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 » Mon Jan 01, 2018 7:15 am

With the dynamotor removed and the switch in operate does the dynamotor relay close when the mic is keyed?In the tune receiver position does the dynamotor relay close and stay closed?


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

Post by Big D » Mon Jan 01, 2018 8:03 am

Hi Radtech

No. With the dynamotor removed and the switch in the operate position, the dynamotor relay does not close when the microphone is keyed (with C162 still disconnected).

The dynamotor relay doesn’t actually operate at all in either tune or operate positions. When I grounded the terminal where C162 has been connected, that was the first time I had seen the relay close and open, properly.

Does that help?
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 01, 2018 10:00 am

Reconnect C162 first with the dynamotor removed,ground pin 25 on the power connector.The dynamotor relay should pull in.If it does install the dynamotor and ground pin 25 again.Is the interphone relay S104 working when you switch between radio and interphone?

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

Post by Big D » Mon Jan 01, 2018 10:49 am

Hi Radtech,

Thanks for that. I will give that a try this afternoon when I’ve finished work.
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 » Mon Jan 01, 2018 6:54 pm

Hi Radtech,

I carried out those checks you suggested.

I connected C162 again and with the dynamotor out, I grounded pin 25 on the power connector. The dynamotor relay S102 pulled in fine – no buzzing at all.

I fitted the dynamotor back into the BC-604. As an aside, I gave the commutator a bit of a clean up beforehand. I didn’t think it looked bad, but the cleaning has brightened it up a bit.

I first switched from Operate to Receive-Tune just to see what would happen. The S102 relay gave the same original problematic buzzing sound, but interestingly the dynamotor armature rotated about half a turn. The dynamotor hadn’t rotated at all previously in the set, so something (albeit small) has changed.

When I grounded pin 25, the S102 relay buzzed and the dynamotor rotated again about a half turn. I noted that when I did this, the S101 antenna relay pulled in (no buzzing) as well, but from looking at the circuit diagram I can see that it is on the same line so I guess that should be no surprise.

I can hear the interphone relay S104 click when I switch between radio and interphone so I’m guessing it is working.

I’m starting to wonder if maybe the dynamotor might have some issues. As I said, the dynamotor did run out of the set, but drew very high current without load. I’m wondering if that high draw of current is overloading the relay in some way when it is in the set.

What do you think Radtech?

The dynamotor armature rotates freely by hand but in the meantime, I will do some resistance checks on the commutator and armature and see if anything looks obviously wrong.
Last edited by Big D on Tue Jan 02, 2018 11:24 pm, edited 1 time 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 » Tue Jan 02, 2018 3:58 pm

Remove the capacitor from the low voltage side of the dynamotor and try it.With no load it should pull about 7 amps.

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

Post by Big D » Tue Jan 02, 2018 4:21 pm

Hi Radtech,

Thanks for the reply. I've just this minute come upstairs from testing the dynamotor.

I pulled the whole thing apart this morning. I ran all the resistance checks - 180 degree - segment to segment - then segment to armature - all measured fine, from what I could see. I put it back together and then cleaned out the bearings re-greased them and experimented with varying preload on the bearings. The armature seems to rotate freely, but is a reasonably heavy armature and when you spin it by hand, it doesn't keep rolling - it stops pretty quick, if you know what I mean. Do you think that is normal?

The brushes measure fine and the surface of the commutator at each end looks good.

I tried another 12 volt source. This time a bigger fully charged car battery which is a better source than the one I've been using. Again, about a half turn rotation on the dynamotor and the leads warm up very fast. It just seems to act like it is a big short circuit.

I had another look at the circuit diagram and I thought it would be worth removing C503 and giving it a test. It occurred to me then that there was no capacitor across the terminal anyway. I can't see it buried anywhere else on the body of the dynamotor. Do you have any clues as to where it is on these? It is right across the terminals on the DM-34 dynamotors.

I guess it is possible it has been removed?
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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

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

Post by Big D » Tue Jan 02, 2018 6:48 pm

Hi Radtech,

I've been looking for another image of the DM-35D online and I came across this one from a guy who got a BC-604 and DM-35 D dynamotor running:

https://i.ytimg.com/vi/2728am3ZPkc/maxresdefault.jpg

https://www.youtube.com/watch?v=2728am3ZPkc

As you can see, his dynamotor has the capacitor C503 connected across the LV terminals, which is what I would have expected on mine.

Are these capacitors used as a startup type device in this mode? If so, can you suggest a likely value that I could use and I'll see if I can source a capacitor that I can try across the terminal and see if that helps the startup.
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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

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

Post by Big D » Tue Jan 02, 2018 8:35 pm

Hi Radtech,

You'll be getting sick of these updates.

