09-05-2011, 09:08 PM
Thanks, chaps, for your additional inputs. To clarify, I was using the manufacturer's (Pye) service sheet for this set (a P75A), as per Jamie's post, but that Trader sheet for the P75 is a lot easier to follow. There are a few differences around the frequency changer valve and elsewhere, but the fundamentals look the same. So thanks to Shaman for that. And Rob T, I now understand which components you were referring to in your earlier post.
Al. thanks for your feedback on my triode section grid voltages and 'scope probe details.
Tony - I take your point about the HT level. I did check it early on and I don't think it was far out. I took some readings yesterday, using the conditions specified on the P75A service sheet (mains in: 230V, MW, tuning condenser fully meshed, Avo 8):
I calculated the anode current (Ia) from the voltage dropped across one of the anode resistors (R4). You'll notice that the service sheet says 27V on the anode of the oscillator section, which surely can't be right, can it?
Your efforts also made me realise that I have a useful resource at hand in the form of a P75 chassis (I nicked the output Tx from this for the P75A). So I thought I would compare the SW osc. coils resistance readings from the P75A with those from the P75. Even better, I then noticed that the P75 Trader sheet gives a resistance value for one of the coils. I also did the checks that Rob T described in his post:
These readings were taken with a DMM. I couldn't get a reliable reading from the trimmer C10 - the reading varied from 0.13 - 0.3 nF, which was the same regardless of whether the test leads were connected to anything or not! To me, that 1.3Ω reading from the grid side of the oscillator coil looks suspect. In fact, I reckon that that is the cause of the fault - I would've thought it should be around the 50Ω level like the P75A. But is it sensible to assume that the SW osc. coils in the P75 and the P75A are the same?
(If you want to put tables in your posts, be prepared for a long stint. These took me ages!)
Al. thanks for your feedback on my triode section grid voltages and 'scope probe details.
Tony - I take your point about the HT level. I did check it early on and I don't think it was far out. I took some readings yesterday, using the conditions specified on the P75A service sheet (mains in: 230V, MW, tuning condenser fully meshed, Avo 8):
Code:
Oscillator section Hexode section
Ea(V) Ia(mA) Ea(V) Es(V)
SS 27 3.5 180 78
MW 67 2.6 193 69
SW 73 6.3 182 82
SS=Pye75A service sheetYour efforts also made me realise that I have a useful resource at hand in the form of a P75 chassis (I nicked the output Tx from this for the P75A). So I thought I would compare the SW osc. coils resistance readings from the P75A with those from the P75. Even better, I then noticed that the P75 Trader sheet gives a resistance value for one of the coils. I also did the checks that Rob T described in his post:
Code:
C11 C10 L5 R4
C(nF) R(Ω) C(nF) R(Ω) Anode side(Ω) Grid side(Ω) R(Ω)
SS 4.7 - 3-50pF - - - 15k
P75A 4.79 ∞ - ∞ 0.9 1.3 17.6k
P75 4.79 ∞ - ∞ 0.8 50.7* 16.4k
* Trader sheet for P75 measured this at 43Ω.These readings were taken with a DMM. I couldn't get a reliable reading from the trimmer C10 - the reading varied from 0.13 - 0.3 nF, which was the same regardless of whether the test leads were connected to anything or not! To me, that 1.3Ω reading from the grid side of the oscillator coil looks suspect. In fact, I reckon that that is the cause of the fault - I would've thought it should be around the 50Ω level like the P75A. But is it sensible to assume that the SW osc. coils in the P75 and the P75A are the same?
(If you want to put tables in your posts, be prepared for a long stint. These took me ages!)
