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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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18th Oct 2020, 10:52 pm | #21 |
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Re: Motorola 5A7A dead.
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?
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18th Oct 2020, 11:29 pm | #22 |
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Re: Motorola 5A7A dead.
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.
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19th Oct 2020, 2:00 am | #23 |
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Re: Motorola 5A7A dead.
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.
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19th Oct 2020, 9:24 am | #24 | ||
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Re: Motorola 5A7A dead.
This set is far from being a scrapper, you haven't located the fault yet.
Quote:
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. Quote:
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.
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19th Oct 2020, 3:35 pm | #25 |
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Re: Motorola 5A7A dead.
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. |
19th Oct 2020, 4:41 pm | #26 |
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Re: Motorola 5A7A dead.
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.
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19th Oct 2020, 4:42 pm | #27 |
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Re: Motorola 5A7A dead.
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. |
19th Oct 2020, 6:10 pm | #28 |
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Re: Motorola 5A7A dead.
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?
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19th Oct 2020, 6:26 pm | #29 |
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Re: Motorola 5A7A dead.
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.
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19th Oct 2020, 6:46 pm | #30 |
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Re: Motorola 5A7A dead.
I think a new set of voltage readings from the output valve would be useful.
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19th Oct 2020, 7:21 pm | #31 |
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Re: Motorola 5A7A dead.
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.
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19th Oct 2020, 7:34 pm | #32 |
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Re: Motorola 5A7A dead.
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.
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19th Oct 2020, 7:37 pm | #33 |
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Re: Motorola 5A7A dead.
Lawrence has beaten me to it.
I hesitate to ask, but are you counting the valve pins correctly?
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19th Oct 2020, 10:54 pm | #34 |
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Re: Motorola 5A7A dead.
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.
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20th Oct 2020, 1:13 am | #35 |
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Re: Motorola 5A7A dead.
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.
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20th Oct 2020, 9:29 am | #36 |
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Re: Motorola 5A7A dead.
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.
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20th Oct 2020, 11:12 am | #37 |
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Re: Motorola 5A7A dead.
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.
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20th Oct 2020, 11:30 am | #38 | |
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Re: Motorola 5A7A dead.
Quote:
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. Last edited by ms660; 20th Oct 2020 at 11:37 am. Reason: Extra info |
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20th Oct 2020, 2:36 pm | #39 |
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Re: Motorola 5A7A dead.
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? Last edited by Station X; 20th Oct 2020 at 2:47 pm. Reason: Electrode names added. |
20th Oct 2020, 2:48 pm | #40 |
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Re: Motorola 5A7A dead.
Looks better. Is that tube in or tube out?
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