[SonicTrance]

Very hard to know whats wrong here, unfortunately. Can't come with any advice except check all connections with your ohm meter (amp off). You can also measure the anode voltages (amp on). There should be about 80 V on the anodes of the 6AS7's. 100 V on the B+, then a 20 V drop across the 330 ohm anode resistor. Also measure voltages on the input stage. If all voltages are good you should have sound.

 

[baronbeehive]

I've measured the voltage at either end of the 330 ohm anode resistors:

B+ side = 98v, other side = 62v on previously faulty circuit
B+ side = 98v, other side = 67v on previously good circuits
I hope I've done this right
I don't know where on the input stage to measure?

I did find a likely touching wire which I thought might have triggered the protection circuit but I've corrected this now and so the readings are taken after this.
Still no sound.

 

[baronbeehive]

I've checked all the resistors and all are OK except for one on the previously faulty circuit which reads 1.9k ohm and should read 106k ohm. This one:

It's the brown one on the right, next to the red cap, just under the Mcap, it's a Dale as well, damn and blast, they're the good ones! Must have blown with the last fault I had.

Do I need to check the resistors in the centre by the fuses I couldn't get stable readings from them as they are connected to caps.

I will order a replacement for the Dale,

 

[baronbeehive]

I'm going to assume that the replacement of the bad resistor will stop the headphone protection circuit tripping ..... I hope.... and go ahead and finish the rest of the mods:

This is my order for this, and the rest of the resistors:

To replace the bad 106k Dale resistor in the pic above:
- Takman metal film 110k .5W or .25W

Impedance mod: (figures from the table by SonicTrance - no switch, I will decide on one or the other for now like Maxx)
- Mills MRA05 120R 5W x 4 - for 421A's (38 ohm headphones)
- Mills MRA05 910R 5W x 4 - for 6AS7G's (38 ohm headphones)
To be soldered in parallel to the 330R anode resistors

Driver Bias mod:
- TKD metal film 33K 2W x 4 to replace existing anode resistors in driver stage.
- Takman metal film 390R .5W or .25W x 2 or:
- Shinko 390R .5W or .25W, x 2 to replace existing cathode resistors in driver stage

Does that look OK?

NB: I know how to measure the resistances in a parallel circuit but is there an easy way to calculate what resistor needed to parallel with the 220k, and 1.5k driver stage resistors, I'm struggling with the maths!

 

[SonicTrance]

've measured the voltage at either end of the 330 ohm anode resistors:

B+ side = 98v, other side = 62v on previously faulty circuit
B+ side = 98v, other side = 67v on previously good circuits

That is 109 mA and 94 mA / tube. It should be around 60-70 mA. Are the anode resistors 330 ohms? Have you measured? Are these measurements on the same channel? If so, have you measured the other channel?

I don't know where on the input stage to measure?

You can measure rail voltage at top of driver stage anode resistors to ground, yellow circle. Also measure across those anode resistors to see the voltage drop. Cathode resistor circled in red, measure across it. Do the same for other channel.

I've checked all the resistors and all are OK except for one on the previously faulty circuit which reads 1.9k ohm and should read 106k ohm. This one:

How do you know that that resistor belongs to your "faulty circuit"? Also, 106k seems very strange. Does it say 106k on the board below the resistor? I think it's a 100k.
I don't know the purpose of that resistor but it's located close to a OPamp so probably have something to do with it. Have you measured DC offset at the outputs?

I'm going to assume that the replacement of the bad resistor will stop the headphone protection circuit tripping

I still think that if the protection circuit is active you should read 0 mA on the meters, i.e. B+ shut off. I don't think your protection circuit kicks in at all.

Does that look OK?

It looks good but I would get the amp working before changing any more parts.

NB: I know how to measure the resistances in a parallel circuit but is there an easy way to calculate what resistor needed to parallel with the 220k, and 1.5k driver stage resistors, I'm struggling with the maths!

http://www.sengpielaudio.com/calculator-paralresist.htm

 

[baronbeehive]

That is 109 mA and 94 mA / tube. It should be around 60-70 mA. Are the anode resistors 330 ohms? Have you measured? Are these measurements on the same channel? If so, have you measured the other channel?

Thanks Sonic! I measured at the anode power resistors, yes they measure 330ohms, same on the other circuit on that channel, I will measure them all again.

