Tektronix 7904 (Problems with the horizontal board) - Page 1
Forum section: electronics / repair

author: Kaveh
author_rank: Newbie
timestamp: December 10, 2020, 09:43:08 am
content: Hello everybodyI recently purchased a Tektronix 7904. The device was probably lying in the attic unused for a long time. After I opened the device, I noticed that the board for the horizontal deflection was quite dirty (see the picture). First I cleaned the circuit board. Now the device can be switched on. The power supply seems to be ok. The vertical deflection works. but not the horizontal. The two vertical channels are visible on the screen when I take the horizontal circuit board out of operation and when I reconnect it, I don't see anything on the screen. I measured the voltages for the horizontal board, they are all there. I suspect that a short circuit occurs in the circuit board as soon as I connect it to the CRT. Perhaps someone here also had this problem with 7904. I measured the output transistors, they are all ok.Maybe someone here has an idea what the fault could be.Thank you

author: richnormand
author_rank: Unknown
timestamp: December 10, 2020, 10:40:18 pm
content: I have fixed several Tek of the same vintage. Fortunately my 7904 has never broken down so I have not opened it.Some guesses:1)get the service manual for it. They are easy to get hold of and will have diagnostic procedures.2)Measure all the input voltages to the card.3)These little coax connectors are notorious to either not connect properly or short while inserting the coax. Check the central wire for alignment with the center hole.4)Tantalum capacitors are an issue (short), better check them.5)Often the amp will swing on one side as the balance is lost. Schematics are your friend here. Looks like some of the adjustments pots have seen a screwdriver blade before.6)Are you sure it is this card but not further down the line. I once had a corroded contact at the clip to the side wire to the horz plate. It was dead and came back to life just moving it a bit!! (HV warning)Best of luck with it.I assume you are familiar with the Tek user group. Lots of people with Tek knowledge there too.

author: Kaveh
author_rank: Newbie
timestamp: December 11, 2020, 06:53:28 pm
content: thanks for your feedback

author: Kaveh
author_rank: Newbie
timestamp: December 11, 2020, 07:16:21 pm
content: Hello,I measured the board. All input voltages are available. I replaced a defective pnp transistor. I think the amp is out of balance. The output voltages are not as noted in the circuit diagram. At the + connection I measure 75 V and at the - connection 117 V against ground. I downloaded the Manuel service from the internet. It is from May 1983 and my device was built in 1972. That is why I noticed that the circuit diagram in this booklet does not match the circuit board 100%. For example, there are components in the circuit diagram that are not present on the circuit board, which makes troubleshooting a bit difficult. For example the documentation and schematics of 7909A are much better.

author: richnormand
author_rank: Unknown
timestamp: December 11, 2020, 10:52:40 pm
content: Wow.That board look grungy. Have you tried getting in touch with the tek group on .io?They may know more about the variations.Can you work your way back to the input stages to find out where the imbalance comes from?

author: Kaveh
author_rank: Newbie
timestamp: December 12, 2020, 05:07:37 am
content: yes this is what the circuit board looked like before I cleaned it.