but alas back in tick mode.

I have put together a test load and I'll be measuring voltages and ripple
shortly.

It just occurred to me that before its most recent death there was
potentially a warning sign: the scope would tick a few times before coming
to life. Then I remember it would tick for longer and longer, but I found
that if I pushed the beam finder button it would 'snap out of it' and get
down to business. This clearly didn't last.

Does anyone have any wisdom on this symptom so perhaps I will have an idea
of the likely problem? When it was running during its 'pre-death' period I
didn't notice anything else peculiar.

Assuming I can rule out the power supply as the cause of the trouble, I
wouldn't know where to begin!

Thanks a million
Simon Owen


OT CRT data wanted

Mark Vincent
 

I have two crts that I would like the data on and schematic of unit(s) they
were in. The numbers are Dumont/Fairchild F7770-0P31 and ETC (Electron Tube
Corp.) 54DGP11. Thanks.

Mark


Re: Is my Tek 468 beyond repair?

Bob Haas
 

The listing already allows international shipping.

--
Bob Haas


Re: Is my Tek 468 beyond repair?

Bob Haas
 

Sorry. I will update the listing to allow international shipping.

--
Bob Haas


Re: TM515 Suitcase 500 Series Cardcage/Power Supply Bail (Wire Stand) Replacement?

Charlie Conger
 

If you have a vise you could bend one from 3/16 piano wire, available at hobby shops.


Re: Is my Tek 468 beyond repair?

Colin Herbert
 

Bit of a setback that they don't ship to the UK. They will ship other things to the UK, so I wonder why not these?
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of robeughaas@gmail.com
Sent: 07 October 2020 18:38
To: TekScopes@groups.io
Subject: Re: [TekScopes] Is my Tek 468 beyond repair?

The vintageTEK Museum now has replacement 468 EPROMs: https://www.ebay.com/itm/114450705368


--
Bob Haas


Re: Is my Tek 468 beyond repair?

Bob Haas
 

The vintageTEK Museum now has replacement 468 EPROMs: https://www.ebay.com/itm/114450705368


--
Bob Haas


Re: TAS465 A1 PCB - why is C437 bi-polar

Siggi
 

On Wed, Oct 7, 2020 at 8:01 AM Jon <jonathan_harrison@hotmail.com> wrote:

Does anyone know why C437 fitted to the TAS465 A1 pcb is bi-polar ? It's
the bulk cap for the -5.2VC supply . Other caps on this board are Nichicon
VX and provide the bulk caps for the +8.6VA , -8.6VB , +5.2VB and -5.2VB
supplies. Looking at the service manual C437 is shown as polar on the
schematic but the parts list shows it as 10VAC. I don't understand why the
rationale for fitting a bi-polar cap here. Does anyone have any knowledge
of this ?
Hey Jon,

That's pretty curious. Note that this decouples -5.2C, which is a somewhat
isolated sub-supply drawn from the -5.2A. It looks like this supply is only
used by U403/U404, which are the transistor arrays responsible for
switching the attenuator relays. Maybe the inductive spike from those
relays is the reason a bipolar capacitor was chosen there?

Siggi


Re: Is my Tek 468 beyond repair?

John
 

Bit more progress today!. The replacement CPU arrived and has been installed. I am now getting all of the correct signals on the CPU board as per the service manual and comparable with Tony's "quick once around the processor" report. I have re-installed the two ICs that were removed and the display came to life on the next power up. It initially shows [ . . . .] then [8.8.8.8] then all of the digits flicker with 0's for a while until it settles to [ 0. 0.]. I am getting no display on the CRT at the moment although I do see a trace when I hit the beamfinder button.

So it seems that the computer is now working. Since the "Lamp Test" is ocurring as shown by the [8.8.8.8.], it seems that the ROM Checksum Test and the Lamp Test have passed. Next comes the "RAM Verificaton Test" and I am unclear as to whether this is succeeding or failing. I have so far been unable to find what [ 0. 0.] means although the manual does suggests that a RAM check will return a zero for "No error". It does also say that the display should blank and the scope should start operating which does not appear to be happening.

