How To: Restoration of Ford M8 armoured car US Ordnance number 7373

[David Dunlop]

Darryl.

Are there any indications the radio mount serves as a ground for any of the connections? That might be why it was simply zinc plated, to maintain good electrical continuity.

If you paint it, be sure to keep good metal to metal contact between the bits where needed.

David

[Big D]

Hi David,

Thanks for the reply.

In all my testing so far, all the power and earthing is done through the three main sockets on the back and sides of the FT-237 tray. I ended up painting it in the end, but that in itself creates some pain in the butt as each time I move the radios on the tray, it scratches the hell out of the paint! Ha!

I have made good progress with getting the radios working and will post another update soon.

[Big D]

Hi all,

This update on the M8 has a bit of a different flavour to the previous stuff but the work I’ve been doing over the last couple of weeks has been a nice change to cleaning, fabricating and painting.

I’ve now made some progress on the radio setup for the M8. After some tidying up of the old wiring, I have the FT-237 tray working. The antenna post still needs replacing and I’ll do this before the tray is finally fitted. I had a BC-604 transmitter and two BC-603 receivers for the radio setup. I needed the BC-604 going for the interphone system in the M8 to work, so I was keen to see if that could be done.

I had two NOS DM-34D dynamotors for the BC-603 radios but before fitting to the receivers, I took the bearing end caps off and put some new grease in there. The old grease was a little thick and might have been okay, but putting some new stuff in was good insurance. When I tested the dynamotors, one worked straight away, while the other one didn’t, bugger it. After some time learning about how they work and fiddling around with it, I found that there appeared to be insufficient gap between the segments on the commutator. I had to scrape the gaps to get the excess of copper out of there. There is a little bit of arcing off the brushes now but the dynamotor is working and hopefully the brushes will settle in. They are quite a neat little device. For anyone else servicing these, I found that when they are working properly, they draw about 1.8 amps with no load.

I have both BC-603 receivers working now. These are French units but they will do until when/if I pick up some original U.S ones. Both had lamps and fuses that needed replacing, and one had an audio problem. After I fitted the new fuses and lamps, and fiddled with the audio tubes on the second unit, both are functioning now.

The BC-604 has been a bit of a challenge so far. In another life, I trained as a radio/TV serviceperson but that was post valve technology and 35 years later I had forgotten all my training. Some of it came back once I started looking at the circuit diagrams, but I had to enlist some help from a Radio forum to get me in the right direction.

I had a used DM-35D 12 volt dynamotor to go with the BC-604. I didn’t know anything about its history but it was the only one I’d seen for sale at the time, so I grabbed it. I put some new grease on the bearings on this before firing it up. The dynamotor fired up initially off a power supply so I fitted it in the set. When fitted in the set though, the dynamotor relay just kept chattering away and the dynamotor wouldn’t start at all. I also couldn’t see any signs of life in the BC-604 itself.

With some assistance from a knowledgeable chap on the other forum, I ran all sorts of tests on the BC-604 but kept coming back to the dynamotor as being part of the problem, so I pulled it out of the set and completely stripped it. I ran all the resistance checks - 180 degree - segment to segment - then segment to armature - all measured fine. The brushes also measured fine and the surface of the commutator at each end looked good. I put it back together and then cleaned out the bearings again, re-greased them and experimented with varying preload on the bearings. The armature seemed to rotate freely, and I figured I was good to go.

I tried another 12 volt source. Again, about a half turn rotation on the dynamotor and the leads to the battery warmed up very fast. The dynamotor was initially drawing over 12 amps (more than my ammeter would go up to) and it just seemed like it was a big short circuit. I had another look at the circuit diagram and I noticed that there was no capacitor across the Low Voltage ends of the dynamotor, unlike the circuit diagram. I wondered if it had been removed at some point and whether that capacitor was supposed to assist as a start-up device. I fitted a capacitor from one of the dynamotors from the BC-603’s, tested the DM-35 dynamotor and away it went.

