Electronics > Repair

BK Precision 1479b vertical amp problem

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Jnkyrdguy:
I recently picked up my first oscilloscope, a BK precision 1479b. (The service manual can be downloaded here: http://elektrotanya.com/bk-precision_1474_1479_a_b_2x5mv_30mhz_delayed_oscilloscope_full_sm.pdf/download.html) Everything seems to respond correctly except for one fault that has me stumped.

The vertical position knob only allows about one division of deflection before the signal appears to saturate. At 1 volt per division the 0.5 volt peak to peak calibration square wave looks ok when it is centered (even though it is obviously not one half of a division tall), but then moving the position knob all the way up turns the square wave into a single line. The same happens when the position knob is turned all the way down. Changing to 100mv or less per division gives a saturated square wave about one division tall. This happens on both channels separately and with a dual trace and happens equally and consistently to both channels. Both channels create an identical output using the same settings with the same input.

Originally I thought it was a problem with the vertical output amp based on the trouble shooting guide, but it all seems to respond correctly so I moved to check the vertical amp board which has four transistors shared by both channels.

Then yesterday morning it all started working perfectly. I was able to look at a simple saw wave vco I have been working on and compare its trigger pulse and waveform. It all looked right, lined up correctly and was even showing about the peak to peak voltages and dc offsets I was expecting. Besides some slight calibration and switch wonkiness, it was 100% usable and predictable.

Then it started having other faults...
-The whole cal waveform was offset so off off the screen that it is hardly visible at 5mv per division, but seemed to respond correctly to the vertical attenuator knobs on both channels.
-Then, at another point, the cal waveform was crushed to maybe only 1/20th of a division max even when set on 5mv/ div, but was offset just above the center of the screen.

I've seen a bunch of posts on other forums pointing to the 2sc458 as the culprit in many old Akai tape decks with high noise and distortion as the transistor is failure prone. This oscilloscope has nine of these transistors including Q125 and Q126 as well as Q405 to Q409, Q418 and Q419.

So far I think I have rounded the problem down to Q125 and Q126. Since I don't have another oscilloscope, I've been using a DMM to test the VAC RMS at the test points, then converting to peak to peak and comparing that with the spec'd waveforms from the service manual. When the scope was working, Q123 through Q126 all consistently had the same and correct VAC reading at their four test points as they should. When not working, Q125 would have some dc offset, too large or a fluctuating or non-responsive AC voltage, or otherwise something wrong or mismatched.

I switched Q125 and Q126 and then the wonky voltages immediately flipped to the other position. So I tried replacing them both with another small signal transistor, but the same problems followed. How critical is the type of transistor? I can get 2sc458C transistors easy enough, but before I order them I wanted to see that that was the problem so I threw in two 2sc732 I had lying around just to see if it would at least fix the faults I keep getting, but they didn't work either. Do Q125 and Q126 need to be matched in any way? They aren't marked and the SM doesn't say anything about it so I figured it wouldn't be a problem.

I don't really understand what Q123 and Q124 do in the circuit, but it seems like they are always outputting the spec'd dc and ac voltages at their collectors when it's working or not so I don't see how they could be contributing.

If it's not these transistors I have no idea what it could be. There's not much else in that part of the circuit that seems like it could be causing these problems and it doesn't seem like anything later in the signal path could be affecting these and only these two transistors so consistently. Considering that the previous stage always looks correct, working or not, I don't see where else the problem could be coming from.


Sorry for the long post and thanks in advance for any ideas! I'm hoping somebody has some insight as I'm pretty stumped at this point.

tautech:
Welcome
Very good first post, congats.
Imagine a flat centered line on your CRO and the Vert Pos knob centered.
At this setting the output to the Vert plates is balanced
Then you should have roughly 1/2 the available HV on each Vert plate.
By reason that deflection is done electrostaticly, high voltages are used and the transistors are known to fail.
You could test with any simple transistor tester that gain meets datasheet spec.

Research that family of devices for one of the same spec, but a higher voltage and if available substitute.
Often these transistors are described as for TV video output or similar.

Others that have had to find replacements may offer suggestions for sourcing/supply.


mij59:
Hi,

Start by checking for mechanical faults, e.g. bad solder joints, connectors, switches, trimmers.
Check the power supply voltages.
Check the voltages as shown in the schematic.
Does the fault apply to both channels ?
Any changes if you tap  the board with a non conducting handle of a screwdriver ?

Jnkyrdguy:
Thanks for the replies!


--- Quote from: tautech on August 12, 2014, 08:43:56 pm ---By reason that deflection is done electrostaticly, high voltages are used and the transistors are known to fail.
You could test with any simple transistor tester that gain meets datasheet spec.

--- End quote ---

I don't think these transistors could be the problem. Even when it is not working right, the output transistors seem to still work the same: It's still possible to short test points P401 and P402 (which gets rid of any errors accrued before the vertical output amp board) and then vertically align the trace with VR402 as per the manuals instructions for vertical alignment. I've never had a problem doing this and getting a full and consistent range of positions for the trace regardless of how the trace looks without P401 and P402 shorted.


--- Quote from: mij59 on August 13, 2014, 06:33:44 am ---Start by checking for mechanical faults, e.g. bad solder joints, connectors, switches, trimmers.
Check the power supply voltages.
Check the voltages as shown in the schematic.
Does the fault apply to both channels ?
Any changes if you tap  the board with a non conducting handle of a screwdriver ?

--- End quote ---

The fault only ever happens to both channels equally and regardless of mode. Both channels react in the same way to all the different specific manifestations of the fault and when one channel starts working again, both channels start working.

I don't see any solder joints or traces that look problematic or any other parts that look to be leaking or mechanically compromised. The supply voltages running into the board all look stable except the -8 volt rail which seems to vary a bit anywhere from -7.75 to -8.05. I checked most of the DC bias points and all AC test points listed in the service manual from the final two output transistors all the way back to the outputs of the separate A and B channel preamps. They all either check out or were off in a way that would make sense given the fault showing up at the time. The only exception being Q123 and Q124 which both have always had the correct/ expected DC bias and AC output regardless of whether the vertical output was working or not.

I tried tapping the board and it does seem to affect things. tapping on the vertical output board doesn't make much of a difference, but tapping on the vertical amp board, especially right around Q125 and Q126 seems to have the most effect. Sometimes tapping lightly will get it working again or will cause it to start to fail or change its failure type.

There is a bunch of a dried resin like substance all over the vertical amp board that I don't see on any of the other boards. It is pretty thick in some places and can be carefully scraped off with an awl without damaging the solder mask. I assume it is rosin left over from the soldering process and it doesn't seem conductive, but I cleaned it off the areas around the transistors that seem to be the problem and it seems to be a bit more stable. Could it be causing problems?

tautech:

--- Quote ---Could it be causing problems?
--- End quote ---
It could well be the problem, you won't know unless you remove it.
Use a solvent like IPA

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