Tek 475A lost beam

shusha · 5th March 2006, 07:46 PM

My scope went dead today. I replaced the mains fuse at the back and it started again but without a beam.

Lamps are showing that measuring part is working. So I opened it and found out that the internal fuse is blown. And it keeps blowing. It is 0.7A fuse F1318.

If I recall correctly this is +15V fuse. Can't really be certain about this, I have to check it again. No smell, no burns.

It is rather an old scope and it might be that some cap is shorted. Has anyone had any experience like this?

This is the fuse:

I did have noticed one pin that looks like it has been ripped off from a little board, which I don't have an idea what it is.

Arrow points to this pin:

Just to have a bigger picture:

Any help would be welcome. I just don't want to throw away this nice piece of equipment.

moamps · 5th March 2006, 08:54 PM

Hi Tomislav:

You can download the service manual from
ftp://bama.sbc.edu/downloads/tek/475/

You may wish to check the power supply voltages first.

Regards,
Milan

shusha · 5th March 2006, 09:41 PM

Tnx Milan. I tried to find serv. manual, couldn't find it. The whole thing is way above my leguaue, but I will surrely check the PSU.

moamps · 5th March 2006, 09:51 PM

Check PM.

shusha · 7th March 2006, 08:44 PM

According to the manual Milan has provided, fault is probably in the High-voltage oscillator which drives high voltage transformer (or the transformer itself - hope not!).

I found out two faulty signal transistors (Q1310 and Q1312 at the sch.), replaced them and now the fuse does not blow. These are 2N3906 and 2N3904. Haven't had any 3904, put BC547 instead. As the manual states, these two forms overvoltage protection circuit.

Have checked all the corresponding transistors. They all seems ok. All PSU rails are OK.

I did some basic measurements, which are noted in the picture below. It seems to me that Q1318 is not open biased.

Link to hi-res image

Any ideas?

glen65 · 7th March 2006, 09:35 PM

A good place to start would be to run a check on the
transformer itself using your ohmeter to check and see
if any of the windings are open. Then start working your way
outward checking the various active and passive devices as
shown on the schematic you posted.

shusha · 11th March 2006, 02:16 PM

Fixed. Found one small signal (protection) diode which was bad, replaced it.

The mess with the voltages was caused by incorectly placed PNP transistor that I have replaced.

5th March 2006, 07:46 PM	#1
shusha diyAudio Member Join Date: Sep 2004 Location: Velika Gorica	Tek 475A lost beam My scope went dead today. I replaced the mains fuse at the back and it started again but without a beam. Lamps are showing that measuring part is working. So I opened it and found out that the internal fuse is blown. And it keeps blowing. It is 0.7A fuse F1318. If I recall correctly this is +15V fuse. Can't really be certain about this, I have to check it again. No smell, no burns. It is rather an old scope and it might be that some cap is shorted. Has anyone had any experience like this? This is the fuse: I did have noticed one pin that looks like it has been ripped off from a little board, which I don't have an idea what it is. Arrow points to this pin: Just to have a bigger picture: Any help would be welcome. I just don't want to throw away this nice piece of equipment.

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5th March 2006, 08:54 PM	#2
moamps diyAudio Member Join Date: Oct 2002 Location: Croatia	Hi Tomislav: You can download the service manual from ftp://bama.sbc.edu/downloads/tek/475/ You may wish to check the power supply voltages first. Regards, Milan

5th March 2006, 09:41 PM	#3
shusha diyAudio Member Join Date: Sep 2004 Location: Velika Gorica	Tnx Milan. I tried to find serv. manual, couldn't find it. The whole thing is way above my leguaue, but I will surrely check the PSU.

5th March 2006, 09:51 PM	#4
moamps diyAudio Member Join Date: Oct 2002 Location: Croatia	Check PM.

7th March 2006, 08:44 PM	#5
shusha diyAudio Member Join Date: Sep 2004 Location: Velika Gorica	According to the manual Milan has provided, fault is probably in the High-voltage oscillator which drives high voltage transformer (or the transformer itself - hope not!). I found out two faulty signal transistors (Q1310 and Q1312 at the sch.), replaced them and now the fuse does not blow. These are 2N3906 and 2N3904. Haven't had any 3904, put BC547 instead. As the manual states, these two forms overvoltage protection circuit. Have checked all the corresponding transistors. They all seems ok. All PSU rails are OK. I did some basic measurements, which are noted in the picture below. It seems to me that Q1318 is not open biased. Link to hi-res image Any ideas?

7th March 2006, 09:35 PM	#6
glen65 diyAudio Member Join Date: Jun 2004 Location: mid Ohio USA	A good place to start would be to run a check on the transformer itself using your ohmeter to check and see if any of the windings are open. Then start working your way outward checking the various active and passive devices as shown on the schematic you posted.

11th March 2006, 02:16 PM	#7
shusha diyAudio Member Join Date: Sep 2004 Location: Velika Gorica	Fixed. Found one small signal (protection) diode which was bad, replaced it. The mess with the voltages was caused by incorectly placed PNP transistor that I have replaced.

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