18-05-2015, 12:15 PM
Nice sets! If the serial is later than 13,001, then it won't have AF117s in the AM sections. You probably won't need to do much with the electronics, but you might want to take it all apart and clean out the mechanics underneath the tuning scale. Depending on how adventurous you are feeling, there is a lot of cosmetic restoration that can be done, including replacing the foam under the Rexine.
Voltage readings in service manuals are always "nominal". You need to think about the context. Firstly, forget about your Fluke 189 for a moment - such instruments simply didn't exist in the 1960s... That thing has a 50,000 count display with 0.025% basic DC accuracy - off the top of my head, I'm not sure if that sort of performance was available to anyone at any cost back then
Instead, measurements were made with the AVO8. And that was a good choice, because that's what most repair workshops had. The basic DC accuracy was 1 or 2%. And of course, you're reading from an analogue scale, so readings like 3.7531 weren't possible.
Next, consider the electronic components available back then. Good resistors had a tolerance of 10%, the cheaper ones might have been 20%, and the snazzy ones were 5%.
Also, some readings would depend on the exact battery voltage. OK, they might have powered the set from a power supply set to 9V, but how accurately could they have set the voltage? And how close to 9V is your battery?
I'm sure there is more I could say, but in short, the readings are an indication. When something is wrong with the set, the readings might be wildly off. So if the manual says 3.45V on the collector of VT4, and you get 3.551V on your Fluke, that's all good. However, if you get 7V, then VT4 isn't conducting, and you need to look further to see if it has died, or perhaps is O/C because it's not being biased correctly by other components. Similarly, if you had less than about 2V on the collector of VT4, then the transistor might be "saturated" because of a biasing problem, or it might perhaps have failed short-circuit.
Of course, the DC voltages are only part of the equation. When you design a circuit - or indeed fault-find - the rule is to get the DC conditions right first, and then see what happens when the signal is applied. So even with the DC voltages being spot on, the set might appear to be dead because C19 has failed open-circuit.
It's all a bit of a game. But mostly it follows simple logic. Mostly
Voltage readings in service manuals are always "nominal". You need to think about the context. Firstly, forget about your Fluke 189 for a moment - such instruments simply didn't exist in the 1960s... That thing has a 50,000 count display with 0.025% basic DC accuracy - off the top of my head, I'm not sure if that sort of performance was available to anyone at any cost back then
Instead, measurements were made with the AVO8. And that was a good choice, because that's what most repair workshops had. The basic DC accuracy was 1 or 2%. And of course, you're reading from an analogue scale, so readings like 3.7531 weren't possible.
Next, consider the electronic components available back then. Good resistors had a tolerance of 10%, the cheaper ones might have been 20%, and the snazzy ones were 5%.
Also, some readings would depend on the exact battery voltage. OK, they might have powered the set from a power supply set to 9V, but how accurately could they have set the voltage? And how close to 9V is your battery?
I'm sure there is more I could say, but in short, the readings are an indication. When something is wrong with the set, the readings might be wildly off. So if the manual says 3.45V on the collector of VT4, and you get 3.551V on your Fluke, that's all good. However, if you get 7V, then VT4 isn't conducting, and you need to look further to see if it has died, or perhaps is O/C because it's not being biased correctly by other components. Similarly, if you had less than about 2V on the collector of VT4, then the transistor might be "saturated" because of a biasing problem, or it might perhaps have failed short-circuit.
Of course, the DC voltages are only part of the equation. When you design a circuit - or indeed fault-find - the rule is to get the DC conditions right first, and then see what happens when the signal is applied. So even with the DC voltages being spot on, the set might appear to be dead because C19 has failed open-circuit.
It's all a bit of a game. But mostly it follows simple logic. Mostly
