K4020 high-q mosfet amplifier kit

Hi I’m new to this forum. Built the above ref’d amp kit about 8 or 9 years ago along with the K4100 Digital Controlled Pre-Amp Kit. Tested it and both and they worked first time and I have been overjoyed with the quality and performance. Just recently the RH channel of the Power Amp has been dropping out. May be a similar problem to one of your other users posts on popping and cracking with this amp. In my case I have extensively examined the problem but still not found the cause to be able to fix it. The -18 V rail collapses after about 45 seconds following power up. Have replaced the zener diodes ZD10 (-18V) -V1 rail. Also replaced ZD12 (10V zener) that defines -V2 and -V rails. Tried to eliminate problems caused by ageing capacitors by replacing all relevant electrolytics (C30, C32, C34, C36, C38) and swopped over C39 & C40. Have also been all over the bad board to check for dry joints and have cleaned up any connections there that looked iffy. The on-board diagnostics provided are a set of 6 leds. Four are for power supply LED’s 31-34, and two are for indication of -ve and +ve dc errors. The fault manifests itself after 45 secs from switch on in the collapse of the -V1 (-18V rail). At this point the B out voltage rises to about +3v which is (with ref to the spec) too high and will therefore cause a dc offset error. Both + & - DC error LED’s LD29 & LD30 light as a result. The biassing to the Power MOSFETS as a result??? Or is it the MOSFETS themselves, or more likely one of them??? is then higher on the T10 & T11 transistors, and the there is no bias on T12 & T13.

I have additionally swapped over all op amps mounted in d.i.l. sockets with no change. Have checked in-circuit diodes and transistors for short circuits and diode functionality with a fluke digital multimeter and all seem OK. In addition to the electrolytics on the PSU side I have replaced most electrolitics on the amp circuitry : C20,C21,C24,C26,C27,C28. I am at a loss to what it is that can be causing the problem other than the POWER MOSFETS. Are they available and do you think it would be a prudent next step to replace them.

Have I missed anything fundamental or glarringly obvious that i should have picked up on much earlier, do you think?

You could disconnect the power MOSFETs and turn on the unit.
Check all voltages and wait for the problem to occur.
If you are unable to locate the cause of troubles, you can return your amp for inspection/repair to:

Velleman Projects Tech Dept.
Legen Heirweg 33
9890 Gavere

Thanks very much for the advice. I got into a cul-de-sac with it and missed the obvious. Yes I have now unsoldered all four power mosfets and the thermal switch. The problem was still there. I have replaced a resistor in the power supply circuitry, R71 680 ohm ! Thought this had solved the problem. Then I got more intermittent occurrence of the fault condition. I decided to change a biassing transistor T7 BC546B. I now do not get any collapsing of the the -18v rail. This is also so now with the power transistors reconnected. However, I still have a rediculously high Bin (+6.2V) and Bout (+13.8v) causing an unnacceptably high D.C. offset which is bourne out by fault condition LED LD 30 ‘+ DC Error’ LED lighting, and the signal path to RH speaker being switched out.

Would I be on the right path if i now replaced the other three biassing transistors T5, T6 & T8, do you think? I have ordered some of these BC546B & BC556B transistors.

We now have current flowing in the POWER MOSFETS. Quiescent current in the Class A being significant as one would expect. Heatsink on the still faulty RH amplifier is, as I would expect, just slightly cooler than the LH amplifier which is processing crystal clear music.

Do you think I am on the right path with swappingb out these transistors or would you suggest something different at this stage. I would prefer to follow the fault through myself, as I find it interesting. Many, many, years ago I was a bench test engineer for Marconi. Am more than a little rusty on fault finding nowadays. Thank you for your continued help with this.

Yes, replacing the transistors is something I would do too.

Thanks I will wait for them to arrive, meantime I am off on holiday for two weeks so I will let you know if it solved the problem when I get back. Thanks once again.

