Tek 547 B intensified by A


Jayadamski728@...
 

Hi there I am currently restoring a 547 and it almost all works … the delay time multiplier seems to work and in alt mode I get both waveforms but the region which is intensified is not any brighter than the rest of the waveform , can anyone point mr in the right direction on where to look ?


Tim Phillips
 

There is a tiny knob on the front panel by the Timebase B controls.
That needs adjusting fpr effect.
Tim

On Mon, 20 Sept 2021 at 12:58, <Jayadamski728@gmail.com> wrote:

Hi there I am currently restoring a 547 and it almost all works … the
delay time multiplier seems to work and in alt mode I get both waveforms
but the region which is intensified is not any brighter than the rest of
the waveform , can anyone point mr in the right direction on where to look
?






Jayadamski728@...
 

Thanks for your response , I have tried this but it does not show an intensified brighter area, it does show the waveform as either all intensified or not


Albert Otten
 

The normal Intensity knob has effect on everything.
The tiny Brightness knob should have a gradual effect in all settings where B is involved (also "B" alone). If not gradual I would expect a fault in the potmeter.
Note that in the modes with intensified trace the Brightness control has effect on the trace parts before and after the intensified zone, not on the intensified part itself.
Albert


Morris Odell
 

Forgive me for stating the obvious, but to get the B intensified by A effect you need to have the A timebase running faster than the B and properly synchronised. The A timebase then intensifies a section of the the trace and the little knob is used to turn the B intensity down before and after it so you can see it. If you have the main intensity turned up on the B trace before you switch to B int by A then you won't see it.

Morris


Jayadamski728@...
 

I have tried both the main intensity which makes everything brighter and the small dial channel B intensity which makes channel B brighter which both seem to work , and I have set the B time base to a higher time scale than the A time base and tried the opposite, but I do not get a region that is emphasised or brighter to indicate the area of the waveform that is being observed. I was wondering what circuitry is responsible for this


Albert Otten
 

I would first of all check whether the A sweep gets running or not during part of the B sweep in "B intensified by A" mode.
On another scope you can create a dual trace display of the 547 front panel outputs "+ GATE B" and "SWEEP A".
Nicest is to trigger at gate B and display at least a full B sweep duration.
You should see the A sweep signal moving along the B sweep when you rotate the delay time dial. Either smooth or jumping, depending on the A trig mode setting AUTO or TRIG.
Albert

On Tue, Sep 21, 2021 at 04:55 AM, <Jayadamski728@gmail.com> wrote:

I have tried both the main intensity which makes everything brighter and the
small dial channel B intensity which makes channel B brighter which both seem
to work , and I have set the B time base to a higher time scale than the A
time base and tried the opposite, but I do not get a region that is emphasised
or brighter to indicate the area of the waveform that is being observed. I was
wondering what circuitry is responsible for this


Morris Odell
 

Referring to the B sweep generator schematic (diagram 5) in the manual, you will see that the intensity voltages for the B sweep go though V193B which is a cathode follower. The small intensity pot is in the grid circuit so if that's altering the B intensity you know that tube is OK. The B intensifying pulse comes through Q173 andQ184 which are both 2N2207 germanium pnp transistors. there has been a lot written here about failures of 2N2207s due to whisker growth. So if the B is intensifying properly the transistors are probably OK. You can tell if that's working properly if you can't see the B retrace as you turn up the intensity.

The A intensifying pulse for "A only" and "B int by A" is generated by the A timebase (diagram 9 in the manual) and comes through Q373 and V393. It goes to the CRT circuit and does not involve the B timebase at all. If there is a pulse at the A Gate terminal then then Q 373 and V393A are OK. Q373 is also a 2N2207 and if there is no pulse you could try replacing it. 2N2207s are hard to find but a fast silicon pnp capable of withstanding at least 65 volts should work OK at all but the fastest A sweep speeds. You could try a BC640. You can also try replacing V393 which is a 6DJ8.

If you get a pulse at the A Gate terminal there should be a similar pulse at the cathode (pin 8) of V393b which feeds the unblanking pulse to the CRT grid circuit. You can check that by looking at a waveform displayed in A only mode. The retrace should not be visible.

Try those things and let us know how you get on.

Morris


Jayadamski728@...
 

Hi there thanks for your help, Q373 was the cause , a 2n2207 had turned into two diodes according to my tester, probably a whisker inside. I replaced it with a BC556 and it worked


Dave Wise
 

Now beef up the speedup capacitor and you’ll be like new. 220pF added should do.

Dave Wise

From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Jayadamski728 via groups.io
Sent: Wednesday, September 22, 2021 1:31 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Tek 547 B intensified by A

Hi there thanks for your help, Q373 was the cause , a 2n2207 had turned into two diodes according to my tester, probably a whisker inside. I replaced it with a BC556 and it worked


Morris Odell
 

I'm really please to see that and glad to have helped. As Dave Wise says, you need to bypass the parallel combination of C370 (200 pf) and R370 (4.7K) with an extra 220 pf to compensate for the lower speed of the BC556 compared to the original 2N2207.

Morris


Jayadamski728@...
 

So you mean just add a 220pf in parallel with C370 and R370 ? Making the capacitance 420pf ? Thanks


Dave Wise
 

That’s right. C370 was sized to speed up a 2N2207. BC556, KSA992, and so on need more. See archived thread “Tek 547 A Sweep display during retrace<https://groups.io/g/TekScopes/message/152069?p=%2C%2C%2C20%2C0%2C0%2C0%3A%3Arecentpostdate%2Fsticky%2C%2Cc370%2C20%2C2%2C0%2C7659032>”, posts 145284, 143288, and 142449.

Dave Wise

From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Jayadamski728 via groups.io
Sent: Thursday, September 23, 2021 4:08 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Tek 547 B intensified by A

So you mean just add a 220pf in parallel with C370 and R370 ? Making the capacitance 420pf ? Thanks


Morris Odell
 

Yes. Dave Wise has done a lot of work on this and if you do a search in the group posts for "Tek 547 A Sweep display during retrace" you will find all the information you need.

Morris