So I found an STR6065 in a pile of junk at our local huge used electronics store with a sticker that said "$30, does not power on!".
Judging by the *huge* layer of dust, I knew it had been there a while so it was dealing time....I ended up with this unit for $15. 😀
Upon initial inspection, I found a massive collection of cat hair and dust, one blown fuse and a shorted output transistor. The fuse was changed, new outputs fitted (drivers tested good) and new bulbs put in. The PSU board was recapped. The unit was thoroughly cleaned. I used 2ST5949 as the new outputs and installed new mica insulators and compound.
Power it on..no bang, no smoke. A soft green glow comes from the dial. Awesome....or so I thought.
The first thing I note is the FM is practically dead. No tuning indicator motion when tuning but if you turn the volume all the way up, you can hear a faint station coming in. The volume is just a hair above the noise but it seems clean. FM stereo doesn't work obviously but the FM muting is operational.
AM works great. I was picking up stations from literally all over the country with just the ferrite rod. I'm impressed.
I then connected speakers. The right channel comes up perfectly, the left has a low distorted output followed shortly by the smell of a burnt resistor. If I disconnect the speakers or use just headphones, the amp doesn't burn the resistor any further.
All semiconductors in the left channel side of the board test OK, caps are good too....this one is gonna be fun.
If you have any tips or better yet, tips and a nice high res schematic that would be awesome. The protection circuit in this looks very strange to me and I'm having trouble following it.
Judging by the *huge* layer of dust, I knew it had been there a while so it was dealing time....I ended up with this unit for $15. 😀
Upon initial inspection, I found a massive collection of cat hair and dust, one blown fuse and a shorted output transistor. The fuse was changed, new outputs fitted (drivers tested good) and new bulbs put in. The PSU board was recapped. The unit was thoroughly cleaned. I used 2ST5949 as the new outputs and installed new mica insulators and compound.
Power it on..no bang, no smoke. A soft green glow comes from the dial. Awesome....or so I thought.
The first thing I note is the FM is practically dead. No tuning indicator motion when tuning but if you turn the volume all the way up, you can hear a faint station coming in. The volume is just a hair above the noise but it seems clean. FM stereo doesn't work obviously but the FM muting is operational.
AM works great. I was picking up stations from literally all over the country with just the ferrite rod. I'm impressed.
I then connected speakers. The right channel comes up perfectly, the left has a low distorted output followed shortly by the smell of a burnt resistor. If I disconnect the speakers or use just headphones, the amp doesn't burn the resistor any further.
All semiconductors in the left channel side of the board test OK, caps are good too....this one is gonna be fun.
If you have any tips or better yet, tips and a nice high res schematic that would be awesome. The protection circuit in this looks very strange to me and I'm having trouble following it.
Emitter resistor measurements.
Resistance is within tolerance, left channel (the bad one) has ZERO volts across the emitter resistors. The right channel has 31mV.
Resistance is within tolerance, left channel (the bad one) has ZERO volts across the emitter resistors. The right channel has 31mV.
I'm not certain how, but FM is operational now. I did some transistor checking on the power board and re soldered them and the FM radio came to life! I blame old glue.
So now I have a fully functional STR6065 except for the left power amplifier. Scope shows low voltage output when loaded (10V max) and what appears to be crossover distortion.
So now I have a fully functional STR6065 except for the left power amplifier. Scope shows low voltage output when loaded (10V max) and what appears to be crossover distortion.
Doh! Bad transistor socket....this is fixed and this bad boy is up and running with 45V into an 8 ohm dummy load with 1 channel driven. That's 253 watts at clipping.
Now to align the FM tuner and oil the wood case.
Ewww.....DC offset of 85mV. Now corrected to 1.4mV 😀
Now to align the FM tuner and oil the wood case.
Ewww.....DC offset of 85mV. Now corrected to 1.4mV 😀
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