This radio loses voltage as it runs. Starts out with 6 VDC going in, drops to about 2.5 volts, warms up and runs about 4.8 volts, and then drops down to under 3 v and the audio goes away gradually under 2.75v. Takes about 5 minutes to drop off. Any suggestions. This model has a spark plate where the 6V enters. Thanks

Do you mean that the radio is dragging down the power supply as it runs? What are you using to supply the power for this radio? Can you measure the current as the radio warms up and plays? By the way, there are two radios listed under Motorola 400 in the Resources section. The old one has the only 0Z4 I have ever seen with a filament inside it, connected to six Volts.
Lewis

This radio loses voltage as it runs. Starts out with 6 VDC going in, drops to about 2.5 volts, warms up and runs about 4.8 volts, and then drops down to under 3 v and the audio goes away gradually under 2.75v. Takes about 5 minutes to drop off. Any suggestions. This model has a spark plate where the 6V enters. Thanks

I am using an ARBE battery eliminator to supply a constant 6 v to the set. And yet it drags down the voltage over a few minutes to the point it has no audio. The Motorola 400 set is the one with the 6X4 rectifier. I have put in a solid state vibrator from AES, lots of capacitors and still drags down. Thanks.
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As a starter , would you try this analysis. Specifically, to see if the problem might be audio output / circuit related.
relevant to the potential other options of power supply or DC leakage therein related. It will be dependent upon the disabling of the output tubes so you will no longer be able to hear the audio.(However, if you have a scope you might watch it upon the plate circuitry of the AF driver stage, should you be feeding in a stable 400 / 1000 ~ modulated signal from a generator...or even AC meter it..in case of the later stable modulated sourcing).
Initilly, trace down your centertap of the output transformer and tack in a 10k 10 watt wirewound to ground from that buss line and then unplug your two AF output tubes and have your metering conected to that B+ point and fire up the set and then clip on and take a B+ DC reading just as soon as the voltage initially stabilizes. Theeeeeeen twidddle your thumbs until "THE" prescribed time approaches, and see if the voltage has / had started dropping. [Also sneak a peek at the your Battery eliminator Power units ammeter, if it has current metering, both at power start and onset of the trouble..if it even occurs this time] This will then differentiate that the prime problem was not related to a gradiated bias shift in the current hungry AF output stage.
We are now waiting for your findings.

73's de Edd

:As a starter , would you try this analysis. Specifically, to see if the problem might be audio output / circuit related.
:relevant to the potential other options of power supply or DC leakage therein related. It will be dependent upon the disabling of the output tubes so you will no longer be able to hear the audio.(However, if you have a scope you might watch it upon the plate circuitry of the AF driver stage, should you be feeding in a stable 400 / 1000 ~ modulated signal from a generator...or even AC meter it..in case of the later stable modulated sourcing).
:Initilly, trace down your centertap of the output transformer and tack in a 10k 10 watt wirewound to ground from that buss line and then unplug your two AF output tubes and have your metering conected to that B+ point and fire up the set and then clip on and take a B+ DC reading just as soon as the voltage initially stabilizes. Theeeeeeen twidddle your thumbs until "THE" prescribed time approaches, and see if the voltage has / had started dropping. [Also sneak a peek at the your Battery eliminator Power units ammeter, if it has current metering, both at power start and onset of the trouble..if it even occurs this time] This will then differentiate that the prime problem was not related to a gradiated bias shift in the current hungry AF output stage.
:We are now waiting for your findings.
:
:73's de Edd

I think that the only reason for the battery eleminator Voltage drop could only be caused by excessive current draw. Excessive current means something is getting hot. I am suspecting a filter capacitor if you haven't changed them yet, and if they are getting hot from leakage, they can build up a dangerous amount of pressure, and give one a face full of hot electrolyte. If you have recapped, check again for proper polarity. A capacitor installed backward will conduct electricity, and act just like a leaky one. Are resistors getting too hot? Remove your vibrator and let just the filaments run for a while, and see what happens. There is a buffer condenser across the secondary of the power transformer, like a 1.2 KV fella. Check this guy, or better replace with a new one. Do you have the old vibrator? Try it and see what happens. These are just some random thoughts, things I would try if I were fixing the radio. Sometimes you fix radios just trying off-the-wall ideas, just doing something and seeing something happen that shouldn't happen.

