醜女大返身之 -- KT88 (Part II)

看過了 Part I, 是否好得人驚呢?

再看看這部份 .... 醜女是否變了靚女呢. 我這隻入行一年的貓手功如何呢?

還要多多向 Modi Sir 和其他人多多學習....

Post 住幾張相先, 部機未完全做完.  :

CAT

Just a few comments and hope you do not mind , I see one of the auricap's connectior had penetrated though several tube's connector , it should be avoid as this will easy pick up noise. The turn pot (VR) should fact towards the bottom plate  so that it is more easy for you to adjust. I am a little bit worry for you to "float" the turnpot as you will find that it will be difficult to adjust the turnpot. I am normally using those normal volume and put the shaft through the top plate so that you can adjust without open the machine. lotherwise, it is a good work.

Thank you very much .... I have no knowledge on that passing through tube socket thing. Thank you so much for telling me .... I will modify it accordingly.

You mean you use a normal potientiometer to do the job? The one that comes along (old and ancient) is 30K. So I should buy a 50K and then adjust to its settings to around 21K (I measured). Am I right?

CAT

If you cannot find a 30k pot, , you can better use a 25K + 5K resistor , this will not change the overall value but the adjustment range is smaller.

Based on the data as per attached , when KT88 work in ultralinaear mode at about 450V, the negative bias should be -59V, you should adjust the pot until the grid voltage of the KT88 to -59v before you plug the tube in, otherwise, you will blow the KT88 easily.

http://tdsl.duncanamps.com/show.php?des=kt88

Dear Accphoto,

I am very glad you tell me about this. The last thing I want to see is to have the KT88 blown. I will watch out for that.

For the time being, I may want to finish the Marantz 7 first ... as I don't have spare time to go over to YMTei to get the remaining parts for the KT88

CAT

Dear Accphoto,

I can't send the schmatics and the drawings to your enquiry@accphoto.com account. It says "unknown user". Can you give me another account instead?

Finally, I got the Marantz 7 working. Lots of efforts paid. Now waiting for the Jensens to break in. I am still suspecting some problem(s) with this M7. The first time I turned it on, I hear some "pak pak" noise once in a while. But as I turned it on the second time, no more.

CAT

"pak Pak noise" mans some shorting or the tube has problem. Check the capacitor and connection on wire......

Dear Accphoto,

The M7 problems fixed. Thank you.

The KT88 problem can't be solved. I recall I swapped the output transformer already. The original problem is from the RIGHT channel with occasion "ssss..ss.." sound.

NOW, the problem is from the LEFT channel. I can hear the sound is distorted (tho' not all the time) and when the distortion occurs, the woofer vibrates seriously and abnormally.

I already changed the 220uf / 100V capacitor. SIGH ..... :cry:

Does this imply the output transformer got problem?

CAT

Did you swap the transformer again ? if the sound go to right channel this time, then it will be definitely  the transformer problem. Look like that you really need a scope to examine the problem.....

If you change most of the component, and measure the voltage on the key point are the same on both channel, then it will be the transformer problem......

Dear Accphoto,

I don't have enough time last night as I need to finish up my other work. Will do this afternoon / evening.

I recall measuring the voltage at all the required points; the huge capacitors, the filament plate for each and every tubes. If I am not mistaken, they give very close readings. I swapped the tubes as well.

If the METAL 25W resistors got problems, there would be a voltage difference at the filament, is that right?

Sigh .... if indeed the output transformers got problems, it is going to cost me a fortune .....

:cry:

CAT

You should measure the voltage across the 25W resistor or the + and - of the 220u 100V, you should got a voltage, Normally between 50V-60V ( 500 ohms x 100- 120 ma for both KT88), if both channel within  5%, it should be ok. It will not affect the filament voltage.

Dear Accphoto,

I am NOT able to cure that problem. The LEFT channel gives off "Buuuu.." noise. This is no like the "deee....deeee" noise of poor grounding. It comes on with the power.

I resoldered almost every point related to the LEFT channel but it didn't help. I have never heard of this "buuuuuu ..... buuuuu" noise before. It is like me 在放屁. Very embarassing ....

The RIGHT channel behaved normally so far. I played two songs and the music came out clear and detailed. So far so good. As I have no idea whether the buuu... buuu noise will damage the amp., I stopped after playing two songs.

