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I recently brought my 7D15 out of storage and found that when plugged into the 7704A it took down the display. A second insertion in to the 7704A brought some smoke, quickly identifying the bad capacitor as C931, a 15 uF 16 volt device. I replaced with a 10uF, 25 volt device and the counter is back alive.

This was one of my favorite 7000 series plugins. When I started out I was doing Digital Signal Processing hardware using serial arithmetic and this counter was invaluable in counting clock pulses associated with each math operation using the delayed sweep gate as an event counter gate. A digital filter with 30 multiply accumulates has a sequence of 30 groups of 32 clocks. Try counting that manually on a 7904 screen. The 7D15 made that easy.

The remaining problem is a non working internal trigger. It does work if I overdrive the vertical input by about 3 screens and a 7B53A works fine in that slot so I believe the problem is in the internal trigger amplifier, Q203, Q213 and Q217 in the counter. It could also be the internal/external trigger switch. I don’t have an extender and the -5 volt power supply board in the counter is over the trigger circuits, making observation of the circuit nearly impossible. The trigger circuit is on the left side of the plug in so I can look at it in a vertical slot but can't feed a trigger signal to it while in a vertical slot.

My plan is to get a connector, Digikey 345-076-520-201-ND, remove the -5 volt supply board and power the counter externally, including the -5 volt supply. Then I can look at the voltages/waveforms in the circuit. Any thoughts on the debug process are appreciated. Has anyone had this problem and found a sore spot in the circuit? The 68 uF capacitor on the -5V supply tests OK. The -5 volt is -5V.

On Fri, Mar 19, 2021 at 09:48 AM, Steve Nossen wrote:

I don’t have an extender and the -5 volt power supply board in the
counter is over the trigger circuits, making observation of the circuit nearly
impossible.

The -5V power supply board can be removed from the main PCB, and
then re-installed perpendicular to it, to allow access to components
underneath. (There should be three spare sockets on the A8 board
mounted at 90 degrees to the main sockets for just this occasion.
Make certain that all three sockets engage their pins properly.)


dan

Steve,

Check the 151-0402-00 transistors. These are the same number as the 151-0367-00 3571TP. These are likely leaky. I had a triggering problem with mine and these transistors were bad from C to E. These check as a diode that way. I put in KSP10BU as replacements. You should see the leakage in-circuit. A VTVM or VOM on R X1 will have about mid-scale deflection on the leaky ones.

Check the ESR of C27, C40, C127 and C140. If high, replace them. Mine were high and were replaced with film types. See if any other of the 1mfd axial condensers are high in ESR and replace those that are high.

Mark

Dan and Mark,

Thanks for the tips. I had the covers off a few times and removed the -5 V board. Never noticed the socket pins allowing perpendicular mounting.

Measured the transistors and Q213 measures 30 ohms from C to E and E to C on my SImpson 260. Much higher on a DVM but the same in both directions. Seems leaky to me. Q203 measured more like a diode, conducts in one direction. I'll add some transistors to my next order. The 1 uF caps both measure 1 uF and D of .02 at 1KHz, about 3.3 ohms. The 2.2 uF caps measure 2.3 uF and D of .1, about 7.2 ohms. I'll start with a transistor swap.

Steve

Steve,

If you get the KSP10BU transistors, these have a pinout of BEC. They are not the common EBC or ECB pinout. I suspect all of your 0402 transistors are leaky from what you already checked. Checking them out of circuit will confirm leakage or not. If you want to have some fun, use a battery of 9 or 12V, a series resistor of about 1000 ohms or so and treat the transistor as a diode (E and C used) and see what voltages you get across each device. Reverse the battery connections or E and C connections and see what happens to the voltages across each of the two. This is done with the transistor out of circuit and base not connected.

Mark

On Sun, Mar 21, 2021 at 01:26 PM, Steve Nossen wrote:

Measured the transistors and Q213 measures 30 ohms from C to E and E to C on
my SImpson 260. Much higher on a DVM but the same in both directions. Seems
leaky to me. Q203 measured more like a diode, conducts in one direction.

