Hi all,
I scored a Tek 475 scope from a car boot sale a few days ago for just £25 - I thought I'd take the risk as it was in immaculate condition externally and is a great scope. It was sold as untested, but I was up for a challenge.
Got it home, plugged it in and powered it on, a few fuzzy traces flashed up on the screen a couple of times and then it went blank - no response to front panel controls and there was a very slight smell of burning from inside the scope.
Pulled the cover off expecting to see something visibly burnt, but nothing seems untoward. Inside is in pretty good condition with just a thin layer of dust on some components. I powered it up a few more times with the cover off, and no burning smell was emitted, but still nothing appeared on the display. The only things that were working were the front panel V/div switch indicators.
I got the scope into the workshop today and had a chance to properly troubleshoot it. I checked the power supply rails first with the following results:
- +50V, voltage and ripple within spec
- +110V, voltage and ripple within spec
- +5V, voltage and ripple within spec
- +15V, no output (~26mV), measured almost dead short to GND
- -15V, no output (~0.45V), no dead short to GND
- -8V, no output (~0.73V), approx. 30? to GND
There is a problem with the +15V, -8V and -15V rails which is stopping the scope from working. As far as I can tell, a lot of the circuits in the scope are powered off the +15V/-8V rails, whilst I read somewhere that the -15V rail is only used for the external probe power option. This would tally with it having a much smaller series regulation transistor than the other rails.
To try and see if there was a short on one of the other boards which was shunting the rails to ground, I disconnected them as best I could without fully disassembling the scope:
- A8 (trigger gen & z-axis logic) - managed to fully remove/disconnect this board
- A7 (timing board) - fully removed
- A6 (fan motor board) - fully disconnected
- A4 (vertical mode switch circuit) - harder to remove, so I disconnected P300, P280 and P160 which appear to be the main (power) connections from A9 (interface board)
Powering up the scope each time after disconnecting each board in turn showed that the rails were still not there, so no luck.
A quick look at the schematic (diagram 11) shows that the op-amps which are used to regulate the -8/-15V rails are powered off the +15V rail, so, with the +15V rail being down, this could explain why those rails are not there either.
I then set about removing and checking the transistors around Q1448 (series pass transistor) for the +15V rail and checking them with my DMM in diode settings. Q1466/1444 (both 2N2222A's) checked out fine. I tried measuring the main pass transistor in circuit, but couldn't get reliable readings.
I have held off removing Q1448 so far as it looks as though it could be quite involved - but would it be recommended to give it a go and measure it out of circuit?
I am also a bit unsure as to what caused the original burning smell and why the rails failed in the first place.
Any help/guidance would be much appreciated. Also, is there anything to look out for in these scopes with their age (thinking along the lines of bad tantalums/electrolytics etc. which other Tek scopes have problems with).
Thanks in advance!
Service manual can be downloaded from Tek here:
http://www.tek.com/manual/475