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Old 27th November 2014, 03:32 AM   #1
trebolo is offline trebolo  United States
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Question Cant adjust bias on rebuild? Please help?

Hello

I have recently completely rebuilt the left channel amp board of my Nikko 1415 receiver

I replaced all components due to a catastrophic failure of the shorted outputs and drivers , burnt resistors etc...

The previous tech had installed drivers in the VA stage that were rated at half the voltage required and possibly fake outputs.

I have replaced the parts with parts recommended by the great people on audiokarma where necessary

I have turned it on following all safety measures and using my old variac. No smoke looks good, comes out of protect ..... one problem so far

I check the voltage at the test points to adjust for 18.0 mv on the bias circuit
And i cant get it to respond - It just jumps up and down thumping

I hooked up some speakers(test pair(cheap but good ) and i hear and see a strong pulse like a heart beat out of the side in question about 80 beats per minuet

Any pointers for me to check? I am applying no signal to the input as per instructions to set bias pot. And amp is not going into protect mode yet. Aslo no dc at speaker terminals

The pulse is about 700mv ac at speaker terminal

Thank You , I am sorry to place such a burden on others , but I am lost
I will post pics of finished amp as soon as finished
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Old 27th November 2014, 01:44 PM   #2
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I find I make a lot of cold solder joints, even with this new 35 W iron. Old steel lead components get a coating of oxide on them, and rosin solder doesn't stick well. Scraping the surface with a screwdriver before soldering helps.
One detects these problems with a DVM, looking for impossible states. Component leads at two ends of a trace at different voltages. Diodes dropping more than 0.7 v forwards. Resistors dropping more voltage than the wattage would allow (v= sqrt(P/R)). Capacitors with the same DC voltage on both ends.
Look for these things, you might find your errors. Good hunting.
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Old 27th November 2014, 02:48 PM   #3
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Expect other people in the forum to think alike but few of them will spent time on this even though intention is otherwise as a starting point .

1) the Data provided do not support a conclusion
2) Why don't you take your question to the great people on audiokarma ?
3) i will have to presume that the question has been already placed there with no results but this doesn't come as a surprise to me
4) I could have a better opinion if i had a list with the replaced parts
5) Are you able to tell if the parts you have are at least original ?


in reality

--from experience i may tell you that what you listen is a wrongly oriented part can be a capacitor but can also be a poor choice or wrong choice of a semi

--Then again a unit that old might suffer from a number of issues except the power amplifier . You need to make sure that the problem is coming from the amplifier unit and is not injected to the amplifier by the preamplifier

--If as said this is only happening to the one ch the best way is to measure in cross reference with the working one but obviously if the choice of semis in the working ch is poor or wrong expect results to be very very wrong ...

Provide more data about the problem , provide a list of replaced semis and may we get somewhere

Real regards
Sakis
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Old 27th November 2014, 02:54 PM   #4
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Quote:
Originally Posted by indianajo View Post
I find I make a lot of cold solder joints, even with this new 35 W iron. Old steel lead components get a coating of oxide on them, and rosin solder doesn't stick well. Scraping the surface with a screwdriver before soldering helps.
Making sure all parts of any joint are clean is a MUST. A screwdriver? You'd get better results with a bit of fine emery paper.
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Old 27th November 2014, 03:16 PM   #5
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is the pulsing/heartbeat occurring only when you're attempting to measure the bias or is it constant?

Last edited by turk 182; 27th November 2014 at 03:18 PM.
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Old 27th November 2014, 06:23 PM   #6
trebolo is offline trebolo  United States
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Default nikko 1415

The pulsating is constant through the speaker. Even when im not probing the test points. I have removed the jumpers from preamp to main out so no signal is going to the main amp

If that helps
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Old 27th November 2014, 07:09 PM   #7
Mooly is offline Mooly  United Kingdom
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Some of these old amplifiers can be very sensitive to transistor types. If you short out the vbe multiplier to reduce Iq to zero does the pulsating stop ?
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Old 27th November 2014, 08:35 PM   #8
trebolo is offline trebolo  United States
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Default schematics and replacements for NIKKO 1415

Here is the schematic of the Nikko 1415

Ive replaced all components

I replaced the
Q701,702,704,705 = 2SA941 BL with 2SA970 BL
Q706,Q707 = 2SC2088 BL with 2SC2240 BL
Q708 = 2SA817 with same
Q709 =2SC1627 with same
Q710 =2SD381(2) with 2SC2073
Q711 = 2SC1627 with same
Q712 = 2sc945 with KSC945
Q713 = 2SA733 WITH KSC733
Q714 = 2SD381(2) with 2sc2073
Q715 = 2SB536(2) with 2SA940

I also replaced diodes
D701 VD1212 with 2 1n4148 in series
D702 VD1212 with 2 1N4148 in series
Attached Images
File Type: jpg nikko croped schematic.jpg (168.0 KB, 71 views)
File Type: jpg nikko schematic 1.jpg (179.9 KB, 70 views)
File Type: jpg nikko schematic 3.jpg (340.7 KB, 69 views)
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Old 27th November 2014, 08:58 PM   #9
trebolo is offline trebolo  United States
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I am getting lower voltage readings than in the manual. At the collectors of the input pairs about 6-7 volts less.

At the reguator input to the maun amp about 8-10 volts low. At the regurator output to the amp the voltage is the same about 10 volts low

I am not sure if this could lead to the problem with the bias voltage not being adjustable , or the constant pulse at the output

Also I am now seeing a 700mv-1.5v ac pulse at the output
And a dc 50 - 150mv pulse as well. The same pulse with both voltages ac and dc

The readings move so fast its hard to get a accurate reading
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Old 27th November 2014, 09:26 PM   #10
trebolo is offline trebolo  United States
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I noticed with no load attached , I measure no pulse. And a decent reading for the bias

Meaning I unhooked the speakers and no dummy load attached
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