Every amplifier secretly wants to be an oscillator 😄 Nice work Trevor, transistor substitution can be troublesome on some amps, and without a curve tracer matching transistor pairs across their full operating voltage can be hit and miss. That 10pF cap will have no effect on the audio performance though. Thanks for sharing, I always look forward to your videos.
Thank you for your time on this video! I always enjoy your troubleshooting approach, oscillation can be a pain. This line of Sansui amps are good ones, they are aged now and this tuneup will make the owner very happy.
Excellent bit of investigative work, Trevor. As for the capacitor, I've done the same thing myself, It's so easy if you get distracted. Anyway, thanks for sharing and have a great weekend.
Hey Trevor, great video as always! I have an AU-519 that I'll be restoring so great timing. Apparently there is a problem with several low value (pF) capacitors used to stabilize the amplifier (prevent oscillation). They are known to fail and can cause serious damage when they do. Do a Google search "Sansui black flag capacitors".
Agree, black flag caps area known problem in the AU 519/719 amps. Pretty cool, Trevor that you tracked down the problem without knowing about the caps. Good to catch the oscillation before it took out the drivers.
Excellent work, Trevor. Too bad Sansui used such a classic, established and well respected name to produce pretty much garbage in their later years. But then lots of companies seem to revert to doing this. For example, I have quite a few Carver components, of note are the Carver C-1 preamp and the Carver AV-64 Magnetic Field power amp. These are solid units and built very well but they weren’t out of the reach of working individuals. Maybe 10 years later, I picked up the Carver DPL-33 surround sound processor/amplifier that was mediocre, not necessarily junk but nowhere near the quality of the other components I had. It was many years later that I found out that Bob Carver had sold the company and this was a unit designed/produced after the sale.👎 Keep up the great work and keep the videos coming…no matter the length! 👍
Hey Trevor, nice detailed video as always. Regarding the touchy trimmers - they really benefit a lot from new multiturn trimmers. Aside from that the trimmers also do go through the same conditions as your other pots like volume, balance, tone etc, so they could also be just as noisy. A new modern hermetically sealed trimmer would be advantageous for the adjustment as well as overall longevity.
Really good analysis Trevor! Do you think the bias and DC offset was bouncing around because of the capacitors in the circuit? You move the trim pot and then have to wait for the caps to settle down. Hopefully those trim pots are 25 turn pots. A less fine adjustment will really drive you mad when adjusting.
Check out LCSC dot com. I buy a lot of caps from them. They sell the cheap garbage caps and but they also sell the better 10000hr @ 105c rated. I buy the latter. The Samxon caps are rated very high, should last the lifetime of the amp. You can also buy the Nicicons, Elnas and Panasonic caps there at a higher cost if that's your choice
Yes and no. The object is to find transistors have have the same or close characteristics as the original. Generally, the sound doesn't change when swapping transistors but it's important to keep the characteristics the same otherwise service life could be shortened. If we select transistors that are too far off on specifications it could effect the sound quality
Hi Trevor. I agree but that capacitor could be at the output of the preamp, and for best signal integrity it should be made with a PTFE dielectric but that would raise the cost considerably. My own system is four * Pass F5 Turbo V3 power amplifiers, the Pass preamp has a 10 uF film cap at the output. Thank you for the videos.
@@leonidbreshnew401 The capacitors are wired in series. To a AC signal, it doesn't matter if it's plus to plus or minus to minus. Electrolytic capacitors are really only for DC circuits either filtering or blocking. If they are used in reverse polarity, they will pass current heavily, sort of like a diode but terribly inefficient unlike a diode. This is why they use them back to back. On a AC signal, one of the two capacitors will block the DC current of the other leaking capacitor (wired in reverse) on the positive swing of the signal. On the negative swing of the signal the roles are reversed and the capacitors switch roles. This allows the AC signal to pass through easily but blocks any DC currents. Using two 10uF capacitors in series gives the both a effective capacitance of 5uF. Also, when using two capacitors in series the ESR of both are added together and is higher that a single 5uF capacitor. I hope that explains it
To measure power output you need to first measure the voltage across the 8 ohm load resistor and then do the math with ohms law I = E/R (27.06 / 8) = 3.38 amps of current to the load Power (watts) = E x I (27.06 x 3.38) = 91.46 watts
@@TrevorsBenchamplifier repairing is both an art and science, requires tremendous patience. I absolutely don't mind your long videos, so much to learn. For others, there are lots of RU-vid shorts
