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 » Wed Jan 03, 2018 2:50 pm

Voltage on the relay should be 12v when it is pulled in(0n the coil not the contacts) of the relay.Any less then you will not have enough filament voltage.The voltage drop on the dynamotor should not be more that 2 volts with no load.The dynamotor windings should not read more than 13 ohms across the terminals with the brushes removed .The dynamotor should run without any capacitor.The reason it doesn't run without the capacitor is that the capacitor allows the voltage to build up which should not be required.It was used on the very early dynamotors to filter the RF signal to ground.The voltage on the mic pin should stay at 12v.Nine volts is not enough to pull in the dynamotor relay.You should have 2v filament voltage in all tubes but the PA tube as long as the set is turned on.You still have issues with the dynamotor.


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

Post by Big D » Wed Jan 03, 2018 7:02 pm

Hi Radtech,

Thanks. You are a hive of knowledge!

When the relay is pulled in there is 11.67 volts on one side of the relay (the side connected to +C162). I guess this is the side you are referring to?

I measured the dynamotor resistance without the brushes in it and it was 13.9 ohms.

I intend trying this again tomorrow with a better battery. I am just wondering if the battery I’ve been using might not be quite up to it and I want to eliminate any possibility that this could be contributing.
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

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

Post by Big D » Thu Jan 04, 2018 2:35 pm

Hi Radtech,

Okay, good progress today. I first got hold of a second battery which was bigger and newer than what I had. I also made up some beefier cables for connecting to the set.

First test and as soon as I hit the Operate button, the dynamotor fired up. No delay in the relay – a nice clean start. A couple of further tests proved I wasn’t dreaming.

I checked the voltage on the dynamotor relay coil. It started at 12.9 volts and dropped very briefly to 11.5 before coming back up to 12.08 volts. Again, a nice clean pulling up of the relay each time.

At that point I noticed that V104, the power amplifier tube was lit. That was a first.

I tried keying the microphone and initially had no success. I went back through and checked voltages here and all seemed fine. I did the test in grounding Pin 25 on PG101 and it seemed to work fine. I went back to the microphone and played around a bit more and worked out that I had a dodgy wire in the lead which was causing intermittent connections. A repair later and the microphone keying now starts the dynamotor.

I then went back to the dynamotor and removed the capacitor. C703. Success. The dynamotor starts now without the capacitor and the meter is now working.

I’m not sure if the power supply was part of the problem all along, but I had felt that the battery was getting a bit tired toward the end and it couldn't have helped. The cable change has made a difference. Looking at the dynamotor, it does say 18 amps on the side so maybe it was just too much for the cables I was using. Either way, I’m very pleased.

I guess the next step is to check the interphone and radio side of it and see if they do actually work. I hadn’t really planned on fixing the transmitter itself that but I guess I’ve come this far. I'll go through and check all the voltages on the tubes and see if everything is in order.

I’ll also have to work out how I can check the Interphone system, bar connecting some cabling to the FT-237 and a BC606 and trying it all together. I figure this makes sense rather than trying to fault find in the vehicle.

Thanks so much for your help so far Radtech. It looks like progress is being made. I can’t promise there won’t be more questions though. Haha.
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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 » Thu Jan 04, 2018 6:57 pm

With the transmitter and receiver mounted in the FT-237 you can test the transmitter interphone by putting the receiver interphone switch in the interphone position and the transmitter switch on interphone then you should have sidetone from the transmitter in the receiver speaker.NEVER key the transmitter without an antenna or A-62 dummy load attached to the antenna post except in the interphone mode.If you have meter movement on each of the 6 tune switch positions you should be good to go if all of your crystals are good and if the antenna current thermocouple has not been fried.

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

Post by Big D » Fri Jan 05, 2018 2:58 pm

Hi Radtech,

More good information, thanks. Can I confirm what I should be hearing? When you say sidetone, is it literally a tone in the receiver speaker when both are operating and in the interphone position when keying of the microphone (in interphone position) ?
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 » Sat Jan 06, 2018 10:22 am

With a transmitter the term side tone means audio from the microphone.If you speak into the mic you should hear your voice in the BC603 receiver speaker.

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

Post by Big D » Sat Jan 06, 2018 11:47 am

Hi Radtech,

Thanks for that and excuse the newbie question.

Okay, I have tried that and there is no audio in the receiver. I have set the sidetone setting on maximum and no change.TM- 11-310 describes a test using a capacitor and the PL-55 headset plug. I'm not sure about this. I only have the T-17 microphone in the BC-604. Are there other ways to check the audio stage?
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 » Sat Jan 06, 2018 1:06 pm

Check tubes V105 and V106 and the voltage on each pin of each tube.It sounds like the audio amplifier stage is not working.
Have you tested all the tubes?

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

Post by Big D » Sat Jan 06, 2018 3:28 pm

Hi Radtech,

I checked the voltages on V105 and V106 as well as the other tubes. V106 does appear to have a problem.

Here are the voltages I found along with the voltages from the manual in brackets. Note that I only had the set in the Operate and Interphone settings, so the dynamotor wasn't running and there is no high voltage there.

Should the dynamotor run when the microphone is keyed in the Interphone position?

