Tek 7603 regulator PCB

[harrison@cs.unc.edu]

I have 2 Tek 7603's with bad regulator boards. Both are missing +5, +15, and +50. I am having trouble locating the bad part(s).
From the schematic, it looks like +50 is necessary for the +5 and +15 to bias the voltages. I have checked most of the parts with a VOM,
but that will not work with a zener diode....I am not sure about that. (my knowledge about electronics is limited...old age!!)
Any help will be appreciated.
Part numbers are: 670-1376 rev 17 and670-1376 rev 26

Thanks.

[Uunoctium]

cleaned up and stiched the two sheets together 

[slbender]

I have 2 Tek 7603's with bad regulator boards. Both are missing +5, +15, and +50. I am having trouble locating the bad part(s).
From the schematic, it looks like +50 is necessary for the +5 and +15 to bias the voltages. I have checked most of the parts with a VOM,
but that will not work with a zener diode....I am not sure about that. (my knowledge about electronics is limited...old age!!)
Any help will be appreciated.
Part numbers are: 670-1376 rev 17 and670-1376 rev 26

Thanks.

I believe some years back, someone mentioned that some 7603 units which used 2N3055 pass transistors where the heat, over time, caused the solder to develop cracks where the base and emitter leads were soldered to the pc board. The use of a solder sucker to remove the old solder, and replace with new 60/40, or maybe 2% or 4% silver solder improved these areas and their connections, as no blown parts needed replacement, the pass transistors just needed to be reconnected into the circuit to get the regulators working again.  Well YMMV. But it’s worth doing.

Steven

[Uunoctium]

Hi Harrison,
I posted the schematic as a subtle cautionary note. It is more helpful to indicate on it what voltages are present. A proprietary manufacturer specific spare part code does'nt.
Are  +55V and -55V unreg present? +18, -18?
Can you measure the reg -50V? According to the detailed description, these are the mother of all other voltages. If present, the Zener diode does not need to be tested. (the only Z-diode in the whole power supply).

Found a 7603 PSU repair report on a blog - similar symptoms and some useful tips on replacement types....

" Then I noticed half of the power rails were missing, as was the 130V rail fuse on the regulator board. Someone had obviously been at it before me. I replaced this fuse, and it blew immediately with a sizeable flash and pop.

It turned out that several transistors on the regulator board had failed, some short, some open. I replaced the TO92 ones with 2N5551 (NPN) and 2N5401 (PNP) and the larger metal can ones with 2N2219s, except for one that looked like it needed to stand a higher voltage, so I got a MJE340 and jammed it into the socket. The smoke had been coming from a crispy-looking 1.2k resistor which I replaced too, even though it still measured 1.19k."
[/qote]

https://scopeboy.com/?p=183

[jonpaul]

First check bus caps, old lytics loose C and increase ESR.

50 V is a REF to all other supplies.

Most likely culprits are the TO-3 2N3055 transistors especially the socket pins.

Beware of fake Chinese 2N3055 knockoffs, if needed find genuine parts not from China

Jon

[harrison@cs.unc.edu]

Thanks for the comments. I tested all the transistors this morning after removing them from the PCB. Even the tO-5 pass transistors.
All are good. The 14 pin DIP with the 4 transistors in it tested good as well. None of the caps are shorted or open. Diodes tested OK.
The voltages from the rectifier are all correct. Plus and minus 18 are there as well as minus 50.
I used my really old VOM to test everything. It is not good enough to test the sub 1 ohm resistors.
I had not thought that bad solder joints could be the problem. I will check those next.
Voltages at the Z axis board, P 1171 show -15, +87 for the +130, and -50.
+5, +15, and +50 show zero.

[jonpaul]

a shorted tantalum capacitor can bring down a bus, suggesting that you check the tantalum caps on all boards and supplies

jon

[Uunoctium]

the  lower level of the 130V supply is correct because the +50V is missing. These are added up to reach the 130V. Similar applies to the +5V.
Does'nt work without +50.

The key seems to be in the +50V supply.

I would concentrate on the current limiter around R875 // Q876A.
Can you measure a voltage drop across R876?
That could be an indication of a short in the following assemblies

[harrison@cs.unc.edu]

I spent some time today on the regulator PCB. Was able to remove 5 of the pass transistors, but a screw on one of them is not coming out.
I will consult a local expert on how to remove that screw without damaging the PCB. ( all 6 pass Q's test OK )
I wanted to get to the back of the board to check the solder joints. I will have to wait a few days for that.
Thank you all for the help.

[harrison@cs.unc.edu]

Good news. I got the stubborn screw out of the pass transistor. That enabled me to get to the back of the board to check solder joints as
suggested by slbender. I did  see some joints that I did not like, so reflowed many joints across the board. Also removed the low ohm
resistors to check them out of the circuit. Two of them were reading more than double their listed value. They have been replaced.
I had to order silicone sink compound and TO-3 insulators which arrived this morning. Put it all back together and the voltages are near
spec, but not quite. Will have to wait for the  unit to warm up for an hour to adjust the reference voltage.
Since I have a trace, I can see that the beam is not blanking properly on retrace and the beam intensity is adjustable, but not all the way to
cutoff. Fortunately, I have another scope to troubleshoot that problem.
Thank you all for reading and making suggestions.

[sean0118]

Since I have a trace, I can see that the beam is not blanking properly on retrace and the beam intensity is adjustable, but not all the way to
cutoff. Fortunately, I have another scope to troubleshoot that problem.

With regards to the trace intensity, try following the 'Adjust CRT Grid Bias' instructions (pg 3-6 of service manual).

I have my own Tek 7603 repair thread, mine keeps getting new faults. 

[harrison@cs.unc.edu]

Sean, you are correct. I adjusted the CRT bias and sprayed some deoxit on the pots and all is well. Many thanks to all. But.....
I still have one 7603 that has a high voltage problem. It was working for a while and then some arcing the in the high voltage enclosure
caused it to shut down. I will need help there too!! Another thread maybe.