jacco vermeulen said:
Yup!
There are enough of those revisions (cut traces, parts mounted off the PCB that were originally going to be on it, large parts mounted on the underside of the PCB because of insufficient space, etc) to suggest that these early examples were not really ready for primetime. A simple PCB rev would have significantly cleaned things up, but who knows. Maybe issues were discovered after the amps made it into the field.
It would not be the first time that's happened with high end electronics!
Yesterday I gave the working monoblock about 4 hours to warm up to a stable operating temperature.
The DC offset hovered between -10mV to +10mV but was not stable.
The target output device bias looks to be around 20mA per transistor.
Here are the voltage drops across the 1R emitter resistors:
NPN Bank:
21.8mV
21.1mV
21.2mV
19.3mV
19.6mV
21.0mV
21.2mV
20.7mV
21.2mV
19.6mV
PNP Bank:
22.2mV
20.6mV
19.2mV
18.0mV
19.3mV
18.7mV
20.0mV
19.6mV
18.3mV
20.2mV
I have sent off an e-mail to Rowland service to see if this bias is correct and if they have any spare output devices available.
Any comments?
Thanks!
The DC offset hovered between -10mV to +10mV but was not stable.
The target output device bias looks to be around 20mA per transistor.
Here are the voltage drops across the 1R emitter resistors:
NPN Bank:
21.8mV
21.1mV
21.2mV
19.3mV
19.6mV
21.0mV
21.2mV
20.7mV
21.2mV
19.6mV
PNP Bank:
22.2mV
20.6mV
19.2mV
18.0mV
19.3mV
18.7mV
20.0mV
19.6mV
18.3mV
20.2mV
I have sent off an e-mail to Rowland service to see if this bias is correct and if they have any spare output devices available.
Any comments?
Thanks!
jacco vermeulen said:
I also think that 20mA is on the low side.
We'll see what Rowland has to say about it...if they ever respond to my e-mail.
I'm pretty sure that these are production units, albeit very early ones. They both have serial number plates on them and look correct with silkscreening, etc.
Serial numbers are below 30!
Hi Jan,
This type of thing is very common with product built in North America. It wouldn't raise an eyebrow with me.
Hi ungie,
My money is on the low bias theory. Let us know what reply you get.
-Chris
This type of thing is very common with product built in North America. It wouldn't raise an eyebrow with me.
Hi ungie,
Nicely matched set of outputs. At lower bias currents, each one will become more dependent on it's beta. This bias current reading is definitely in the class AB enclosed in a feedback loop area.
My money is on the low bias theory. Let us know what reply you get.
-Chris
No reply yet from Rowland.
I'll likely have to give them a call when I have a chance.
In the meantime I am on the lookout for a few pcs of the original 2SD555/2SB600 output devices to replace the damaged units.
My intent here is to get the bad amp up and running and do some listening.
If I like what I hear I am likely going to replace all of the output devices with MJ21193/94 and strip the chassis and heatsinks completely and have all of the metal parts stripped, re-brushed and re-anodized.
This will likely give me a better than new pair of amplifiers for little investment.
If I do replace the outputs with modern devices, should the drivers be replaced as well, or can the original 2SD555/2SB600 drivers be kept?
I'll likely have to give them a call when I have a chance.
In the meantime I am on the lookout for a few pcs of the original 2SD555/2SB600 output devices to replace the damaged units.
My intent here is to get the bad amp up and running and do some listening.
If I like what I hear I am likely going to replace all of the output devices with MJ21193/94 and strip the chassis and heatsinks completely and have all of the metal parts stripped, re-brushed and re-anodized.
This will likely give me a better than new pair of amplifiers for little investment.
If I do replace the outputs with modern devices, should the drivers be replaced as well, or can the original 2SD555/2SB600 drivers be kept?
Hi ungie,
Spark it up with a couple transistors missing, but don't drive it hard. The amp would have sounded better with more outputs, so if you like it now, you'll really like it later! You can determine sound quality better at normal listening levels anyway.
-Chris
Unless you get these from a very reputable source, they will be fakes. No ifs, ands or buts on that. Those parts were discontinued many, many years ago!
Spark it up with a couple transistors missing, but don't drive it hard. The amp would have sounded better with more outputs, so if you like it now, you'll really like it later! You can determine sound quality better at normal listening levels anyway.
-Chris
Hi Unqie
To repair that amp I would follow the advice given by anatech and other members here regarding MJ21193/94. There are other transistors you could use too but that is a good choice. To find 2SD555/2SB600 nowadays is very difficult, I have repaired amps using this and managed to find some spares but this was some years back. You could try contacting a member here called sakis, he found a store in greece which had a lot of older japanese transistors and if I can recall correctly these were in the list he provided in a thread here some months back, i cant seem to find the thread now. Another reason to go with the onsemis is that if you do find them you gonna pay maybe 10 us dollars for each, if you have 3 or 4 to replace it would be cheaper to replace the whole lot with the mjl, better also because maybe you could get them from same batch with better matching. The last time I found
2SD555/2SB600 was in spain in 2006 and I paid 9 euros each, that is about 12 us dollars.
Try criclewood electronics and audiolab of ga, they might still have, but warning stay away from fakes and so called second source suppliers, these transitors havent been made in years. Insist on the real part or youll end up with more headaches and emty pocket.
