Quote:
Originally Posted by PJL
As the DC conditions are close I would be cautious about your conclusions. Touching a pin does not always result in a response as it depends on the impedance of the circuit at that point. The grid is connected to the IF secondary which has very low resistance, you will only be able to inject noise at the resonant frequency and even that is going to be affected by the probe.
Do you get any noise when you switch between the MW and LW bands? With the waveband switch on MW, measure the voltages on V2.
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I don't get any noise while switching. The buzz is constant and gets louder when I switch in the FM circuit.
I took your point and measured voltages around V2 (and re-checked around V3, V4 and V6). The are fairly close with p6 at 159v and p1 at 94v which is around 6v low. I also noticed that V6 p7 has 60vAC though, which probably explains the buzz, even when the volume is turned right down.
The voltage at V7 p3/C50 is low at 228v, but C49 and C42 have close to correct voltages of 196v and 158v. The caps have been replaced and there seems to be no AC here, although I do get a momentary few volts which fades away over 3-4secs which I presume is down to the soothing effect of the caps (and possibly my digital multimeter)? C47 has also been replaced but there is 15v instead of the expected 11.8 at p3. R27 is high though at 340ohm instead of 270ohm.
This circuit has not transformer and puts the heaters in series on the AC side of V7 so I wondered whether AC can bleed through the heater to affect the valve, or could it simply be down to the higher than expected voltage at p3?