Tube tech LCA 2B crackling noise

[mihailojocic]

Hello everyone!

My tubetech have some problem and I can't find what it is so please help me.

Firs short history of this device.

I got it from a company which wanted to throw it out. I look at it and figured out that power supply was burned and decided to take it (for free, hate me as much as you want) Someone plugged it in 230V while switch was on 110V. I managed to find what was burned, changed few parts, I changed all tubes, did calibration and had a perfectly working LCA 2B.

Recently I noticed crackling noise which is totally random on both channels and have no idea what can be. For me it sounds like a dry capacitor but I am confused because it is on both channels.

Please help. Does anyone have idea where to start?

My electronic knowledge is limited so be gentle.

 

[scott2000]

Just put the service manual in the tech docs... Thought there were already schematics in there but didn't see any...?

https://groupdiy.com/threads/tube-tech.78427/

 

[guy_4]

Hello mihailojocic,

A few months ago I posted a reply at servicing Tubetech stuff, to a member that had similar problems as you.
( You can search for my post to get more details )

Mainly I recommend to replace all the tube sockets with hight quality ceramic sockets with gold contacts.
The original tube sockets are cheap quality and with the time and heat they oxidize and create all sorts of issues, and hard to detect.
You also MUST allow more air to flow in, to help decrease the high temperature inside the chassis
That's cooking here !
I throw away the top cover and replace it with some solid metal " chicken fence " to help keep the shielding efficient.
And keep some free space ( 1rack unit ) above the Tubetech to let hot air escape freely.
Just my experience.

Best,
Guy

 

[mihailojocic]

Hello mihailojocic,

A few months ago I posted a reply at servicing Tubetech stuff, to a member that had similar problems as you.
( You can search for my post to get more details )

Mainly I recommend to replace all the tube sockets with hight quality ceramic sockets with gold contacts.
The original tube sockets are cheap quality and with the time and heat they oxidize and create all sorts of issues, and hard to detect.
You also MUST allow more air to flow in, to help decrease the high temperature inside the chassis
That's cooking here !
I throw away the top cover and replace it with some solid metal " chicken fence " to help keep the shielding efficient.
And keep some free space ( 1rack unit ) above the Tubetech to let hot air escape freely.
Just my experience.

Best,
Guy

Thank you so much for your reply!

I managed to find posts about that quite similar problem. I will follow your advice and start changing parts, but I will do "one by one" just to see where exactly was the problem.

Cheers
Mihailo

 

[mihailojocic]

Just an update.

At this moment i am testing both channels with highest gains possible and interesting thing is that both have same random crackling noise but not simultaneously. Also, it's more often to hear noise when cold.

I have found here in my country some Teflon based gold plated tube sockets, I hope they are good.

 

[beatnik]

Before replacing components it would be very helpful to probe the signal path with an oscilloscope to see where the noise is generated.

Be very careful with desoldering tube sockets it can easily damage the pcb, particularly if it's not good quality.
It's a lot safer to cut off the legs and remove the pins one by one. You are probably gonna trash the old sockets anyway.

A friend had a similar noise problem on a tube tech compressor, I think it was the optical model.
It turned out to be a damaged output transformer.
Random popping and crackling noises are common symptoms on failing audio transformers, as the wire insulation deteriorates and creates internal shorts in the windings.

Knowing that the unit has been stressed under double mains voltage, it's possible that also caused damage to the audio transformers.

 

[guy_4]

Hello beatnik,

I've been lucky enough to never encounter a damaged output transformer on a Tubetech.
I found a PT one time, with a heater coil opened.

Most of the random noises, hums and cracklings that I've experienced came from the bad old tubes sockets.
I can't remember having any issue with desoldering the old tube sockets.
Your suggestion to cut the legs is fine, too.

Best,
Guy

 

[mihailojocic]

Before replacing components it would be very helpful to probe the signal path with an oscilloscope to see where the noise is generated.

Be very careful with desoldering tube sockets it can easily damage the pcb, particularly if it's not good quality.
It's a lot safer to cut off the legs and remove the pins one by one. You are probably gonna trash the old sockets anyway.

A friend had a similar noise problem on a tube tech compressor, I think it was the optical model.
It turned out to be a damaged output transformer.
Random popping and crackling noises are common symptoms on failing audio transformers, as the wire insulation deteriorates and creates internal shorts in the windings.

Knowing that the unit has been stressed under double mains voltage, it's possible that also caused damage to the audio transformers.

Thank you Beatnik!

Unfortunately I do not own oscilloscope. "Friend" who have it don't have will and time to help so I'm forced to go with trial and error.

In the meantime I changed 4 capacitors and resoldered all sockets, just to try before I buy new tomorrow.

Nothing has changed after capacitors change but after resoldering sockets right channel is completely silent. I really hope that transformers are ok. They probably cost a kidney.

 

[mihailojocic]

Hello again!

I have changed tube sockets and I still have the same problem. Tried to replace tubes, swap them, used some old ones... Nothing...

I didn't loose patience, just stuck.

It sounds like a static electricity, somewhere between that and noisy potentiometer. Closest I can explain with my English.

Any additional advice will be highly appreciated.

Thanks!

 

[Matt Syson]

Some output transformers (particularly the older grey plastic cased ones TR242/1) sometimes had an intermittent fault as I believe the internal winding was a bit 'tight' and particularly with mains on the high side of 'tolerance' they could arc inside. The later issue /2 or even /3 were corrected .Of course many hundreds of these older ones are just fine. I suppose that if the unit in question had suffered an 'overvoltage event' (230 Volts applied to it when set to 115, the HT could have been VERY high for a while which may have caused the winding insulation to break down which in turn made it prone to internal arcing or 'tracking' (partial short circuit through carbonised varnish).

 

[mihailojocic]

Hello Matt!

Yes, mine are old grey ones. I understand what you are saying, It's just strange that after repair unit was working fine for 4 years. Right channel was affected more (closer to power supply) with overvoltage, it had two burned resistors. Left and right are doing the same thing just not simultaneously. That telling me, from my amateur point of view, that problem isn't in power supply, so most likely that I have faulty transformers like you and Beatnik are suggesting. Can this be checked?

 

[Matt Syson]

The power supply feeds both channels so if that were the source of the fault it would appear on both channels. Checking the transformers is not at all easy but you coul;d find a way to drop the HT voltage down to say 200 Volts and see if it crackles less, or maybe not at all. HT is intended to be 270 Volts but if yopur mains is higher than 230 Volts it can get closer to 290/300 Volts as it is not regulated. I would find it unusual for two output transformers to fail although I suppose your unit had been subjected to a serious over voltage by plugging 230 Volts into it when it was set for 115.

 

[mihailojocic]

Thank you Matt for your effort!

I do have the same problem on both channels, maybe you didn't understood me because of my questionable English. Just that stupid static crackling is not appearing simultaneously on both channels.

Your tip for checking transformers is good. I will definitely try to do that. In the meantime I will replace some capacitors and voltage regulators which I changed when I took device broken.

Thank you one more time
Mihailo

 

[beatnik]

I am gonna repeat myself. You can find an old school oscilloscope 2nd hand for relatively little money. It's a very valuable tool for troubleshooting electronics.
If you need to order replacement transformers you wanna make 100% sure these are the problem.
They are going to be expensive and the manufacturer likely won't accept returns.
Although the test suggested by Matt could give you a further hint.