SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Radio Telephone and Telegraph Transmitting and Receiving Equipment
Radtech
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Re: SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Post by Radtech » Sun Jan 07, 2018 5:04 pm

If you run a jumper from pin 20 in the transmitter to the tip of a 1000 ohm headset plug and ground the shaft you should be able to hear any side tone from the transmitter in interphone mode.Make sure you have the correct plate voltage on V105 and V106.
Last edited by Radtech on Sun Jan 07, 2018 7:31 pm, edited 2 times in total.


Big D
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Re: SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Post by Big D » Sun Jan 07, 2018 5:41 pm

Thanks for the continued support Radtech!

Talk about a rollercoaster of multiple problems. As you say, this latest fault is in the FT-237. The mic keying on the BC-604 works fine when I remove it from the mount.

I will see what I have here to try that latest check you suggest. I have some HS-30's here. Not sure if they will do it. I will need to check.
Darryl Lennane
NZ

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Radtech
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Re: SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Post by Radtech » Sun Jan 07, 2018 7:24 pm

Upon further research,in the interphone mode,the dynamotor runs constantly which is normal.When the transmitter and receiver are mounted in the FT-237 when in the interphone mode the receiver is disabled and only the side tone of the transmitter is heard in the receiver speaker.It has been nearly 4 years sense I last worked on one of these.An HS30 will work to test the audio output of the transmitter in interphone mode.

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Re: SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Post by Big D » Sun Jan 07, 2018 8:52 pm

Hi Radtech,

It’s all good. Thanks for continuing to keep me supplied with ideas. It is more progress so I am happy.

I have had the BC-604 back in the FT-237 tray. I plugged the HS-30 headphones into the socket on the BC-603 I also had on the tray. I had a CD-604 high impedance adaptor and a CD-307 extension cord connected to the headphones.

When keying the microphone I could hear my voice in the headphones. The background noise was very loud, especially as the microphone was keyed and when the voice came through the voice quality was not great, but it was working.

Is this type of setup sufficient for testing? I had the sidetone control on the BC-604 to maximum so I will back that off a bit and try it again.

Testing chews up the battery pretty quick so I have to keep test sessions fairly short.
Darryl Lennane
NZ

1943 Willys MB
1941 LP2A MG Carrier
1943 White M3A1 AOP
1942 Willys MBT
1944 Ford M8 Armoured Car
1945 Ford M20 Armoured Car

Radtech
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Re: SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Post by Radtech » Mon Jan 08, 2018 2:35 pm

Is your mic good?A lot of times a poor quality mic is the problem.The T17 is a carbon mic and can go bad with age.Also if you have new in the box 1619 tubes to swap V105 and V106 with new tubes and see if audio gets better.The set should pull only 12 amps in interphone mode.And 22 amps in transmit mode.You need to use a good car battery or 30 amp power supply.

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Re: SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Post by Big D » Mon Jan 08, 2018 8:44 pm

Hi Radtech,

Thanks for that. I guess the microphone might not be helping. I see there is the odd NOS one around so I will check them out.

I tried the setup again today with the interphone level turned down a bit. I felt the sound was a bit better this time and I gave the audio tubes a bit more time to warm up and the sound got better as they warmed.

I tried the only other spare tube I had here first in V106. The filament test was okay on it was okay but I don't know if it is a good tube as such. There was no real change in the audio quality with this tube in there.

I swapped the tube I had taken from V106 into V105. At about this time I noticed that I was back with the situation where the dynamotor wasn't running all the time in the interphone position. I now need to key the microphone again to get it to run. I swapped the tubes back between V105 and V106 including the tube I took out but no change.

The interphone still works but because the dynamotor only runs now when you key the microphone (in the interphone position) it takes a couple of seconds for the audio to come in.

I took the set out of the FT-237 and tried it again with the same result.

I'm guessing that maybe I have a faulty tube in their now and this is causing it (unless I am missing something). The only thing I noted is that tube V106 feels warm to the touch (not hot) but V105 feels cold. I hadn't felt these before so I don't know whether that is normal or not.

I called it quits at that point so haven't done any measurements or anything.

I will get my hands on a few new tubes that have been tested as spares. I see there are plenty around for sale and it can't hurt to have a few.

