Marantz 250M
Hello there;
Here are some pic's to help you locate the 39 ohm resistors (upper-right and down-right thick resistors on the picture) and the bias-connector.I also noticed the 2 small transistors that are thermally connected on mine,the latter seems to be missing on yours=double check that.I do not own this amplifier anymore but still had some pics left.I think i still have 2 spare TO-66 driver transistors laying around somewhere.Hope this helps.
Have fun!
Grtngs;
Johan
Hello there;
Here are some pic's to help you locate the 39 ohm resistors (upper-right and down-right thick resistors on the picture) and the bias-connector.I also noticed the 2 small transistors that are thermally connected on mine,the latter seems to be missing on yours=double check that.I do not own this amplifier anymore but still had some pics left.I think i still have 2 spare TO-66 driver transistors laying around somewhere.Hope this helps.
Have fun!
Grtngs;
Johan
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Marantz 250
Interesting thread.
I have two Marantz 250s I am rebuilding.
The easy fixes to make the amplifier more stable are as follows:
1) Degenrate the diff amp's emitters with 200-390 ohm resistors.
2) Add in Collector to base capacitors on the pair of VAS transistors (Q507, Q508).
3) Insert base stoppers between the VAS and the first driver transistors (22 ohm) and also base stoppers in the 4 output transistor base leads (2.2-3.9 ohm).
This cures instability.
The short circuit issue can never be resolved as the OC circuit is crude. Simple remedy - do not load this amplifier below 4 ohms per channel UNLESS you put in higher power devices.
With the MJ15024/MJ15025 pairs this amplifier will work well and epi base devices are not required as long as you have the mods in place as above.
The TO-66 drivers can be replaced with a multitude of devices (MJ15030/MJ15031) work well.
Steve Mantz
Zed Audio Corp.
Interesting thread.
I have two Marantz 250s I am rebuilding.
The easy fixes to make the amplifier more stable are as follows:
1) Degenrate the diff amp's emitters with 200-390 ohm resistors.
2) Add in Collector to base capacitors on the pair of VAS transistors (Q507, Q508).
3) Insert base stoppers between the VAS and the first driver transistors (22 ohm) and also base stoppers in the 4 output transistor base leads (2.2-3.9 ohm).
This cures instability.
The short circuit issue can never be resolved as the OC circuit is crude. Simple remedy - do not load this amplifier below 4 ohms per channel UNLESS you put in higher power devices.
With the MJ15024/MJ15025 pairs this amplifier will work well and epi base devices are not required as long as you have the mods in place as above.
The TO-66 drivers can be replaced with a multitude of devices (MJ15030/MJ15031) work well.
Steve Mantz
Zed Audio Corp.
Jorre - take a look at my picture (post 40). I must have a significantly older model 250 than yours. My layout is completely different. I have no connectors and the connections to the driver transistors is crude point-to-point. I can clearly see the 39's on your arrangement. What S/N is that? This one is 1537.
Moer - RE: your suggestion to use MJ15024/MJ15025; they have a significantly lower Hfe (15) than OEM. Is this a part of the stability strategy? I thought the MJ21194/95 was a better OEM match.
Moer - RE: your suggestion to use MJ15024/MJ15025; they have a significantly lower Hfe (15) than OEM. Is this a part of the stability strategy? I thought the MJ21194/95 was a better OEM match.
Sorry, I missed this from quite some time ago.
Where to start? How familiar are you with this design?
The 250M I had in for repair last year checked out o.k. after repair. The customer said it blew out after a few months.
As I said I think that this is a seriously flawed design, especially with regard to the output protection. I wonder if the owner shorted it again.
I'm looking at Nelson's very simple Citation 12 MOSFET and the outputs are very high voltage devices (200V) and if the front end devices were substituted with ~150V devices it could be used as a replacement in this amp - would want to double up the output devices and perhaps increase the VAS bias.
I also considered the APT1 Holman power amp:
http://www.diyaudio.com/forums/solid-state/70261-apt-1-power-amp-undeservedly-forgotten.html
I wish I found this thread earlier and did some of the stability mods. I just blew my Marantz 240 and it fried one of my Klipsch bookshelf speakers. I was using the 'preamp' out on my Pioneer SX-626 to feed the Marantz 240. All was well for about 10 minutes until I heard a long 'screech and buzz' sound, then silence and then black smoke. Since I was too far from the amp to unplug it in time it had about 10 seconds to do it's damage.
Firstly, is there an easy way to tell if the transformer shorted, or is that very unlikely with this? All fuses are in tact (not blown).
Also, is it possible some voltage went back the opposite way into my pioneer through the pre-amp and did some other damage to components attached to my Pioneer? I had all my devices still plugged into the Pioneer receiver preamp like my record player, EQ's, cd player, and a set of main speakers stll in the receiver terminals. Please tell me 'not likely'. Man, this blows...pun intended.
Firstly, is there an easy way to tell if the transformer shorted, or is that very unlikely with this? All fuses are in tact (not blown).
