Tram HUMMM

[raider]

I have a 201a that is a great radio. I replaced both tall cylinder caps however I have a very slight hum during transmit. AC 60hz hum. Any suggestions would be much apprciated.

raider

[Sandbagger]

Oct 21, 2008 17:21:30 GMT -5 raider said:

I have a 201a that is a great radio. I replaced both tall cylinder caps however I have a very slight hum during transmit. AC 60hz hum. Any suggestions would be much apprciated.

raider

I just got done redoing a point wired D201 for a friend that had a hum on transmit when the mic gain was set to max. As the gain was turned down, the hum gradually went away. Filter caps were all changed out. Drove me nuts. But I finally found the problem. It was a ground loop. I ran a 1 inch piece of braid from the ground pins on the mic jack to the ground pins on the first mic amp tube socket and the hum disappeared.

[Tombstone (R.I.P.)]

Sounds like you still have a bad capacitor in the transmit section, where to check I have no idea since I'm not a Tram expert. If it were still in the power supply I think that the hum would be in the receive too, just a guess??

Tombstone

[Tombstone (R.I.P.)]

Ha! When I made my post a couple of minutes ago Sandbagger's post hadn't shown up on this computer yet. There you go, could be a ground problem.

Tombstone.

[2600]

Sandbagger, I do have a solution for your mysterious mike-audio hum.

It's caused by a ground loop from the heater power to the tube right behind the mike socket. Too lazy to look up the callout number for the mike preamp. Maybe V601? The heater is on pins 4 and 5. One of these is grounded to the metal ring on the tube socket. Can't remember which. A bare wire from the socket's center post also passes through this lug, leading to a ground lug on the socket's ring. Take this wire loose from the grounded heater pin. Be sure the socket's center post still has that ground wire to the ring. Might have to insulate it or make it longer so it won't touch the lug you just took it loose from. But that center post has to be grounded.

Now solder a piece of hookup wire to the now-loose heater lug, and run this wire to a ground lug near where the fat heater-winding wires come out of the power transformer. I usually just suck off the solder and clip one corner of the socket lug, then bend the bare ground wire out of the lug's eye, and slide a sleeve over the new wire once it's soldered to the socket lug. The sleeve prevents that bare ground from touching the lug we just took it loose from.

Now the tube's 60-Hz AC heater current won't be causing a tiny voltage drop across the ground rivets on this tube socket. This is the only tube in the radio that seems to develop this problem.

Usually works.

As for this other radio, the filter caps on the negative bias voltage are probably causing his hum on transmit. That voltage does NOT feed into any part of the receiver. A 'scope would narrow this down, if you have a look at the negative bias supply on that cap.

Or the 10 uf 450-Volt cap on the audio board.

Or the 4.7uf cap in the mike amp's screen grid.

Or the 14-Volt filters on the BA board. Current draw on the 14-Volt supply increases slightly during transmit, so it could be quiet on receive, and start to hum on transmit.

Still too lazy to look up the schematic callouts for these right now. Mostly the cost of all the remaining small filters is a LOT less than the labor to decide which ones are still good, and which ones are not. Replace them all at the same time and this kind of nagging gremlin usually goes away. Nearly any one of the electrolytics in that radio can cause a hum on transmit, or receive, or on SSB only, etc.

73

[Sandbagger]

Oct 22, 2008 1:08:12 GMT -5 2600 said:

Sandbagger, I do have a solution for your mysterious mike-audio hum.

It's caused by a ground loop from the heater power to the tube right behind the mike socket. Too lazy to look up the callout number for the mike preamp. Maybe V601? The heater is on pins 4 and 5. One of these is grounded to the metal ring on the tube socket. Can't remember which. A bare wire from the socket's center post also passes through this lug, leading to a ground lug on the socket's ring. Take this wire loose from the grounded heater pin. Be sure the socket's center post still has that ground wire to the ring. Might have to insulate it or make it longer so it won't touch the lug you just took it loose from. But that center post has to be grounded.

Now solder a piece of hookup wire to the now-loose heater lug, and run this wire to a ground lug near where the fat heater-winding wires come out of the power transformer. I usually just suck off the solder and clip one corner of the socket lug, then bend the bare ground wire out of the lug's eye, and slide a sleeve over the new wire once it's soldered to the socket lug. The sleeve prevents that bare ground from touching the lug we just took it loose from.

Now the tube's 60-Hz AC heater current won't be causing a tiny voltage drop across the ground rivets on this tube socket. This is the only tube in the radio that seems to develop this problem.

Took me a while, but I came to the same conclusion about the ground loop. I wondered if the riveted ground tabs were losing their once-solid connection. My solution was a little different from yours but it worked. I just ran a short piece of braid from the mic socket ground to the center ground pin on the mic amp tube. The hum went away. I like your solution, and I'll probably try that as well.

I take it that this problem is somewhat common on the point-wired D201's.

Thanks for the tip.

[Marc]

I am Having the same issue on my D-201.
But it is the PCB version just a slight ac hum on the carrier.

changed all caps in the radio over the last 3 weeks.
could adding the extra ground help on this radio as well?

[husker]

I have noticed in the last week or so a slight humm AFTER I transmit and I too have changed out every single cap... Marc, I think your ground issue could be a very good guess..that's where I am going to look

[Marc]

Hi Bob

Going to look at the traces on that tube tonight. make a jumper from mic socket ground to the center pin.

other than that just need to get new tubes and get it aligned and it is done!!!

[husker]

This radio is working perfect except for this very slight issue. It sound awesome and works perfect ( knocking on wood!)....but you know these old radios...fix one thing and three more pop up