The old rowland amps are very good in design, if your amp is something like the model 9 in design it sure is worth repairing even if it costs some heavy cash.
To repair that amp I would follow the advice given by anatech and other members here regarding MJ21193/94. There are other transistors you could use too but that is a good choice. To find 2SD555/2SB600 nowadays is very difficult, I have repaired amps using this and managed to find some spares but this was some years back. You could try contacting a member here called sakis, he found a store in greece which had a lot of older japanese transistors and if I can recall correctly these were in the list he provided in a thread here some months back, i cant seem to find the thread now. Another reason to go with the onsemis is that if you do find them you gonna pay maybe 10 us dollars for each, if you have 3 or 4 to replace it would be cheaper to replace the whole lot with the mjl, better also because maybe you could get them from same batch with better matching. The last time I found
2SD555/2SB600 was in spain in 2006 and I paid 9 euros each, that is about 12 us dollars.
Try criclewood electronics and audiolab of ga, they might still have, but warning stay away from fakes and so called second source suppliers, these transitors havent been made in years. Insist on the real part or youll end up with more headaches and emty pocket.
The old rowland amps are very good in design, if your amp is something like the model 9 in design it sure is worth repairing even if it costs some heavy cash.
anatech said:
I had a chance to put the output stages back together (what a job with all those TO-3s!!). Everything was connected up and I switched on with 8 devices per side.
No smoke, always a good sign, so I checked DC offset and let the amp warm up for about 30 mins.
Strangely, unlike the other monobloc, this unit was showing output device bias curents between 77-84mA which seems MUCH more likely to me! The driver bias was around 220mA which is similar to what I was getting with the "working" unit. Seems to me like that 20mA on the other unit WAS too low.
Once I get all the parts together to fully refurb these things I'll check with Rowland to ask what the correct bias current should be.
So far no weird noises, hum, or oscillation...all good signs!
Hi homemodder,
From what I have heard, djk is giving good advice. I do not have any direct experience with them.
It is important to buy semiconductors from the normal, authorized supply chain. Once these parts get out into the smaller shops, it's much harder to get matching as the date codes on real devices can be all over the map. That's if they didn't get less expensive devices from very scary places. I'm looking at a pair of faked 2SD424 and 2SB554 transistors right now that were sourced by another shop from Global Semiconductors in Ontario. This amp arrived dead on that same channel (apparently twice now).
I remember paying $25 ~ 30 CDN each for 2SD555 and 2SB600 in the late 70's. Back in the bad old days of house numbers and no Japanese parts distributors in North America.
Hi ungie,
That's about what I expected to hear from you.
Now, listen to it and make your mind up. The bias figures I gave you earlier should be very close to what the designer intended.
-Chris
From what I have heard, djk is giving good advice. I do not have any direct experience with them.
It is important to buy semiconductors from the normal, authorized supply chain. Once these parts get out into the smaller shops, it's much harder to get matching as the date codes on real devices can be all over the map. That's if they didn't get less expensive devices from very scary places. I'm looking at a pair of faked 2SD424 and 2SB554 transistors right now that were sourced by another shop from Global Semiconductors in Ontario. This amp arrived dead on that same channel (apparently twice now).
I remember paying $25 ~ 30 CDN each for 2SD555 and 2SB600 in the late 70's. Back in the bad old days of house numbers and no Japanese parts distributors in North America.
Hi ungie,
That's about what I expected to hear from you.
Now, listen to it and make your mind up. The bias figures I gave you earlier should be very close to what the designer intended.
-Chris
I don't how many of the 555/600 outputs you need, but I still have at least 50 of each. These were purchased at least 15 years ago and have all been tested. The gain spread for a box off 100 was within 5 %. I'm pretty sure they are real NEC parts because they were purchased from an NED authorized distributor. These were very good devices and I don't understand whey they were disconitued. They were widly used by High end companies because they sounded good an by Pro companies because they were rugged and held up well to abuse.
I can't guarantee the ones I have will match the ones you have gain wise. I guess I could send you a sample or all you need and if they don't match you can send them back and pay postage both ways. Find a price for the ones available that are of questionable parantage and I will match that price.
Steve
I can't guarantee the ones I have will match the ones you have gain wise. I guess I could send you a sample or all you need and if they don't match you can send them back and pay postage both ways. Find a price for the ones available that are of questionable parantage and I will match that price.
Steve
Hi Steve,
Thanks for your offer.
Another diyAudio member is sending me a few pieces to test.
If I run into a problem with them I will contact you.
In the end I will likely just replace all of the outputs with new devices as that seems to make the most sense.
Chris,
I won't have a chance to do much listening for a bit, but I have drawn up a schematic of the amplifier minus the input stage for now.
I'm sure I have made some mistakes but unless I'm incorrect VR3 is used to adjust bias.
Your Model 5 bias of 55mA-80mA per device seems to jive nicely here.
Thanks for your offer.
Another diyAudio member is sending me a few pieces to test.
If I run into a problem with them I will contact you.
In the end I will likely just replace all of the outputs with new devices as that seems to make the most sense.
Chris,
I won't have a chance to do much listening for a bit, but I have drawn up a schematic of the amplifier minus the input stage for now.
I'm sure I have made some mistakes but unless I'm incorrect VR3 is used to adjust bias.
Your Model 5 bias of 55mA-80mA per device seems to jive nicely here.
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