Motorola 5A7A dead.

[daveo23]

I have to check that measurement again but the bottom line is with the audio tranny removed all of the plates are perfect at 67vdc. Tranny and speaker checkout perfect. Any thoughts?

[Station X]

Er...With the output transformer removed there can't be any anode voltage on the output valve. It's fed via the primary winding of the output transformer.

Don't run the output valve with no voltage on the anode. All the current will pass via the screen grid and destroy the valve.

Make sure that the output valve's cathode is positive wrt the heater or the anode current will be excessive and could destroy the output valve and output transformer.

[daveo23]

now you tell me. I hope its not headed for the clapper. So it would appear the transformer is bad because when removed all of the plate voltages look great but it tests out fine. I am at the end of the line, literally, at the output transformer so where do I go from here. Not sure it this matter but if I life the red transformer wire and measure to "common" ground I get 800ohms.

[Station X]

This set is far from being a scrapper, you haven't located the fault yet.

I have to check that measurement again but the bottom line is with the audio tranny removed all of the plates are perfect at 67vdc.

Look at the circuit extract which I've attached to this post. B+ voltage is applied directly to the screen grid on pin 4. To reach the plate the path for the voltage goes along the red lead, through the primary winding of the output transformer, along the blue lead and thence to the plate on pins 2 and 6.

You've said that with the output transformer taken out there's 67VDC on the plate. I do not see how this can be so. I suggest that with the output valve removed you check the voltages on all the pins of the output valve holder except pins 1, 5 and 7 which are the heater. Post the results here for analysis.

if I lift the red transformer wire and measure to "common" ground I get 800ohms.

It's not clear whether you're measuring the resistance on the red lead or on the tag it was removed from. If the latter, it's possible that the output transformer has developed a short-circuit from the primary winding to the laminations and thence via the fixing screws to the chassis. You can check for this by disconnecting the red and blue wires from the primary winding, unbolting the transformer from the chassis and measuring the resistance from the primary winding tags to the transformer's frame.

If however you're measuring the resistance on the disconnected red wire then the fault is elsewhere. This resistance has appeared before during testing with values of 700R, 5000R and now 500R. The only way to trace it is to progressively disconnect components from the B+ line and note when the resistance (measured at the power supply for example across the 20uF capacitor) increases to infinity.

[daveo23]

When I was doing your checks I think I found a problem. A hidden wax cap connected to pin 2 of the 3S4 to common. It measures 200ohms across it. BAD. This could be the issue.
as to your tests on 3S4: pin 2 anode=13V, pin 3 control grid=9.5V, pin 4 screen grid=22V. That's with the tube out.

[Station X]

Is that 200R across the capacitor with the capacitor disconnected? If it's still connected you'll be measuring everything connected to the B+ supply.

[ms660]

9.5 volts on pin 3 is also bad, should be 0 volts, I would first check C15 for that.

The audio output valve bias on battery is developed across R22 by the HT current flowing through it and on mains by the voltage developed across part of the filament chain, on battery the filaments are connected in a combination of parallel/series and on mains they are connected in series only.

Lawrence.

[daveo23]

I replaced it with a .0068 400v and the voltage is there and the radio buzzes. No signal though. Could that be 1. because the case is open with no shields or 2. I replaced r-16 with two 2.2meg resistors (didnt have a 4.7m) and now it needs an alignment?

I am getting close!!!

[ms660]

With the 3S4 either in or out there should be no DC voltage between pin 3 of its valve socket and ground except perhaps a few mV.

Lawrence.

[Station X]

I think a new set of voltage readings from the output valve would be useful.

[daveo23]

yea, for some reason its really messed up. pin 3 is 5v now which is wrong and there is a factory 270 ohm resistor that goes from 3 to 7. Thats way off the schematic. This radio was working a few days ago. I wonder if the schematics are wrong or different version.

[ms660]

The receiver would never have worked with a 270 Ohm connected between pin 3 and pin 7 of the 3S4, in the schematic a 270 Ohm resistor is connected across one half of the 3S4's filament, it's there to shunt the valves anode and screen current that flows through the other half of the filament, maybe check again.

Lawrence.

[Station X]

Lawrence has beaten me to it.

I hesitate to ask, but are you counting the valve pins correctly?

[daveo23]

I know it sounds crazy but proof is in the pudding. Blue wire is tranny to pin #2, red is tranny to pin #4, guess whats in between. I did not add this 270r, its factory.

[Herald1360]

The pins underneath run from 1-7 clockwise with 1 and 7 adjacent to the gap. That would put red on pin 4 and blue on pin 6 which is fine. The 270R is where it should be, between 5 and 1.

[Station X]

Well spotted Trevor! It looks like the OP was numbering the pins anti-clockwise.

Unfortunately incorrect pin numbering throws doubt on earlier test results.

A new set of voltage readings for the 3S4 output valve is needed.

[daveo23]

I thought that at first but pin #2 has nothing connected to it yet the instructions give resistance readings for it. Sorry for the confusion, will get new voltage readings.

[ms660]

Originally Posted by daveo23

I thought that at first but pin #2 has nothing connected to it yet the instructions give resistance readings for it.

Pins 2 an 6 are both anode connections, they have used pin 6 to connect the transformer to, the voltage and resistance tables show that the voltage and resistance figures for pins 2 and 6 are exactly the same which is what would be expected.

The valve socket orientation and pin numbers are as shown in the under chassis view in the service info that gives the voltage and resistance figures.

Lawrence.

[daveo23]

So now that I am going the right way I checked the voltages on the 3s4 tube. Will check others today.
1 filament=4.6vdc
2 anode=66vdc
3 Control grid=0 some mv but I think its my dvm
4 screen grid=67vdc
5 suppressor grid =5vdc
6 anode=66vdc
7 filament=6vdc

Could I not be receiving any signals because the radio is not shielded now?

[Station X]

Looks better. Is that tube in or tube out?