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Tek 7603 regulator PCB

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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:

--- Quote from: harrison@cs.unc.edu on May 23, 2022, 05:11:08 pm ---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.

--- End quote ---


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....


--- Quote ---" 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


--- End quote ---

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

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