Hello!
I purchased a beautiful Musical Fidelity B1! Now I have a many "noob" questions! If you don't like noob questions, please, go to another thread!
So, I'm not into electronics (I like it, but I'm not a specialist). I want to do a overhaul to my machine!
Can I replace the original 2N3055 with 8 2N3055? I mean, I hear that I need matched pairs, can't i just ask for 8 from the same lot? Do they need to be XPTO "audiophile" or whatever, or can I buy regular ones?
I can get them for 1€ each (at my local shop). Too cheap?
Can I use regular (white) mass to put them in the dissipators?
I just need to desolder and solder the new ones in the position of the older, right?
And how about the electrolytic capacitors. What brand should I use? Should I replace them like the originals or can I upgrade it (like to 105º degrees).
I already gave a good cleaning (with a contact cleaner) and also replaced the on/off!
One thing: The highs are too high... this happen after the clean...
Another thing: This beats is hot! I mean, really hot.
Very much appreciated!
I purchased a beautiful Musical Fidelity B1! Now I have a many "noob" questions! If you don't like noob questions, please, go to another thread!
So, I'm not into electronics (I like it, but I'm not a specialist). I want to do a overhaul to my machine!
Can I replace the original 2N3055 with 8 2N3055? I mean, I hear that I need matched pairs, can't i just ask for 8 from the same lot? Do they need to be XPTO "audiophile" or whatever, or can I buy regular ones?
I can get them for 1€ each (at my local shop). Too cheap?
Can I use regular (white) mass to put them in the dissipators?
I just need to desolder and solder the new ones in the position of the older, right?
And how about the electrolytic capacitors. What brand should I use? Should I replace them like the originals or can I upgrade it (like to 105º degrees).
I already gave a good cleaning (with a contact cleaner) and also replaced the on/off!
One thing: The highs are too high... this happen after the clean...
Another thing: This beats is hot! I mean, really hot.
Very much appreciated!
They will just be standard 2N3055. According to Mark Hennessy, the later revision used 2N5882 instead. Consider using MJ15003 instead.
Use good quality 105c rated capacitors. No particular brand other than a good quality one - Nichicon, Rubycon, Nippon Chemicon, Panasonic, etc...
Beware of fake transistors. If a supplier has them much cheaper than reputable suppliers such as Farnell, RS, Digikey etc - they are probably fake. This site is a good reference on this subject: Counterfeit Transistors
Use good quality 105c rated capacitors. No particular brand other than a good quality one - Nichicon, Rubycon, Nippon Chemicon, Panasonic, etc...
Beware of fake transistors. If a supplier has them much cheaper than reputable suppliers such as Farnell, RS, Digikey etc - they are probably fake. This site is a good reference on this subject: Counterfeit Transistors
The B1 is not a large amp, only 32W in class AB so it will get quite warm with 100 mA bias current, as the following attached thread in another forum advises. Is it possible you moved the bias adjustment pot. settings when cleaning? That could well increase bias and consequently heat dissipation quite a lot. However, its an old amplifier and the bias problem could even be caused by the the previous owner fooling about with the bias setting.
Musical Fidelity B1 - UK Vintage Radio Repair and Restoration Discussion Forum
https://www.hifiengine.com/manual_library/musical-fidelity/b1.shtml (yes you have register for schematics on that site but its free).
BTW, I hope you didn't spray or slosh the solvent contact cleaner over the PVB to clean it. That is not what it's for.
I will try to get the 2N5882 or the MJ15003! And thanks for the page warning about fake transistors!
I wiil try to get one of those brands!
Maybe. But I believe the source selector is falling, because audio is leaking, specially in the right channel!
I will try to find out that "set bias" in the PVB! (I haven't a clue where it is)
I did. Is it bad? But I mostly did it around the contact area, since it had LOTS of dust and dirty! Now I feel bad for doing it!
I leave a photo of the internals of my MF B1 MK1!
An externally hosted image should be here but it was not working when we last tested it.
I do not recommend changing any transistors.
If they are working leave them alone.
The four big electrolytic capacitors may be nearing the end of their life.
Try measuring the Vripple on the supply rails at the capacitors.
I don't know what Vripple this amplifier gives, but there are many here who may have measured it with good capacitors in place.
The small electrolytic capacitors may be nearing the end of their life. There may be a benefit in replacing these.
Both these and the big ones should probably get replaced with 105°C, or 125°C versions, if they fit the PCB.
Do not replace any of the plastic film, or ceramic capacitors.
If they are working leave them alone.
The four big electrolytic capacitors may be nearing the end of their life.
Try measuring the Vripple on the supply rails at the capacitors.
I don't know what Vripple this amplifier gives, but there are many here who may have measured it with good capacitors in place.
The small electrolytic capacitors may be nearing the end of their life. There may be a benefit in replacing these.
Both these and the big ones should probably get replaced with 105°C, or 125°C versions, if they fit the PCB.
Do not replace any of the plastic film, or ceramic capacitors.
Some solvents are OK on a lot of plastics, coatings and labels used in electronics. Without testing first, you can't know what will happen. A fairly safe general purpose solvent for consumer use is IPA (isopropyl alcohol) but don't substitute methylated spirit (industrial alcohol) which usually contains other solvents which can do damage to plastics and you.
However, there's nothing wrong with removing the pcb and brushing under hot soapy water, rinsing and drying in a warm oven.
The main concern is not to disturb the settings or contaminate the resistance tracks of the 4 open trimpots seen just above the power transistors in your pic. If so, they could need resetting but first measure the voltages across the large 0.47 ohm resistors connected to each 2N3055 transistor's emitter lead in the upper transistors and the collector (case) of the lower pairs and then calculate the bias current for each transistor, using ohms law.
Compare with the recommended and actual measurements posted and then consider if the parallel pairs are acceptably close in their individual currents to the suggested current for all the output transistors.
If you look a the schematic I linked above and read the Vintage Audio Forum thread, you'll be able to follow the procedure of bias setting, which isn't straightforward, given the unusual circuit.
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I find it easier to use two DVMs. One monitoring approximately 80mV between the emitter of one 3055 on the top and one emitter of the lower 3055 whilst checking the DC offset is as near to 0v as possible.
If both presets are a little high with respect to ground, your DC offset will be too high but the quiescent current will look OK.
I get a few that have been twiddled and one pair run hotter than the other due to DC on the speakers.
If both presets are a little high with respect to ground, your DC offset will be too high but the quiescent current will look OK.
I get a few that have been twiddled and one pair run hotter than the other due to DC on the speakers.
I had a B1 in the shop not too long ago. It took some major fiddling to figure out how the two bias trimmers interacted. Using multiple DMMs and a distortion analyzer I got the procedure down pretty well, don't ask me now though. It was very obvious that the stock output transistors weren't matched in this particular unit but I was able to get the idle current and DC offset adjusted for minimum distortion without a lot of heat. Can't wait to get my paws on another one as it sounded pretty damn good.
Just remembered all of the adjustments were done without a load.
Craig
Just remembered all of the adjustments were done without a load.
Craig
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This one says it's safe to use on very delicate electronics and I solder right after...
OKAY... I maybe "touched" on those... Now I know why one channel is louder than the other! (I believe you mean those 4 "pins", right?)
I will try to "look" and understand... since I'm very noobish in these stuff!
B1 Multisim 12
Here is Multisim12 model of the output stage of the B1. This may help to understand the operation better.
Here is Multisim12 model of the output stage of the B1. This may help to understand the operation better.
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