Thank you all! But I tried with the bias pot in full clockwise and full counterclockwise, they were always getting hot. I asked a local repairman that dealt with Marantz in the golden age, and he said that on the drivers you have limited adjustment of the bias pot, and you have to replace devices with the correct hfe. At least I'ts what he told me... He says that without signal, it shouldn't get hot. And yes, the thermal contact is good, the transistors and the heatsink were at the same temperature.
And yes, I measured that with my DMM on the 2000mV scale. It gave me 0.2. So it is 0.2mV... I was measuring on R748, but I can't say if it is an emitter resistor... maybe you're referring to R736?
So, any numbers?
And yes, I measured that with my DMM on the 2000mV scale. It gave me 0.2. So it is 0.2mV... I was measuring on R748, but I can't say if it is an emitter resistor... maybe you're referring to R736?
So, any numbers?
Both channels getting hot? I guess if all four transistors are on the same heatsink it might be hard to tell.
You need to measure the voltage across (not with respect to ground or the chassis) all four 0.24 ohm resistors...R747 and R749 on the one channel, and R748 and R750 on the other channel. Like I said, if the outputs are getting hot, then there is too much current flowing through these. Report back with what you measure.
As far as your friends suggestion that 'you must find the proper hfe transistor', perhaps he's a wonderful person and doesn't beat his dog and loves his wife and goes to church, but...he's wrong.
You need to measure the voltage across (not with respect to ground or the chassis) all four 0.24 ohm resistors...R747 and R749 on the one channel, and R748 and R750 on the other channel. Like I said, if the outputs are getting hot, then there is too much current flowing through these. Report back with what you measure.
As far as your friends suggestion that 'you must find the proper hfe transistor', perhaps he's a wonderful person and doesn't beat his dog and loves his wife and goes to church, but...he's wrong.
Yes Echowar: he might also be wrong. I will put again in the BC639/640, solder a few wires to the resistors, and check. I should move the bias pot until I find an approssimate 7.5mV reading as suggested in the service manual.
(you say: why wires? 'cause when I tried measuring the bias with my DMM clips, I shorted something and my amp got on flames! Just the two BC's destroyed, the others are still alive)
Let's hope it will go this way! I can get BC639s for 0.10 euros... they're cheap!
Both channels were hot: because even after just 1 minute everything was at the same temperature (heatsink, each transistor), but I may be wrong.
Tekko: I don't have a pic of this amp, but I may search... here it is: http://electronics.listings.ebay.co...gItemListQQsacatZ94898QQsocmdZListingItemList
Mine is the Model 1060...
And I already replaced everything on the board! It is a long project...
BUT ECHOWARS! i JUST REMEMBERED! I MEASURED THE BIAS FROM GROUND! I know I missed something, sorry.
Next time I will be less stupid...
I will go soon to the shop and purchase the things I need. See you!
(you say: why wires? 'cause when I tried measuring the bias with my DMM clips, I shorted something and my amp got on flames! Just the two BC's destroyed, the others are still alive)
Let's hope it will go this way! I can get BC639s for 0.10 euros... they're cheap!
Both channels were hot: because even after just 1 minute everything was at the same temperature (heatsink, each transistor), but I may be wrong.
Tekko: I don't have a pic of this amp, but I may search... here it is: http://electronics.listings.ebay.co...gItemListQQsacatZ94898QQsocmdZListingItemList
Mine is the Model 1060...
And I already replaced everything on the board! It is a long project...
BUT ECHOWARS! i JUST REMEMBERED! I MEASURED THE BIAS FROM GROUND! I know I missed something, sorry.
Next time I will be less stupid...
I will go soon to the shop and purchase the things I need. See you!
It seems u have done lot of work...
Have u checked for solder bridges, mistakes, component interchanged,dry solder,wrongly inserted transistors, etc,etc
Check whether u have shorted two nearby solder pads thinking that it is one track.
A hairline crack on a copper track is the most difficult to find.
Gajanan
Have u checked for solder bridges, mistakes, component interchanged,dry solder,wrongly inserted transistors, etc,etc
Check whether u have shorted two nearby solder pads thinking that it is one track.
A hairline crack on a copper track is the most difficult to find.
Gajanan
Yes! I've checked everything! In fact the amp works now, but gets hot, and I've understand that's a bias problem. By the way, the last time I dismounted the amp, I broke a little black component on the heatsink: in the schematic is H006, and it should be 2 diodes in series. How I can replace it? Two 1N4002 in series mounted on the heatsink? Or what?
Thank you all for the help you're giving to me.
Thank you all for the help you're giving to me.
I can't believe it even works without the diode...
The schematic says it's a SV-3A, which implies that it is the approximate equal of three diode drops, regardless of what the symbol is.
Odd that you didn't mention this before...
Very very carefully, measure the forward voltage drop with a meter set to diode test function. Should read about 1.3V or so. I'd do this with power off, and if you can't get a good reading, I'd carefully desolder it and check it out of circuit.