I will wiggle the chips on the RAM board, but if anyone has seen [ 0. 0.] and can tell me what it meas that would be appreciated.


Re: Is my Tek 468 beyond repair?

Michael W. Lynch
 

On Tue, Oct 6, 2020 at 01:17 AM, jrseattle2002 wrote:


Since there was a request for this in the above discussion, I uploaded an
image of the Tektronix 468 service rom to the files section. It's in a
directory called "Tektronix 468 Service Rom"
THANK YOU! I have been searching for this SERVICE ROM for some time. Now I have to find the chips, figure out how to load this file and use them. I sincerely appreciate this!

--
Michael Lynch
Dardanelle, AR


Re: 7623A Storage problem

Colin Herbert
 

It appears that I am a nit. I means to type "it" and a gremlin made me add
an "n" to the beginning. Sorry.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Colin
Herbert via groups.io
Sent: 07 October 2020 14:23
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7623A Storage problem

Would you believe that I have a 7CT1N curve-tracer, but of course nit
operates in the 7623A mainframe, which isn't working? I don't have another
7000-series mainframe, sadly.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Thomas
Garson
Sent: 04 October 2020 22:37
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7623A Storage problem

The junctions of that type of counterfeit power transistor will have a
higher forward voltage than a genuine part. Usually they read as much as
.1 to .15 volts higher on a typical DMMs diode test range. They will
also have different junction capacitance. I can't give numbers as I'm
not where I have proper tools. However a data sheet for the McCoy will
provide at least some of that information.

A curve tracer will reveal a lot. Small signal transistors have very
different family of curves than most any power transistor. While the Tek
curve tracers are the tool of choice, I've gotten a lot of good mileage
from the Heathkit variety. The Heathkit curve tracer, which does not
have a display, will work with any DC coupled 'scope and can sometimes
be found on eBay for a low price that belies its usefulness. Even if
broken, they are relatively simple and easy for a moderately
knowledgeable tech to fix.

Thomas Garson
Aural Technology, Ashland, OR
By my calculation, the dynamic range of the universe is roughly 679dB,
which is approximately 225 bits, collected at a rate 1.714287514x10^23 sps.

On 10/4/20 12:59 PM, Tom Lee wrote:
There are also notorious Chinese knock-offs which contain low-power
(2N2222?) dice in what appear to be period-authentic packages. They'll
pass the standard multimeter diode test, but don't survive actual use.
The ft test that Brad T. suggests is a very good way to identify
non-genuine 3055s.

--Tom


Re: 7623A Storage problem

Colin Herbert
 

Would you believe that I have a 7CT1N curve-tracer, but of course nit
operates in the 7623A mainframe, which isn't working? I don't have another
7000-series mainframe, sadly.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Thomas
Garson
Sent: 04 October 2020 22:37
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7623A Storage problem

The junctions of that type of counterfeit power transistor will have a
higher forward voltage than a genuine part. Usually they read as much as
.1 to .15 volts higher on a typical DMMs diode test range. They will
also have different junction capacitance. I can't give numbers as I'm
not where I have proper tools. However a data sheet for the McCoy will
provide at least some of that information.

A curve tracer will reveal a lot. Small signal transistors have very
different family of curves than most any power transistor. While the Tek
curve tracers are the tool of choice, I've gotten a lot of good mileage
from the Heathkit variety. The Heathkit curve tracer, which does not
have a display, will work with any DC coupled 'scope and can sometimes
be found on eBay for a low price that belies its usefulness. Even if
broken, they are relatively simple and easy for a moderately
knowledgeable tech to fix.

Thomas Garson
Aural Technology, Ashland, OR
By my calculation, the dynamic range of the universe is roughly 679dB,
which is approximately 225 bits, collected at a rate 1.714287514x10^23 sps.

