Hi there,
The Marantz receiver I'm currently resurrecting (I hope) has 0.18 ohm 3W Noble dual emitter resistors. I can't find this value - would this have been a custom non standard value made for Marantz?
Will a 0.22 ohm suffice, and if so, what needs to be changed if anything (other than the bias being reset).
Can anyone name any suppliers other than mcminone.com ?
Cheers and thanks
Stuey
The Marantz receiver I'm currently resurrecting (I hope) has 0.18 ohm 3W Noble dual emitter resistors. I can't find this value - would this have been a custom non standard value made for Marantz?
Will a 0.22 ohm suffice, and if so, what needs to be changed if anything (other than the bias being reset).
Can anyone name any suppliers other than mcminone.com ?
Cheers and thanks
Stuey
Hi,
you are likely to find that the voltage across the higher value emitter resistor is the same, or very nearly the same, as the original without making any change in the bias setting.
The crossover distortion should be virtually unchanged if you leave the bias setting as the manufacturer set it.
However, if the original setting was poorly chosen, then an improvement is sound quality may come from a slight adjustment. But, do not risk running the output stage hotter than the manufacturer intended.
you are likely to find that the voltage across the higher value emitter resistor is the same, or very nearly the same, as the original without making any change in the bias setting.
The crossover distortion should be virtually unchanged if you leave the bias setting as the manufacturer set it.
However, if the original setting was poorly chosen, then an improvement is sound quality may come from a slight adjustment. But, do not risk running the output stage hotter than the manufacturer intended.
I hope my terminology makes sense...I'm just starting out on this stuff.
Yes, the resistors across the output transistors. The output and bias transistors and emitter resistors are open circuit on one channel and are being replaced, so the bias current is necessarily being readjusted via the trimmer resistors (also open and being replaced) anyway. This is what I meant by reset - not a different value to the spec. value.
Thanks for the replies. If anyone else has some suggestions for suppliers, I'd appreciate it, especially any Aussies...
Cheers
Stu
Yes, the resistors across the output transistors. The output and bias transistors and emitter resistors are open circuit on one channel and are being replaced, so the bias current is necessarily being readjusted via the trimmer resistors (also open and being replaced) anyway. This is what I meant by reset - not a different value to the spec. value.
Thanks for the replies. If anyone else has some suggestions for suppliers, I'd appreciate it, especially any Aussies...
Cheers
Stu
Hi Stuey,
you misunderstood my post.
It's not the current that should be set. It's the voltage across the emitter resistor that should be set. This Vre is a constant for all normal emitter resistor values.
If the manufacturer specifies Vre then copy it irrespective of what value of emitter resistor you finally fit.
If they specify a bias current for 0r18 then convert that to an equivalent Vre and use that same voltage across the new emitter resistor.
Yes, I realise that gives a different bias current, but current is apparently not the deciding factor, it is the voltage that is important and for a BJT output stage Vre should be between 15mV and 25mV.
you misunderstood my post.
It's not the current that should be set. It's the voltage across the emitter resistor that should be set. This Vre is a constant for all normal emitter resistor values.
If the manufacturer specifies Vre then copy it irrespective of what value of emitter resistor you finally fit.
If they specify a bias current for 0r18 then convert that to an equivalent Vre and use that same voltage across the new emitter resistor.
Yes, I realise that gives a different bias current, but current is apparently not the deciding factor, it is the voltage that is important and for a BJT output stage Vre should be between 15mV and 25mV.
Thanks, all of you. I appreciate the comments.
Yep, Andrew, I did misunderstand!
That's a good idea Radtech. Simple, too. Failing finding the proper parts, it might be the way to go.
I won't have the opportunity to check samples, unfortunately.
I looked at Noble's US website and they still list the 0.18 (and lower) under their type number RGC33 (the three watters) so I've fired off an email to see if they know of one of the bigger online suppliers that stock them.
They also have a sample program, so I may go down that route, above board, and ask if they'll supply direct (paid for of course).
Thanks again
Stuey
Yep, Andrew, I did misunderstand!
That's a good idea Radtech. Simple, too. Failing finding the proper parts, it might be the way to go.
I won't have the opportunity to check samples, unfortunately.
I looked at Noble's US website and they still list the 0.18 (and lower) under their type number RGC33 (the three watters) so I've fired off an email to see if they know of one of the bigger online suppliers that stock them.
They also have a sample program, so I may go down that route, above board, and ask if they'll supply direct (paid for of course).
Thanks again
Stuey
Stuey,
the easy way would be to replace a dually 0.18 by 2 single metal plate ceramic resistors.
