Hello everybody!
My 7104 started having problems in displaying traces and readout characters, it did not happen suddenly, sometimes after switching off and on again everything went back to normal operation.
ISSUES DESCRIPTION
Traces could be moved both vertically and horizontally, the horizontal part was all OK.
With (no difference between LEFT or RIGHT) and without a vertical plug in installed the traces were positioned half way between the top and the middle of the screen.
Focus had effect on traces and readout characters but it was difficult to focus them.
The readout characters were all aligned at the bottom line, no matter if related to LEFT/RIGHT vertical or A/B horizontal plug ins; bottom line was not at the bottom of the screen, but 1 inch above.
The readout characters were not readable, changing plug in settings had effect, something changed.
In some cases some weird readout characters were displayed.
The front panel readout controls were working correctly.
FIRST CHECKS AND FIXES
I¡¯ve changed all the electrolytic and tantalum capacitors and also 2 MKT in the Main Interface board, in total 77 capacitors;
in the PSU I could not find replacements for the 3 blue big ones (C1216, C1217 and C1354) that could fit so I only tested them for capacitance and leakage current, capacitances are in spec for all of them, the high voltage rated ones are OK while the low voltage one (for the +5V) rail has a slightly high leakage current but it can still do its job;
in the Low Voltage Regulators board there was a 40K (R1446) resistor of the -50V regulator out of tolerance that prevented to have a correct value when setting the +50V (it was at -52.6V with +50.0V), I changed both 40K resistors with 0.1%, 0.5W metal film ones;
in the Low Voltage Regulators board there was a missing nut on Q1494 (-15V power regulator) preventing a good thermal contact with the heat sink, it probably run (very) hot but it was still working fine;
I¡¯ve adjusted the two PSU voltages (+109V and +50V) and checked all other ones, they are all in spec within 0.05V;
there was a leaking electrolytic capacitor in the Vertical Amplifier on the -15V line (C897), I cleaned with IPA all the area around it;
I¡¯ve checked all transistors for shorted/not functioning ones, not with a curve tracer as I do not have one;
I¡¯ve fixed a couple of issues of the moving connectors in the right (B) horizontal plug of the Main Interface board;
I¡¯ve changed 5 contact pins of the plug in connectors because they were some irreparably damaged;
I¡¯ve fixed a shorted cable from the right (B) horizontal plug of the Main Interface to the Horizontal Amplifier, there was no problem before removing the Main Interface board, short circuit appeared after reinstalling it, the internal wire was touching the cable shield;
I¡¯ve checked all coaxial cables for shorts/broken connections and sprayed their connectors with contact cleaner before replacing;
I¡¯ve cleaned with IPA all HYPCON and hybrid ICs contacts and tested for good electrical connection, by the way I found as a fix for contact issues to leave the elastomer in IPA for some hours and then to bend the contacts a little from both the inner and outer side, this fixed a problem with the 7B15 contact failure raising after 20 minutes of use;
I¡¯ve cleaned all transistors and ICs sockets and their pins with contact cleaner (I had troubles with IPA), then tested for good electrical connection;
I¡¯ve cleaned all connectors and PCBs;
I¡¯ve changed the left vertical plug in slide guide because the outer edge was broken preventing a secure locking in the bay of the plug in module;
I¡¯ve changed all bulbs of the trigger source push buttons and that of the power on indicator, there were only two broken bulbs, but while changing them the filament of other two broke so I decided to change all of them.
ANALYSIS
At this point the 7104 was really shiny, with all lights working and half a can of contact cleaner used to clean contacts but the issue was still there.
If I swapped the two Vertical Amplifier input signal wires the trace did not move from its position, so the issue was not in the Vertical Channel Switch.
Disconnecting the Readout board had no effect, the trace did not move from its position, so the issue was not in the Readout for what is related to trace position.
SOLUTION
At this point I focused on the vertical amplifier, given that all transistors were OK, I started checking the opamps: I begun from the readout signal amplification and phase inversion LF351 (U705).
I did not have on hand a LF351 so, because it is not too critical, I used instead a LM318. With the new opamp in place the readout traces were now fine, not anymore all aligned at the bottom, but all of them in the right place.
But still the signal traces were not in the right position, with strange spikes and with a limited bandwidth.
I changed the feedbeside LF315 (U782) with another LM318 and the signal traces were now clear, noiseless, not bandwidth limited and in their correct position.
I then decided to change all opamps on that board just to be sure to avoid future issues, so I purchased 3 x LF351 and 1 x MC1458.
The broken connectors in the Main Interface board were dedicated to the readout signals, this explained the fuzzy/strange characters sometimes displayed.
I had real hard times with poor contacts after reassembling the boards. Before I experienced no issues in use. Fortunately I decided to work on one board at a time so, if something went wrong I could focus on only one board and its connections. The strangest issue came up when I worked on the Display Control one (A/B intensity, focus and astigmatism), after replacing it intensity controls were swapped (A controlling B and B controlling A) and not properly functioning¡ it was just a bad connection between this board and the Mode Switch one but it took me hours to figure out.
7104 finally fixed after 3 months since I started repairing/restoring it and 1.5 years since I stopped using it.
THOUGHTS
This repair ended up in an almost full restoration (only calibration is missing), but what I still have to understand is what caused two LF351s failing at the same time on the same board, I¡¯m afraid there is something wrong somewhere else I still have to find, the leaking electrolytic capacitor was on the -15V rail that might explain U705 failure, but not that of U782 that only has +15V supply, maybe the missing nut on the -15V power transistor caused spikes on this rail¡ no idea.
Hope this helps someone having same troubles.