V105 pins:
1 = 0 (0)
2= 0 (2.5)
3 = 0 (220)
4 = 0 (40)
5 = 0 (0)
6 = not measured
7 = 0 (5)
8 = 0 (0)

V106 pins:
1 = 0 (0)
2= 13.17 (12.5)
3 = 0 (370)
4 = 0 (220)
5 = 0 (0)
6 = not measured
7 = 12.9 (10)
8 = 0 (0)


V102 pins:

1 = 0 (0)
2= 8.5 (7.5)
3 = 0 (400)
4 = 0 (50)
5 = 0 (-55)
6 = not measured
7 = 10.5 (10.5)
8 = 0 (0)


V108 pins:

1 = 0 (0)
2= 7.9(7.5)
3 = 0 (400)
4 = 0 (100)
5 = 0 (-40)
6 = not measured
7 = 5.5 (5.5)
8 = 0 (0)

V103 pins:

1 = 0 (0)
2= 2.5 (2.5)
3 = 0 (220)
4 = 0 (40)
5 = -0.05 (-0.5)
6 = not measured
7 = 5 (5.5)
8 = 0 (0)


V104 pins:

1 = 2.6 (2.5)
2= 0 (210)
3 = -0.26 (-50)
4 = 0 (40)
5 = 0 (0)

I'm off to work soon I'll check out the circuit diagram tonight myself when I finish. I have some spare tubes here so I will see what I have got but it appears to me there is a leak somewhere on that line to pin 2 of V106.
Last edited by Big D on Sun Jan 07, 2018 2:52 pm, edited 2 times in total.
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 » Sat Jan 06, 2018 3:55 pm

The dynamotor must run in order for the interphone to work.When you key the mic the dynamotor must start.It sounds like You have 1 or more bad tubes.You need to pull them all and test them.
Last edited by Radtech on Sat Jan 06, 2018 4:12 pm, edited 1 time in total.

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

Post by Big D » Sat Jan 06, 2018 3:58 pm

Hi Radtech

Okay, I still have a problem in the microphone/interphone circuit somewhere then as keying the microphone only starts the dynamotor when the Radio/Interphone switch is in the Radio position.
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 » Sat Jan 06, 2018 7:21 pm

Flip the interphone switch back and forth a few times and test the tubes in a tube tester.

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

Post by Big D » Sun Jan 07, 2018 3:54 pm

Hi Radtech,

Okay, you are on the money again. For starters, I realised that I had transposed the voltage measurements for V105 and V106 on the last post I did. I have corrected that last post but I managed to confuse myself with that in chasing potential causes. It was V105 with the problem, not V106.

Anyway, I had no access to any tube tester so I used my ohmmeter to test the filaments on all the tubes. All the tubes tested fine with the exception of V101, where the filaments were open. I had some spare tubes so I replaced V101 with one that looks unused and tested okay on my ohmmeter. I ran all the voltage tests on the tubes and there is now filament voltage on V105:


V101 pins:
1 = 0 (0)
2= 9 (10)
3 = 0 (400)
4 = 0 (180)
5 = 0 (-30)
7 = 6.8 (7.5)
8 = 0 (0)

V102 pins:
1 = 0 (0)
2= 8.5 (7.5)
3 = 0 (400)
4 = 0 (50)
5 = 0 (-55)
7 = 10.5 (10.5)
8 = 0 (0)


V103 pins:
1 = 0 (0)
2= 2.5 (2.5)
3 = 0 (220)
4 = 0 (40)
5 = -0.05 (-0.5)
7 = 5 (5.5)
8 = 0 (0)


V104 pins:
1 = 2.6 (2.5)
2= 0 (210)
3 = -0.26 (-50)
4 = 0 (40)
5 = 0 (0)

V105 pins:
1 = 0 (0)
2= 2.5 (2.5)
3 = 0 (220)
4 = 0 (40)
5 = 0 (0)
7 = 4.89 (5)
8 = 0 (0)

V106 pins:
1 = 0 (0)
2= 13.17 (12.5)
3 = 0 (370)
4 = 0 (220)
5 = 0 (0)
7 = 12.9 (10)
8 = 0 (0)


V107 pins:
1 = 0 (0)
2= 2.5 (2.5)
3 = 0 (100)
4 = 0 (40)
5 = 0 (-9)
7 = 0 (0)
8 = 0 (0)

V108 pins:
1 = 0 (0)
2= 7.9(7.5)
3 = 0 (400)
4 = 0 (100)
5 = 0 (-40)
7 = 5.5 (5.5)
8 = 0 (0)


This makes sense as I see on the circuit diagram there is some sharing of the lines between V101 and V105. On start up, I keyed the microphone again but still no luck with it starting the dynamotor.

I didn’t think it would make any difference but I thought for the hell of it, I would put both the BC-604 and BC-603 into the FT-237 and see if there was any change. This time there was and each time I keyed the microphone in the interphone position, the dynamotor in the BC-604 would start.

As far as audio in the BC-603 goes, I’m not sure yet. There is just hissing in the speakers in the BC-603s when they are running (no antenna). When I key the microphone, it cuts out the hissing but I don’t actually hear my voice.

I had to stop then as the load was killing the battery and I was back to a chattering dynamotor relay in the BC-604. I will charge the battery again and give it another try.

I’m still wondering if there is something not quite right with the microphone cable as I don’t see why keying it with the BC-604 didn’t make the dynamotor run, while when I tried it in the FT-237 it did make it run.

I guess as you’ve said it could still be something not quite right with the contacts on the interphone relay. I don't see any easy way to get at those contacts though.

We are getting there!
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 » Sun Jan 07, 2018 4:36 pm

The receiver does not need to be turned on for you to hear the side tone from the transmitter.Just because the tube filaments are good the tube can still be bad.

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

Post by Big D » Sun Jan 07, 2018 4:49 pm

Hi Radtech,

Yes, I know what you are saying about the tubes. I figured this was a 'quick and dirty' test that would show me any immediate problems.

Okay, I wasn't aware the BC-603 didn't need to be turned on.

I have noticed now that the dynamotor is running all the time in the interphone mode now, bugger it. I removed the microphone and have flicked the radio/interphone switch back and forth but it now runs continuously in interphone mode.

Now I'm confused. Any thoughts from here?
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


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