Author Topic: B&K 1472B scope info to repair  (Read 3095 times)

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Offline Radio Tech

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B&K 1472B scope info to repair
« on: July 18, 2014, 02:36:25 pm »
This was my first "real" scope many years ago when I got into business with a friend during the 80s in radio. Before that I had a few real old scopes that just displayed waveforms with no settings like volts/Div and time base.  This scope quit a long time ago. Really no need to repair it other than it being my first scope and has a bit of sentimental value to me.

I lost trace and the power indicator does come on.  Checked various power supply voltages and capacitors in different circuits. I have the schematic for this unit but is missing key information like voltages and such. Been a few months since I have had time to even look at it again.

 This morning is my first Friday off in a while and is when I mostly work on projects and clean the shop. But started thinking maybe that there was another scope out there that was re-badged and similar to the 1472b. I spent an hour looking on the web and found a Kenwood CS-1560a   that looks identical to my 1472b.  So I downloaded the service manual and saw it was very similar and has voltages and a nice trouble shooting flow chart. One difference I did see was that the Kenwood unit has X/Y mode on the time base setting that the 1472b does not.

If anyone here has worked on one of these old scopes and have any info that would help I would gladly appreciate it. Going to try and use the Kenwood flow chart and see what I can find. Would love to get this old unit up and running again..

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Re: B&K 1472B scope info to repair
« Reply #1 on: July 18, 2014, 03:07:00 pm »
will see if it will let me attach the schematic

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Re: B&K 1472B scope info to repair
« Reply #2 on: July 18, 2014, 04:29:29 pm »
PROGRESS!
That was FUN!
Got to love flow charts!!
I did find a few that would not apply to this scope so just skipped over them and moved on to the next.
Following the flow chart it asked me to check for certain voltages on the DC supply. All checked good so went to the “OK” side of the chart. It then asked me to the collector waveform at Q317. After a quick search on the board I found it was a 2SD381-M TO-220.  The transistor had a large aluminum heat sink attached to it. After the check I found no waveform. What I did find was the leads of the transistor were cracked. Most likely due to years of stress and vibration.  Loosen the board up, re-soldered the connection. Mounted the board, applied power and bingo, there was now somewhat of a trace on the CRT.  It is not much but a start.

Now other issues to address.
Focus control does nothing.
Turning the time base control does nothing until you go far clockwise to “B” position then the trace moves to the right side. Above I said there was no X/Y position on the 1472 like the Kenwood scope. Guess I was wrong.
Trigger control very erratic, dirty.
Funny after all these years I find a very simple issue that caused the original problem.
For some time the scope was stored in an uncontrolled environment so will have to do quite a  bit of cleaning and getting corrosion out of the controls and connectors.
Sure is good to see a trace again.

Offline tautech

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Re: B&K 1472B scope info to repair
« Reply #3 on: July 19, 2014, 12:04:47 am »
Good start. Q317 will have been subject to thermal cycling more than anything else.
But apparently you have no sweep. Manual refers to the collector of Q217.
But first carefully check all regulated DC power rails for ripple. You might need to search for a full service manual, that should list each rail V and tolerance and ripple spec.
It may be likely Ecaps need replacing.

Get the PSU spot on first.  :-+
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Re: B&K 1472B scope info to repair
« Reply #4 on: July 19, 2014, 01:49:51 am »
Good start. Q317 will have been subject to thermal cycling more than anything else.
But apparently you have no sweep. Manual refers to the collector of Q217.
  manual, that should list each rail V and tolerance and ripple spec.
It may be likely Ecaps need replacing.

Get the PSU spot on first.  :-+

Agreed on the PSU. Will work on that tomorrow. Scope all the supplies and see what is going on.
I cleaned a few controls and starting to get some sweep now. The trace is about 5 mm high and 12 mm long. With a probe connected it moves from left to right now in about a 30 mm span.

After I get all the power supplies fully functioning I will look at the power resistors in the focus and intensity control. Turning the intensity control does nothing.
Thanks for the input.

Offline tautech

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Re: B&K 1472B scope info to repair
« Reply #5 on: July 19, 2014, 02:05:18 am »
Grab the first document in: https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/
It will help you a lot with fault finding CRO's.
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Re: B&K 1472B scope info to repair
« Reply #6 on: July 19, 2014, 11:11:37 am »
Looks like the same one I printed out a few years ago. Forgot all about that doc.
Thanks

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Re: B&K 1472B scope info to repair
« Reply #7 on: July 19, 2014, 01:58:53 pm »
tautech
I do want to say you were spot on about the supply. When the CRT would not show a waveform and was blank the voltages on the supply looked ok.  Now that there is a display the voltages are way down.

P309
Pin 1 = 190v  at 154
Pin 2 = + 5v at 2.4
Pin 3 = +10v at 4.3
Pin 4 = ground
Pin 5 = -10v at -9.8
Pin 6 = +13v at 7.7
Pin 7 = +20v at 9.27

Looks like there are a load of bad caps that need replacing.

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Re: B&K 1472B scope info to repair
« Reply #8 on: July 19, 2014, 02:14:57 pm »
Unplugged P309 to check voltages with no load on them.
This is the results in [   ]:

P309
Pin 1 = 190v  at 154  [+203]
Pin 2 = + 5v at 2.4      [+9.4]
Pin 3 = +10v at 4.3    [+8.1]
Pin 4 = ground
Pin 5 = -10v at -9.8    [-9.9]
Pin 6 = +13v at 7.7    [+11.2]
Pin 7 = +20v at 9.27  [+10.6]

Pin 2 is almost doubled
Pin 7 is half


Offline tautech

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Re: B&K 1472B scope info to repair
« Reply #9 on: July 19, 2014, 08:08:25 pm »
Study the PSU schematic, it seems several V+ rails are referenced from the +10V.
Some are not regulated(+22V & 20V), so they can be overlooked for now.

Recently there was a post of a similar "shelved" piece of gear that was so long since operated the Ecaps had to be re-formed to smooth correctly.
I think I would replace many, based on further testing.

Then the HV for CRT operation will need to be looked at.
The TEK doc has many clues of faults in that area.
DC restorer caps are prone to failure.
In my experience, the high value resistors can drift or go open.
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Re: B&K 1472B scope info to repair
« Reply #10 on: July 19, 2014, 11:48:32 pm »
I agree with what you are saying. I pulled the power supply board out today. Had to pull the transformer first. Good thing I was able to power the board up outside the scope and test more voltages that I could not get to when installed.
I am going to replace all the Ecaps on the power rails. I looked at the schematic Also a little concerned over those small bridge rectifiers in there.

Also got the 195v rail up. Checks at 200v now. Yep, bad cap. I also noticed a lot of 10 v caps on the horizontal and vertical boards. They are a must.
Thanks for taking time to respond on this.

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Re: B&K 1472B scope info to repair
« Reply #11 on: July 20, 2014, 12:33:49 am »
Yep, it will be a labor of love and not much else.
But you have a good start and I'm quite sure it will all come together for you.
Keep posting your findings for others to learn from.  :-+
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