?

Hi all,
a few weeks ago i bought a 547 wich was in a very good shape. Almost no dust inside, no dents in the casing and the solder reel untouched. I think he (she ;-) has never seen a soldering iron inside.

The only thing was that the previous owner take almost all the tubes out. But because I have three 547 and one in cosmetically in a very bad shape this was not a real problem so I put the tubes from the scrap 547 in my new one and in an afternoon I got the "new" 547 worked.

I think it has always spend his life in a climate room or so because even the dreaded HV Transformer problem was not an issue. I can let it work for 6 or 10 hours with a total stable trace. My third 547 wich is also in a very good shape had the HV Xformer potting problem so I took the HV transformer out and let it rewound by Chuck, really a master piece of work, but still has to be build in.

So, after that I checked all the voltages from the powersupply and everything was in specs, only the +350V had an enourmous ripple on it.
After a check it had a bad cap (C730, 125u/250V) so I changed it. Before there was almost 4.5V ripple on the +350V line and now it?s reduced to a normal value, 3mV.
Also one of the 5642 HV tubes (V842) I changed. I think it was gassy because it was a bit blueish so I put another good one inside. Before that repair the HV was -1894V and with the new tube (strange!?) it was a bit lower -1880V so I adjusted the HV to -1850V, as the manual said.

But it seems to have two other problems inside...

Before the repair there was a bright green spot in the beginning of the trace, always at the lower part of the trace but also with a bit lower intensity at the upper part of the trace.
This occures on both A and B timebase. But after the repair of C730 they where gone, but a few days later they returned :-(

There is also sometimes a line visible in the upper part of the trace with a bit lower intensity then the original trace line. Especially when I turn up the intensity a bit.

The other problem is that a 1Khz signal (with the timebase setting at 0.2mS) comes on the screen with 6 divisions, thus 1.2mS. So that is 20% too much because it should be 5 div. Strangely this is on the A and B Timebase.
I tried to correct this as explained in the calibrating procedure but there is not enough "adjustment room" to put it on 5 divisions.

So the question is what are the possible causes for these faults, and maybe it?s one component wich is the main reason for these two phenomenoms.

I put a few pictures in the Photo Albums (Rene 547) so you can see the strange bright dots.

Rene

You see the retrace, so it is not blancking.
Fred PA4TIM

---
Before the repair there was a bright green spot in the beginning of the trace, always at the lower part of the trace but also with a bit lower intensity at the upper part of the trace.
This occures on both A and B timebase. But after the repair of C730 they where gone, but a few days later they returned :-(

There is also sometimes a line visible in the upper part of the trace with a bit lower intensity then the original trace line. Especially when I turn up the intensity a bit.
---
I put a few pictures in the Photo Albums (Rene 547) so you can see the strange bright dots.

Rene

You were just a few minutes faster than me to mention this Fred!
Rene, what happens if you trigger with negative slope? Bet you see the dot at the upper side. And if you switch trigger to Ext (and not Auto) you probably see 2 dots at the start position of the trace and a faint vertical line between the dots.
Did you check +350 V again after the dot reappeared? I didn't look at the schematics but I suppose the +350 V is heavily used in the Z-axis circuits.
Albert

You see the retrace, so it is not blancking.
Fred PA4TIM

---
Before the repair there was a bright green spot in the beginning of the trace, always at the lower part of the trace but also with a bit lower intensity at the upper part of the trace.
This occures on both A and B timebase. But after the repair of C730 they where gone, but a few days later they returned :-(

There is also sometimes a line visible in the upper part of the trace with a bit lower intensity then the original trace line. Especially when I turn up the intensity a bit.
---
I put a few pictures in the Photo Albums (Rene 547) so you can see the strange bright dots.

Rene

?

?
It?s now already a bit late here in Holland so tommorrow I check the +350V line again.
Thanxxxx so far!

Reply to sender | Reply to group | |

(4)

.

Hi Albert and Fred, people are really quick on this forum :-)
I put the scope on a few minutes ago and I have now two dots; one below and one above in the trace picture.