Lewis

::As a starter , would you try this analysis. Specifically, to see if the problem might be audio output / circuit related.
::relevant to the potential other options of power supply or DC leakage therein related. It will be dependent upon the disabling of the output tubes so you will no longer be able to hear the audio.(However, if you have a scope you might watch it upon the plate circuitry of the AF driver stage, should you be feeding in a stable 400 / 1000 ~ modulated signal from a generator...or even AC meter it..in case of the later stable modulated sourcing).
::Initilly, trace down your centertap of the output transformer and tack in a 10k 10 watt wirewound to ground from that buss line and then unplug your two AF output tubes and have your metering conected to that B+ point and fire up the set and then clip on and take a B+ DC reading just as soon as the voltage initially stabilizes. Theeeeeeen twidddle your thumbs until "THE" prescribed time approaches, and see if the voltage has / had started dropping. [Also sneak a peek at the your Battery eliminator Power units ammeter, if it has current metering, both at power start and onset of the trouble..if it even occurs this time] This will then differentiate that the prime problem was not related to a gradiated bias shift in the current hungry AF output stage.
::We are now waiting for your findings.
::
::73's de Edd
:
:
:
:I think that the only reason for the battery eleminator Voltage drop could only be caused by excessive current draw. Excessive current means something is getting hot. I am suspecting a filter capacitor if you haven't changed them yet, and if they are getting hot from leakage, they can build up a dangerous amount of pressure, and give one a face full of hot electrolyte. If you have recapped, check again for proper polarity. A capacitor installed backward will conduct electricity, and act just like a leaky one. Are resistors getting too hot? Remove your vibrator and let just the filaments run for a while, and see what happens. There is a buffer condenser across the secondary of the power transformer, like a 1.2 KV fella. Check this guy, or better replace with a new one. Do you have the old vibrator? Try it and see what happens. These are just some random thoughts, things I would try if I were fixing the radio. Sometimes you fix radios just trying off-the-wall ideas, just doing something and seeing something happen that shouldn't happen.
:
:Lewis

Guys,

Are we sure the ARBE eliminator can supply enough current to properly operate the radio. If the ARBE III is being used it is rated 1.25 - 6.5 volts at 3A. The schematic shows the radio being fused at 10A. I would suspect the radio draws some measure over 3A possibly shuting down the output of the eliminator from overload.

Radiodoc

:::As a starter , would you try this analysis. Specifically, to see if the problem might be audio output / circuit related.
:::relevant to the potential other options of power supply or DC leakage therein related. It will be dependent upon the disabling of the output tubes so you will no longer be able to hear the audio.(However, if you have a scope you might watch it upon the plate circuitry of the AF driver stage, should you be feeding in a stable 400 / 1000 ~ modulated signal from a generator...or even AC meter it..in case of the later stable modulated sourcing).
:::Initilly, trace down your centertap of the output transformer and tack in a 10k 10 watt wirewound to ground from that buss line and then unplug your two AF output tubes and have your metering conected to that B+ point and fire up the set and then clip on and take a B+ DC reading just as soon as the voltage initially stabilizes. Theeeeeeen twidddle your thumbs until "THE" prescribed time approaches, and see if the voltage has / had started dropping. [Also sneak a peek at the your Battery eliminator Power units ammeter, if it has current metering, both at power start and onset of the trouble..if it even occurs this time] This will then differentiate that the prime problem was not related to a gradiated bias shift in the current hungry AF output stage.
:::We are now waiting for your findings.
:::
:::73's de Edd
::
::
::
::I think that the only reason for the battery eleminator Voltage drop could only be caused by excessive current draw. Excessive current means something is getting hot. I am suspecting a filter capacitor if you haven't changed them yet, and if they are getting hot from leakage, they can build up a dangerous amount of pressure, and give one a face full of hot electrolyte. If you have recapped, check again for proper polarity. A capacitor installed backward will conduct electricity, and act just like a leaky one. Are resistors getting too hot? Remove your vibrator and let just the filaments run for a while, and see what happens. There is a buffer condenser across the secondary of the power transformer, like a 1.2 KV fella. Check this guy, or better replace with a new one. Do you have the old vibrator? Try it and see what happens. These are just some random thoughts, things I would try if I were fixing the radio. Sometimes you fix radios just trying off-the-wall ideas, just doing something and seeing something happen that shouldn't happen.
::
::Lewis
:
:Guys,
:
:Are we sure the ARBE eliminator can supply enough current to properly operate the radio. If the ARBE III is being used it is rated 1.25 - 6.5 volts at 3A. The schematic shows the radio being fused at 10A. I would suspect the radio draws some measure over 3A possibly shuting down the output of the eliminator from overload.
:
:Radiodoc

Good idea there, Doc! The delay could be caused by temperature protection of the output transistors in the eliminator. We need to know what is the measured input current of the radio when it is first swithced on and working properly.

