K8020 not working anymore

I bought the K8020 Valve Control Amplifier kit many years ago as a pre-amplifier, with two K8010 Valve Amplifier kits as finals.
After many years of perfect functioning the 8020 has stopped working (while the 8010s work fine).
I opened it and did not notice anything strange. I found out with the help of a tester, that the power was not flowing to the circuit board because the fuse in the power plug was burned, so I tried to change it. I could not find a 2.50A fuse like the original, so i put a 3.15 fuse, but nothing changed even if now the power is reaching the circuits and the transformer on the circuit board is now warming up. The valve don’t heat up and the leds and buttons on the front panel are dead.
I am not a technician and so I don’t know how to test all the components on the circuit board.
I need your help to solve this problem.
Thank you for your kind attention.

Hello Nencio68,

I think the best way is to bring it back to the shop, where You bought the K8020 for RMA REQUEST.
You can also ask for a Cost Estimate. Further You can add this topic as complaint.

Best regards,
Velleman Support.

I am in Italy. Actually I bought the kit online directly from you, along with the two K8010 kits, because there was no physical or online resellers which carried that product in catalogue.
But it was long time ago as I said: you were the only ones at that time to offer quality valve amplifier kits for a reasonable price. I think it was early years 2000. Sad you discontinued the products… they were just amazing.
And that’s why I want to try repair it, because even now I can’t find a pre with input selection like yours…

Hello Nenico68,

As Your explanation, We thought that the 12V AC of the transformer is no longer present.
Because, for example, a tube is worn out: for example a short circuit in the filament, as a result of which the winding of the transformer is defective.

Can You try to measure the following voltages on circuit board?
and let us know what you measure.

Attention! Be careful, there is also a very high voltage on the PCB ==> 300V, can result in immediate death or very serious injury. So be extra careful!!

Measure these voltages indicated on the diagram.

You can find it here on PCB:

Attention: these marked components on circuit diagram and PCB are under high tension (voltages)!!

Best Regards,
Velleman Support

Thank you very much, I will test the voltages and let you know. Do i have to remove the tubes before testing or should I leave them on?
Regards,
Andrea

Hello Nenico68,

You can perform a measurement first with the tubes still placed in there socket on PCB.
Then take another measurement without the tubes. So we can see the difference in voltages, With or without the tubes placed on PCB. In this way, making a diagnosis is easier.

Best Regards,
Velleman Support

Hello, I definitely think it’s the transformer which is burned…
This morning I disassembled the pre, plugged the power with tubes on: after 2-3 minutes a soft noisy sound and burned smell started to appear, I was starting to measure point 4 on your image (since I first tested AC current at the mains) and it showed 0.2VAC (instead of 12VAC).
Could not measure anything more since then the the life saving switch took current off all the apartment, like there has been a short circuit. Unplugged the circuit, touched the transformer surface and it was burning hot.
Switched again on the current in the flat, powered again the circuit and these are the measurments I could take with the tubes on the board:
Point 1: 0,3V DC [should be 12V]
Point 2: no current (0V DC) [should be 5V]
Point 3: no current (0V DC) [should be 12V]
Point 4: 0,3V AC [should be 12V]
So practically there is no current anywhere but the trasformer heats too much (burning at touch).
Since you suggested a problem with a tube could have coaused the transformer failure, I took the tubes apart and tried to sort if there was a short circuit in one of those. I don’t have a proper instrument but just a normal tester. I tested the resistance between each pin on the tubes, and I found some values only between pins 9-4 & 9-5 (6,8 ohm on tube A and 6,5 ohm on tube B). I guess they still work, or they are both damaged if there should be no open circuit between pins 9-4 and 9-5.
Is it possible to have a substitude for the transformer if this is the problem?
Could its failure have caused damage to any other component on the circuitboard?
Thanks,
Andrea

Hi, While waiting for your answer, I unsoldered the transformer today and this is wht I found… Guess it’s burned as I thought.
Is it possible to have a replacement?
Andrea

TR8020_1 !

Hello Nenico68,

According to your sent picture, and the life saving switch took current off all the apartment,
it seems that the transformer is defective, indeed.

According to we suspected earlier, the failure of the transformer is a possible consequence of defective tube lamps. => A short circuit between pin 4 - 5 of the tube lamp (filament). see circuit diagram.

We currently have a Spare transformer TR8020 in stock.

If you can send us your details in “private message”, we can give you information on how to order the spare transformer for K8020 from us.

However, we no longer have the tube lamps in stock.
However, since according to the diagram the tube lamps are of the ECC82 type, they are still available here: https://www.thomann.de/nl/tad_jj_12au7_ecc82_tube.htm.

Before trying the new transformer, it’s better that you test it without the lamps mounted.
If the transformer would also become very hot, there may be another problem.

If the transformer does not get very hot (normally less than hand warm) without the tubes,
Then you can install the new tube lamps in the sockets.

Best Regards,
Velleman Support