On 10/4/20 12:59 PM, Tom Lee wrote:
There are also notorious Chinese knock-offs which contain low-power
(2N2222?) dice in what appear to be period-authentic packages. They'll
pass the standard multimeter diode test, but don't survive actual use.
The ft test that Brad T. suggests is a very good way to identify
non-genuine 3055s.

--Tom


Re: 7623A Storage problem

Colin Herbert
 

I have just checked all of my 2N3055 stash using a magnet and they all have
steel casings.

I am afraid my electronics knowledge is too limited for me to be able to
knock-up any test-rigs to do electrical checks on them. Any hints?

TIA, Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Thomas
Garson
Sent: 04 October 2020 20:10
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7623A Storage problem

If your 2N3055s are in aluminum packages, they are likely Motorola or
Fairchild fab, and will be epitaxial (high gain). If they are in steel
alloy cans, they could be RCA, which would be hometaxial, or slower and
lower gain. There were a lot of smaller fabs making 2N3055 because that
part number had become ubiquitous. Most, perhaps all, used hometaxial
process, and steel cans. Possibly those foundries had acquired obsolete
lines which had been surplussed out by the bigger operations when they
changed over to epitaxial.

You didn't specify what collector current your tester operates at. I
suspect its in the low ma range. At 10 ma, hometaxial parts would show
hFE close to your 30 & 34. hFE maxes out at about 70 at 3a. I wouldn't
use the part with hFE of only 12, or the high gain part.

I think that when RCA was killed by GE, their power devices were still
primarily (exclusively?) hometaxial. The RCA 40436 speced identical to
the 3N3055. I used a number of them in one off power amps and linear
supplies during the early 1970s. Another RCA option would be 2N3772.

I agree that it is not a good idea to install newer epitaxial devices in
a circuit whose design hails from before about 1975. There is a good
chance the part will not behave nicely. As the unwashed might say:
"Magic smoke may appear".

Somewhere, I've got a bag of old, but mostly unused, 2N3055s, should it
become an issue. All the ones I just found in a Google search were
planar epitaxial, which work fine in newer designs that expect the
higher speed.

Anecdote: 1972. To get cheap high power for small time rock bands, I
would take 2 Peavey PA120, which was a 2 ohm capable 1ch power map
entirely built on a 5u rack panel. PA120 used house branded 2N3055
equivalent devices. Circuit probably copied from RCA Transistor Manual.
I put two of them in one cabinet, back to back with end to end fan
added, configured for bridged mono operation: Easy 500w.r.m.s into 4
ohms. If one blew up (not easy), I used RCA 2N3055s as replacement
devices. They were very reliable.

Thomas Garson
Aural Technology, Ashland, OR
By my calculation, the dynamic range of the universe is roughly 679dB,
which is approximately 225 bits, collected at a rate 1.714287514x10^23 sps.

On 10/4/20 8:26 AM, Colin Herbert via groups.io wrote:
Thanks for that, guys. I have three old 2N3055 in my "junk box". One
doesn't have a manufacturer's name but is also labelled 3055H and has (using
my MK-328 transistor tester) what I assume is a beta ("B") of 30, another
labelled ITT has a beta of 34 and one marked RS has beta of 12. Which of
these would be appropriate, if any? I also have a newer one with a much
higher beta, which I assume is not good in this application. Please forgive
my ignorance on this.
TIA, Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Tom
Lee
Sent: 04 October 2020 12:12
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7623A Storage problem

As David Partridge says, it's risky to pop in a modern replacement. It's
not just a matter of gain (I assume you meant beta). The original 2N3055
was made by a now-extinct "hometaxial" fabrication process, which
produced surprisingly rugged, inexpensive, and very slow devices. Modern
planar versions of the 3055 are much, much faster, but also less rugged.
The higher speed is not always a virtue -- it can provoke wild and not
infrequently destructive oscillations in circuits that rely upon the
slowness of original 3055s for stability. It is often possible to add
capacitances to make modern devices slower in the right ways, but it's
an ugly circuit-specific kludge that doesn't fully reproduce the
glorious sluggishness of the original.

-- Tom

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