I fitted the dynamotor back into the BC-604 and although the dynamotor ran, the set still wasn’t doing anything and keying the microphone didn’t start the dynamotor like it should have. I removed the dynamotor again and gave it another going over but could not see any reason why it shouldn’t do what it was supposed to do. I put it back in the BC-604 and made up another set of cables for connecting to the set. These were beefier than the ones I used before and I figured it couldn’t hurt. First test and as soon as I hit the Operate button, the dynamotor fired up as it should. No delay in the dynamotor relay; a nice clean start. A couple of further tests proved I wasn’t dreaming. At that point I noticed that the power amplifier tube in the set was lit. That was a first.

I tried keying the microphone and initially had no success. I checked everything and worked out that I had a dodgy wire in the microphone lead which was causing intermittent connections. A repair later and the microphone keying now starts the dynamotor. I had been told these dynamotors should run without the capacitor (a late war change) so I went back to the dynamotor and removed the capacitor. Success. The dynamotor starts now without the capacitor and even the meter on the BC-604 set 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 I had been using was getting a bit tired toward the end of my hours of testing and that couldn't have helped. The cable change definitely 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, a few lessons learnt.

The next step is to check the interphone and see if it works. I hadn’t really planned on fixing the transmitter itself that but I guess I’ve come this far so I may persevere with that. After that, I’ll tidy up the old paint and freshen the sets up a bit.

[Big D]

More photos.

[James P]

Happy new happy Big D. Small baby steps but each and every one brings you closer to seeing a new and complete M8 in time. Breathtaking amount of in depth work you are putting into this project.

[Big D]

Happy New Year to you to James P.

Thanks for that. I have to admit that I wasn't holding my breath about getting all the radio gear going but it looks like I am getting there. I've made more progress in the last couple of days so will post another update soon.

[Big D]

Hi all,

I have had some limited success with the interphone system in the BC-604 transmitter. I had to do some more fault-finding and with the assistance of a radio forum I worked out that one of the vacuum tubes in the set was faulty. I replaced this and the audio stages in the set started to work.

The BC-604 mounted in the FT-237 does allow you to test sidetone, which is a test on the interphone circuit. The BC-604 now works but it has a wee problem which I may or may not be able to resolve in the short term. I am told that in the Interphone mode, the dynamotor in the BC-604 should run continuously (can anyone confirm this?). This makes sense as the dynamotor powers up the audio stages in the set which are required for the interphone audio to operate.

On my set however, I seem to have an intermittent problem which is likely to be caused by the S104 radio/interphone relay. I had the BC-604 in the FT-237 radio tray and for a short time the dynamotor was running continuously in the interphone position (which I’m told is correct). However, then the dynamotor stopped running continuously and now it won’t run in interphone mode unless I key the microphone. I figure that one of the contacts in the radio/interphone relay is the cause of the problem but there seems to be no easy way to get at the contacts to clean them. It is a complicated relay with lots of connections and I don’t really fancy the idea of trying to replace it, even if I could source a replacement!

As it stands, when using the interphone, one has to key the microphone and pause for a second or two before talking to allow the dynamotor to fire up the audio. At this stage, I’ve found the audio is a bit noisy and there is some dynamotor whine in the audio so I intend replacing the capacitors on the dynamotor to see if that helps. I have also ordered some replacement vacuum tubes for the audio stages, just in case one of the existing ones is not quite right. Not an ideal situation, but when it does work, it does work! If I’m not happy with the performance, I do have a NOS BC-605 that I could fit instead of one of the BC-603’s and this will provide interphone capability.

I have tidied up the BC-604 and the two BC-603’s. I didn’t do a complete strip down and paint of all the panels on the sets. There was just too much involved in removing all the control panels and besides, the appearance of these was not too bad as it was, and I may replace the French sets with US ones at some point anyway. The new paint on the exterior panels has freshened up the appearance of the three sets though.

I am still having some issues with the DM-34D dynamotor that I needed to work on to get it to run and I am ready to admit defeat on this one. More often than not it won’t start. The resistance measurements I’ve done on it suggest that there are some open windings on the armature, so I will keep an eye out for a replacement. If anyone has a good spare they want to part with, I’d be interested.

In the meantime, a few other bits and pieces have arrived in the post (thanks to Brent and Jean-Marc).

Next week I hope to have some progress photos on the hull sandblasting.