You just never know sometimes when something you cause yourself can throw a monkey wrench in the gears, you are right about that. I was just polishing up a Pioneer SX-636 receiver, fixed a bad bias pot, adjusted the bias, checked the offset, did a full power run and everything looked good. Then I had watched a video on the same one someone else had done and they mentioned the problematic Sanyo coupling caps on the power amp input, so I thought I should check the esr on those caps on mine before buttoning it up. I use a MESR-100 ESR meter, which uses these alligator clips with rubber booties on them. The receiver was unplugged, and I did short the capacitor that was to be tested. I was connecting the meter leads to the capacitor on the component side of the board. The clip popped off and bumped into the base lead of the bias transistor and shorted that to the metal body, which happens to be the collector. There was a pop as there was a capacitor in the -41 volt supply that still had a charge on it. I plugged it in to my dim bulb tester, and good thing, because it lit up brightly. The 2SA777 driver transistor was now shorted collector to emitter. I replaced the complementary pair with some substitute transistors, but not the bias will not come down to 20mv, it is at 28mv with the bias pot cranked all the way down to minimum (CCW). The hardest most time consuming part of working on this old transistor gear is trying to find replacement transistors that will work.
When I have this issue, I check the bias transistor beta, and find one with a higher gain. You can also decrease the feed resistor(s) in the bias adjustment circuit or lower the bias pot value to compensate. All you're trying to do is control the bases of the AB driver transistors.
@@JordanPierYes that would work, but I think if I can find some driver transistors with lower gain that would be an easier fix. The beta on the originals were hFe 110, thereabouts. The replacement subs were 208 and 209 hFe. Using KSA1013YTA/KSC2383YTA as subs for the 2SA777/2SC1509 drivers. The bias regulator transistors are known to be problematic in these and should be changed with T0220 devices which are more reliable, but really more than I wanted to get into with only a 25 wpc receiver. Thanks for your response, your expertise is much appreciated.
I like how you modifed your transistor checker, to get away from those annoying small clip. Do you have any video’s on using an oscilloscope? ALSO is it easy to modify the transistor checker?
I looked at this receiver when it was brand new in 1978 and the fake plastic heat sinks and plastic back panel were a real turn-off. Kenwood started making some very stupid build and marketing decisions, I think because Pioneer was killing them in market share. Luckily the quality of their seperate amps and tuners stayed high for a little longer than their receivers.
Any thoughts on HK AVR 8000 no sound out of left and right channels. it powers up and does Not go in safe mode. its not speakers all have been checked with cables, Thanks in advance
Dear Mr. Pier, Just an excellent video on these repairs, question, what is the bias adjustment for, and how does this effect the operation of the transistors. Thank you kindly sir.
Think of the bias as like the idle speed on a car. Too low and the car will stumble or stall. Too high and you can damage the engine. Bias is the same way. It keeps the transistors at ready to minimize distortion from the time it takes them to go from off to on. Too low and the transistor will not come in quick enough, causing notch distortion. Too much and the transistor is full on all the time, which wastes energy by dissipated heat and can also cause distortion because of time from full on to off state.
I have a kenwood KR-3600 with a left "fuzzy" channel. It's not a dirty pot or selector switch, how can I tell if it's potentially a bias issue and any recommendations on what to check without a scope? Is there a common component that fails on these Kenwoods that cause a fuzzy channel ? Thanks
@@tbone9694 bias can be checked across the base-emitter junction of the offending channel. Should be around .56v Too low and you'll get notch distortion which could be described as "fuzzy". Also, with a scope connected to the output (under load) you'll see the notch in a sine waveform
If you are testing transistors which are only marginally bad for hfe and leakage, then I think you could make very good use of a curve tracer to save time. Transistors like that will always or almost always be easily revealed to be unambiguously bad by their collector breakdown voltage and their saturation characteristics.
They don't come in for service often. Common problems include failure of power switch, noisy pots and switches, bad contacts on the speaker selector switch, failed embedded resistors. Wanna talk about fixing yours? Inbox me vintageavrepair at gmail dot com
Great Jordan! that was very interesting. I have a general question: I have a Sherwood RA-1142 receiver, it has no bias or dc offset adjust, how does this kind of amp regulates itself?
Precision matched transistors for the differential amplifier stage, low tolerance parts in the bias circuit and proper feedback to monitor what the amp is doing.
Yep. Tuner needs cleaning. Uses CRC QD to clean it, run the tuner back and forth several times and let dry for a couple hours. Do t use anything else to clean it
@@michaelmeyer1310 no, the condenser assembly. Requires disassembly of the receiver. Look for a good tech to send it to familiar with vintage audio. It can be very easy to make things worse if you do not understand
