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 Post subject: Motorola 5C1 Hum and No Stations
PostPosted: Apr Wed 08, 2020 6:42 am 
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Posts: 134
Location: Round Rock, TX
I've been working on a Motorola 5C1 and fixed a number of issues. Now the radio has a hum that is not heard at low volume but increases steadily with increased volume. It does not pick up stations at all. The radio has been recapped and out of spec resistors replaced. I've checked that the local oscillator is working properly. When alignment is done, the radio doesnt play the 400 hz tone, but loud motorboating. The motorboating increases or descreases when adjusting the if coils, antenna, and oscillator on the tuner. I've gone over every component I've replaced and dont see anything out of the ordinary, no shorts etc. Has anyone had this issue before?


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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Wed 08, 2020 8:17 am 
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Location: SoCal, 91387
What method did you use to check the osc? No station reception would seem to point to it as the primary culprit. Does it have an air loop, or do you have a line connected to it as an antenna?

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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Wed 08, 2020 2:43 pm 
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Joined: Mar Fri 23, 2018 2:12 am
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Location: Round Rock, TX
fifties wrote:
What method did you use to check the osc? No station reception would seem to point to it as the primary culprit. Does it have an air loop, or do you have a line connected to it as an antenna?

I used an oscilloscope to look at the sine wave and warched the frequency change as i manipulated the tuner. I have the loop antenna that came with the radio attached. The antenna picked up the signal when placing a loop next to it when aligning.


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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Wed 08, 2020 5:08 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3619
Location: Lexington, KY USA
Use your oscilloscope to find where the hum begins, and where the signal is lost. Just as you would use a signal tracer.

You might check the chassis against the schematic. Just takes one error to make a radio not work. Use an ohmmeter to check connections and resistances as you go. Be sure all coils and windings have continuity.

Did the radio play before you began work on it?

Does this set have the power switch on the back of the volume control? Has the wiring and lead dress around this area been changed in any way?

Ted


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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Wed 08, 2020 9:28 pm 
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I would suggest checking osc function by a tried and true method.


Attachments:
Second Radio Test.Jpeg
Second Radio Test.Jpeg [ 110.74 KiB | Viewed 515 times ]

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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Wed 08, 2020 9:44 pm 
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Joined: Mar Fri 23, 2018 2:12 am
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Location: Round Rock, TX
fifties wrote:
I would suggest checking osc function by a tried and true method.

Ok I tested and the oscillator works just fine. I could see that it was working with the oscilloscope, this just justified my conclusion. Next is to use the oscilloscope stage by stage to find which section this hum is coming from as Usually Lurking has suggested. This sounds like 60 hz or a harmonic of it. Not that noticeable at low volume but increases dramatically with volume increase.


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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Wed 08, 2020 10:34 pm 
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reubanks1 wrote:
It does not pick up stations at all.

When alignment is done, the radio doesnt play the 400 hz tone, but loud motorboating.

I would suggest that you tackle the issue of no station reception to begin with, and then worry about hum.
The IF frequency is normally 455 kilocycles, so that's what you want to adjust the IF filters to. The osc coil, if adjustable, should be done at 600 Kc. Perhaps do these and report your findings.

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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Thu 09, 2020 7:34 am 
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Joined: Mar Fri 23, 2018 2:12 am
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Location: Round Rock, TX
Its late and I will tackle this again tomorrow. I did start measuring with the scope starting with the input to the converter from the LO. Nice sine wave going in. Now.. the output at pin 5 going to the 1st IF is a 60Hz signal (hence the hum). I exchanged the tube, but same result. Sleepy and too tired to look at it anymore tonight. I cant for the life of me figure out where that signal is coming from.


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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Thu 09, 2020 8:33 am 
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Use your meter to see if there's a negative voltage on the control grid of the converter. If not, that's the start of the problem of no reception.

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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Thu 09, 2020 8:45 am 
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Joined: Apr Thu 09, 2020 7:33 am
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I had a Crosley E15SL that had a cathode to heater short on the IF tube 12BA6. It sounded like 60 cycle ac and changed with volume. It was putting 60 cycle ac on the avc


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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Thu 09, 2020 6:27 pm 
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Joined: Mar Fri 23, 2018 2:12 am
Posts: 134
Location: Round Rock, TX
fifties wrote:
Use your meter to see if there's a negative voltage on the control grid of the converter. If not, that's the start of the problem of no reception.

All voltages are low. There is a negative 12 v on the control grid, should be -15. The voltage coming off the output pin 7 of the 35W4 is 121, should be 130. I have changed out the 35W4 and the voltages remain low. I also checked for shorts to the filaments and there are none.


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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Thu 09, 2020 6:28 pm 
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Joined: Mar Fri 23, 2018 2:12 am
Posts: 134
Location: Round Rock, TX
fifties wrote:
reubanks1 wrote:
It does not pick up stations at all.

When alignment is done, the radio doesnt play the 400 hz tone, but loud motorboating.

I would suggest that you tackle the issue of no station reception to begin with, and then worry about hum.
The IF frequency is normally 455 kilocycles, so that's what you want to adjust the IF filters to. The osc coil, if adjustable, should be done at 600 Kc. Perhaps do these and report your findings.

The IF is 455Khz and the LO is working normally.


