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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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20th Oct 2020, 2:59 pm | #41 | |
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Re: Motorola 5A7A dead.
Quote:
Err.... Trevor?
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20th Oct 2020, 3:00 pm | #42 |
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Re: Motorola 5A7A dead.
tube in
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20th Oct 2020, 3:05 pm | #43 |
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Re: Motorola 5A7A dead.
The filament supply voltage looks too low.
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20th Oct 2020, 3:18 pm | #44 |
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Re: Motorola 5A7A dead.
Here is the 1R5, looks bad, - = some mv
1 Filament = - 2 Anode =66vdc 3 Screen grids =44vdc 4 Grid of triode = - 5 Filament = - 6 Grid 3= .6vdc 7=Filament =.9vdc |
20th Oct 2020, 3:52 pm | #45 |
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Re: Motorola 5A7A dead.
Wot no filament voltage? The voltage on the screen grid suggests the tube is passing current though.
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20th Oct 2020, 4:31 pm | #46 |
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Re: Motorola 5A7A dead.
I wiggled the tube around and was able to get .9vdc but thats still too low, right
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20th Oct 2020, 5:04 pm | #47 |
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Re: Motorola 5A7A dead.
Check the voltage between pins 1 and 7.
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20th Oct 2020, 6:00 pm | #48 |
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Re: Motorola 5A7A dead.
I checked all the tube filaments, 1&7, same at .9vdc so I looked at the voltage at the end of r-6(double 1000ohm res) it was 6vdc.
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20th Oct 2020, 9:34 pm | #49 |
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Re: Motorola 5A7A dead.
Before tackling the heater problem I'd check the voltages on the other two valves to make sure plate and screen grid voltages are present. Let's not forget that the original problem was the the B+ voltage was being pulled down.
If the 1S5 (audio amplifier) and 3S4 (output tube) are working correctly you should hear a buzz or click from the speaker if you touch the centre tag (wiper) of the volume control with the metal blade of a screwdriver. Don't forget that the service manual linked to in post #3 gives resistance values from every tube pin to chassis. It would be useful to check these.
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20th Oct 2020, 10:56 pm | #50 |
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Re: Motorola 5A7A dead.
That screw driver trick worked..bzzzz
Now the crazy question about the 1s5, how can you have 2 different voltages for pin 1 and 2 when they are connected? Here are the 1u4 and 1s5 voltages: 1U4 1 Filament=4 2 Anode =67 3 Screen grid =67 4 = .6 5 Internal connection=4.1 6 Grid 1=1.3 7 Filament =3.2 1S5: 1 Filament=.9 2 NC =.9 3 Anode of diode=.7 4 Screen grid =20 5 Anode=20 6 Grid 1=.4 7 Filament=1.8 |
20th Oct 2020, 11:41 pm | #51 |
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Re: Motorola 5A7A dead.
Pin 2 isn't connected to anything inside the tube. It's connected to pin 1 externally to provide a convenient termination point. US terminology is Tie Lug or TL.
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21st Oct 2020, 1:14 am | #52 |
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Re: Motorola 5A7A dead.
Look at the sams photofacts for the 1s5 and you can see what iam talking about
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21st Oct 2020, 9:06 am | #53 |
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Re: Motorola 5A7A dead.
There's probably been a production change at some and the routing of wires has been changed. There are many different ways of connecting a number of points together.
The bottom line is that the set worked before with pins 1 and 2 linked together, so it should work now barring any other faults. The main problem at the moment is that filament voltage is low at 0.9V when it should be 1.4V. There are several resistors in the filament circuit which could be causing the problem. I would suggest taking resistance readings on pins 1 and 7 of all valves at the very least and comparing them with the service sheet. Note that there are two sets of readings, one for the set switched to mains supply and one with the set switched to battery supply.
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21st Oct 2020, 1:20 pm | #54 |
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Re: Motorola 5A7A dead.
Given that you only have 6V on pin 7 of the 3S4 when you should have 7.9V, a likely scenario is that R17 1000R + !000R or R4 150R have gone high in value.
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21st Oct 2020, 1:42 pm | #55 |
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Re: Motorola 5A7A dead.
Before taking the filament resistance, pull the tube right? The directions say not to measure the filaments with an ohm meter.
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21st Oct 2020, 1:52 pm | #56 |
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Re: Motorola 5A7A dead.
Yes pull the tubes.
I'd suggest moving straight on to checking R17 and R4 though.
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21st Oct 2020, 2:36 pm | #57 |
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Re: Motorola 5A7A dead.
r-4=184ohms
r-17=1meg r-8=980 each side resistance on 1r5 looks good except for pin 6=0 1r5 resistance 1=0 2=5k 3=29k 4=106k 5=0 6=0 7=9 |
21st Oct 2020, 2:49 pm | #58 |
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Re: Motorola 5A7A dead.
0 what? If it's ohms the pin is short-circuit to ground.
There are a whole load of resistors in series from 1R5 pin 6 to ground ie R1, R10, R12, R13 and R14. Their total value is about 9 Mohms.
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21st Oct 2020, 3:05 pm | #59 |
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Re: Motorola 5A7A dead.
The valve filaments should draw 50mA which will give a volt drop of 98V across R8 (1960R). There'll also be a volt drop in R4 (184R) of 9.2V. In practice the volt drop will be a bit more as R4 also carries the B+ current. Even so with 120V mains there should still be enough voltage for the heaters.
Check the volt drop across R8 and the voltage wrt ground at the junction of R4 and R8.
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21st Oct 2020, 3:36 pm | #60 |
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Re: Motorola 5A7A dead.
The current flowing through R4 would be typically at least twice the value of the total load current.
Lawrence. |