[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.