kevzep
Its all about the Music
The amplifier is not operating correctly, lets do one thing at a time......lets get the amplifier operational and then worry about the protection circuit.have you checked it ?
The amplifier is not operating correctly, lets do one thing at a time......lets get the amplifier operational and then worry about the protection circuit.have you checked it ?
can you point out the coupling caps ? maybe my schematic is wrong .Yes, and do check the coupling caps. They're at the age where they need replacement. Biased up properly once serviced, you should be good for a long time. Do begin with voltages at fuse 3 and 4.
Ok, they are both on the same channel tho? I can check the voltages on all 4.Yes, and do check the coupling caps. They're at the age where they need replacement. Biased up properly once serviced, you should be good for a long time. Do begin with voltages at fuse 3 and 4.
They are part of the service bulletin...can you point out the coupling caps ? maybe my schematic is wrong .
I was able to set the bias to 25mv measuring at emitter resistors ...is the bias ok?
Well you can try to measure DC offset on a capacitor coupled output stage, but you might get erroneous results without a suitable load connected.You can't measure DC at the speaker terminals
The unit has no DC offset adjustment,Correct me if I'm wrong, but the 3000A is capacitor-coupled. You can't measure DC at the speaker terminals. You need to measure off of the collector of the output transistors. There are 2 for each channel, and one should be set 1/2 voltage of the the other one. For example, start with the left side. One should be somewhere around 70v or so, then set the other left one at 35v. Then do the right side transistors.
Also, the DC must be set before setting bias because adjusting DC balance WILL affect bias. If you turn up the DC balance voltage, the bias will lower, and vice-versa.
Yes, Kev touched on this in post #16 - seems like 2 meters would be a very good idea.I believe by adjusting both rails of each channel equally, the DC should be minimized.....I think?? You need 2 meters to adjust bias, as both adjustments affect each other.
That is my understanding. (personally I wouldn't even consider putting one of these into regular use without performing the Service Bulletin modifications).Original design wouldn't be considered cap coupled ,would it? The service bulletin safety mod added in the caps afterwards to prevent speakers from catching fire.
If you don't have a load on the speaker terminals the caps can have a DC voltage there because there is nothing to discharge the cap except the input resistance of your meter which could be over 10M Ohms. You have to check the DC offset at the input side of the output coupling caps. There are 2 adjustments, which are really both bias adjustments, and they affect each other. You get the bias close on both transistors, then tweak one of the adjustments for the DC offset to be 0V.We realise this, my point was how on earth is the DC getting through the output caps.....
The 4000 is totally different. Bias has no effect on offset on the 4000.On my 4000, adjusting the bias didn't affect the DC balance all that much.
IIRC, the DC affects the bias directly. (Although conversely. )
I only had one MM, so it was a pain going from the fuse holder to the transistor collectors, but it can be done. Work carefully.
Ahhh. Ok ! Now I have something . Makes good sense.... I initially had a load on speakers, but I was also checking the dc at speaker outputs....If you don't have a load on the speaker terminals the caps can have a DC voltage there because there is nothing to discharge the cap except the input resistance of your meter which could be over 10M Ohms. You have to check the DC offset at the input side of the output coupling caps. There are 2 adjustments, which are really both bias adjustments, and they affect each other. You get the bias close on both transistors, then tweak one of the adjustments for the DC offset to be 0V.
I thought he had a load on it which is why I was wondering whether the mod was actually implemented, how else would there be that much DC on the output?If you don't have a load on the speaker terminals the caps can have a DC voltage there because there is nothing to discharge the cap except the input resistance of your meter which could be over 10M Ohms. You have to check the DC offset at the input side of the output coupling caps. There are 2 adjustments, which are really both bias adjustments, and they affect each other. You get the bias close on both transistors, then tweak one of the adjustments for the DC offset to be 0V.
I did have dummy loads attached for the first measurements, but from what Dr.Audio states, I need to be measuring st the inputs of the coupling caps?I thought he had a load on it which is why I was wondering whether the mod was actually implemented, how else would there be that much DC on the output?