These are tough to get, but Pacific Coast Parts shows them for about $10 each. If you want to rig up your own diode string (cheezy, but it ought to get the job done), I'd use 1N4148's.
The schematic says it's a SV-3A, which implies that it is the approximate equal of three diode drops, regardless of what the symbol is.
Odd that you didn't mention this before...
Very very carefully, measure the forward voltage drop with a meter set to diode test function. Should read about 1.3V or so. I'd do this with power off, and if you can't get a good reading, I'd carefully desolder it and check it out of circuit.
These are tough to get, but Pacific Coast Parts shows them for about $10 each. If you want to rig up your own diode string (cheezy, but it ought to get the job done), I'd use 1N4148's.
Thanks Echowars! Yes it measures around 1.7V, but I don't need to replace it since I found a way to repair it (the strings were broken on the connection with the plastic body, just soldered them) and to mount on the heatsink. No replacement needed!
I didn't know that 3a means 3 diodes... Thank you!
I just get the parts and soldered them. The last two BC639/640 and an output transistor (blown on that bias measurement, I still feel bad about that...
). I set the bias pots at 12 o'clock just to be sure and soldered two wires to the resistor I am measuring, to do the bias safely. About the center voltage adjust, where I can measure it? What is the 30V (or 35 in my case, I got a 70V positive rail don't ask me why) point?
I wish I was braver to fire it up... but I'm not addicted to flames and sparks yet. That amp is gonna be my nightmares' main character....
Maybe tomorrow...
I didn't know that 3a means 3 diodes... Thank you!
I just get the parts and soldered them. The last two BC639/640 and an output transistor (blown on that bias measurement, I still feel bad about that...
). I set the bias pots at 12 o'clock just to be sure and soldered two wires to the resistor I am measuring, to do the bias safely. About the center voltage adjust, where I can measure it? What is the 30V (or 35 in my case, I got a 70V positive rail don't ask me why) point?I wish I was braver to fire it up... but I'm not addicted to flames and sparks yet. That amp is gonna be my nightmares' main character....
Maybe tomorrow...
Cool...those things usually break off so close to the plastic body that you can't do anything with them. Glad it's back.
You need to put together a current limiter with a light bulb in series with the power cord, then you could put a bulb in to save the amp is something goes wrong. Use a 40W or a 60W bulb for a small amp, and a 100W or so for large ones. Then you could see if it's drawing too much current. When I replace drivers and outputs, I fire up the amp with the bulb to check for shorts, and then set bias to something very very low, then power it up on full line voltage and set it to specs.
The Marantz is capacitor-coupled at the output, and is powered on only one positive rail, so to get maximum swing out of the amp, the point where the output signal meets the output capacitor should be set to exactly 1/2 the supply voltage. Ideally this should be done with the amp under full load and set with a scope so clipping happens on the positive and negative part of the wave at the same time, but unless you run the amp at full power all the time it's not strictly necessary. Check the voltage at the '+' terminal of the output capacitor and set it to 1/2 of the supply voltage.
You need to put together a current limiter with a light bulb in series with the power cord, then you could put a bulb in to save the amp is something goes wrong. Use a 40W or a 60W bulb for a small amp, and a 100W or so for large ones. Then you could see if it's drawing too much current. When I replace drivers and outputs, I fire up the amp with the bulb to check for shorts, and then set bias to something very very low, then power it up on full line voltage and set it to specs.
The Marantz is capacitor-coupled at the output, and is powered on only one positive rail, so to get maximum swing out of the amp, the point where the output signal meets the output capacitor should be set to exactly 1/2 the supply voltage. Ideally this should be done with the amp under full load and set with a scope so clipping happens on the positive and negative part of the wave at the same time, but unless you run the amp at full power all the time it's not strictly necessary. Check the voltage at the '+' terminal of the output capacitor and set it to 1/2 of the supply voltage.
Hi Giaime,
You will do fine. Don't let that diode short to the heatsink. The center voltage will end up being slightly less than 1/2 V+. You could just set it for 33V and you wouldn't be very far off. Besides, you shouldn't be that close to clipping and be picky at the same time.
You are already "baptized in fire" so you have no worries.
-Chris
You will do fine. Don't let that diode short to the heatsink. The center voltage will end up being slightly less than 1/2 V+. You could just set it for 33V and you wouldn't be very far off. Besides, you shouldn't be that close to clipping and be picky at the same time.
You are already "baptized in fire" so you have no worries.
-Chris
When you test for symetrical clipping under full power, the supply will drop a little. Therefore, the half voltage point will be lower than if the supply is just sitting there unloaded. A droop in the mains voltage will also cause an error in the downward direction. The upshoot of all this is that the setting isn't extremely important. You just want to be in the ballpark. By the same token, if you set using a 'scope and a 4 ohm load, your setting will be lower still. As you know, a speaker impedance is not constant, so again the adjustment is approximate.
Translation: Don't sweat this adjustment too much, just put it in the ballpark.
-Chris
Translation: Don't sweat this adjustment too much, just put it in the ballpark.
-Chris
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