Okay, some success. I have the DM-35 running. The screws were a little hard to get at but I was able to remove the C704 capacitor from the LV of one of the DM-34 dynamotors. I connected this across the LV terminals on the DM-35 and it now runs. It draws over 12 amps (over the scale on my ammeter) on start up before dropping right back to very low. The battery cables get warm but not hot like they did before. It seems there is some inertia in getting that armature rotating and the capacitor prevents what is effectively a short circuit through the brushes and the commutator.

What do you think? A reasonable theory? Haha.

Perhaps I still need to revisit the bearings on the unit. The armature rotates freely but not as freely as the armatures in the DM-34's. This might help that initial rotation.

The label on that capacitor that I put on there is 0.005 (mf?). Will that value suffice? What should I specify in getting a replacement?

I will put the dynamotor in the BC-604 next and see what happens.
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 » Tue Jan 02, 2018 11:34 pm

Hi Radtech and others,

Here is yet another update.

I fitted the dynamotor back into the BC-604. In the Receiver-Tune position, the dynamotor now starts up, which is a step forward. It is not exactly a clean start as relay S102 does chatter a bit as the dynamotor starts and spins, but it does go.

I tried keying the microphone in the Operate position. The dynamotor didn’t run. I checked the microphone plug with a multimeter and the switch inside does seem to be working, so I’m not sure why this isn’t working.

Radtech – I tried your suggestion of grounding pin 25 on the power connector again. S102 chattered and the dynamotor started up. Again S101 pulled in as well.

I couldn’t see any other obvious signs of life in the set but then I’m not sure what I’m looking for. Are any of the tubes obviously lit when the set is operating?

I was running out of time at this point and called it quits for the day. I will do some more voltage checks on the system tomorrow now that the dynamotor appears (?) to run.

I've just worked out that the value of the capacitor across the LV on DM-35 is .003MF. Can anyone confirm the type of capacitor this should be?
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 » Wed Jan 03, 2018 10:27 am

Hi all,

Is there a preferred option to replace the Mica capacitors used on the dynamotors?

I'm looking at some options:

https://www.jaycar.co.nz/3-3nf-50vdc-ce ... 2/p/RC5342

https://www.jaycar.co.nz/3-3nf-100vdc-p ... r/p/RG5033

https://www.jaycar.co.nz/3-3nf-100vdc-m ... r/p/RM7033

I'm thinking the polyester variety might be the best bet, but I am well out of touch with this.

What are your thoughts?

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 » Wed Jan 03, 2018 10:45 am

Don't worry about C-503 it only filters RF noise the same with C-501.With the set on you should have filament voltage to all tubes but the PA tube 1624.Do you have 12v on the dynamotor relay as the dynamotor runs?Does the dynamotor relay pull in clean or continues to chatter as the dynamotor is running?Also try switching the radio interphone switch back and forth to operate the switch as there may be a dirty contact in the wafer switch.

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

Post by Big D » Wed Jan 03, 2018 10:55 am

Hi Radtech,

It was only after I put C503 on the dynamotor that the dynamotor would run. Up until then it only briefly worked outside the set and then not at all. Once I hooked up a capacitor from one of the DM-34 dynamotors across the LV of DM-35, then the dynamotor started. With that capacitor on there it now runs in and out of the set and doesn’t heat up the cables to the battery.

I will check those voltages today. In my testing yesterday after I got the dynamotor to run, I found that the dynamotor relay would only chatter for a second or so until the dynamotor got up to speed. It was fine then. Each time I flicked from Operate to Receive the relay would chatter for a second and then pull in cleanly.
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 » Wed Jan 03, 2018 11:50 am

The dynamotor chatters because there is not enough current to pull it in clean.It sounds like you have a heavy current draw somewhere in the set.unscrew the cover on the mic plug and slide it back,plug it in and check the voltage at the part that connects to the plug tip and see what voltage you have there with the switch in the operate position.C503 which is just a RF bypass cap was eliminated in late 1943 so the dynamotor should operate normally without it.Most of the DM35's did not have it.You may have damaged the windings in the dynamotor when you put 6 volts to it.I will see if I can dig out a dynamotor and see what the resistance should be.

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

Post by Big D » Wed Jan 03, 2018 1:33 pm

Hi Radtech,

Thanks. Very interesting…

Here’s what I found this morning:

The microphone tip voltage is 12.9 volt in the operating position. It drops from 12.9 volts to about 9.4 volts when the button on the mic is keyed.

The other terminal on the mic plug goes from 0.6 volts to zero when the mic is keyed.

As far as the dynamotor relay goes, it is barely a second that the dynamotor relay chatters for before the dynamotor starts.

While the dynamotor is running the voltage at the relay is about 10.8 volts.

On starting the dynamotor, the battery side of the relay starts at about 12.9 volts, drops to about 7.8 volts and then comes back up to about 10.8 volts.

Bear in mind that my battery might be getting a bit beat up at this point.

When the dynamotor is running, I am now detecting voltages on V105, V101 and V107. Prior to this I had nothing there on these tubes at all. I haven’t checked every pin yet, but that must be progress.

Does this narrow things down at all?
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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