Also, is it possible some voltage went back the opposite way into my pioneer through the pre-amp and did some other damage to components attached to my Pioneer? I had all my devices still plugged into the Pioneer receiver preamp like my record player, EQ's, cd player, and a set of main speakers stll in the receiver terminals. Please tell me 'not likely'. Man, this blows...pun intended.
I'd be really surprised if you did any damage to 'upstream' equipment unless the problem was brought on by some sort of AC supply problem or ground fault.
You didn't blow the AC fuse? That seems odd, maybe it was the xfmr. Some testing will verify. Isolate the bad amp by removing its V+ & V- supply lines at the amp circuit card and measure the DC supply then - right at the capacitors.
You also made it sound like the problem happened with only one side(?) If so, at least you have a reference for measurements on the good side. Also, you mentioned smoke. I'd be interested to hear what items show physical damage, as this is really where you should start. This amp is similar to our 250, so you might also be interested in this thread: http://www.diyaudio.com/forums/solid-state/190127-marantz-model-250-power-amp-repair.html should you choose to carry on with a repair.
You didn't blow the AC fuse? That seems odd, maybe it was the xfmr. Some testing will verify. Isolate the bad amp by removing its V+ & V- supply lines at the amp circuit card and measure the DC supply then - right at the capacitors.
You also made it sound like the problem happened with only one side(?) If so, at least you have a reference for measurements on the good side. Also, you mentioned smoke. I'd be interested to hear what items show physical damage, as this is really where you should start. This amp is similar to our 250, so you might also be interested in this thread: http://www.diyaudio.com/forums/solid-state/190127-marantz-model-250-power-amp-repair.html should you choose to carry on with a repair.
Thanks for your help. That makes me feel a little better regarding the chances of damaging other components. It seems like one thing down the line did get affected; my Ipod. It was plugged in through the auxilary and was playing at the time. The right channel on it sounds dull and weak now. When the amp blew both speakers did stop working. There was a sound that literally sounded like electricity waves going through the speakers. The speakers smelled like burnt electronics afterwards. The right speaker is dead, the left speakers works but sounds pretty bad.
So, I'm not sure what exactly shorted on the amp, both side? I will do the testing you suggested and see what's going on. Thanks again for your help.
I recently found that there were several revisions to the service manual, and tech bulletins,
one has improved driver and output transistors installed, another lowers the output bias current
in half. One warns about commercial use and says to send the unit in for mods in that case.
Seems they were having a lot of trouble with these.
Found this, from:
Vintage Marantz Audio Equipment - Journal - A Toasty Marantz 250 is Delivered to theShop
"Of all of the amplifiers the were made by Marantz, the 250 seems to have quite a mystique about it with enthusiasts. The desirability of the 250 might only be surpassed by the model 500. Part of the mystique of the 250 is with it's reputation for power and possibly its unreliability due to its design.
There are rumors that the 250 is an inherently unstable design and perhaps this is true. There are many Factory Service Bulletins which outline modifications a for the 250. There are also no less than 3 versions of the 250 /250M which seems like a high numbers for the 6 years of production for these models.
The closest cousin to the model 250 in the Marantz line up is the 1200 / 1200B. On first glance these models share the same basic design and share many of the same boards and parts. The primary difference is the rated output, the 250 is rated at 125 WPC and the 1200 is rated at 100 WPC."
one has improved driver and output transistors installed, another lowers the output bias current
in half. One warns about commercial use and says to send the unit in for mods in that case.
Seems they were having a lot of trouble with these.
Found this, from:
Vintage Marantz Audio Equipment - Journal - A Toasty Marantz 250 is Delivered to theShop
"Of all of the amplifiers the were made by Marantz, the 250 seems to have quite a mystique about it with enthusiasts. The desirability of the 250 might only be surpassed by the model 500. Part of the mystique of the 250 is with it's reputation for power and possibly its unreliability due to its design.
There are rumors that the 250 is an inherently unstable design and perhaps this is true. There are many Factory Service Bulletins which outline modifications a for the 250. There are also no less than 3 versions of the 250 /250M which seems like a high numbers for the 6 years of production for these models.
The closest cousin to the model 250 in the Marantz line up is the 1200 / 1200B. On first glance these models share the same basic design and share many of the same boards and parts. The primary difference is the rated output, the 250 is rated at 125 WPC and the 1200 is rated at 100 WPC."
Simply amazing. Ran into this thread and old thread and it made me imagine how lucky I must have been when I repaired a number of these inherently unstable M-250 amplifiers and much to my surprise a quick check reveals they are still functional and still working fine.
Reading these three different sections on the 250 has me thinking that some would prefer the Crown DC300 or 150 amplifier over the Marantz design. This has me puzzled as to how some that consider themselves to be audiophiles and yet cannot hear the differences between the two designs. You talk factory service bulletins? I have I believe four of the Crown manuals sitting on my shelf in my garage. Want to guess how many factory service bulletins there are on the DC150 and DC300 power amps? There are a few more than the Marantz 240, 250, and M250.
As a misfit that has enjoyed them over the years my opinion is they are quite musical and have a clarity and sound that hasn't been duplicated by other amps out there on the market at the same time or since. I guess if your seriously looking for faults you can find them in any power amplifier you crack open.