Q213 pins are connected to GND and +5V rails with very low value resistors, so
I would be suspicious of any in-circuit resistance measurements.

A non-invasive test of the basic health of these transistors is to
check the operating point voltages of Q203, Q213 and Q217, and compare
them against the expected values provided in schematic <1>. This check
should not need a signal applied to A20/B20, so the plug-in can be operated
in the vertical compartment for ease of access. (Unless, of course, you are
one of the luck few who have a nice 7000 plug-in extender.)

Also, did you remember to clean all attenuator cam switch contacts with
isopropyl alcohol? This would be the first order of business before
attacking the plug-in with a soldering iron.


dan

I just did a quick measurement on my 7D15, with the plug-in installed in a horizontal slot,
and being fed a 1 MHz 5 div p-p sine wave from a vertical amplifier. The 7D15 was set to
measure frequency, TRIGGER B selector set to TRIG SOURCE.

The 5 div signal from the vertical amplifier becomes a 840 mV p-p signal measured
at R117 with a 10X high-Z probe. It is symmetrical about 0V. Observing the signal
here will tell you the effect of the Q203/Q213/Q217 amplification stage, and you
should be able to see any contact resistance issues with cam switch contact
number 12.

R117 is readily accessible with the right mainframe cover removed if you install
the time base in Horizontal slot A, and the 7D15 in the Horizontal slot B.


I hope this helps,

dan

I had a chance to get back to the 7D15 after a new grandchild arrived. Following group suggestions (mount -5V supply perpendicular to board), I measured the voltages on the 7D15 internal trigger amplifier while in the second vertical slot and all matched the voltages on the schematic. Looking at the scope I realized all of my observations were with the 7B53 in the right H slot (B) and 7D15 in the left H slot (A). I swapped the equipment and the counter now triggers and the timebase does not! The 7D15 trigger seems OK. Measured voltages on the main frame trigger select ICs, U2404 and U2424, 155-0022-00. The voltages were not the same on pin 4 of the two ICs under the same conditions. I swapped the chips and the problem and voltage measurements followed the chip. I could get a trigger to occur with the bad chip intermittently and dependent on the setting of both slot A and slot B trigger select switches, both had to be in vertical mode position. The voltage on pin 4 of the good chip is the same if the right slot is selected as trigger independent of how the right slot is selected. For the bad chip the voltage is dependent on how the right slot is selected, right slot or mode and vertical mode right. That combination changes if pin 4 is driven through the transistor, Q2090, or though the resistor, R2093. Additionally, the voltage on pin 11 is -1.3 volts instead of -4.1 volts.

I located a trigger board from a parted 7603 that claimed to have a failed CRT and am waiting on delivery to harvest the IC. Any thoughts on the IC failure are appreciated. Anything else I should be concerned with in replacing the IC? Other than trying a few plugins in this particular scope I don't have any history with this 7704A so I don't know if this is a preexisting condition or something that happened on my watch. I have read that these ICs are subject to damage doing hot swaps.

On Wed, Mar 31, 2021 at 09:50 PM, Steve Nossen wrote:

I have read that these ICs are subject to damage doing hot swaps.

The 7000 series do not support hot swapping. A warning to that effect is shown on many plugins.

Raymond

Steve,

Check R2409. I suspect it is high or open by the voltages you measured. A bad IC will also make the voltages off. Good you will get a replacement 0022. If bad, use a 1/2W resistor with the size about 3,5x10mm. The original is 1/8W. More wattage is being dissipated than the resistor is rated, ,133W dissipation of a ,125W resistor. I am glad you found your 7D15 is working. That is a nice plug-in.

Raymond Frank is exactly right! Do NOT hot-swap the plugins. This advice should be observed on the 5000 and 11000 series as well.

Mark

Received the 7603 trigger select board and swapped the suspect IC, U2404. My 7704A is now fully functional for trigger. R2409 measures 910 ohms as it should. The voltage on pin 11, the common substrate voltage for the 2 ICs is now -4.1V as on the schematic.