When it?s not trigering there are (ofcourse) no lines but only two dots. Also on Ext. gives the same two dots.
But the retrace line as Fred called it is in the lower part of the trace when the trigger is set to negative and at the upper part when trigering is set to positive.
It?s now already a bit late here in Holland so tommorrow I check the +350V line again.
Thanxxxx so far!

Hi Rene,
If you slowly change the sweep time/div with the VAR knob then you will see the retrace jumping from lower to higher level and vice versa. You will see why when you adjust Horizontal Position such that you can see the end of the trace.
Albert

PS Fred: some people have to work tomorrow;=)


--- In TekScopes@..., "tek_547" <R.Kelbling@> wrote:

Hi Albert and Fred, people are really quick on this forum :-)
I put the scope on a few minutes ago and I have now two dots; one below and one above in the trace picture.

When it?s not trigering there are (ofcourse) no lines but only two dots. Also on Ext. gives the same two dots.
But the retrace line as Fred called it is in the lower part of the trace when the trigger is set to negative and at the upper part when trigering is set to positive.
It?s now already a bit late here in Holland so tommorrow I check the +350V line again.
Thanxxxx so far!

The unblanking signals from the A and B sweeps are combined in the
horizontal display switch (SW530 5R and 5F located on schematic 2). I
would start looking for a problem there. Does wiggling that switch
make any difference?

Since it is so easy, you may want to check for the presence of the A
and B gate outputs which are copies of the two separate unblanking
signals before the horizontal display switch.

On Mon, 08 Oct 2012 13:14:41 -0000, "tek_547" <R.Kelbling@...>
wrote:

You are dead right Albert about changing the sweep time/div with the VAR knob. It seems the retrace comes from the last section of the end of the trace and changes up and down while turning on the knob.

I also checked the +350V and it?s still correct after the repair of C730. It is +353V with about 5mV ripple, completely in specs just as the other voltages from the power supply. All of them have only a few millivolts of ripple and the voltages are comfortable in specs.

When I turned the scope on today only the B timebase have the "dot problem", the A time base functioning correct. Very weird!

Suggesting that the problem is in the unblanking circuit in wich part of the circuit the defective component can be?

Maybe a capacitor or a tube, ideas?

Grtz, 搁别苍é


--- In TekScopes@..., "aobp11" <aodiversen@> wrote:

Hi Rene,
If you slowly change the sweep time/div with the VAR knob then you will see the retrace jumping from lower to higher level and vice versa. You will see why when you adjust Horizontal Position such that you can see the end of the trace.
Albert

PS Fred: some people have to work tomorrow;=)


--- In TekScopes@..., "tek_547" <R.Kelbling@> wrote:

Hi Albert and Fred, people are really quick on this forum :-)
I put the scope on a few minutes ago and I have now two dots; one below and one above in the trace picture.

When it?s not trigering there are (ofcourse) no lines but only two dots. Also on Ext. gives the same two dots.
But the retrace line as Fred called it is in the lower part of the trace when the trigger is set to negative and at the upper part when trigering is set to positive.
It?s now already a bit late here in Holland so tommorrow I check the +350V line again.
Thanxxxx so far!

-----Original Message-----
From: TekScopes@...
[mailto:TekScopes@...] On Behalf Of tek_547
Sent: Tuesday, October 09, 2012 10:17 AM
To: TekScopes@...
Subject: [TekScopes] Re: Bright dots at start of the trace on a 547

Thanx for all the reactions guys, it was an old germanium
transistor Q184 in the unblanking circuit. Chuck did me the
suggestion to look specific at that transistor and he was damn right.
I replaced it for a 2N3906 and also his brother on the A
timebase Q373. They are silicium transistors but functions perfectly.
So in a few days, problem solved :-)

The only thing what?s left is the timing from the sweep rates,
they are about 20% to long. All of them on both timebases, so
if somebody knows...

grtz and thanx so far for all the help, 搁别苍é



--- In TekScopes@..., David <davidwhess@...> wrote:

The unblanking signals from the A and B sweeps are combined in the
horizontal display switch (SW530 5R and 5F located on

schematic 2). I

would start looking for a problem there. Does wiggling that switch
make any difference?

Since it is so easy, you may want to check for the presence of the A
and B gate outputs which are copies of the two separate unblanking
signals before the horizontal display switch.

On Mon, 08 Oct 2012 13:14:41 -0000, "tek_547" <R.Kelbling@...>
wrote:

You are dead right Albert about changing the sweep time/div

with the VAR knob. It seems the retrace comes from the last
section of the end of the trace and changes up and down while
turning on the knob.

I also checked the +350V and it?s still correct after the

repair of C730. It is +353V with about 5mV ripple, completely
in specs just as the other voltages from the power supply. All
of them have only a few millivolts of ripple and the voltages
are comfortable in specs.

When I turned the scope on today only the B timebase have

the "dot problem", the A time base functioning correct. Very weird!

Suggesting that the problem is in the unblanking circuit in

wich part of the circuit the defective component can be?

Maybe a capacitor or a tube, ideas?

Grtz, 搁别苍é

Good catch there Dave. I have noticed when testing my curve tracer on
2N3906 and 2N3904 transistors that they often operate significantly
higher than their rated breakdown voltage but it sure is not something
I would want to rely on without grading them. The specifications I
see for the 2N2207 show it to be operating close to its breakdown
voltage.

Offhand I would try a 2N5401 or 2N6520 as a replacement and check all
of the 2N2207 transistors.

On Tue, 9 Oct 2012 10:47:35 -0700, David Wise <david_wise@...>
wrote:

Q184 sees up to about 75V from collector to emitter. The 2N3906 is
only rated 40V. Even if it doesn't break down, you may find that the trace
unblank is a little bit slow, because the original transistor had quite a low Cob.
I can't suggest a better one, though. Maybe something for video amp service?

Dave Wise

-----Original Message-----
From: TekScopes@...
[mailto:TekScopes@...] On Behalf Of tek_547
Sent: Tuesday, October 09, 2012 10:17 AM
To: TekScopes@...
Subject: [TekScopes] Re: Bright dots at start of the trace on a 547

Thanx for all the reactions guys, it was an old germanium
transistor Q184 in the unblanking circuit. Chuck did me the
suggestion to look specific at that transistor and he was damn right.
I replaced it for a 2N3906 and also his brother on the A
timebase Q373. They are silicium transistors but functions perfectly.
So in a few days, problem solved :-)

The only thing what?s left is the timing from the sweep rates,
they are about 20% to long. All of them on both timebases, so
if somebody knows...

grtz and thanx so far for all the help, 搁别苍é

?

Good catch there Dave. I have noticed when testing my curve tracer on
2N3906 and 2N3904 transistors that they often operate significantly
higher than their rated breakdown voltage but it sure is not something
I would want to rely on without grading them. The specifications I
see for the 2N2207 show it to be operating close to its breakdown
voltage.

Offhand I would try a 2N5401 or 2N6520 as a replacement and check all
of the 2N2207 transistors.

On Tue, 9 Oct 2012 10:47:35 -0700, David Wise <david_wise@...>
wrote:

>Q184 sees up to about 75V from collector to emitter. The 2N3906 is
>only rated 40V. Even if it doesn't break down, you may find that the trace
>unblank is a little bit slow, because the original transistor had quite a low Cob.
>I can't suggest a better one, though. Maybe something for video amp service?
>
>Dave Wise
>
>>-----Original Message-----
>>From: TekScopes@...
>>[mailto:TekScopes@...] On Behalf Of tek_547
>>Sent: Tuesday, October 09, 2012 10:17 AM
>>To: TekScopes@...
>>Subject: [TekScopes] Re: Bright dots at start of the trace on a 547
>>
>>Thanx for all the reactions guys, it was an old germanium
>>transistor Q184 in the unblanking circuit. Chuck did me the
>>suggestion to look specific at that transistor and he was damn right.
>>I replaced it for a 2N3906 and also his brother on the A
>>timebase Q373. They are silicium transistors but functions perfectly.
>>So in a few days, problem solved :-)
>>
>>The only thing what?s left is the timing from the sweep rates,
>>they are about 20% to long. All of them on both timebases, so
>>if somebody knows...
>>
>>grtz and thanx so far for all the help, 搁别苍é

