Tektronix 7603 Power Supply Question - Page 1
Forum section: electronics / testgear

author: wlzed279
author_rank: Contributor
timestamp: August 06, 2023, 07:41:40 pm
content: First off, let me qualify that I am a relative newbie at repairing test equipment. I have restored a number of vintage tube and solid state audio amplifiers and hence the need for a decent oscilloscope. I purchased a Tek 7603 for relatively cheap but soon discovered it had a number of issues. I recapped the rectifier board with all new filter capacitors and had to replace a number of transistors, a diode and a resistor on the low voltage board. All the voltage rails are now back and the scope has come back to life with a trace. All the rails with the exception of the 130v rail. I am getting 145v on that rail. I have a dodgy time base plug in so it is difficult to diagnose but everything appears to be working as it should. I understand that the 130v rail is summed of the +50v and +80v from the rectifier. I am getting 50v but not sure where to measure the 80v to be that they are being summed correctly. Any assistqance for the Tek 7603 experts out there woul be very helpful.

author: Hexley
author_rank: Regular Contributor
timestamp: August 06, 2023, 10:48:59 pm
content: Quote from: wlzed279 on August 06, 2023, 07:41:40 pmThe 130V supply is divided by R855 and R856 in a 2.6:1 ratio to produce exactly 50 volts at the junction of the two resistors. Q852 then compares that with the fixed 50V output of the 50V supply, and if the two do not match then buffer Q850 raises or lowers the voltage applied to the bottom of the "130V" bulk supply (which is actually more like 90V if measured across its filter cap, C806.)When all is operating normally, Q852 holds the bottom of the "130V" bulk supply at about 40V, so that the top of that supply sits at 130 volts after the addition of the 90 volts from the bulk supply. If the voltage at the base of comparator Q852 is too high, then Q850 turns off a bit to lower the voltage at the bottom of the bulk suppy. It the comparator voltage is too low, the opposite happens. Thus, the output of the 130V supply is regulated at exactly 130V.If you are seeing too much voltage out of the 130V regulator, it may be that something in the Q852/Q850 chain has failed. If Q850 has shorted, for example, then the bottom of the bulk supply will go from its nominal 40V to a fixed 50V, causing the total output rise 10 volts and become about 140V. Other faults could cause similar misbehavior, so some investigation will be needed.Check the schematic at W140.com for reference, btw.https://w140.com/tekwiki/images/b/b4/Tek_7603_power_supply_A10_and_A11_schematic_%28redrawn%29.pdfGood luck.

author: wlzed279
author_rank: Contributor
timestamp: August 06, 2023, 11:52:33 pm
content: Greetings Henley, Thank you so much for the quick response. I pulled Q852 as you suggested and sure enough, it is a shorted. Do you know if there is an easily accessible sub for a 151-0337 transistor?

author: Proto
author_rank: Contributor
timestamp: August 08, 2023, 08:27:48 am
content: The common part number is in the schematic listed as 2N5087. Mouser and probably several other distributors have them in stock. The transistor package may be different meaning you may have to identify the base, collector and emitter leads in a datasheet, correlate them with the schematic and make sure you insert the leads in the correct PCB plated thru holes. The datasheet will show you the lead connections as b, c and e (and / or 1, 2, 3) along with a picture. One catch is the the parts in stock are from Central Semiconductor whose datasheet has no lead identification information. So look at the ON Semi data sheet for that. Both suppliers are using epoxy packages with inline leads and should be identical in their pinout. Data sheets are attached for your convenience.

author: med6753
author_rank: Super Contributor
timestamp: August 08, 2023, 02:11:56 pm
content: If you can't find the exact replacement an NTE159 is listed as a substitute.

author: wlzed279
author_rank: Contributor
timestamp: August 09, 2023, 12:31:31 am
content: Thanks Proto, I messed up in my previous post about which transistor was shorted. It is Q852, the big power T0-3 transistor. I was able to find it at Talon Electronics. Just wanted to be sure I didn't overpay for the part.