UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Hameg HM307-3 Cleaning and Repair

WaveyDipole · 25th May 2019, 3:04 pm

The fact that you are getting a voltage at the base of T121 suggests that the resistor chain is not o/c otherwise you would be getting zero.

At this point I have to make an apology. I made a mistake here. That 100Ω pot (R223) is the astigmatism adjustment, nothing to do with the Y amp balance. It can't possibly have an effect on the DC balance of the plate voltages from the collectors of T123 and T124. The section subtitled "Astigmatism Correction" makes this clear.

The subsequent section "Troubleshooting the instrument" gives a voltage range of 81-89v for the Y plates so there is a bit of leeway in terms of what the output voltage should be, however, ideally the voltages should be the same on both plates to within a fraction of a volt or so.

The fact that the emitter and collector voltages are both slightly different means that something is not quite balanced, but not that far off either. Any variance between R190 and R191, also R197 and R198 as well as the two coils L101 and L102 is going to throw the balance. You said you measured those resistors. You are confident that each of those two pairs measures exactly the same (to within a small fraction) on both sides of the circuit? Although it seems unlikely there would be a fault with the coils, I suppose it might be worth checking them as well. They also should have a near identical DC resistance. After that, we are left with just the transistors themselves and one of them may have been been stressed for a period of time. Easily proven or ruled out I suppose by temporarily substituting the pair.

Hope all goes well with sorting the FETs.

25th May 2019, 3:04 pm	#26
WaveyDipole Nonode Join Date: Mar 2013 Location: Leicester, UK Posts: 2,535	Re: Hameg HM307-3 Cleaning and Repair The fact that you are getting a voltage at the base of T121 suggests that the resistor chain is not o/c otherwise you would be getting zero. At this point I have to make an apology. I made a mistake here. That 100Ω pot (R223) is the astigmatism adjustment, nothing to do with the Y amp balance. It can't possibly have an effect on the DC balance of the plate voltages from the collectors of T123 and T124. The section subtitled "Astigmatism Correction" makes this clear. The subsequent section "Troubleshooting the instrument" gives a voltage range of 81-89v for the Y plates so there is a bit of leeway in terms of what the output voltage should be, however, ideally the voltages should be the same on both plates to within a fraction of a volt or so. The fact that the emitter and collector voltages are both slightly different means that something is not quite balanced, but not that far off either. Any variance between R190 and R191, also R197 and R198 as well as the two coils L101 and L102 is going to throw the balance. You said you measured those resistors. You are confident that each of those two pairs measures exactly the same (to within a small fraction) on both sides of the circuit? Although it seems unlikely there would be a fault with the coils, I suppose it might be worth checking them as well. They also should have a near identical DC resistance. After that, we are left with just the transistors themselves and one of them may have been been stressed for a period of time. Easily proven or ruled out I suppose by temporarily substituting the pair. Hope all goes well with sorting the FETs.