Lewis
:

I am back again. I studied the ARBE III battery eliminator, and it was designed to replace batteries in battery operated radios, not replace a car battery. Here's something to try: remove the vibrator and rectifier tube. Connect six volts to the input power lead, and power the filaments only. Connect approximately 200 Volts B+ to pin 8 of the rectifier socket, and the B- to pin 3 or 5 of the rectifier socket. This should give your B- return through the transformer secondary to the center tap, and it can bias the output tubes properly. Your 6 Volt 3Amp A supply should be happy handling the filaments, and the B supply should run the plates OK.The negative sides of the A and B supplies MUST me seperate from each other. The ARBE supply has a LM350 chip for a regulator, and it will lower its output if it starts to overheat, and it will start to overheat at about 3A. These old six Volt radios had peer-lenty of current to operate on, so they made no attempts to build any efficency into them. They had plenty of current available, and they used plenty of current.

Lewis

::::As a starter , would you try this analysis. Specifically, to see if the problem might be audio output / circuit related.
::::relevant to the potential other options of power supply or DC leakage therein related. It will be dependent upon the disabling of the output tubes so you will no longer be able to hear the audio.(However, if you have a scope you might watch it upon the plate circuitry of the AF driver stage, should you be feeding in a stable 400 / 1000 ~ modulated signal from a generator...or even AC meter it..in case of the later stable modulated sourcing).
::::Initilly, trace down your centertap of the output transformer and tack in a 10k 10 watt wirewound to ground from that buss line and then unplug your two AF output tubes and have your metering conected to that B+ point and fire up the set and then clip on and take a B+ DC reading just as soon as the voltage initially stabilizes. Theeeeeeen twidddle your thumbs until "THE" prescribed time approaches, and see if the voltage has / had started dropping. [Also sneak a peek at the your Battery eliminator Power units ammeter, if it has current metering, both at power start and onset of the trouble..if it even occurs this time] This will then differentiate that the prime problem was not related to a gradiated bias shift in the current hungry AF output stage.
::::We are now waiting for your findings.
::::
::::73's de Edd
:::
:::
:::
:::I think that the only reason for the battery eleminator Voltage drop could only be caused by excessive current draw. Excessive current means something is getting hot. I am suspecting a filter capacitor if you haven't changed them yet, and if they are getting hot from leakage, they can build up a dangerous amount of pressure, and give one a face full of hot electrolyte. If you have recapped, check again for proper polarity. A capacitor installed backward will conduct electricity, and act just like a leaky one. Are resistors getting too hot? Remove your vibrator and let just the filaments run for a while, and see what happens. There is a buffer condenser across the secondary of the power transformer, like a 1.2 KV fella. Check this guy, or better replace with a new one. Do you have the old vibrator? Try it and see what happens. These are just some random thoughts, things I would try if I were fixing the radio. Sometimes you fix radios just trying off-the-wall ideas, just doing something and seeing something happen that shouldn't happen.
:::
:::Lewis
::
::Guys,
::
::Are we sure the ARBE eliminator can supply enough current to properly operate the radio. If the ARBE III is being used it is rated 1.25 - 6.5 volts at 3A. The schematic shows the radio being fused at 10A. I would suspect the radio draws some measure over 3A possibly shuting down the output of the eliminator from overload.
::
::Radiodoc
:
:Good idea there, Doc! The delay could be caused by temperature protection of the output transistors in the eliminator. We need to know what is the measured input current of the radio when it is first swithced on and working properly.
:
:Lewis
::

I am going to use a 6v car battery fused to 10 amps and see what happens. A local fellow told me the ARBE 3 does not do well on these old radios as a power supply. Thanks for confirming what he said. Best wishes.

: I am going to use a 6v car battery fused to 10 amps and see what happens. A local fellow told me the ARBE 3 does not do well on these old radios as a power supply. Thanks for confirming what he said. Best wishes.

I think you will be very pleased with the results. As I said, those old car radios were power hungry monsters.

Lewis

Del,

Don't forget to let us know the results you get when you try the radio on a 6 volt car battery.

Radiodoc

:: I am going to use a 6v car battery fused to 10 amps and see what happens. A local fellow told me the ARBE 3 does not do well on these old radios as a power supply. Thanks for confirming what he said. Best wishes.
:
:I think you will be very pleased with the results. As I said, those old car radios were power hungry monsters.
:
:Lewis