After 50 years getting my feet wet. I bought a Dynaco ST-150 and have a 9db difference between channels on output. I have reset the bias and dc offset and they’re right on. My question is really about the oscilloscope and the design of this amp. The negative speaker terminals are grounded to the chassis(2 wire cord). When I read the chassis/negative speaker terminal to the ground on my oscilloscope lead, I get 20 M ohms. Am I safe using this probe on the output terminals of this amp given its design? My apologies in advance if this belongs in equipment but I wanted to reference the design of this particular amp in the question.
Oscilloscopes typically have a ground clip that attaches to the probe about 4" from the end. The ground clip needs to be attached to speaker ground. The closeness of the ground to the signal is very important at 10 mhz and above, but not very at audio frequencies. I typically attach an alligator clip on a banana plug cable to the signal ground jack on the scope. Not the case ground jack. Then connect that alligator clip to the speaker return. As I don't use a scope, haven't done this in 25 years. I use a DVM on DC & an analog VOM on AC scale.
Dynaco practice up to 1966 included no 3rd pin on the power cord. If your unit is similar as you said, then there should be no connection through the wall plug to the chassis of your oscilloscope.
High quality oscilloscopes also have the signal ground isolated from the case ground, which should be connected to the 3rd pin of the power cord. Some oscilloscopes have a metal bridge between two binder banana jacks to swing down and connect signal ground to earth ground. This device sometimes gets lost before a scope is sold on to an amateur.
Extremely bad ST150 could build up charge between the case ground and the wall plug 3rd pin, but my ST120 and ST70 have shown no such tendency to build up static charge.
As far as safety, remember the rules. Never touch the unit or any metal with two hands. Electricity >25 v from one hand to the other can stop your heart. Never wear jewelry on hands wrists or neck when working on electronics. Even 1 v @ 25 A through a ring could burn your flesh to charcoal. Don't touch any metal until you measure it as <1 v to case ground. Wear safety glasses, overstressed components sometimes explode. Melted solder is a liquid and can splash in your eyes.
Finding this problem will likely involve probing the unit with the power on, so train yourself to stay away from the end with the power switch & fuse back terminals. I use a battery FM radio with a Y cable to the two RCA inputs to get the same signal on both channels. If you tune to a rock station you can see the beats of the music in the signal. Look to see where the voltage differences start between the two channels. The problem is right before there.
A 60 w incandescent bulb in series with the AC plug can limit the damage if you miss probe and short something to case ground. I have my bulb socket in a grounded metal case with a circuit breaker. That way if the wire on the light bulb socket pops loose, it blows the breaker, not set fire to my coffee table.
Happy hunting.
Dynaco practice up to 1966 included no 3rd pin on the power cord. If your unit is similar as you said, then there should be no connection through the wall plug to the chassis of your oscilloscope.
High quality oscilloscopes also have the signal ground isolated from the case ground, which should be connected to the 3rd pin of the power cord. Some oscilloscopes have a metal bridge between two binder banana jacks to swing down and connect signal ground to earth ground. This device sometimes gets lost before a scope is sold on to an amateur.
Extremely bad ST150 could build up charge between the case ground and the wall plug 3rd pin, but my ST120 and ST70 have shown no such tendency to build up static charge.
As far as safety, remember the rules. Never touch the unit or any metal with two hands. Electricity >25 v from one hand to the other can stop your heart. Never wear jewelry on hands wrists or neck when working on electronics. Even 1 v @ 25 A through a ring could burn your flesh to charcoal. Don't touch any metal until you measure it as <1 v to case ground. Wear safety glasses, overstressed components sometimes explode. Melted solder is a liquid and can splash in your eyes.
Finding this problem will likely involve probing the unit with the power on, so train yourself to stay away from the end with the power switch & fuse back terminals. I use a battery FM radio with a Y cable to the two RCA inputs to get the same signal on both channels. If you tune to a rock station you can see the beats of the music in the signal. Look to see where the voltage differences start between the two channels. The problem is right before there.
A 60 w incandescent bulb in series with the AC plug can limit the damage if you miss probe and short something to case ground. I have my bulb socket in a grounded metal case with a circuit breaker. That way if the wire on the light bulb socket pops loose, it blows the breaker, not set fire to my coffee table.
Happy hunting.
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Replace small electrolytic capacitors in the signal circuit. Check trimmers, switches and regulators.
Thx guys. Will set up the light bulb and start with the capacitors. From another source, R9, R10 and C4 determine the gain of the amp so if your offset and bias are good, measure R9, R10 and change C4. This is for a PC-43
My bias offsets are 75 and 76 ma and are supposed to be 75ma so good there. The dc offset at the speaker terminals is 1.5 and 3.5mv so within spec there as well. I did go ahead and test R9 and R10 on both channels and they are good. I’ll have to pull C4 and test.
Back on the scope, why doesn’t the resistance test between that ground link on the scooe probe and the chassis/negative soeaker terminal on the amp show as an open circuit? Why dies it show 20Mohms?
My bias offsets are 75 and 76 ma and are supposed to be 75ma so good there. The dc offset at the speaker terminals is 1.5 and 3.5mv so within spec there as well. I did go ahead and test R9 and R10 on both channels and they are good. I’ll have to pull C4 and test.
Back on the scope, why doesn’t the resistance test between that ground link on the scooe probe and the chassis/negative soeaker terminal on the amp show as an open circuit? Why dies it show 20Mohms?
You could do a current leakage test on the dynaco transformer. Hammond organ transformers with cotton insulation tended to leak in their old age & shock the player if he touched the metal bars. They were installed in churches with leaky roofs frequently. A circuit consisting of a parallel 10k 2W resistor and a .01 uf ceramic cap was put between a test probe and a known earth ground (like a water or gas pipe). A AC voltmeter of >5000 ohm/volt was put between test probe and known earth ground. The test probe was touched to chassis ground while the unit was turned on to AC. More than 4 volts, the transformer was to be replaced.
My dynaco transformer has never tingled my fingers but I have always maintained my roof. What brand scope is this? I might suspect more likely power transformer in a scope not made by HP or Tek.
Note ST150 has no volume pot, tone control pot, or source selector switches to go high resistance. Does have various e-caps.
My dynaco transformer has never tingled my fingers but I have always maintained my roof. What brand scope is this? I might suspect more likely power transformer in a scope not made by HP or Tek.
Note ST150 has no volume pot, tone control pot, or source selector switches to go high resistance. Does have various e-caps.
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I’ll get the parts and test leakage. In the meantime is it a problem to connect that oscilloscope to the speaker terminals with that 20Mohm resistance? Will I short the amp output?
20 megohms is a long way from a short. But if your measurement is 20 milliohms then that is a short. If you sense a problem the AC current leakage test to earth should come first.
Haven’t tried the leakage test but had some great results today. R9 and R10 were identical on both channels but C4’s were not. In fact they were both bad when I pulled them, just one worse than the other. I replaced them and now both channels are perfectly balanced. I was going to swap output transistors tomorrow but I wanted to check waveforms before and after so I guess I’ll have to wait for parts to do the leakage test.