Sorry Radtech - you will be regretting answering my call for help!
Darryl Lennane
NZ

1943 Willys MB
1941 LP2A MG Carrier
1943 White M3A1 AOP
1942 Willys MBT
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1945 Ford M20 Armoured Car

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Re: SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Post by Radtech » Tue Jan 09, 2018 9:38 am

I think the problems are in the S 104 interphone switch.If the set is working properly all tubes should be very warm except for the PA tube as it is not activated until transmit mode.

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Re: SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Post by Big D » Tue Jan 09, 2018 10:01 am

Hi Radtech

Thanks for that. I don't see any easy way to get to the contacts to clean them so I'm not sure where I will go with that.

I will run a few more tests tonight and see if all tubes are getting warm.

I meant to say that while testing the side tone I noticed a lot of dynamotor noise in the headset. Are there filtration capacitors that are supposed to dampen this noise down?
Darryl Lennane
NZ

1943 Willys MB
1941 LP2A MG Carrier
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1944 Ford M8 Armoured Car
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Radtech
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Re: SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Post by Radtech » Wed Jan 10, 2018 3:57 pm

The filter caps are on the end of the HV side of the dynamotor.

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Re: SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Post by Big D » Thu Jan 11, 2018 10:56 am

Hi Radtech,

Thanks. I will pick up a couple of polyester capacitors of the same value and try them and see if there is any difference.

There are plenty of the vacuum tubes around so I will order some that have been tested from the US.
Darryl Lennane
NZ

1943 Willys MB
1941 LP2A MG Carrier
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Radtech
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Re: SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Post by Radtech » Thu Jan 11, 2018 7:32 pm

Also check the brushes in the dynamotor for excessive arcing.

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Re: SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Post by Big D » Thu Jan 11, 2018 7:40 pm

Hi Radtech,

Thanks. It is interesting you say that as I have just had one of the brushes on the HV side out. It is arcing a bit, whereas the other side has no arcing. The brush is in good condition and slides freely. The commutator is nice and shiny. Is it just a case of me needing to reseat that brush with a bit of very fine sandpaper? Say 400 grit or 600 grit?
Darryl Lennane
NZ

1943 Willys MB
1941 LP2A MG Carrier
1943 White M3A1 AOP
1942 Willys MBT
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Radtech
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Re: SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Post by Radtech » Fri Jan 12, 2018 8:18 am

You can reface the brushes with 220 grit sand paper and a small carpenters square.If you do this do both brushes.This must be done with a square other wise they could be faced uneven and you will have arcing.

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Re: SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Post by Big D » Sun Jan 14, 2018 12:06 am

Hi Radtech,

Thanks for that. I cleaned up the brushes today and that has improved the arcing. Both brushes now arc for barely a split second and then the arcing stops. There is still dynamotor noise in the earphones though. It comes through as a loud whine, and changes pitch as the dynamotor speeds up and slows down.

I will order some new ceramic capacitors (if that is the best option to replace the Mica ones?) and replace those on the HV end of the dynamotor and see if that improves things.

I have ordered a replacement microphone that is known to work well. I am not confident that the one I have here is working correctly. I have also ordered some replacement tested tubes just in case there is a problem with one of the audio tubes. These will take a few weeks to get here so it might be a while before my next update.

I mentioned the temperature of V106, the 2nd audio tube. During long testing with the microphone and the dynamotor running, this tube gets very hot. All the other tubes in the set are what I'd describe as lukewarm, but V106 is hot enough that you would only leave your fingers on it for a couple of seconds. I'm not sure if that is normal during audio operation, or whether I should be investigating that.

Many thanks.
Darryl Lennane
NZ

1943 Willys MB
1941 LP2A MG Carrier
1943 White M3A1 AOP
1942 Willys MBT
1944 Ford M8 Armoured Car
1945 Ford M20 Armoured Car

Radtech
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Re: SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Post by Radtech » Mon Jan 15, 2018 9:21 am

If the set is working properly the tubes should be hot enough so you can't touch them.Some will be warmer than others.Metal tubes run hotter than glass tubes.
Last edited by Radtech on Wed Jan 17, 2018 2:42 pm, edited 1 time in total.


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