01-05-2012, 08:46 PM
If you can discover a cct. diag., obviously that will help. But looking at your results, there are some clues as to where to look for faults.
Your assumption about setting up the 'Set Zero' & 'Set Inf' is the same as my understanding.
As it stands, the 50 v. F.S.D. has the most linear scale, albeit reading very low. As the gain of the amplifier within the instrument is increased, indicated by progressing to more and more lower F.S.D. settings, the linearity slowly degrades. The fact that on the 50v F.S.D. range, the reading are all too low implies that high-value resistors in anode circuits have probably gone high in value. The increasing non-linearity as the gain is increased could be due to defective bias on the amplifier valve, leaky capacitors or possibly a low-emission valve. Finally there is the question of the HT: if that is seriously too low, then that could easily explain all those symptoms!
So treat it like any other valve-based item of kit that's in for a repair: check the HT; check the value of all the high ohmic value resistors; check for leaky capacitors; clean switch contacts and finally replace a valve or two. The instrument will certainly need re-calibrating via its internal adjustable pre-set resistors: even these might be defective, although unlikely.
Al.
Your assumption about setting up the 'Set Zero' & 'Set Inf' is the same as my understanding.
As it stands, the 50 v. F.S.D. has the most linear scale, albeit reading very low. As the gain of the amplifier within the instrument is increased, indicated by progressing to more and more lower F.S.D. settings, the linearity slowly degrades. The fact that on the 50v F.S.D. range, the reading are all too low implies that high-value resistors in anode circuits have probably gone high in value. The increasing non-linearity as the gain is increased could be due to defective bias on the amplifier valve, leaky capacitors or possibly a low-emission valve. Finally there is the question of the HT: if that is seriously too low, then that could easily explain all those symptoms!
So treat it like any other valve-based item of kit that's in for a repair: check the HT; check the value of all the high ohmic value resistors; check for leaky capacitors; clean switch contacts and finally replace a valve or two. The instrument will certainly need re-calibrating via its internal adjustable pre-set resistors: even these might be defective, although unlikely.
Al.
