06-05-2013, 11:52 AM
I made a very wet trip to pick up one of these late last year.
I had a quick look at it and knowing little or nothing about it ran a check to make sure there was nothing that would cause deterioration of it.
I discovered the AC probe and it was time to go and Google it.
Next day off came the back and yes at this stage the AC probe was put in a safe place to one side and the first dodgy parts were spotted.
The first was a killer waxy that was well fried and had spat all over the place making a mess on the cabinet floor. The good thing is that it is not a grid coupling part but all the same not a good sign.
I put it on the Variac while keeping a sharp eye on the killer waxy and was surprised to see the little 6V bulbs bright with the Variac set to 100V.
It must be 110V mains or is it.
I tried the ohms ranges and got a very dodgy flickering on the dial lights. So this is the first hard fault to find.
Next the voltage range to assess the movement.
So 2.5 volt range selected and on goes the diode tester and the pointer swept over to two volts.
Day two with a handbook with a circuit diagram downloaded the chassis is out and the grub-screw threads re-tapped after drilling the seized screws out a short circuit selenium rectifier is the cause of the shorting POT in the ohms zero circuit is ready to be sorted out.
The fried waxys are mostly in the AC measurement circuits and are OK during this phase of the restoration.
R1 is the dodgy POT and point Y goes to the capacitance range chain while Z goes to the resistance chain so we now know that the diode is duff.
The fried waxy is C9 up top left so I don't hold too much hope for the double diode in the removable AC probe shown in the dotted box as it looks like another waxy has gone off inside it.
Then once I have sorted out the diode and caps there is a Bakolite repair to sort out.
The first thing I have found is that the transformer has no voltage selector on it and instead there is what the comm's guys call a "bathtub" capacitor in series with the primary winding.
I donor chassis was acquired. From this I have liberated some of the original resistors and a few small mechanical parts.
I was looking at a small transformer with a single turn secondary with a meter shunt to bring it down to the correct voltage with the original transformer still in there. I think the "bathtub" is only there to stop it being plugged in on DC by mistake. With that out there will be room inside for the extra part.
The panel inside does not have the links and also the transformer only has two primary connections.
The probe on mine may well need a new double diode and a couple of caps.
I will sort out the main chassis first as the probe can be fixed after the chassis is all back together again.
There are only two primary tags and the voltage selector is not present.
I have now tested the AC probe and the 470pf capacitors are very slightly leaky compared to modern types but below 5ua at 300V.
There has been an offset on the meter during testing and the time has come to sort this out. Armed with the pin-out for the double triode and a continuity buzzer I set too and located all the tags they go to it and traced it to an open circuit cathode resistor shown as R23.
It is two wire wound 20K parts that look like cotton bobbins. It is 40K.
It looks like I might be able to hide something to replace it in circuit. I think it is open circuit in the middle.
There is also an issue with the low terminal continuity to sort out too.
The worst waxy is the input coupling to the AC probe and will be replaced before I test the AC ranges.
I have fixed the resistor for now so that I can test the rest of it.
Then I found another that was also open circuit and have guessed the value and a temporary 33k is in there for now. It is in the coarse zero adjustment chain.
meaningful readings are now showing on voltage and resistance but not amps.
There is a third open circuit bobbin resistor in the amps chain and the amps chain makes up part of the very low voltage chain so it will have to be replaced too.
There are a lot of dodgy offsets on some ranges and I get the feeling this is the double triode as it is showing some cathode vapor stains above the cathodes on the glass.
Double triodes have a nasty tendency to be expensive.
We are gradually heading for an analogue meter that does not load the circuit under test.
I had a quick look at it and knowing little or nothing about it ran a check to make sure there was nothing that would cause deterioration of it.
I discovered the AC probe and it was time to go and Google it.
Next day off came the back and yes at this stage the AC probe was put in a safe place to one side and the first dodgy parts were spotted.
The first was a killer waxy that was well fried and had spat all over the place making a mess on the cabinet floor. The good thing is that it is not a grid coupling part but all the same not a good sign.
I put it on the Variac while keeping a sharp eye on the killer waxy and was surprised to see the little 6V bulbs bright with the Variac set to 100V.
It must be 110V mains or is it.
I tried the ohms ranges and got a very dodgy flickering on the dial lights. So this is the first hard fault to find.
Next the voltage range to assess the movement.
So 2.5 volt range selected and on goes the diode tester and the pointer swept over to two volts.
Day two with a handbook with a circuit diagram downloaded the chassis is out and the grub-screw threads re-tapped after drilling the seized screws out a short circuit selenium rectifier is the cause of the shorting POT in the ohms zero circuit is ready to be sorted out.
The fried waxys are mostly in the AC measurement circuits and are OK during this phase of the restoration.
R1 is the dodgy POT and point Y goes to the capacitance range chain while Z goes to the resistance chain so we now know that the diode is duff.
The fried waxy is C9 up top left so I don't hold too much hope for the double diode in the removable AC probe shown in the dotted box as it looks like another waxy has gone off inside it.
Then once I have sorted out the diode and caps there is a Bakolite repair to sort out.
The first thing I have found is that the transformer has no voltage selector on it and instead there is what the comm's guys call a "bathtub" capacitor in series with the primary winding.
I donor chassis was acquired. From this I have liberated some of the original resistors and a few small mechanical parts.
I was looking at a small transformer with a single turn secondary with a meter shunt to bring it down to the correct voltage with the original transformer still in there. I think the "bathtub" is only there to stop it being plugged in on DC by mistake. With that out there will be room inside for the extra part.
The panel inside does not have the links and also the transformer only has two primary connections.
The probe on mine may well need a new double diode and a couple of caps.
I will sort out the main chassis first as the probe can be fixed after the chassis is all back together again.
There are only two primary tags and the voltage selector is not present.
I have now tested the AC probe and the 470pf capacitors are very slightly leaky compared to modern types but below 5ua at 300V.
There has been an offset on the meter during testing and the time has come to sort this out. Armed with the pin-out for the double triode and a continuity buzzer I set too and located all the tags they go to it and traced it to an open circuit cathode resistor shown as R23.
It is two wire wound 20K parts that look like cotton bobbins. It is 40K.
It looks like I might be able to hide something to replace it in circuit. I think it is open circuit in the middle.
There is also an issue with the low terminal continuity to sort out too.
The worst waxy is the input coupling to the AC probe and will be replaced before I test the AC ranges.
I have fixed the resistor for now so that I can test the rest of it.
Then I found another that was also open circuit and have guessed the value and a temporary 33k is in there for now. It is in the coarse zero adjustment chain.
meaningful readings are now showing on voltage and resistance but not amps.
There is a third open circuit bobbin resistor in the amps chain and the amps chain makes up part of the very low voltage chain so it will have to be replaced too.
There are a lot of dodgy offsets on some ranges and I get the feeling this is the double triode as it is showing some cathode vapor stains above the cathodes on the glass.
Double triodes have a nasty tendency to be expensive.
We are gradually heading for an analogue meter that does not load the circuit under test.
