Troubleshooting loud hum from valve set after a few minutes.

[Scotorvm]

Guy's

I've hit a snag with a set I restored recently.
The set works fine and tunes into a few stations well. Just a faint hum from the speaker when the volume is at minimum.

However after around 7 mins the speaker hum becomes louder and 2 valves become too hot to touch.

If I touch a metal can inside the set the hum returns to the initial turn on level.
Let go loud hum returns. Is this a capacitance issue that can be resolved?

The set is plugged into mains, I've tried using different voltage tappings on the rear of the set using the little screw (unplugged while doing) made no difference. I've attached a pic with the relevant info. The set is RAP transatlantic, not too much info re this set.

Thanks.

Lee

[newlite4]

The output and rectifier valves (arrowed) would get too hot to touch after about five minutes anyway since these are carrying the most power so this is normal. Look for a dry joint under the IF can, thermal effects could show up a weak connection leading to the hum.
Neil

[HamishBoxer]

Could be a lot of things from the o/put valve to the smoothing caps and a good few things in between.

[Scotorvm]

Originally Posted by newlite4

The output and rectifier valves (arrowed) would get too hot to touch after about five minutes anyway since these are carrying the most power so this is normal. Look for a dry joint under the IF can, thermal effects could show up a weak connection leading to the hum.
Neil

Thanks Neil, will have a look at the joints in that area.
Cheers
Lee

[Scotorvm]

Originally Posted by HamishBoxer

Could be a lot of things from the o/put valve to the smoothing caps and a good few things in between.

Cheers Hamish, I did deviate from the caps originally fitted, the big can was a combined 8 and 16uF, I fitted 2 off 8uF, will replace the 8 for a 16 to see if that helps.

All valves have been checked for continuity only, so maybe an iffy output valve.

Thanks.
Lee

[Glowing Bits!]

If the 16uF has been replaced with an 8uF, I'd expect a slight hum but nothing drastic.
What output valve is this set using?
My first thought would be the output valve getting knackered in it's old age, is there a blue glow coming from the output valve when the set has the onset of hum?
If the glow is turquoise colour, that's bad news.
Just some little pointers to work with.

[Glowing Bits!]

Just looked up the set and it uses a pentode, it seems these valves are not too common, let's hope it's not that that's gone iffy.
It's a shame the OP is living at the top end of the country and not somewhere around the midlands, I could have done a quick valve test.

[Heatercathodeshort]

Maybe a clue here given the time for the fault to occur. The only components that are going to heat up within five minutes are the valves. Is it a distinct hum or more of a low beat/motor boating/oscillation and does it still hum with the volume on minimum?
Could be the output valve developing a heater cathode short, the double diode triode if it uses one, or IF instability if it appears to be sensitive around the I.F. amplifier valve. J.

[cathoderay57]

If it's a faulty valve, why would touching the can shield of the IFT restore normal working only while touched? If the symptom remained absent after the touch had been released I could just about buy the vibration affecting a valve, but not otherwise. Sounds like a bad chassis earth joint underneath the can shield, especially if the mounting screws are also retaining the earth terminal of a tag strip? Jerry

[Glowing Bits!]

Looking again at the valve line-up, the front end is typical American valves, it may be necessary to screen these, unless they are already screened and the screen itself has lost it's chassis connection.
Just a thought.

[Scotorvm]

Originally Posted by Glowing Bits!

Just looked up the set and it uses a pentode, it seems these valves are not too common, let's hope it's not that that's gone iffy.
It's a shame the OP is living at the top end of the country and not somewhere around the midlands, I could have done a quick valve test.

Thanks for the offer Rick, a pity I live up in the sticks. Not 100% I have the valves in the right place.

[Scotorvm]

Originally Posted by cathoderay57

If it's a faulty valve, why would touching the can shield of the IFT restore normal working only while touched? If the symptom remained absent after the touch had been released I could just about buy the vibration affecting a valve, but not otherwise. Sounds like a bad chassis earth joint underneath the can shield, especially if the mounting screws are also retaining the earth terminal of a tag strip? Jerry

Hi Jerry

I have tested for continuity from the top of the can to various points on the chassis frame, seems to be ok, going to reply to another post with the issues I have found.

Cheers
Lee

[Scotorvm]

Originally Posted by Glowing Bits!

Looking again at the valve line-up, the front end is typical American valves, it may be necessary to screen these, unless they are already screened and the screen itself has lost it's chassis connection.
Just a thought.

Hi Rick, the issues I have found -

Not sure re the valve line up...

When I got this set it was V1 - 6A7, V2 - 75, V3 - 78, V4 - 42, V5 - 80

Not sure if this is correct, assuming the valves are numbered in the sequence I have marked on the attached pic.

On the drawing PDF the line up is V1 - 42, V2 - 75, V3 - 78, V4 - 6A7, V5 - 80

Radio museum has V1 - 6A7, V2 - 78, V3 - 75, V4 - 42, V5 - 80

I think the drawing is wrong, there's only 2 valves that can be switched 75 and 78, the others have different pins or top caps so can only go in specific holders.

Assuming the valves 75 and 78 were in the wrong place would this make a difference?