The double resistor ceramics have the same height and same thickness, but are only slightly shorter than 2 single ones placed head to tail.
These resistors are meant to be positioned high above the boards for best cooling, spreading the leads to fit into the pad holes would be no problem.
Doesn't cost much more either than using the original ones, the duallies usually cost a little less than two singles.
Using dually resistors instead of single ones saves placing a few components on the boards, a benefit for high volume production.
ps:
The sizes of MPC series Fukushima Futaba resistors and the Nobles ceramics are as good as identical.
MPC722 dually 0.18 ohm ceramics may be easier to track, and those are 5-watters instead of 3.
the easy way would be to replace a dually 0.18 by 2 single metal plate ceramic resistors.
The double resistor ceramics have the same height and same thickness, but are only slightly shorter than 2 single ones placed head to tail.
These resistors are meant to be positioned high above the boards for best cooling, spreading the leads to fit into the pad holes would be no problem.
Doesn't cost much more either than using the original ones, the duallies usually cost a little less than two singles.
Using dually resistors instead of single ones saves placing a few components on the boards, a benefit for high volume production.
ps:
The sizes of MPC series Fukushima Futaba resistors and the Nobles ceramics are as good as identical.
MPC722 dually 0.18 ohm ceramics may be easier to track, and those are 5-watters instead of 3.
Sony use dual emitter resistors in some of thier subs, I checked some of my sevice manuals and the SA-WP890 and SA-WMS145 use 0.22 5W, the part number is:
1-234-182-11
I wonder what sony will charge, if they have them in stock?
I have some 2x 0.18 5W single metal plates, these are pulls with short leads though, can send them if you want?
Regards
James
1-234-182-11
I wonder what sony will charge, if they have them in stock?
I have some 2x 0.18 5W single metal plates, these are pulls with short leads though, can send them if you want?
Regards
James
Well, it appears that the emitter resistors measure OK now that I have them out of circuit. Although my meter won't measure accurately down to 0.18 ohms, they're around 0.1 - not open or shorted.
The bias pots were both shorted and I assume this was what caused the destruction of the transistors.
This amp was repaired professionally before, and these semi-open style 1K pots are notable in not having been replaced. The owner said the amp worked for a while when broke again, but it was used for too long a time to take it back to the repairer. I would have assumed it imperative that they were replaced, being mere 0.25W jobs at a guess.
The Marantz SR-50 seems to be known for this fault of blowing outputs (it's not one of their best!
) so I wonder if these bias pots were of a poor quality batch. Anyway, I'm thinking of replacing them with a Spectrol multi-turn cermet style - would these likely be more reliable, or merely more easy to set accurately?
Cheers
Stuey
The bias pots were both shorted and I assume this was what caused the destruction of the transistors.
This amp was repaired professionally before, and these semi-open style 1K pots are notable in not having been replaced. The owner said the amp worked for a while when broke again, but it was used for too long a time to take it back to the repairer. I would have assumed it imperative that they were replaced, being mere 0.25W jobs at a guess.
The Marantz SR-50 seems to be known for this fault of blowing outputs (it's not one of their best!
) so I wonder if these bias pots were of a poor quality batch. Anyway, I'm thinking of replacing them with a Spectrol multi-turn cermet style - would these likely be more reliable, or merely more easy to set accurately?Cheers
Stuey
Yes, it's like talking to myself, but I may as well add to the story...in case some other nutter decides to repair one of these!
I've used the multi-turn pots, and all was going well until I tried to remove some of the smaller transistors. The solder pads are VERY thin on this unit, and as it had been repaired before, they were weakened. Anyway, some pads all but disappeared, and I had to use some thin copper wire to jumper the transistor legs to the tracks. Arrgh!
I was going to replace all of the SS TO-92 transistors (2SA970, 2SC2240) but many had been replaced before and I didn't want to ruin the board, so I'll have to leave them. They seem to measure OK in circuit.
Looks like a 50/50 issue as to whether this thing will go again. At least it's a Marantz from the disposable era!
Stuey
I've used the multi-turn pots, and all was going well until I tried to remove some of the smaller transistors. The solder pads are VERY thin on this unit, and as it had been repaired before, they were weakened. Anyway, some pads all but disappeared, and I had to use some thin copper wire to jumper the transistor legs to the tracks. Arrgh!
I was going to replace all of the SS TO-92 transistors (2SA970, 2SC2240) but many had been replaced before and I didn't want to ruin the board, so I'll have to leave them. They seem to measure OK in circuit.
Looks like a 50/50 issue as to whether this thing will go again. At least it's a Marantz from the disposable era!
Stuey
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