My opinion here.....once properly restored/rebuilt these amps have enjoyed many years of usage and work properly including the protection circuit.
Again I should thank DJK for his mentoring 25 years ago when he took the time to train me properly in the art of the M250, 250, and 240 power amps.
Reading these three different sections on the 250 has me thinking that some would prefer the Crown DC300 or 150 amplifier over the Marantz design. This has me puzzled as to how some that consider themselves to be audiophiles and yet cannot hear the differences between the two designs. You talk factory service bulletins? I have I believe four of the Crown manuals sitting on my shelf in my garage. Want to guess how many factory service bulletins there are on the DC150 and DC300 power amps? There are a few more than the Marantz 240, 250, and M250.
As a misfit that has enjoyed them over the years my opinion is they are quite musical and have a clarity and sound that hasn't been duplicated by other amps out there on the market at the same time or since. I guess if your seriously looking for faults you can find them in any power amplifier you crack open.
My opinion here.....once properly restored/rebuilt these amps have enjoyed many years of usage and work properly including the protection circuit.
Again I should thank DJK for his mentoring 25 years ago when he took the time to train me properly in the art of the M250, 250, and 240 power amps.
I would not want a Crown amp either.
You seem to be taking this personally, you have some love for these amps, lol?
I'm just reporting the facts as I see them - I hate amps that blow up.
Have you tried shorting the outputs of the amps you repaired when running at full power into
a dummy load? They have short circuit protection after all ... I dare you!
You seem to be taking this personally, you have some love for these amps, lol?
I'm just reporting the facts as I see them - I hate amps that blow up.
Have you tried shorting the outputs of the amps you repaired when running at full power into
a dummy load? They have short circuit protection after all ... I dare you!
" I hate amps that blow up."
The D150/150A Crown was one of the worst I have ever seen in that regard, and (when working) one of the worst sounding amps available.
The DC300 service manual says (on the front cover) "300 watts and a cloud of smoke".
(former authorized Crown service center)
Owning Crown is a love/hate kind of thing (I still own several).
The D150/150A Crown was one of the worst I have ever seen in that regard, and (when working) one of the worst sounding amps available.
The DC300 service manual says (on the front cover) "300 watts and a cloud of smoke".
(former authorized Crown service center)
Owning Crown is a love/hate kind of thing (I still own several).
Well I guess shear stupidity would dictate that I attempt such a moronic feat.
Maybe I view the idea of a protection circuit differently that you do. My idea of a protection is it will accomodate a accidental short circuit one that a owner/consumer might actually have instead of a deliberate attempt to see if the amp can fend off an attempt to ark weld. I believe that I am correct in assuming the original intent was to ward off an accidental short. In my experience a properly restored 240,250 ,M250 will ward off an accidental unintentional short. One needs to realize and come to terms with the idea that the common owner of such a piece of equipment isn't going to turn the amp up(feed a signal capable of obtaining full power) and then short he hell out of it on purpose.
Sir, what I have seen so far is an attempt by you to elevate yourself to a level in which you are trying to make yourself superior to the designer of this amplifier. My last 25 years has had me in the field actually using a majority of the commercial amplifiers that are available in todays market.
I have found none to date that are incapable of some type of damage associated with a full power deliberate short into a dummy load or a shorted speaker load.
I will respect any piece of equipment that is capable of working correctly thru its warranty period. I live in the real world and I use commercial amplifiers in large format systems and no I'm not talking the junk that is associated with musicians or their primitave workings. I have no desire to see or to have to use a Crown piece of equipment simply because I know that before the warranty is up I will be back pulling at least a few of the amps for a warranty type repair. I've seen up close just how good their short circuit protection is also and you wouldn't want to bet on them either.
Based on what you have written on a number of threads I will make the following assumption and that is you have no real world experience in the field. You have probably never gotten any further away from you cushy chair than possibly a trip to the rest room or the coffee machine. This is why I dispise most engineers.
It seems that you are irked by the fact that you think "attempt by you to elevate yourself
to a level in which you are trying to make yourself superior to the designer" where do you
get this idea? All I want is an amp that doesn't blow up.
There are many reports of this amp blowing up, even after a repair, on this forum when in
NORMAL use by the owner.
Why don't you help us and tell what exactly you do to "properly" repair them - or is that
some closely held secret?
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So you are better because you are in the field. The problem here is that in your own words:
"I dispise most engineers" so you will never get over this emotional reaction to
engineers. I happen to be a pretty damn good engineer who does do a lot of hands on
work. No I'm not a tech in the field. Perhaps you should get over the fact that you
have something against engineers and then we can have a productive conversation.
Your comments make no sense, you've jumped to many conclusions. No I do not like
Crown amps. No people here with 250 amps are not deliberately shorting the outputs.
Many 250s have failed in normal use.
Do you have anything productive to add about how to get these back working properly,
it seems rather that you are using this thread to put down engineers.
Why don't you put me on ignore if you have an irrational dislike for engineers?