475 Z-Axis Amp Woes

agwnorden@...
 

Dear TekScopes

Could someone a bit more competent than me help with analysing the problem I am having with the Z-Axis Amp on my beloved 475 which has served me incredibly well for over 10 years.

The initial symptom was no trace. All power voltages and ripple were fine.

Using the XY setting to debug, I found the following. A spot will only appear in XY mode when Beam Finder (BF) is activated. BF, as I understand it, overrides the Z-Axis amp and sets the Amp O/P to about 25 Volts (from 6V) which then goes into the DC Restorer. Voltages at TP1364 confirm this works OK. (please see CRT circuit and Z Axis Amp circuits attached; apologies for quality but the best I have). This is also confirmed by simply putting -8V on the junction of R1342 and R1346; the spot reappears. X and Y position controls work fine with BF on.

The operation of the Z-Amp is described as follows in the manual:-

Z-Axis Amplifier
The Z-Axis Amplifier circuit controls the CRT intensity level from several inputs. The effect of these input signals is to either increase or decrease the trace intensity, or to completely blank portions of the display. The input transistor Q1338 is a current-driven, low input impedance amplifier. It provides termination for the input signals as well as isolation between the input signals and the following stages. The current signals from the various control sources are connected to the-emitter of Q1338 and the algebraic sum of the signals determines the collector conduction level.

Q1344, Q1352, Q1354, Q1358, and Q1362 compose a feedback amplifier stage. R 1369 is the feedback element, C 1352 provides high-frequency compensation. Q1344 is an emitter follower that provides drive to the output complementary amplifier made up of Q1352, Q1354, and Q1358. Q1358 is a device with higher frequency characteristics than Q1354 and is used to improve the overall frequency capabilities of the Z-Axis Amplifier. On the fast positive going output signal peaks, Q1358 depletes the charge on C1358. Then, on the negative-going signal peaks, Q1362 is pulsed on to renew the charge on C1358. CR 1367 provides protection to the Z-Axis Amplifier circuitry in the event of short duration arcing in the CRT High-Voltage Power Supplies.

My problem is, I think, that I do not really understand the circuit! When I rotate Beam Intensity I see no change in the voltage on the emitter of Q1338 (even with all other inputs (Q1332 etc.) disconnected. The voltages on Q1338 are Ve -2.498, Vb -1.729 and Vc 0.467 so Vbe is ~0.75 forward biaseed. Perhaps this always pulls Ve down when the Intensity Control is changed? Voltage measurements perhaps of little use in a summing amp like this?

If CR1341 is lifted as per the troubleshooting instructions below I do get the specified change in Vc of Q1338 which swings from +7.8 volts to –1.3 volts while turning INTENSITY control from full counterclockwise to full clockwise

Troubleshooting the Z-AXIS Circuit (CRT Circuit). Set TIME/DIV to 1 ms (lock knobs) and set TRIG MODE to SINGL SWP. Disconnect and lift one end of CR1343. Check the voltage at TP1364 for approximately +6 volts (OK). Press the BEAM FINDER pushbutton and check the voltage at TP1364 for approximately +25 volts (OK). If these voltages cannot be obtained, check the voltage across VR1362 for approximately +6.2 volts. Remove Q1362 from its socket and check the voltage at the base of Q1352 for approximately +1.3 volts. Check the base of Q1344 for approximately +0.6 volt.

If the above-mentioned voltages can be obtained, disconnect and lift one end of CR1341. Check the voltage swing at the collector of Q1338 for approximately +7.8 volts to –1.3 volts while turning INTENSITY control from full counterclockwise to full clockwise. (OK). If these voltages cannot be obtained, check the voltage at the base of Q1338 for –1.75 volts. Remove Q1332 from its socket.
Disconnect J1333 and J1334 (coaxial-type end-lead connectors) from their sockets. Repeat the voltage swing check at the collector of Q1338 as performed previously. Connect J1333, J1334, R1343 and CR1341 to the circuit board. Install Q1362 and Q1332 in their sockets.

Replacing all parts and confirming initial voltages around Q1338 I see no response to the Intensity Control at Ve of Q1344, the emitter-follower driving the Z Amp. That is surely wrong?

This places the fault around Q1338/1344 but I cannot find a problem. Junctions of the two test OK. The only obvious fault about the board is some discolouration of CR1360 but it still rectifies.

I apologise for the density of this email but any help much appreciated. I have no more hair to tear out.

Anthony M0WWV

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