What do you think about that noise? What would have caused that? Will that be the problem of the 30K variable resistor?

CAT

Since you have one channel working now, it is easy, just compare all the major voltage of each point to see there is a big difference. Most likely some connection or component problem. If possible, get a scope and found the sound of "buu" sound. It is possible the 30K resistor had changed value as the machine is already +10 year. It is quite easy to found out by measure the actual value of the working channel and compare to the non-working channel, of course you need to soldier it out first.

Dear Accphoto,

The 30K is a NEW one. It is the TINY Blue Box you see in the photo. I tuned it to 11K for both channels (got the value from my friend's KT88). However, the two variable resistors are from different brand.

I also measured the orignial 30K I dis-assembled. They give a reading of around 21K. Just wonder whether adjusting the value will correct the "buuuu" sound problem.

Second. I need to feed in a say, 1Khz 0db signal before I can do the voltage measurement for the Left and Right channels, right?

CAT

For measurement of voltage,  no need to feed in signal, just measure it at idle.  For the 30K volume, basically, it is adjusting the bias for the upper and lower wave, if your 11K work in the left channel, it should also work in the right channel. If you got the scope, it is better you should feed in 1Khz sine wave, and adjust the pot until the upper wave as close as to the lower wave.

Just talk to a buyer of my Sp-10 per-amp, I just found out that there are  lots of CD-ROM in this forum.  

I just sold my SP-10 per-amp and all proceeds had gone to Charity  for China Earthquake.  

Dear Accphoto,

Where should I take the voltage? AC or DC? One probe to the ground or ..
I have tried with CD Player but not an amp.

I will have to borrow a oscilloscope from the lab to do that. And I have to have a CD player to play that 1KHz sound. Will try that later.

For the time being, I just hope to cure that "buuu..." noise.

CAT

Oh, I am glad that you can find a good owner for the SP-10. And generous of you to have the proceeds donated to the victims of the disaster. Our school raised over HK$ 22x,000 just from the boys and their parents as well. I am glad that many of our boys are willing to donate as well.

I don't understand what you mean by "there are lots of CD ROMS in this forum" ....

CAT

CD-ROM means read only........

For the bu bu sound,  I talk to 棠師傅,I believe that there must be some wrong connection, missing resistor or wrong resisitor value, Since you have already got one channel working, you should able compare two channel  to find the difference.

For measurement of voltage, it must be DC not AC, just compare the voltage point by point ( get the circuit and mark it). you can also find the difference. You can also connect one resistor 8 ohms to the output transformer, if the resistor getting hot for a while, then the wire for the tranformer is not correctly wired.

Dear Accphoto,

Sigh ... I have double, triple checked .... I think it is better for me to leave it for a while. Perhaps really 當局者迷 ... maybe I overlooked something.

Tonight, I changed the LAST used resistor on the left channel. For a while, the buuuu noise is gone. I was more than pleased. Then I turned it off and connect the other wires back .... when I turn it on again, the noise came back in 10 seconds.

Very disappointing.

CAT

BTW, how should I connect the 8 Ohm resistor to the Output transformer? Except the Speaker Terminal RED & BLACK, there are 5 wires; Brown, White, Grey, Blue, Black. Which TWO should I connect?

Connect to the speaker terminal, red and black, this one is loading. Look like that it is really either cold soldering and connection problem as the blu sound has disappeared, you should check what you should have done on after last resistor. Properly something is shorted.

Don't be too upsetting , this is DIY but you do enjoy when you complete the project and solve the problem......

I think the problem occurs because I did the LEFT channel at different periods of time. For the Right channel, I completed the whole thing in ONE past (2 hours or so).

The worst case, I dis-assemble everything on the LEFT channel and do it again in one pass... .ha..ha..

Just wish I can spare the time this weekend.

CAT

Still no good.

This time, even if I pull away the cable, it still gives off buuu ... buuuu noise.

It is strange. I am 100% sure about this. I have tried just connect the speaker terminals to the KT88. Turn it on for 2 mins ... no noise. The second time I turn it on, the noise comes back. If I connect the cable, noise comes back rightaway.

Sigh .... looks like I need to spend a day to dis-assemble the whole LEFT channel and try re-constructing it again.

CAT

Dear Accphoto,

I dis-assembled everything on the LEFT channel and re-assembled it again. This should get rid of any mis-match of resistors, cold soldering, etc.

The problem is still there with the LEFT channel. Very depressed .... :cry:

CAT

Sometimes, next week, we can meet together and see what happen.

Dear Accphoto,

I think we have to meet somehow. I was happy for just a while.

The 1200uf Huge Capacitors were swapped before I tuned down the 30K resistor. The left channel now is perfect.

However, the bumping of the speaker woofer comes back with the RIGHT channel after I played 3 or 4 songs. I thought it was the Right 30K problem so I tuned it down to 8K as well but it was not. After turning the amp on, the bumping of the woofer comes back right away.

As I swapped the 1200uf capacitors, now I wonder whether it is the capacitor problem. The output transformers remained cool.

CAT

Dear Accphoto,

I swapped the 1200uf capacitors again just now. The problem still exists.

I think there were a couple of mistakes I made during the previous testings

I focused too much on the "buu...buuu" sound in the LEFT channel. I did not connect the CD signal to the Right. So I presume the RIGHT channel is okay. Eventually it is not.

The funny thing is, if I don't input a signal, there is NO problem with the Right Channel. It is silent and quiet. But if I feed a signal in, very soon, music become distorted and the woofer moved abnormally. It is like the output from the amp is not stable / irregular.

My last try to confirm ... swap the Output transformer again later. If the problem now goes to the LEFT channel with CD signal in ... then it MUST be the Output Transformer.

If that is so ... a Tango pair cost > HK$ 3,700 ...... should I change them....


CAT

I think it is the 30K resistor. By guess a value of 8k may not solvethe problem as we cannot see the problem. I am still thinking of checking the whole thing with a scope to confirm the problem. Buying a Tango is too expensive if the transformer has some problem. I   suggest that you can buy a PO shan which is really of good quality and should cost you about $ 1,400. Let meet this weekend , I bring it the scope and let the machine had a full check.

Dear Accphoto,

You are right. There is something fishy about that setting.

Last night, I swapped the Output Transformers again. This time, NO obvious trouble. I played the thing for > 3 hours and everything seemed to be normal. It is only when I turn it off, the right woofer bounced mildly once (like lost of power).

I will be free on Sat > 2:00pm. You don't need to bring the scope (I think it is heavy) as we have plenty of them here in the Physices Lab.

What time will you be okay?

CAT

It shall be fine. Call you when I back to Hong Kong.

Dear Accphoto,

I am glad you can come. Just tell me what is required except the scope.
Many thanks.

CAT

A signal generator and some multi-meter should do the job.

The most important is the 1KHz signal, right?

I will have the equipment ready

CAT

Dear Cat & accphoto,

Kindly share with us the finding after u had done the troubleshooting..

TQ

I am thinking of taping it down if Accphoto doesn't mind.

As we don't do this often, it is easy for us to forget about it totally after a month or so.

CAT

Really need to thank Accphoto. He spent the whole afternoon checking the CARY SLA-70 Signature for me !

The problem of "sss...sss" noise in the Right channel is now gone. No more "buuuu.....buuu..." noise as well. The temperature for BOTH output transformer is more or less the same (but still at 107F without air conditioner turned on).

There is NO funny noise when I switch to STANDBY mode.

BUT still got one problem left. When I switch it back on from Standby mofe, the RIGHT woofer is bouncing mildly like a heart beat. No sound, just see the RIGHT woofer moving back and forth gently.... Really funny.

Do you think it is the 1200uf capacitor?

CAT

If this  happens only  when switch on the machine from standby mode? did you wait for a while after turning on the main power ? It is quite common as the tube were not stable in the first 2-3 minutes when turn on.
The reason why the speakers move may due to some DC current goes into the signal path. This may be resulted from either instable tubes or excessive current high voltage from the main power.

This should be avoided as the current is harmful to the the power supply.
You should try the followings:
1. You should swap the rectifier tube to see whether it will goes to the other channel if one rectifiier tube responsble for the each channel. If it happens again on the other channel, then the rectifier tube is likely to be faulty soon; If it repeats on the same channel, then it will be the capacitor problem.
2. You should also try not to use the standby button, which means that turn on the main power supply when the standby button on. The problem will not happen as the rectifiying tube will heat up slowly to avoid the high voltage flush when using the standby button. If the problem happen again, then likely to be the instable  rectifier tube.

Dear Accphoto,

Yes, that is exactly what Ah Yuen suggested; no more than 200uf. He said no need to go for this 1200uf high capacitance.

I will try your suggestion for a few days. The heat is now even across the two output transformer ... but is it a bit hot? Around 40 to 42C.

CAT

40-42C is very normal, Because the tube's heat will also heat up the transformer. I have made some changes in my last comment, please check.

Dear Accphoto,

I read your re-posted comments already.

The problem won't exists if I just turn off the MAIN switch. At most (once or twice) the woofer moves a little bit.

I swapped the rectifier tubes, looks better. Seldom got any trouble. The two output transformers' temperature are very closely matched now.

At stated in the last posting, no more "distortion", "sss...sss..." or "buu..buu" now.

THANK YOU very much indeed!!!!

8)

CAT

Problem fixed........  

Dear Accphoto,

I think I will go to Ah Yuen's place to get some good KT88. These Chinese tubes got lots of limitation, I suppose.

CAT

Dear Cat,

So finally what's the main cause of the "sss" & "bu bu bu" sound on your KT88? How you both solve it? is it because of faulty part or connection problem or grounding problem?

[quote:c4ceefea32="alfcat"]Dear Accphoto,

I think I will go to Ah Yuen's place to get some good KT88. These Chinese tubes got lots of limitation, I suppose.

CAT[/quote]

Yes, they sound harsh to my ears. Get a Gold Lion Replica KT88 , they are good but a little bit expensive.  For EH, forget about it , you should just stick to CHina KT88.

Dear Accphoto,

I got a QUAD of new issue Gold Lion KT-88 from Ah Yuen this afternoon. Really understand why you use the word "HARSH" to describe the Chinese KT88 tubes. The Gold Lion give much better sound BUT the problem comes back again ..... pumping of the Right Channel Woofer.

If I switch back to the Chinese KT88, no problem at all. I think I have to adjust the 30K thing again for this pair of new Gold Lion.

The LEFT channel is perfectly okay

CAT

No worry, can adjust it again.

Dear Accphoto,

I went up to the lab yesterday and tried out that signal generator. It seems to be it is erratic. Can generate a wave but can't display properly at the oscilloscope.

Today, I use the scope with a CD player, playing that 1khz 0 db CD. The wave can be shown without trouble.

The Re-issue KT88 should be matched QUAD as stated by the label attached to them. Now, the Right Channel got this woofer problem whereas the LEFT one is perfectly okay. Do this suggest the problem of the Capacitor? Should I change them as well? They had been used for over 15 years already. If I indeed need to change them, what should I use instead?

CAT

That sort of distortion is not caused by capacitor, it will be either the 6SL7 or the transformer. Try to swarp the 6SL7 to see the problem still exist. If yes, then need to use the scope to check which stage has the distortion come out. When you use the scope to check the distortion, mainly put the scope on the signal path of each tube input and ouput, you should able to see whether the wave was distorted. Unfortunately, I cannot come on Thursday, otherwise, I can give a quick look with you.

Of course, the capacitor need to be replace soon but we should fix this distortion problem. I aussme that you check all component value and sometimes, it will be possible for some component and connection problem.

Dear Accphoto,

I swapped the 6SL7. Problem still there. I discovered something new.

NO problem exists if I don't switch on the Pre-Amp. I therefore swapped the Connecting cables from the Pre-amp. Problem still there. That suggests whenever the KT88 receives anything from the pre-amp, the RIGHT Channel will have problem; no matter the source is from the LEFT or the RIGHT channel of the PRE-AMP.

I haven't turn on the CD player. There is no signal into the Pre-amp. What could have come out from the pre-amp and cause the woofer to vibrate?

CAT

Dear Accphoto,

As it is already late, I dare not call you anymore.

Spent an hour listening to the KT88 and then let it idle for another 30 mins or so after our last conversation.

PROBLEM FIXED!

This is one interesting exercise for me instead. The curve for the Right Channel is now smooth. I haven't test the left channel yet. Will do that tomorrow. The two output transformers are of more or less the same temp again.

MANY THANKS!!

CAT