I am now back off holiday and have found time to work on the amplifier again. Have replaced the biassing transistors and this has solved the problem. Thank you for your help. Am now listening to the amplifier giving it a good soak test. But it would appear to have solved the problem.

Thanks once again for your help.


Hi Sorry I’m on again,

The Amplifier has functioned fairly well since fixing it back in November. However, a similar problem has now again developed on the same RH channel power amplifier. Switching on the Amplifier it will run ok for a couple of minutes until the -18V rail again collapses, signalled by the extinguishing of LED LD32. Simultaneously LED’s 29 & 30 indicating -DC & +DC error illuminate.

When I replaced the biassing transistors T4 & T6 : BC546B, and T7 & T8 : BC556B back in November, I used just standard transistors. I don’t have an hfe tester. Should these transistors be matched pairs to ensure correct operation, or is the design such that it is non-critical?

Switching the amplifier off for about 30secs and then back on again produces the same result. Both channels run ok for a minute or two and then the RH channel drops out due to a -18V rail collapse.

What would you suggest?

The transistors do not need to be matched.
Are you sure there is no thermal (mechanical) problem. It is kind of strange that it happens a few minutes after turn on.

Well this one could run and run as it would appear to be a very, very intermittent fault. Can’t bring on the problem by changing temperature : I have used freezer spray to no effect! The problem occurred last friday and seemed like a hard fault, dropping out continuously. Yesterday and today I have the amp on for hours at a time and the fault has not re-occurred. Will let you know if/when I get a recurrence of the fault and/or have any ideas/hit upon the solution. I remember from my days as a test engineer many moons ago that intermittent faults were the most challenging!

What about the led ? Has it been replaced ?
I’ve noticed that the post has been deleted.

I did see that post when I looked in yesterday and I too noticed that it had been deleted. It wasn’t me that deleted it, it’s a mystery. Thank you to the person who put it on there. I will try anything. The very next time it fails I shall try this and see if this is the problem. No doubt if I had a scope I would have solved the problem ages ago. One idea I had wondered was it possible that this RH amp channel was unstable and veering into high frequency oscillation, (above audibility) and that the amplifier’s design was such that it copes with this and shuts down.
Yes, It is distinctly possible that the LED could be causing the problem. In one of my earlier posts above I stated that I replaced pretty much everything around it in the power supply, capacitors, diodes, even LD32’s dropper resistor R71. Yeah it’s possible that I took it’s apparent functionality wrongly, and it could indeed be the culprit. At the moment, since the weekend, I have it on prolonged soak test. It was on yesterday for several hours with no problem. It’s been on full blast today from 9.30-(now) 1.00, and counting, on full blast, and I can’t make it misbehave. No doubt it will fail again, sometime. If and when it does, i’ll get it out on the bench again and replace the LED. It’s quite a pain getting all the transistors unsoldered, everything disconnected from the board, and getting the board out. When it’s functioning correctly, it’s such a good quality amplifier, which is why I WILL eventually find out the problem, however long it takes. It’s a mission now! I will keep you posted on developments, and thanks to you and the gentleman from the States who posted the LED idea. Thanks very much again guys.

As the fault is intermittent, I slightly hesitate to say, but I do now believe it to be fixed! The fault did re-occur, (it was very randomly intermittent). The answer : The culprit was the LED LD32. I took it all apart again and replaced the suspected LED and it has not failed, is really solid. Have had it on a long soak test yesterday and working perfectly again today. Thanks go to VEL 417 and to ‘Helping Hand’ from the USA for applying perfect logic to the information I have given in these posts and seeing the obvious that I had overlooked. A salutatory lesson in keeping my ‘eye on the ball’, so to speak when fault finding. I had replaced every other component around that in the power supply and stupidly disregarded the LED as I was treating it as a fault indication device and omitted to consider the possibility that it too could be faulty and so cause the very problem that I was experiencing. Thank you once again. This is a really good forum. :smiley:

Happy to read you solved the problem.

Had same problem with a power supply LED. Replacing the LED solved the problem. Thanks for the tip ! :wink: