Tek 7603 regulator PCB - Page 1
Forum section: electronics / repair

author: harrison@cs.unc.edu
author_rank: Contributor
timestamp: May 23, 2022, 05:11:08 pm
content: 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 26Thanks.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: May 23, 2022, 06:22:34 pm
content: cleaned up and stiched the two sheets together

author: slbender
author_rank: Regular Contributor
timestamp: May 24, 2022, 07:55:07 am
content: Quote from: harrison@cs.unc.edu on May 23, 2022, 05:11:08 pmI 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

author: Uunoctium
author_rank: Frequent Contributor
timestamp: May 24, 2022, 03:16:42 pm
content: 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

author: jonpaul
author_rank: Super Contributor
timestamp: May 24, 2022, 03:33:50 pm
content: 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 ChinaJon

author: harrison@cs.unc.edu
author_rank: Contributor
timestamp: May 24, 2022, 09:34:46 pm
content: 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.

author: jonpaul
author_rank: Super Contributor
timestamp: May 25, 2022, 01:06:07 pm
content: a shorted tantalum capacitor can bring down a bus, suggesting that you check the tantalum caps on all boards and suppliesjon

author: Uunoctium
author_rank: Frequent Contributor
timestamp: May 25, 2022, 05:55:23 pm
content: 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

author: harrison@cs.unc.edu
author_rank: Contributor
timestamp: May 26, 2022, 12:42:39 am
content: 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.

author: harrison@cs.unc.edu
author_rank: Contributor
timestamp: May 30, 2022, 07:54:15 pm
content: 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 assuggested by slbender. I did see some joints that I did not like, so reflowed many joints across the board. Also removed the low ohmresistors 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 nearspec, 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 tocutoff. Fortunately, I have another scope to troubleshoot that problem.Thank you all for reading and making suggestions.

author: sean0118
author_rank: Frequent Contributor
timestamp: June 04, 2022, 09:15:50 am
content: Quote from: harrison@cs.unc.edu on May 30, 2022, 07:54:15 pmWith regards to the trace intensity, try following the 'Adjust CRT Grid Bias' instructions (pg 3-6 of service manual).I have my ownTek 7603 repair thread, mine keeps getting new faults.

author: harrison@cs.unc.edu
author_rank: Contributor
timestamp: June 07, 2022, 09:17:20 pm
content: 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 enclosurecaused it to shut down. I will need help there too!! Another thread maybe.