Rebuilt 7904A / 7L12 and bad Forum advice
David Campbell <k_8_b_y_p@...>
...and except for flaky ' drum' switch contacts..
Scope picture- Perfect. Shows under- damped ringing on the Cal signal at
Full bay of plugins work perfectly. None of this
" power supply cant handle all the plugins..." nonsense.
SA perfect. No spurs whatsoever exc for 2.x L.O. and 2.4 GHz as expected,
and those are half the max. allowed level.
Coarse tuning on SA right on exc for wear in the dial mechanism.
All alignments that were checked were in spec.
Fortunately I ignored the BAD ADVICE from
" Forum Experts" here and replaced all the electrolytics in the mainframe
and all tantalums in the 7L12.
The worst advice was " youll risk damaging circuit boards" said by persons
who knew nothing about me.
Odd. No such thing happened. Those boards are very well made.
Maybe 50 years soldering experience, Manufacturing engineering and SMT
experience helped...And 15 years hi volume repair bench experience helped
Notes, and a couple unforgiveable, manufacturing errors found and fixed.
Compared to HP gear, this Tek looks like a High School class assembled it.
1. SIX ELECTROLYTICS in the scope leaking badly. One in the HV board under
rated (160V on a 130Vsupply) It was bulged and on its way to catastrophic
The 11th Commandment is:
" Thou shalt rate ' lytics at 2X WVDC!"
Tek didnt do it. Had I taken the advice of " Forum Experts" here, Id have
ended up with the typical
" power supply hand grenaded" situation.
2. Threaded Standoff too short in 7L12, badly warped A1300 board near the
large transistor. Standoff too short, the screw fastening the board to
standoff bent the board. Let the board relax, then put spacers between
board and standoff.
3. Large tantalum in 7L12, A1300 bd causing wires to be pinched. Replaced
all the old tants with new ones laid down, plenty of space to spare.
4. coax cable from A500 marker to front panel badly stretched. Poor
assembly work. Cable incorrectly routed.
5. HV multiplier in 7904 right up against grounded metal panel. If that HVM
starts to leak current and it finds an arc path to that shield, itll be
catastrophic destruction. Moved the board back and put a HV barrier
between. Saw lots of potted HVM fail this way in old TV sets.
6. Put extra insulation on the CRT HV lead. That wire insulation is 30
years old. Arcing can damage the HV board. HV lead was hanging down near
the switch and cal generator board causing interference. Poor assembly
work. HV lead much too long.
Easy to do, exc. for being multi layer boards, ONCE THE BOARDS ARE OUT.
Getting the boards out without damaging any wires, cables or headers is the
The buried power planes make great difficulty cleaning the PTHes out.
Solder wick wont work. Heat the PTH and stick a wooden bakers toothpick in
the hole. Push the molten solder thru.
30 watt iron on a variable voltage. Turn heat up and down as needed. This
was some of the easiest soldering Ive done bc of thick boards, well made
with plenty of copper. Youll have to try to damage these boards...but dont!
Tantalums. Cut one lead, unsolder the un- cut lead and remove component.
Then remove cut lead.
Electrolytics in the scope, not so easy. Cant cut one lead easily.
Power supply is not so static sensitive and is easily removable. Heat sink
mounted devices removed and old heat sink grease cleaned out and replaced.
Screws set with low strength Loctite so they cant loosen. DO NOT just shove
the PS assy. back in the chassis, move the cables if necessary so it slides
in easily without pressing on the boards behind the backplane.
DO NOT replace caps in a SMPS with caps not rated for high frequency
filtering. No junk from Jameco or hamfest.
Whomever wired this scope did a pathetic job of wiring routing.
Other boards in mainframe...DONT REMOVE THEM.
1. Static sensitive, custom ICs.
2. Flex ribbon connectors to CRT
3. Coax cables. Old plastic, formed in place over 30 years, dont bend them.
Replaced all the ' lytics without removing the boards. Not easy, but
eliminates much chance of damage.
.DO NOT just pull center section out. Old wiring and cables dont like being
bent and twisted.
CAREFULLY pull the connectors off on A1100 and without bending the wiring,
slip the board out.
That gives access to A1300. Same with it..DO NOT bend wiring or coax
cables. One intermittent connection caused by torquing a cable will make
you pull your hair out....
Dont remove the other boards. No large filters on them. One cap on A1100
can be replaced from top side, cut the cap body off and solder new cap to
Replace the three caps in the microwave area, 2.2 on the board with ICs and
two on the +/ - 15V supplies to the mixer block (mixer?).
DO NOT use electrolytics in the 7L12. They are tantalum for a reason!
Especially do not install tantalums backwards! Boom. Crying. Throwing
Caps and polarity not marked on the boards. DO NOT power it up without
verifying polarity against schematic.
(Note: Recall.. NO SPURS in the SA display...none. .thats what new state of
the art filter caps buys! Clean power lines!)
DO NOT PUT OVER SIZED CAPS IN.( capacitance wise, over voltage is fine).
Highly experienced engineers sized them. Excess capacitance also means
higher pulse charging currents. No reason to do so and old connectors do
not need high currents.
Carefully break loose and reseat as many connectors as possible. No
twisting or bending.
STATIC SAFE WORKSTATION MANDATORY!!!!
Human body can generate 3Kv.
Unless its the manual for the SN range you have dont trust the parts list
for ordering caps.
Older 7L12 manual said front panel pots are fastened by swaged fittings.
Not on mine, had to remove front panel to access panel nuts. Manual said
electrolytic caps...not in mine- all tantalum. Alignment procedure for
vertical gain was wrong. Fairly minor details.
This scope and plugins were $20,000- ish , new cost. If it isnt worth
doing right, dont do it.
These are not ham radio toys. Word to the wise.
Any questions, Email me. Wont accept any more interference, or ignorant
lectures from supposed Forum Experts here that gave such bad advice.
Appreciate the repair notes. Agree that it's worth doing this right,toggle quoted messageShow quoted text
and that there's no reason to keep power supply capacitors "vintage".
We've all seen what happens when those "vintage" tantalums give up the
ghost. It's like taking a torch to the board. Have you considered
replacing the coax or the HV lead? How did you add new insulation onto
the HV lead?
On Tue, Jun 8, 2021 at 3:45 AM David Campbell <email@example.com> wrote: