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

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

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

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

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

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

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

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

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?

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

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

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

Dear Accphoto,

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

CAT

The most important is the 1KHz signal, right?

I will have the equipment ready

CAT

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

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

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

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 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

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

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

Dear WINDWSS,

I can't exactly tell what is wrong. I think Accphoto can give more details and amend my sayings if I am wrong.

It seems to me that the tubes are NOT MATCHED (the original Chinese KT88) and the Bias not set right. It is not surprising as the Cary uses other KT88 tubes in the first place. My friend swapped the Chinese tubes in for me to do the test run. He kept the expensive KT88 at his place.

An incorrect BIAS setting makes the wrong voltage flowing into the tubes. If you hook the amp onto a scope, the wave from the 1KHz CD source and the Amp output do NOT match.

Accphoto told me to adjust the BIAS (30K resistor) until the wave form is clean and sharp and the two can approximately overlap with each other.
I posted the photos here for your reference.

Accphoto said many forgot to perform this step after they finished constructing their amp. I do agree with him that this is a very important stage. I recall some time ago, another Westerner also told me the same thing --- check the amp using a scope before the actual run in.

Hope this helps.

CAT

Dear Accphoto,

This KT88 indeed brings a lot of new experiences to me.

I totally agree with you, it is very important to test out the performance of the amp by means of a scope after you finished it.

Today, I also test out the rectifying stage using the scope, to see if it is a straight line or got lots of ripples. Fortunately, after the capacitors fully charged up and the tubes warmed up as well, the scope showed a more or less straight line with no jagging.

The 6SL7 I am using right now are mine. I put them in before I do the adjustments. However, I haven't get hold of the 5u4 yet. Still trying to get some from eBay.

Since you can't come today and one of my colleagues suddenly got a meeting to attend, we don't have enough "feet" for the game. We ended up playing continously for two hours with just 5 people ... totally exhausted, ha..ha...

My colleague teaching Physics suggested me to change the 1200uf caps as well ... as they tend to deteriorate after a few years. The two on board are well over 15 years ... What do you think?

Anyways, thanks again for all the help. I am now enjoying the amp as the new KT88 runs in and the caps as well.

CAT

Dear Accphoto,

I will call Ah Yuen to see what he has to offer this afternoon or tomorrow. My Physics colleague said the amp should sound much better if those filtering caps are of better quality. He said with good caps, even if there is a high current draw, the output is still stable and you can see on the scope the line is relatively smooth.

Just ONE question. How to fill up the HOLES? Ha..ha..

I am searching for RCA & GE 5U4 already. Just no luck as bidding ends at either 3am or 5am ... You typed 5R4 GE. Can I use 5AR4 as well?

CAT

Dear Accphoto,

I am glad I waited for your response first.

I won 2 GE 5U4 just now. The price is okay. I ALMOST bid on a pair of 5AR4 ... ha.ha.. Now, I should go for 5R4 instead.

There are TWO holes at the rear of the Amp Chasis to hold the 1200uf capacitors. If I use Jensen 200+200uf, the size won't be the same as the Chemi-con 1200uf caps ... Therefore, I need to find some way to stuff the space in the two holes.

CAT

Dear Accphoto,

Do you know where I can get these sheet metal? They need to be at least 3" wide as the holes were quite large.

I was hoping to go over to Yau Ma Tei yesterday but it was raining terribly. This morning it was even worse and I was almost caught in the car as the water surrounding the road is well over 2 feet in certain areas.

I tried something but I am not sure whether I did the right thing. If the cap and the rectification process is good, I should have a almost FLAT wave shown from the scope, right?

What I did was I switched the scope to AC. I put the probe to the +ve and -ve terminals of the 1200uf cap. Then I turned on the power of the amp, let the cap charge up for 2 - 3 mins. Finally I input a CD signal to pull the current from the cap. The scope showed very UNEVEN waves; like that of Hang Seng Index. This is NOT good, am I correct?

CAT

Dear Accphoto,

I have drawn the diagram here. The original layout is the blue lines. The Red and Pencil markings are my thoughts for the Jensen caps layout.

Please point out my mistakes if they exist.

What should I do with the Original Cap (0.1nf / 600V round one)  + Transistor (27K 2W for draining voltage after power down)?

CAT

Dear Accphoto,

Is this cap already dried? The paper seems to be a bit dry though I can still tell it has oil on it.

The yellowish thing is the solidified oil. If I use a cigarette lighter to heat it up for 5 - 10 seconds, the oil will melt back into liquid form. Then it becomes solid again.

So, this cap is no good, right?

CAT