How do you know that that resistor belongs to your "faulty circuit"? Also, 106k seems very strange. Does it say 106k on the board below the resistor? I think it's a 100k.
I don't know the purpose of that resistor but it's located close to a OPamp so probably have something to do with it. Have you measured DC offset at the outputs?

I measured each group of 6 resistors in that position for each circuit: each group of 6 measured - .56k, .56k, 10k,10k, 1.9k and 106k, except for the previously faulty circuit where the resistor that should read 106k measures 1.9k or 1.8k near enough. I can't see what the board says but the resistor is marked 107 so that sounds right. As they are all the same ie. 106k I am wondering if I need another resistor with that exact reading rather than, say 100k, or 110k?

This resistor is on the previously faulty circuit I'm sure because it is close to the problem solder joint that I had to repair, when one leg of the WCF cap became disconnected from the trace.

I will take some more measurements where you suggest later.

Edit: Hey thanks for that parallel resistor calculator, just the job lol!

 

[klnglim]

I bought MK8SE 6 months ago, unlucky it come out smoke & smell burn, so now I have to modify & change some circuits.
After change all those circuits, have noticed sound signature & bass improvement, the amp run more stable, better performance & lower temperature.

 

[SonicTrance]

Looks like you still have the stock coupling caps and then put the RIFA 330n caps in parallell? Why?

Good thing you got it working again! From the looks of one of the old cathode bypass caps there has been some serious heat inside that amp! I would also replace those burnt resistors in the ps.

 

[klnglim]

Myself actually is not very good with solder or unsolder iron on board, so I have to spend few days to watch & learn from youtube channel and other electronic websites regard about calculatie and change electronic component.
RIFA caps is for bypass which I read from the first page, I understand some people may be disagree with my idea, please explain to me or correct my mistaken anytime as I like to learn and want to know more.
In my opinion the caps is so cheap, why not have a try & experience with it?
I've been already using this amp about 6 months without any mod, run with the original high quality Electro Harmonic "Gold pins" (Driver tube & Power tube).
When notice smoke & smelling bad burn happened from inside the amp, that means is time to upgrade.

So I Replaced all the Power Resistor from original 150 Ohm / 3Watts with wirewound 150Ohm / 5Watts, upgrade Capacitor 220uF/63Volt to Audio Grade Nichicon 220uF/100Volt
Install 2 x silent PC fan cooler with blue LED under the case, I used and modified my very old EXT HDD 12Volts adaptor to drive those fans.
Swap the original Electro Harmonic driver tube with Russian's Gold Lion tube (12AT7)
Totally, I spent less than 2 week began since channel, then slowly try replace every bad burn circuits inside the board.
May b this mod is not perfect sound for everyone, but I m still satisfy with the sound audio performance improvement, since after few hour's amp burn in, then next day I used high gain & run balance connection to my HD800S, I get smoother + stronger + more bass , notice the amp runs very stable and well balance when read from the from meter.

Unluckily, within 2 days of 10 hours used, half way there seeing a little bit lite smoke come out from inside the amp again but no bad burn smell no over heat happen this time.
Now I have to get involve deeper to investigate where and why the hell smoke still there.

 

[Maxx134]

I bought MK8SE 6 months ago, unlucky it come out smoke & smell burn, so now I have to modify & change some circuits.
After change all those circuits, have noticed sound signature & bass improvement, the amp run more stable, better performance & lower temperature.

Very nice job regardless of having paralleled coupling caps.
Think about replacing the cathode follower caps also..
Break the red caps with a cutters to expose thier legs, then solder the new caps to them.
That's if you not want to remove the board.
For fun buy a pair of different brand power tubes and mix.
My favorite on the MK8 was Sovttek on left & EH on right of each side.(all 6n30 type just different brand)

Unluckily, within 2 days of 10 hours used, half way there seeing a little bit lite smoke come out from inside the amp again but no bad burn smell no over heat happen this time.
Now I have to get involve deeper to investigate where and why the hell smoke still there.

Most likely it is from the resistors in your PSU after the caps and they can get burnt over time they should be replaced with the 5w wirewound as stock resistors are fake asss 2w garbage resistors that can't even hold 1watt without gettn Brown so over time they will burn.
No worries though just check and replace whenever but be careful if taking board out as the cheap adds garbage tin stranded wire that connects board to power transistors will detach...
So take pics when opening up...

