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

Radio Telephone and Telegraph Transmitting and Receiving Equipment
Big D
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SCR-508 radio - BC-604 Transmitter - diagnosis tips?

Post by Big D » Wed Dec 27, 2017 3:43 pm

Hi all,

I am in the process of trying to get a working SCR-508 system for my M8 armoured car restoration:

viewtopic.php?f=82&t=264128&start=225

I have/had no real plans to get the radios working, and was really just after a working interphone system for the M8. However, after some tinkering I have one of the BC-603 radios working and the other one works, albeit with no audio at this stage. I will keep playing around with the second unit and see if I can get the audio going on it as well. Both of the BC-603’s are French units.

I have an FT-237 radio tray which I have restored and which now appears to be working fine. I haven’t connected the new interphone wiring yet.

I am now working on the BC-604 (a BC-604L). The condition of this seems to be pretty good but I don’t know anything about its history. I have a DM-35D dynamotor which I have fitted to the BC-604. This dynamotor was used and I’m unsure whether it was working, but it appears to be in tidy condition. I put some new grease in the bearings and the armature spins freely and the brushes look good.

I initially fired up the BC-604 with a 6 volt supply until I was sure there was nothing too dodgy going on. Since then I have tried 12 volts on it.

When I switch the unit on, the lamp glows but only glows weakly. I initially thought the bulb was faulty and removed it as you can’t see the light going with the protective cover fitted. From what I’ve seen of videos of these running, that doesn’t seem normal, and they should be quite bright.

When switching from the Radio to the Interphone, I can hear the S104 relay click (once). When the radio/interphone switch is in the radio position, the lamp dims a bit more.

When switching from the Receiver tune to the Operate position, the S102 relay buzzes constantly.

The dynamotor doesn’t fire up at all at any stage. I have plugged in a microphone and keyed it but still no change.

I have all the TM manuals now and schematics so I am trying to digest all that. I trained as a Radio/TV technician in another life, but that was post valve technology and it was nearly 40 years ago now, so I am a bit scratchy on that side of it. I can still work my way around a circuit diagram though.

Does anyone have any pointers on a good starting point for a diagnosis? I did a bit of searching on the G503 and I see the odd mention of relay problems on the BC-604 but I’m not sure that is all that is wrong with this though.

As an aside, I have a second problem with the radio setup. I have two NOS DM-34D dynamotors for the BC-603’s. I put some new grease in the bearings of these, although the grease in them was not too bad. One of these dynamotors worked fine straight off but I can’t get the other one to go. They are slightly different in design with different commutators and brushes. I am getting different resistance readings across the windings but from what I can see in the manuals, resistance checks are not a reliable test on these dynamotors and they recommend a dynamotor testing device which I certainly don’t have. The 12 volt supply is getting to the windings on the faulty dynamotor but it won’t fire up.

Does anyone have any quick tips on what I should look for with the faulty dynamotor to identify the problem?

Any bright ideas on both problems would be appreciated.
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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 Dec 28, 2017 4:15 pm

What is the amp output of your 12 volt power supply?

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

Post by Big D » Thu Dec 28, 2017 4:23 pm

Hi Radtech,

Thanks for the reply. In my initial tests, I just hooked it up to a fully charged good quality 12 volt battery. However, I can get my hands on both a 10 AMP and 20 AMP bench power supply which I was going to try next.

I guess the 20 AMP supply would be the better option?

By the way, I have managed to get the second BC-603 working with audio.
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 Dec 28, 2017 7:58 pm

A good preferably new 12 volt battery is best.Otherwise it seems you are loosing filament voltage. It is also possible you damaged something attempting to operate it at 6v.NEVER attempt to operate a DC powered radio at less than the operating voltage.

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

Post by Big D » Thu Dec 28, 2017 8:22 pm

Hi Radtech,

Thanks for that. I wasn't aware of the dangers of operating it at a lower voltage. That's good to know!

I didn't get a lot of time to tinker today apart from taking some resistance and voltage measurements.

Most of the resistance checks as per the TM 11-310 manual were pretty close to what the manual outlined apart from these:

* A1 pin on PG101 measured 0.7 ohms versus the 37 ohms as per the manual
* pins 5 and 6 of the second audio tube both measured about 50k versus 14.5k in the manual
* pin 4 of the first RF tube measured 316k versus the 260k in the manual

Everything else was within cooey to my way of thinking. I wouldn't think the differences at the audio and RF tubes would be the problem but the A1 pin measurement got me curious.

I did a few quick voltage checks (with the 12 volt battery!) and all the voltages on the dynamotor plugs were correct, bar me checking the 600 volt output.

I measured the filament voltages on the rectifier, doubler and power amplifier tubes and the filament voltages were correct there. There doesn't seem to be any appreciable load on the battery when running the tests.

I ran out of time at that point.

Any thoughts on what I am seeing here?

Thanks for your 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

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

Post by jesse » Fri Dec 29, 2017 7:10 am

It’s hard to say without a thourough tracing and testing. One thing that your description brought to my mind was a power shortage. The relay chattering, and the dynamotor not starting could be caused by insufficient connections to handle the huge amount of current drawn from the low voltage source when transmitting. Voltage may read normal with just the filiments burning. All pins in the connectors should be clean. A good connection on the battery is required too. Alligator clips are not good. Jesse.

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

Post by Radtech » Fri Dec 29, 2017 8:01 am

If the dynamotor runs continuously in the receiver tune position your dynamotor relay is good then try different microphones.If not you have a dynamotor relay problem.Check the contacts on the dynamotor relay.You need to make sure first that you have sufficient filament and plate voltages at each tube.Also keep in mind the circuit resistance readings in the manual were taken with a vacuum tube volt meter like the I107 used to trouble shoot these sets.

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

Post by Big D » Fri Dec 29, 2017 1:03 pm

Hi Jesse and Radtech,

Thanks for the replies.

Jesse - good point. I will fit some better connectors just to be sure that is not the problem.

Radtech - the dynamotor has not run as yet, in any switch position or with testing with the microphone. The dynamotor relay S102 buzzes away when the set is in the Operate position but the dynamotor doesn’t run. I had checked the contacts on this relay and they seemed to be in good condition. Should I be looking for anything in particular apart from signs of burning etc?

Is it possible this relay is simply faulty? I read on another thread where someone had to replace the dynamotor relay.

I will take some more measurements of the filament and plate voltages across all the tubes later today.

Good point about the resistance checks. I hadn’t considered that.
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 » Fri Dec 29, 2017 1:52 pm

Disconnect capacitor C162 from the dynamotor relay circuit it may be bleeding current to ground.It has been a few years sense I worked on one so I am going by memory.The dynamotor relay should do nothing in the operate position until the mic button is pressed.In the receiver tune position the relay will buzz if it is not getting enough current to close.

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

Post by Big D » Fri Dec 29, 2017 11:36 pm

Thanks Radtech.

I did a wee bit more testing after work today, after fitting some better connectors to the battery.

Out of curiosity I checked how much current the unit was drawing from the battery. When the switch was in the Receiver/Tune mode, it was drawing 2 amps. When I flicked the switch to the Operate position, the relay began buzzing again and it starting drawing 8 amps. Would this figure be correct?

If I can get hold of another multimeter, I will check it again. This multimeter is nearly 40 years old so I want to be sure, and the other meters I have only measure in milliamps and microamps.

I disconnected C162 as you suggested. That stopped the relay buzzing but the dynamotor still would not run (should it run with the cap disconnected?), with keying the microphone. Out of interest, I checked the current being drawn then and it was about 2 amps (if that figure is correct) in either switch position.

I checked all the filament voltages at the tubes, with C162 still disconnected. There was no filament voltage at tubes V107, V101 and V105. The others seemed okay.

I also noticed that the pilot lamp is a 28 volt bulb. Would these not normally be 12 volt? Does that explain the lack of brightness, meaning the unit has other issues?

I need to get a good magnifying glass and get my head around the circuit diagram a bit more. It is starting to come back to me but the valve technology is still foreign for me.

I appreciate the ongoing advice.
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 Dec 30, 2017 3:47 pm

The lamp should be 12v.Yes the dynamotor should run with the capacitor out of the circuit.In the operate position there is no voltage on the dynamotor relay until the mic switch is pushed.In the receiver tune position the dynamotor should run continuously.In the operate position the relay should have no power and should not buzz.How are you getting power from the battery to the BC604?

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

Post by Big D » Sat Dec 30, 2017 4:04 pm

Hi Radtech,

I initially had the BC-604 in the FT237 mount but have since been connecting directly through the external connector, PG101.

I will get the correct bulb and fit that. I will also do some voltage measurements on the relay while keying the microphone.

I’ll keep you posted!
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 » Sun Dec 31, 2017 8:21 pm

Hi Radtech, Jesse and others,

Before I got started today, I pulled the DM-35 dynamotor out and did a couple of very quick checks on it with 12 volts into the LV side. It does run, but draws a heavy current and I wasn’t prepared to brave the light cabling I had temporarily connected it to the battery source for too long.

My ammeter suggests it was drawing over 12 amps (and I believe the ammeter to be correct based on other checks I’ve done). On the casing it does say 18.5 amps. Any idea what a correctly running DM-35 dynamotor draws? The manuals don’t seem to show this and I wanted to be sure a possible dynamotor issue wasn’t adding to the problem I’m having with the BC-604.

I put the dynamotor back into the BC-604 and I checked the voltages on S102. With the unit switched on, there is 12 volts on each of the four terminals on S102 regardless of the position of the Receiver-Tune/Operate switch, or for that matter, any other switch on the unit.

Out of curiosity, I took the dynamotor out again and did another check on S102. This time, there was 12 volts on three of the four terminals; the exception was the terminal that provides the line to pins 1 and 3 of the dynamotor.

Again, it made no difference what position the Receiver-Tune/Operate switch was in.

The relay itself seems to move freely and the contact pads appear to be in reasonable shape. I’m leaning now though into thinking it might have an internal problem which makes it always ‘on’.

What do you think? Am I on the right track?

On another question – you might recall I was having difficulties getting the second NOS DM-34 dynamotor running. I was suspicious about the resistance readings I was getting at the LV end and it appears there is a problem with the gap between the commutator segments. I lightly scraped between the segments and cleared out some crud and managed to get the armature briefly rotating. I was only doing very quick checks with the power connected to it, but I am still seeing a bit of arcing across a couple of these segments in particular and the armature won't consistently spin.

Is the capacitor across the positive and negative terminals supposed to prevent this or is it just likely to be fine pieces of copper stuck there from the manufacturing process? If it is the latter, do you have any bright ideas on fixing this? The gaps on this commutator are fairly small.

Thanks for anymore assistance you can offer.
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 Dec 31, 2017 10:16 pm

Will the dynamotor run if you manually close the dynamotor relay?Remove the dynamotor and ground the dynamotor relay terminal where the C162 is connected and see if the relay closes.The dynamotor should not pull that much amperage with no load.

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

Post by Big D » Sun Dec 31, 2017 11:37 pm

Hi Radtech

I have just tried what you suggested, and the relay does manually close when that terminal is grounded (with C162 disconnected).

I guess that throws my initial theory out the window.

Yes, the dynamotor made me wonder. Admittedly the cables I used were light but with no load on it, the cables warmed up fairly fast. Perhaps the dynamotor itself has an issue?
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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