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Luxman R-1500, chasing left channel hiss!

Starbender

Starbender

Gathering dust...
I'm struggle with this beautiful receiver for a while. Fully recapped. It has tons of tantalum caps everywhere. All replaced with Nichicon low leakage UKL caps.

Sound of the receiver is improved after recap. But one problem remains; left channel has white noise, amplifies with volume control. If I connect efficient speakers, it becomes unbearable.

Tried different things to narrow it down. R-1500 does not have pre-power amp jumpers. I swapped power amp channels internally, hiss remains on the left channel. IMO, this indicates the problem probably lies within the pre-amp section (or power supply rails for preamp?).

3300uf 25v & 1500uf 50v caps connected to second winding of the transformer (red-brown-red).
Between these caps and the transformer there is two 2A fuses. Fuses connected to caps via 0.1uf film caps&1N4003 parallel diode. According to SM, this rail power up pre-amp and tuner sections. I decided to replace these diodes, because one of them measures strange. After diode replacement, 2A fuse connected to 1500uf caps negative leg smoked. Checked for shorts, nothing. Second attempt, fuse smoked again. According to SM rectifier diode cathode(parallel with 0.1uf cap) goes to capacitor.

What can cause problem like this. I'm sure diode replacement was correct, all surrounded parts and chassis isolated before power up.

Any idea?






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I would suspect transistors in the pre-amp, some of the transistors Luxman used in those days dont age well.
 
Alienz said:
I would suspect transistors in the pre-amp, some of the transistors Luxman used in those days dont age well.
Click to expand...

Replaced 4 preamp transistors last night. No change. I will replace other 4. One other thing is; power amp boards have two trimpots, i think one is for offset other is bias adjustments. Service manual does not have any info about test points for bias adjustments, only info at the manual is it must be 50mA.
 
I think it can be done by putting some your DMM clips on both sides on one of the big emitter resistors in the output stage. Then you have to calculate how much mV (mVOLTS not mA!) the voltage drop for that resistor would be at a current of 50mA. But I'm very bad at formulas and calculating so I hope someone else can confirm this.
 
Sounds like something has blown down the line of that supply. Perhaps a not fully discharged capacitor got unloaded on a semiconductor when you started working on the diode replacement stuff.

Your noise problem also sounds like bad transistors.
 
Baron K Rool said:
Sounds like something has blown down the line of that supply. Perhaps a not fully discharged capacitor got unloaded on a semiconductor when you started working on the diode replacement stuff.

Your noise problem also sounds like bad transistors.
Click to expand...


Noted. Also r-1500 has tons of tantalum caps everywhere. I never seen that much tantalum in any gear. Replaced them all with UKL's anyway..
 
I fixed the short at preamp rails. All rail voltages in spec. Removed L&R power amps. Adjusted dc offset to zero. Both channels dc offset bounces around 0-3mV. Service manual suggests 50mA bias current. I measured bias between emitter resistors. It was around 5-8mA! Adjusted to 30mA and waited more than an hour to tweak it a little bit. After two hours bias was stable at 30mA for both channels. I didn't want to make it 50mA, power amps gets really hot without any load at 30mA.

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I removed preamp and replaced all 2sc1345's with 1845's. This did not eliminate hiss at the left channel. When I measure the frequency from the speaker outputs,no source, volume at zero; right channel reads 50hz, left channel goes to 1.2khz after 10-15min.

I tried to narrow it down with freeze spray. Looks like left channel input stage parts is the suspect. When I spray the left channel input stage parts, frequency reading goes ~50hz. I will check caps&resistors tonight. As a note; this beauty recapped completely month ago.
 
Baron K Rool said:
Did you replace the input stage transistors (differential pair)? Common cause of issues.
Click to expand...

I replaced input stage transistors first, then the rest. Right now i'm pulling caps one by one and replaced them. Without signal tracer or scope, this kind of task is like shooting in the dark.
 
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