I replaced all these things and just because I complain , dont think I am putting this amp down...
I can name ALL tube amps out there and call them out for using cheap non-optimal parts for cost cutting.
I'm not talkin about quality, I'm talking about parts that not as design intended like 2w resistors that are probably 1w capacity or stranded wire which should be copper but instead using cheaper tin or other garbage.

I seen $2k-$8k amps using lowly $10 caps in critical bypass areas.

The problem is the design. Most amps is using old topologies,
not like Sonic modern amp which not old school design.

 

[Maxx134]

How do I locate and fix it?

You don't fix a resistor that has lowered in value.
Something in the circuit made it lower in value.
Resistors can only increase and burn open,
They never go down in value..
Check your dip switch gain to make sure it is not stuck in middle ..

Edit: I had the board out again to check the trace on the back but I was super careful when I put it back to make sure the wires were OK first so I'm confident nothing happened there.

The garbage stranded tin wire is guaranteed to break off after about two times opening board out..
Even so, you would not have proper voltage if any problem there.

I've checked all the resistors and all are OK except for one on the previously faulty circuit which reads 1.9k ohm and should read 106k ohm. This one:

You probably "over" soldered and spilled solder somewhere there,
Or a bit of solder can be lodged in all that wiring...
Go get a can of compressed air to blow out debris from the board...

*********


Now to test if the protection circuit actually engaged...:::


1-Make sure all ready to turn on, have amp off.

2-have source already continuously playing before turning on amp.

2-Have a cheapo headphone (that not worry about breaking ) already plugged into the balanced socket before turn on amp...

3-Place headphone on head but NOT directly on your ear, in case you hear a pop..

Turn on amp...!
It Will warm up.
Listen for music from driver stage to travel thru the output tubes before they get their power, when the second meter clicks on.

The amp should have weak music playing on both sides evenly..

Once the delay finish and the second meter clicks in , try to notice if a second relay immediately clicks in (protection circuit).

If the protection circuit actually did click in,
There may be a loud pop sound before this that may or may not damage your headphones lol.

A pop sound is not enough to damage a high impedance headphone but can damage a small portable...

So the point in this test is to verify if the driver stage is working and if the protection circuit actually engaged to shut off the power tubes..

OR...

You can just buy an oscilloscope ha..

 

[klnglim]

Yes, the picture shown is just temporary as I m planning to get some good quality capacitors & resistors.
Can you write me the quantity needs and list of brand capacitors & resistors recommendation for replacement?
Anyway, I appreciates & Thanks for your kindly help

 

[Maxx134]

Yes, the picture shown is just temporary as I m planning to get some good quality capacitors & resistors.
Can you write me the quantity needs and list of brand capacitors & resistors recommendation for replacement?
Anyway, I appreciates & Thanks for your kindly help

Did you check the first page instructions?
Much info there.
You did great and I would not bother with the resistors right now...

Just the WCF bypass caps replace with same size & brand gold Square ones you used for coupling cap to make match...

Then I would skip all else and do the driver bias mod upgrade.
It is talks about but I haven't checked if we posted it.

An optimal mod would be a CCS there but haven't tried it yet.

Those are sound quality mods that are immediate.

As for the rest,
Don't bother with output mod on your mk8 yet but can try as a last mod to do to match to your headphones..

 

[coinmaster]

Oh boy, so many words to read through on this page. Baron I told you a long time ago I would skype you to help you fix it. My offer still stands. It would be very simple to deduct where the issue is if I could guide you in real time.

 

[klnglim]

I have read the first page at least 10 times more, still confusing a bit cause the mod between MK6 & MK8SE are mix up togather, that's why I only replace all those burnt capacitors & resistor
Before I had changed resistors 150R 3W to 300R 5W (Vishay brand), the amp turn on like normal but no sound receive from headphone, then again I changed to 150R 5W (Vishay brand as well)
at last, the amp run very stable and heat temperature is lower compare with the original brand new out from factory.
The reason I bought this amp is to drive my headphone HD800S as I didn't like the sound drive by solid state amp, which missing low end bass. Honestly, I like the design of the amp but very disappointed with some crap components inside the board.
In my opinion, modding this amp will improve the performance and get the better sound quality more than I expected especially the bass