Tram D201A parts help

[Deleted]

My friend's Tram arrived. This was the one with the receive died. It is the first Tram I have ever seen or touched. I can immediately see where the heat problems are in this radio.I have 2 questions:

1. One of the carbon composition resistors (2.7K) underneath, rear, center, is bubbling a flux like substance. If this a sign of it failing? I measured it and it is more than double its banded resistance.

2. The 4 cap can C624 is rated at 10uf 500V (x4). I cannot locate caps this high in voltage at Mouser. They have 450V. I know you are not supposed to go lower. Is this really 500V neccessary?

The radio had much of the 100,000 mile tune up already done. Most caps were replaced. Several resistors were replaced. Looks like good work, too (for a change).

[Sandbagger]

Jan 17, 2009 20:45:52 GMT -5 @vinnies said:

My friend's Tram arrived. This was the one with the receive died. It is the first Tram I have ever seen or touched. I can immediately see where the heat problems are in this radio.I have 2 questions:

1. One of the carbon composition resistors (2.7K) underneath, rear, center, is bubbling a flux like substance. If this a sign of it failing? I measured it and it is more than double its banded resistance.

2. The 4 cap can C624 is rated at 10uf 500V (x4). I cannot locate caps this high in voltage at Mouser. They have 450V. I know you are not supposed to go lower. Is this really 500V neccessary?

The radio had much of the 100,000 mile tune up already done. Most caps were replaced. Several resistors were replaced. Looks like good work, too (for a change).

Those high voltage multi-gang caps are very expensive. What I usually do is just use 4 single caps and tie them in under the chassis. They're a good bit cheaper and work just as well, although if you're a purist and want to replace the exact part as original, this may not be appealing.

I would not worry about the cans if they've already been replaced, unless the ripple on the power supply is excessive.

I would be concerned about those 2 watts carbon resistors though. I've already seen a bunch of them change value and a couple open up completely. Both Trams that I worked on with dead receivers were due to open resistors, so that's a good place to start.

[Deleted]

Thanks. I don't replace the cans. I replace them with indivdual caps. But there are only 450V caps available at Mouser.

These radios were poorly designed for heat dissipation.

[Sandbagger]

Jan 18, 2009 8:35:29 GMT -5 @vinnies said:

Thanks. I don't replace the cans. I replace them with indivdual caps. But there are only 450V caps available at Mouser.

These radios were poorly designed for heat dissipation.

After my experiences with D201's, I have come to the conclusion that they should have had another winding off of the transformer to make a 120 to 150 volt supply to feed all the small signal tubes, rather than having to drop so much voltage from the 400V supply on each stage. Dissipating that much current in those resistors is what generates so much heat and also strains the resistors and eventually causes them to drift in value.

But the extra winding in the transformer and the associated parts in the power supply would have added cost, so I'm sure that's why they chose to do it the way they did.....

None of the D.C. voltages on C624 should ever exceed 450V so a 450V rated cap should be ok.

[2600]

The 450-Volt caps work okay for a while.

The higher rating is due to the UNLOADED voltage on the B+ supply while the tubes are still cold.

Once they warm up, everything drops to the normal voltage readings you see on the schematic.

But for the first minute the radio is powered up, the tubes draw NO current from the high voltage. As a result, the B+ will run between 460 and 480 Volts for this period. The tubes warm up, begin to draw current, and then safely drops to the normal 400 to 420-Volt level.

That one-minute interval of excess voltage shortens the life of a 450-Volt part, but won't clobber it right away. Not very often, at least.

This is one reason given for the "Standby" switch seen on the rear of many tube-type guitar amplifiers. You turn on the amp, wait a minute, and then flip it from standby to "Operate".

I suppose a time-delay relay would fix this, but adding more moving parts to this radio just doesn't seem attractive. Coolest Idea I ever saw was to use one of the unused tube sections to control this relay. When that tube warms up, it turns on the B+ relay. By then, the other tubes are also warm enough to pull the B+ voltage down to a safe level.

73

[Sandbagger]

Jan 18, 2009 22:43:02 GMT -5 2600 said:

Coolest Idea I ever saw was to use one of the unused tube sections to control this relay. When that tube warms up, it turns on the B+ relay. By then, the other tubes are also warm enough to pull the B+ voltage down to a safe level.

73

THAT is an excellent suggestion, and one that would not be all that hard to do. At least not in a 201A, where there are a couple of unused tube sections. Not so for a 201 though..........

Although I don't understand why the main filter cap (C5) is only rated for 450V while the smaller can used mostly in the audio circuit (C625) is the one rated for 500V. Logically I'd think it should be the other way around......

[Deleted]

One of those carbon comp resistor in the center-rear, has a wire jumping it, but is coiled with about 1.5 turns around the resistor. What is that? Is that a jumper, or a futile attempt at a coil? I will have to post a picture.

[FIXR]

That is an 82 ohm resistor that goes to the plate of the final output tube. The wire wrapped around it makes a parasitic suppressor. The same as on the plate lead of a linear amplifier.

Fixr

[Deleted]

If I replace that resistor, how should I wrap it (turns?). Use the same wire?

[2600]

Yep. Make a replacement look as much like the original as you can.

It's a coil. The resistor makes it a very low-Q (spelled 'broadband') RF choke. Like Fixer said, it's there to keep the final tube stable, so it won't oscillate at 100 or 150 MHz. A replacement would need to be as close to the original as you can get it.

73

[Deleted]

Does anyone have a component picture and list for the underside? The service manual does not appear to have a picture and list for the underside. It had everything else.

[mrgumby OT21]

www.tech-systems-labs.com/other_manuals.htm

scroll down to tram don't know if it will help or not

[Deleted]

I have the full service manual, but they don't have a diagram for the underside or the parts ID for it. Unless I missed it.