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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Thu 09, 2020 11:29 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3619
Location: Lexington, KY USA
The voltages reported above are close enough for the radio to play.

An urgent concern is that voltages might be low because something is drawing too much current. Is anything getting hot that shouldn't?

To be safe, can you check the AC line current, and the B+ current?

Note that a radio must be working before an alignment can be done. Attempts at alignment of a non-functional radio can just add more reasons the radio does not work.

That said, some alignment steps can be an aid to troubleshooting.

Otherwise, sounds like a straightforward signal tracing job.

Ted


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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Fri 10, 2020 12:54 am 
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Joined: Mar Fri 23, 2018 2:12 am
Posts: 134
Location: Round Rock, TX
Usually Lurking wrote:
The voltages reported above are close enough for the radio to play.

An urgent concern is that voltages might be low because something is drawing too much current. Is anything getting hot that shouldn't?

To be safe, can you check the AC line current, and the B+ current?

Note that a radio must be working before an alignment can be done. Attempts at alignment of a non-functional radio can just add more reasons the radio does not work.

That said, some alignment steps can be an aid to troubleshooting.

Otherwise, sounds like a straightforward signal tracing job.

Ted

I've tried that and I've gotten nowhere. The hum overwhelms any other signal coming in to the various stages. This hum starts right at the converter and just travels through to output at the speaker. Nothing is getting hot at all. I know there has to be some sort of shorting to get the 60 hz coming through but cannot find it.


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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Fri 10, 2020 1:10 am 
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Location: Cromwell, Connecticut
A few things come to mind.

I did not see mention of tubes being tested or swapped. As mentioned before, a tube with heater to cathode leakage can cause hum.

I may have missed the Schematic, but does the radio call for metal tubes and there are glass ones in their place? Pin 1 on the sockets should be grounded if a metal tube is being used.

This may seem off the wall, but could there be a tube or tubes in the wrong socket?

Not trying to be rude here, but I have seen this happen :oops: Don’t ask.

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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Fri 10, 2020 2:27 am 
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Joined: Mar Fri 23, 2018 2:12 am
Posts: 134
Location: Round Rock, TX
Tbone wrote:
A few things come to mind.

I did not see mention of tubes being tested or swapped. As mentioned before, a tube with heater to cathode leakage can cause hum.

I may have missed the Schematic, but does the radio call for metal tubes and there are glass ones in their place? Pin 1 on the sockets should be grounded if a metal tube is being used.

This may seem off the wall, but could there be a tube or tubes in the wrong socket?

Not trying to be rude here, but I have seen this happen :oops: Don’t ask.

Hi, yes I've swapped out every tube one by one and no change. No metal tubes, just the normal AA5 tubeset. Below is a copy of the schematic


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5C1.PNG
5C1.PNG [ 396.48 KiB | Viewed 402 times ]
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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Fri 10, 2020 2:45 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3619
Location: Lexington, KY USA
Thanks for posting a schematic!

More stuff to check:

Have you checked all the coils and windings for continuity?

What's the resistance between pin 7 of the 12BE6 and pin 1 of the 12BA6, for instance?

Howabout the resistance between pin 7 of the 12BE6 and B minus?

Where is your scope ground connected?

How much ripple is there on the B+ supply to the signal tubes?

What is your isolation transformer / test equipment grounds hookup like?

Ted


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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Fri 10, 2020 2:55 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 34853
Location: SoCal, 91387
reubanks1 wrote:
I've been working on a Motorola 5C1 and fixed a number of issues. Now the radio has a hum that is not heard at low volume but increases steadily with increased volume.

Did the set have the hum before you worked on it?

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\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\He Who Dies With The Most Radios Wins/////////////////////////


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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Fri 10, 2020 3:26 am 
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Joined: Mar Fri 23, 2018 2:12 am
Posts: 134
Location: Round Rock, TX
fifties wrote:
reubanks1 wrote:
I've been working on a Motorola 5C1 and fixed a number of issues. Now the radio has a hum that is not heard at low volume but increases steadily with increased volume.

Did the set have the hum before you worked on it?

Yes, it had a loud pervasive hum that did not change with volume increase. That went away after putting in new filter caps. The hum now apears at the mid to upper volume level and increases with volume.


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 Post subject: Re: Motorola 5C1 Hum and No Stations
PostPosted: Apr Fri 10, 2020 3:31 am 
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Joined: Mar Fri 23, 2018 2:12 am
Posts: 134
Location: Round Rock, TX
Usually Lurking wrote:
Thanks for posting a schematic!

More stuff to check:

Have you checked all the coils and windings for continuity?

What's the resistance between pin 7 of the 12BE6 and pin 1 of the 12BA6, for instance?

Howabout the resistance between pin 7 of the 12BE6 and B minus?

Where is your scope ground connected?

How much ripple is there on the B+ supply to the signal tubes?

What is your isolation transformer / test equipment grounds hookup like?

Ted

I have checked continuity prior to working on the radio. I'll check that again in case something I did changed that. I'll also check resistances and reply back with the results. The scope is plugged into house outlet. The radio is plugged into an isolation transformer that is plugged into a variac. As far as ripple it is in the mV range. I checked the ripple prior to the filter caps and also checked it after the filter caps. I've changed out the 35W4 twice just in case but no change in results.


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