Bc-610 modulator issues

[ve8xj]

I have been running the bc-610H  a bit on 20 meters lately and having a lot of fun . But as of the last few qso's I have been running into a couple of issues . Thought I would run it by a panel of the smartest guys on the block. Since my knowledge is a small fraction of yours.

  I won't be trying to do any major stuff  right away as I am still healing from a herniated disc and working on the 610 is a major pain when one doesn't have their back. Hoping it's something simple like a tube . Worth running it by you guys before I wrestle it out of the corner to get at the back of it.
  What is happening is every odd transmission when I key down on AM the bias on the modulator plate meter which according to the manual should be 40ma is jumping up to 100ma and stays there till I key up. But only sometimes. Also sometimes after I have finished transmitting the mod plate meter needle will be pinned in the negative (all the way to the left). One last symptom is when I am talking/transmitting  I will hear the 60 cycle hum change pitch for a second or two on voice peaks and then back to a regular 60cycle hum after what almost sounds like a short oscillation.

  There was apparently no change in audio characteristics according to the people I was talking to during these incidents .

   I know my 2a3's are getting on the weak side and barely tested good the last time I had them out. I have a new pair that I will be putting in there when I do the next servicing.

  Any Ideas fellas ??
  
Tim Ve8xj

[W2PFY]

Well I'm no expert here but i have been playing with 610's for about 25 years. The first thing I would do is run that bias pot back and fourth for it's full travel about 12 times and then see if the bias holds steady. Do this with the power off. It may have a open or dirty spot on the pot. If the meter still goes negative, replace one of the 100TH tubes and see if it happens again. If it recurred, replace the other one. You may have a filament to grid short on one or both of them. Another thing to look for is loose hardware on the CW-Fone relay. The nuts on the screws that route the HV and the nuts on the screws that supply 115 volts AC to the coil may be loose. Make sure you have the transmitter unplugged and make sure there is no HV on the contactor by shorting the HV supply. I usually use a clip lead to short out the HV while I'm working on it. Be sure to remove it when your done!    

[ve8xj]

Hello w2pfy

   Thanks for the reply . These are things I will definitely try when I get around to pulling it out. Think I will get a friend to come over and give a hand this week. Got to get this thing on casters.

  I hope it's not the 100ths I just put brand new ones in there and they seemed to be working fine. But we will see.
  There is always something with a 60 year old transmitter I guess . For sure gonna give that relay the once over.
  Have to check the schematic but I also think there is a tube rectifier for the bias perhaps it has failed. .

 I will let you know what I find.

[W2PFY]

Well you can exercise the pot (not the kind people smoke) now and check the loose hardware later   If I remember correctly, the bias rectifier also supply's B + to the 2A3's so I don't think that is the problem.

Terry

[ve8xj]

Well I got brave today and pulled the 610 out to take a look .

 Terry, good call it was one of the 100th mod tubes . It had developed a intermittent short . Luckily it was just the one and I have a few more good ones . I replaced the 100th's about 4 months ago with NOS ones bought off of Ebay . A matching pair of Jan spec Eimacs . They worked great up until last week,when the one developed a problem it seems. Shame as one pays through the nose for these things today. But with 60 year old tubes whether they are used or not something can always go wrong . NOS does not mean your going to get full life out of them when they are that old. Could have had a problem right from the factory all those years ago.

  Anyways all works ticky boo now and will be back on 20m soon enough just have to wait for the band to recover from that X-class flare.

 Cheers and Thanks for the advice.

Ve8xj

[WA1GFZ]

Maybe you can see the wire sag inside the tube and lay it on its side power it up and get it good and hot to sag it back into place. Gravity could be your friend.
I onch bought a vacuum variable that had the stator off to one side. Just for the heck of it I lightly hit it against the living room couch arm rest about 300 times. It came back in line and I didn't let the vacuum out.

[W2PFY]

A good way to check tubes for shorts is to hook your meter to the grid and the other to the filaments. Tap the tube and if your meter has a beeper on it, it will sound if there is a short while taping on the tube. Setting it on the ohms scale to read the short is often imposable due to the small time the tube is shorted. I learned the trick about the meter on the rig going negative with 4-400's tubes. Anytime I get an unknown tube, I test them in the way mentioned above! Sometimes people ask me how much a tube costs that I may have purchased. I tell them you have to add the price to the tube for all the duds that you purchased to get a good one. One caution, it's best to stick with Eimac. Most other brands are gassy! Glad I helped you

Another tip to see of you have a matched pair in there is to set you bias so the tubes are drawing about 125 mils and check to see if both tubes are showing equal redness, if they are not, pull the dull one out and try another.

 If they are imbalanced, you'll splatter all over the band just as I did about a week ago on my BC-610D!

BTW, the next time I'm at my camp where the 610 is located, I would like to try to work you. What do you do up there? Freeze??

[ve8xj]

The 100th s in there now are both Eimac though they are different . One has a metal base the other a ceramic . The metal one is JAN. Unfortunately they are about as match a pair as I have for the time being . I will be sure to ask the people with whom I talk to check my width. Could be a problem.

  Listen out for me on 20m be great to talk to another 610 . As far as what  do here . I am a wood turner ,carpenter,trapper. Though I have been off for a while due to my back issues. But starting to get back on my feet again which is a good feeling .

 I put in the new 2a3's into the 610 as well. They are Chinese so I am interested to get reports from people I have talked to in the past to see if they can hear any difference .  I know ,I know Chinese tubes are not my first choice but 2a3's NOS are getting bloody expensive and like this recent 100th experience ,proves one just doesn't know what your getting . At least I know these are new. They did get good reveiws. Made by Shuguang they have a  pretty good rep for audio tubes.

Good advice on the tests .

W1GFZ I will try that on the 100th and see if I can't fix it. After all not much to loose at this point. Maybe it was stored on it's side one way for 50 years and all it needs is  some time on the other side.