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From: Henry Brown <htbrown@...>
To: Tektronix Scoipes <TekScopes@...>
Sent: Mon, Jan 23, 2012 21:35:12 GMT+00:00
Subject: [TekScopes] 7704A Readout Board

?

Hello all, I have a Tek 7704A with no readout. Does any one have a readout
board (No. 34) on lower left rear or scope? Is there a source I could
contact for the board? Thanks much -- this group looks very interesting
with plenty of information on scopes. 73 de Hank

I can only think I'm not interpreting what I'm seeing on the 'scope screen correctly.

I'm probing TP286 (output of TTH) with Sweep Range set to 50nS, 5nS/div in sequential mode. I have the viewing timebase set for 5uS/div. The signal I see at high scan rates is similar to waveform capture 11 in the 7T11 and 7T11A manuals except that the top of the bright zone starts ABOVE 0V, and the bottom of it is about 5.6V below there. The picture of this signal manuals however clearly has the top of this signal at 0V with nothing above

I can't reconcile what I see with the descriptions in the manual that say the TTH ramp starts at 0V and goes more negative until stopped by the slewing ramp comparator.

How can the TTH ramp voltage ever be +ve in sequential mode???? I could possibly understand the bright zone moving down as you turn the Time Position controls CCW if the individual trace line in the brightened zone represents the value of the TTH ramp after it has been stopped by the slewing ramp comparator for this sample (I now think this is the case). But I cannot see how it can end up above zero!

If you turn the Scan control fully CCW, you can actually see the voltage ramp down and then jump back up.

D.

I don't understand how the TTH sweep voltage start point is affected
by the front panel Time Position controls as they connect into the
circuitry around U572D on the Analog Logic board. When Q294 in the
TTH Miller circuit conducts, my dumb logic says that the TTH ramp
voltage will reset to zero, but clearly I would seem to be missing
something crucial, because adjusting the Time Position controls DOES
appear to change the start voltage for the TTH ramp as measured at
TP286, but I can't work out how :-(


Regards,
David Partridge

Hi,

Am looking for 3 or 4 knobs for the lower front intensity, focus, readout intensity and scale illumination controls for my 2445A. Tek part number is 366-2041-03. Anyone have any spares I can buy?

Thanks,
Chris
fazeka (at) gmail (dot) com

Hi there,
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Hi,

On a tek 2230 in non-store mode, the displayed signal becomes compressed when position is moved to the left.

So if I have a 5K square wave displayed at .1 Ms the first 1/2 of the wave will be 60% the width if I move it to the right. It also looks like brighter.

+==+==+==+==+==+==+==+==+==+==+
---___---___--- <moved to right>

+--- Short and brighter.
V
==___---___---

What needs adjusting

Hi Tan,
This morning I remembered about an interesting 2235 repair that was done by
a very crafty person by the name of Irwin Zosa during 2007 and 2008.
Irwin lives in your part of the world in the Philippines where it is almost
as hot as in Singapore. His basic problem was the same as yours, the 2235
would shut itself down with a thermal problem.
There was not much interest in his 2235 problem on Tekscopes at that time,
so the email exchanges we had went off group as he worked his way through
the problem. Once he had the problem solved, he posted a nice summary of his
findings to the Tekscopes group.
Irwin's repair summary is in message number 36518 and it was posted on Jan
6, 2009.
He found a number of problems as he went along, but his final discovery
might amaze you.
It sure amazed me because I would have never found it!
tom jobe...
PS I will paste in Irwin's 2235 repair summary below to save you the trouble
of getting it from the Tekscopes Message archive.

Hello to all: Way back in the first quarter of 2007, I had this
problem of my 2235 shutting down after a few minutes from switching
it ON. After a lot of interaction from forum members I finally
nailed down the problem. Here is a brief summary of this process:
First, I checked all the voltages according to the service manual and
they were OK. I also checked the ESR of the capacitors and they read
good but the unit being about twenty years old or so, I decided to
replace the secondary caps (C960, C961, C962, C963, C968, C970) with
ultra-low ESR, 105-degree C Nichicons. I could not find 840uF units
so I used 1000uF. The problem still did not go away although the
length of time-to-shutdown increased. I also replaced the FET
switcher Q9070 (IRF710)with a higher rated device (IRF840). I then
replaced Q946 and Q947 with TIP41 units. At every change that I
made, slight improvements were observed. I also replaced R912 (357
Ohms)with a 390 Ohm part to increase the shutdown threshold of this
2235. Then I replaced the secondary rectifiers CR954, CR955 (MR814),
CR956, CR957 (MR812), CR960, CR961, CR962, CR963 (MR812). The
replacements that I used were the MUR160 ultra-fast rectifiers. All
of these were not done "shotgun" style and in the exact order as
described but rather, followed logical steps of checking one section
at a time. I even thought of thermal runaway because T944, T948, and
Q9070 would heat up so much (really hot!). I also noticed early on
that the frequency of the pre-regulator section (U930) was about
72KHz and in the service manual it is listed as about 60KHz. It
suddenly dawned on me to try to reduce the operating frequency of
this section so I replaced R919 and now it is running at 60KHz. The
excessive heat has gone and no more shutdown. I tried to run the
scope for about half a day and still no shutdown. This 2235 has all
of the power supply improvements already installed from the factory
(but why was the pre-regulator section running at 70KHz ?). I would
like to thank the forum members who gave their insights. Special
thanks goes to Tom Jobe, who stayed with me all the way in this
repair project. He took the time to take measurement readings for
me, gave his analyses, and even offered parts that I may need for
free. At some point I was considering another brand of oscilloscope
but I held on to this (even keeping it in storage for almost a year
because of this problem) because from my research, these TEK scopes,
during their time, were among the best. I think they still are.

I don't understand how the TTH sweep voltage start point is affected by the front panel Time Position controls as they connect into the circuitry around U572D on the Analog Logic board. When Q294 in the TTH Miller circuit conducts, my dumb logic says that the TTH ramp voltage will reset to zero, but clearly I would seem to be missing something crucial, because adjusting the Time Position controls DOES appear to change the start voltage for the TTH ramp as measured at TP286, but I can't work out how :-(


Regards,
David Partridge

-----Original Message-----
From: TekScopes@...
[mailto:TekScopes@...] On Behalf Of Don Black
Sent: Saturday, January 21, 2012 7:55 PM
To: TekScopes@...
Subject: Re: [TekScopes] Re: Nuvistors.

Perhaps it was just easier to provide a DC heater supply of 12
volts at
half the current of a 6 volt tube in a tubes scope for hum reduction.

Don Black.

On 19-Jan-12 10:30 PM, phosphorphile wrote:

Yes, it may be a 13 volt filament is easier to run at

slightly lower voltage because perhaps the voltage being twice
and the current being half of a 6 volt filament would make for
a more constant current with line variations. Probably the way
tungsten filaments work?

The 8393 was used in early 453's and later 503's, I know.

Mebbe other models too.

Thinking rather more about this, the start voltage of -2.22V for the TTH Miller circuit doesn't sound right - from all my reading in the manual I think it should be starting at 0V, and the screen shots in the manual also suggest this. The waveforms shown for the Horiz. Amp. also suggest that there's no (or very little) DC offset.

Comments?

Thanks again
Dave

.......It was around 1971 that the 8080 came out and the world was about to really change. .....