Re: Tek 2215 vertical gain


 

Hi,

I've fixed the issue with the traces being too low, the cause was a pair (or maybe one) of failed transistors in the vertical drive for the CRT (-) (minus) plate, on the A10 mainboard. These were Q387 (Tek 151-0127-00) which I replaced with a 2N2369A, and Q386 (BFR96) which I replaced with a BFR96S.

Here's the list of checks and measurements I made and notes on the circuit.

Following the service manual first checks were to the power supply voltages (PDF pages 75 and 176 of the manual).

Test points and voltages measured were:

TP500 -8.62V
W985 5.08V
W975 8.60V
W965 30.1V
W966 102V

So all well within spec. I set the scope controls as per the manual troubleshooting guide (so ch1 only), and measured the voltages in the vertical amplifier circuit schematic (p.159 of the PDF). This shows two very similar circuits feeding the CRT (+) and CRT (-) vertical deflection plates. The figures in parenthesis are the voltages given on the schematic, followed by actual measured voltages.

CRT + R398 (+9.0V) +9V after adjusting ch1 vertical position control pot.

CRT - R399 (+9.0V) +18V

CRT + R376 (top +2.6V bottom -4.3V) +2.0V and -5.5V

CRT - R386 (as above) +8V and -2.6V

CRT + R338 (0V) 150mV (adjusting vertical position changes from 400mV with beam highest, to -300mV beam lowest).

CRT - R348 (0V) -270mV (adjusting vertical position changes from -370mV with beam highest, to +200mV beam lowest).

The inputs to the amplifiers were at R338/R348 and the outputs to the plates are at R398/R399.

Looking at the circuit and the voltages as the ch1 vertical position pot is adjusted it became clear that the vertical deflection uses differential amplifiers, so that CRT+ voltages go positive whilst CRT- voltages go negative(and vice versa), providing noise and interference immunity, as it is the differential voltage between CRT+ and CRT- that is important, not any exact voltage of either. The beam of electrons hitting the CRT are negatively charged so when passing between the CRT+ and CRT- plates are deflected towards the most positive plate.

I checked the DC behaviour of the output amplifiers and compared CRT+ and CRT- sides. First checked CRT+ side, setting voltage at input (R338) from -100mV to +100mV (by adjusting vertical control) and measuring corresponding voltage at output (R398)

Input Output
-100mV 2.9V
-50mV 4.6V
0V 6.0V
+50mV 7.1V
+100mV 8.0V

Then did same with CRT-, input R348, output R399:

Input Output
-100mV 20V
-50mV 21V
0V 21V
+50mV 21.5V
+100mV 21.5V

Seemed very clear now the issue with the scope was in the CRT (-) side of the output amplifier, so I decided to fix it so it worked fine at DC and worry about AC later if needed. There is some coupling between the CRT - and + sides (eg the sweep separation circuit from Q370 and Q380), but nothing seemed to be amiss.

Checked a few more voltages against schematic, R378/R388 = (+30V) +30V (ok), R390 (-5.0V)
-5.3V, pressed beam find then -3.6V so ok too.

To find which part of the CRT- circuit was at fault I traced voltages through from input to output. If the voltage measured was same in CRT- and CRT+ sides then assume OK.

Again I varied the input voltage (at R338 and R348), this time only 3 values, -100mV, 0V and +100mV.

CRT- measurements(at transistor Base Emitter Collector):

R348 Q360C Q386B Q387B Q386C/Q387E R399
-100mV -0.7V -2.9V 9.9V 9.22V 20V
0V -1.7V -3.5V 9.9V 9.22V 21V
+100mV -2.7V -3.9V 9.6V 8.95V 21.5V


CRT+ measurements (* - not measured)

R338 Q350C Q376B Q377B Q376C/Q377E R398
-100mV * * -0.95V -1.5V 2.9V
0V * * 0.63V -60mV 6.0V
+100mV * * 1.6V +2.0V 8.0V

Issue appeared to be in Q386 or Q387, I couldn't tell which. First I replaced Q387, but this had no effect on the CRT- output voltages. Then I replaced Q386 and this fixed the issue. Earlier I wrote I measured R378/R379/R388/R389 as 315 ohm each (should be 340 ohm). I realised these were connected between the +30V and -8.6V power rails so I lifted the leg on one and it measured 340 ohm, so all good.

I could now move the trace from below the bottom of the CRT to above the top. I tried the front panel test square wave plus an external sine generator, and waveforms seemed fine, nice and symmetrical and no visible change when moved to top or bottom of the display. I assume when I first noticed issue with waveform peak-to-peak varying as I moved the display up and down that Q386 was on the way out, finally failed and stopped acting as a transistor.

I re-did some of my earlier measurements to wrap things up and show what typical voltages in a working 2215 scope should be. First set trace to a horizontal line exactly mid-screen.

CRT(-) R348= -64mV R389= 9.2V
CRT(+) R338= -93mV R379= 9.4V

I tested the maximum range of the vertical position control. R389 range was from +20V (trace off bottom)to -3V (trace off the top). The range for R338 was from -1V to +19V.

CRT- measurements:

R348 Q360C Q386B Q387B Q386C/Q387E R399
-100mV -1.9V -4.3V 1.7V 1.1V 7.3V Trace at +1.4 div
0V -2.5V -4.3V 4.7V 3.6V 11.9V Trace at -2.2 div
+100mV -3.1V -4.3V 6.7V 6.0V 16V Trace off bottom of CRT


CRT+ measurements:

R338 Q350C Q376B Q377B Q376C/Q377E R398
-100mV -2.0V -4.3V 2.6V 1.9V 8.8V Trace at -0.4 div
0V -2.7V -4.3V 5.4V 4.7V 14V Trace at +4.0V div (very top)
+100mV -3.6V -4.6V 7.0V 6.3V 17V Trace way off top

Interesting how much higher the CRT+ voltage measurements were following the fix to CRT- circuit, clearly a bit more coupling (eg emitters of Q376 and Q386 connected to R590) between the two sides than I anticipated.

Resulting traces on square and sine waves are now fine again, I haven't attempted any calibration but works well enough for my use.

Regards.
John

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