The other issue is there is a screened cable in one of the pics coming out the top of a can, the screening is soldered and crimped to the can. When i did continuity between the cap for the valve and the screen it was a short. I had to strip the screen back to just after it exits the can to clear the short. I insulated the internal wire and re-fitted the cap.

Same issue with the screen for valve 3 but I did not have to strip back as far.

Will the missing screen be detrimental?

The speaker wires are 4 0ff. black and red go to the same point on valve 4 (42), green to a different pin on valve 4 (42), white terminates at a pin on valve 5 (80).

Hope this makes sense, would any of this be the cause, the hum when it does get louder stays the same level when using the volume control.

Cheers
Lee

[Richardgr]

I had a similar issue with a reel-to-reel recorder. I replaced all the electrolytics, but sound deteriorated slowly after switch on after having done that. It was coupling caps that were causing that, another sympton being scratchy controls. Replacing every cap transformed the sound.

[Glowing Bits!]

Now it's been established there are screens involved, use a multimeter set on the lowest setting and test the valve can metalwork to chassis, one picture is showing a bit of rust on the bottom of the can, this will make a bad connection, it should be clean.
Another test would be to use some croc-clip leads from the cans to chassis then try the set again, it will prove a point somewhat.
With regards to top cap leads and their screens, the third valve is the detector/drive stage for the output valve, this lead will need a good screen, the second valve top cap may need a screen but I can't be sure of that at the moment, the front end (first valve) doesn't always need a screen, the gain is quite low in this region.
This is quite possibly looking like a grounding issue at the moment.
Hopefully these pointers may help to narrow things down.

[Scotorvm]

Originally Posted by Richardgr

I had a similar issue with a reel-to-reel recorder. I replaced all the electrolytics, but sound deteriorated slowly after switch on after having done that. It was coupling caps that were causing that, another sympton being scratchy controls. Replacing every cap transformed the sound.

Thanks Richard, I have replaced all caps except the ones rated at pF, which I think should be left as is. Going to replace one of the 8uF caps for a 16uF .

[Scotorvm]

Originally Posted by Glowing Bits!

Now it's been established there are screens involved, use a multimeter set on the lowest setting and test the valve can metalwork to chassis, one picture is showing a bit of rust on the bottom of the can, this will make a bad connection, it should be clean.
Another test would be to use some croc-clip leads from the cans to chassis then try the set again, it will prove a point somewhat.
With regards to top cap leads and their screens, the third valve is the detector/drive stage for the output valve, this lead will need a good screen, the second valve top cap may need a screen but I can't be sure of that at the moment, the front end (first valve) doesn't always need a screen, the gain is quite low in this region.
This is quite possibly looking like a grounding issue at the moment.
Hopefully these pointers may help to narrow things down.

Hi Rick, thanks for this.

Going to check the points you have made, the croc clip leads are a great tip.
Recently bought a job lot of parts that had the same screening, so will look at fitting this

Cheers
Lee

[stuarth]

Post 13 refers to uncertainty about valves being in the correct position, particularly the type 75 and 78 valves.

The 78 valve is a variable mu pentode. This should be the IF amplifier, and you would expect it to be fitted between the two IF cans.

The 75 valve is a dual diode triode. This should be the detector and first stage AF amplifier, and you would expect it to be fitted between the 2nd IF can and the output pentode.

The listing in post 13 for when the OP got the set appears to have these two valves in the wrong position, and the Radiomuseum list looks correct.

The pinout of these two valves is the same for heaters, anode, grid1 and cathode. The pins for grid2 and grid3 on the pentode valve are used as the two diode anodes on the dual diode triode valve. The set may work with them swapped, but not very well. With a triode as the IF amplifier, there may be stability issues as the set warms up.

Stuart

[Scotorvm]

Originally Posted by stuarth

Post 13 refers to uncertainty about valves being in the correct position, particularly the type 75 and 78 valves.

The 78 valve is a variable mu pentode. This should be the IF amplifier, and you would expect it to be fitted between the two IF cans.

The 75 valve is a dual diode triode. This should be the detector and first stage AF amplifier, and you would expect it to be fitted between the 2nd IF can and the output pentode.

The listing in post 13 for when the OP got the set appears to have these two valves in the wrong position, and the Radiomuseum list looks correct.

The pinout of these two valves is the same for heaters, anode, grid1 and cathode. The pins for grid2 and grid3 on the pentode valve are used as the two diode anodes on the dual diode triode valve. The set may work with them swapped, but not very well. With a triode as the IF amplifier, there may be stability issues as the set warms up.

Stuart

Hi Stuart

That's great, thanks for a comprehensive answer, think even I understand, I will set up as you mentioned and follow your advice and as per the Radiomuseum setup.

Cheers
Lee

[Scotorvm]

Thanks guy's for all help given.

Have had running for 6 hours following works completed on my nightshift last night.

The items I addressed were -

1. Valves in correct location as per Radio Museum.
2. 8uF cap replaced for a 16uF regardless of the only schematic I can find.
3. Shielding wire replaced on IF cable.
4. The valve encapsulated with metal was not grounded as was an IF can, discrete wire run to underside of chassis to help.

Now OK without issue, hope this helps others. Certainly a learning curve for me.

Thanks
Lee