The only thing what?s left is the timing from the sweep rates, they are about 20% to long. All of them on both timebases, so if somebody knows...

grtz and thanx so far for all the help, 搁别苍é

Hi David,

you say "operating close to its breakdown voltage", then you are joking.
V_ceo is 50 volts and in the schematics, the difference between E and C
is over 67 volts !!
They are working "beyond the cliff" !

Herbert

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

From: G. K.

To: TekScopes@...

Sent: Tuesday, October 09, 2012 4:31 PM

Subject: Re: [TekScopes] Re: Bright dots at start of the trace on a 547

?

How about the MPSA42 and 43? (?). Both are high voltage and reasonable fast. Mouser has stock, and they are cheap. - George

From: David <davidwhess@...>
To: TekScopes@...
Sent: Tuesday, October 9, 2012 3:27 PM
Subject: Re: [TekScopes] Re: Bright dots at start of the trace on a 547

?

The specifications I found for the 2N2207 were Vceo = 50 volts and
Vcbo = 70 volts. When the blanking signal is applied to the emitter
and the base driven by a relatively low impedance -0.6 volts, the Vcbo
is a better match.

But even a Vcbo of 70 volts is not much greater than the -68 volts
marked on the schematic. Maybe Tektronix graded them for a higher
minimum breakdown voltage.

On Tue, 09 Oct 2012 21:31:25 -0000, "Albert" <mailto:aodiversen%40online.nl>
wrote:

>No, for 151-063 (= 2N2207) Vceo = 70 V, but even then ...
>Albert
>
>> Hi David,
>>
>> you say "operating close to its breakdown voltage", then you are joking.
>> V_ceo is 50 volts and in the schematics, the difference between E and C
>> is over 67 volts !!
>> They are working "beyond the cliff" !
>>
>> Herbert
>

Hi George,
Those NPN transistors are a nice find!
Thank you for the link to the datasheet.
Is there a PNP version?
I think that is what they are looking for.
tom jobe...




----- Original Message -----
From: G. K.
To: TekScopes@...
Sent: Tuesday, October 09, 2012 4:31 PM
Subject: Re: [TekScopes] Re: Bright dots at start of the trace on a
547




How about the MPSA42 and 43?
( ). Both
are high voltage and reasonable fast. Mouser has stock, and they are
cheap. - George


From: David <davidwhess@...>
To: TekScopes@...
Sent: Tuesday, October 9, 2012 3:27 PM
Subject: Re: [TekScopes] Re: Bright dots at start of the trace on a
547




The specifications I found for the 2N2207 were Vceo = 50 volts and
Vcbo = 70 volts. When the blanking signal is applied to the emitter
and the base driven by a relatively low impedance -0.6 volts, the
Vcbo
is a better match.

But even a Vcbo of 70 volts is not much greater than the -68 volts
marked on the schematic. Maybe Tektronix graded them for a higher
minimum breakdown voltage.

On Tue, 09 Oct 2012 21:31:25 -0000, "Albert"
<mailto:aodiversen%40online.nl>
wrote:

>No, for 151-063 (= 2N2207) Vceo = 70 V, but even then ...
>Albert
>
>> Hi David,
>>
>> you say "operating close to its breakdown voltage", then you are
joking.
>> V_ceo is 50 volts and in the schematics, the difference between E
and C
>> is over 67 volts !!
>> They are working "beyond the cliff" !
>>
>> Herbert
>

-----Original Message-----
From: TekScopes@...
[mailto:TekScopes@...] On Behalf Of larrys@...
Sent: Tuesday, October 09, 2012 4:57 PM
To: TekScopes@...
Subject: Re: [TekScopes] Re: Bright dots at start of the trace on a 547

Comp PNP should be MPSA92 / 93
-ls-