Hi Bob,
I just got one in (Marantz 250). An idiot was loose inside it. It is well worth restoring and that is what I intend to do with it. I got it as scrap, and the case is rough. The amp will sound great when it's done.
Just rebuilt a 170DC just now. It's still on the bench. That is another really nice amplifier. I'm in the middle of a Marantz 500. I wish I owned one, and I have a 300DC for my main system that I've improved and restored. Lovely equipment.
Most amplifiers do not deserve what happens to them. The better techs just try to pick up the pieces. You're right about one thing Bob, you will enjoy that 250 for decades. Note that there were service bulletins on the 250. You should install higher rated outputs and drivers. So I can understand why yours died once. Did you do both channels? You should.
-Chris
I just got one in (Marantz 250). An idiot was loose inside it. It is well worth restoring and that is what I intend to do with it. I got it as scrap, and the case is rough. The amp will sound great when it's done.
Just rebuilt a 170DC just now. It's still on the bench. That is another really nice amplifier. I'm in the middle of a Marantz 500. I wish I owned one, and I have a 300DC for my main system that I've improved and restored. Lovely equipment.
Most amplifiers do not deserve what happens to them. The better techs just try to pick up the pieces. You're right about one thing Bob, you will enjoy that 250 for decades. Note that there were service bulletins on the 250. You should install higher rated outputs and drivers. So I can understand why yours died once. Did you do both channels? You should.
-Chris
I'm in the same boat with the chassis. When I bought it got some bad shipping damage, the face plate is all bent up. I'm still looking for a replacement years later.
There something to be said about the Marantz sound. Its fantastic and something that I haven't heard anywhere else although my Sansui AU 7700 comes closest. Meant to say 1200B but I think they're pretty much the same. Was not to bad to recap and replace the components although if I were to do it again I would have just shotgun approached everything (would have saved some time). I did both channels, it bothers me to even think there might be an imbalance or a time bomb.
I haven't done any mods to it although I plan on it. I have tried to find the service bulletin but the sights I have been to I get an error and can't access them. Do you have any sources for mods?
There something to be said about the Marantz sound. Its fantastic and something that I haven't heard anywhere else although my Sansui AU 7700 comes closest. Meant to say 1200B but I think they're pretty much the same. Was not to bad to recap and replace the components although if I were to do it again I would have just shotgun approached everything (would have saved some time). I did both channels, it bothers me to even think there might be an imbalance or a time bomb.
I haven't done any mods to it although I plan on it. I have tried to find the service bulletin but the sights I have been to I get an error and can't access them. Do you have any sources for mods?
Ha, That's a great question!
I am now considering that they are selling for $500 - $1500 and that there
is at least one person who has been able to reliably restore them over at Audio Karma,
Bob Speece (wlhd1610 at AK):
Marantz 250M rebuild by Bob Speece | Audiokarma Home Audio Stereo Discussion Forums
I might try to do the restoration with a some careful selection/upgrade of replacement transistors.
I've also found most of the service bulletins on AK:
Marantz 250M Power Amp Service Bulletins | Audiokarma Home Audio Stereo Discussion Forums
The main changes were upgrades to the driver and output transistors which makes
sense since they were improving fast around that time.
Hi Pete,
There should be a bunch of people who can do an excellent job rebuilding these and other Marantz amplifiers. Maybe because I've worked on them my entire life it seems easy, but it isn't that hard. Take you time and take care with matching and you'll be fine. That is as long as your replacement parts actually fit where the old ones came from.
I've rebuilt everything from Marantz tube equipment (model 7 on the bench right now, sitting beside a model 500 with an intermittent fault) to modern equipment. It is designed and built well (except for the Philips years) so that apart from these updates from Marantz, you can pretty much rebuild them as they left the factory and be rewarded with excellent performance. I just rebuilt a 170DC for my bench amp. It's sitting up there now. I guess I'm pretty much surrounded with Marantz equipment. So I find it odd when its mentioned that only certain people can fix them properly. I sure hope that isn't true!
-Chris
There should be a bunch of people who can do an excellent job rebuilding these and other Marantz amplifiers. Maybe because I've worked on them my entire life it seems easy, but it isn't that hard. Take you time and take care with matching and you'll be fine. That is as long as your replacement parts actually fit where the old ones came from.
I've rebuilt everything from Marantz tube equipment (model 7 on the bench right now, sitting beside a model 500 with an intermittent fault) to modern equipment. It is designed and built well (except for the Philips years) so that apart from these updates from Marantz, you can pretty much rebuild them as they left the factory and be rewarded with excellent performance. I just rebuilt a 170DC for my bench amp. It's sitting up there now. I guess I'm pretty much surrounded with Marantz equipment. So I find it odd when its mentioned that only certain people can fix them properly. I sure hope that isn't true!
-Chris
Hi Chris, I have a 250 (not 250M) and blown the outputs trying to bias according to the service manual using the Wattmeter reading for 10W increase ie... There was no changing in the Wattmeter when adjusting the bias pot, I just replaced the outputs and but now I am afraid of messing with it again, any hint to do this adjusting other than this 10W procedure?
Recently I bought another one that was not working, but it was a really steal, complete with wood cabinet. It did not arrived yet, the owner said that it was blowing fuses after Turning on, so I might look at the outputs also.
I love Marantz, the 510M I also had, comparing it is an inferior amp, the fun is very noisy, also as for the audio quality comparing with the 250 it is not as good for my ears.
I have also the 1300DC masterpiece, that is a 300DC and a 3650 preamp on a single chassis. This amp is awesome. 2385, 2270, 2330, 4400, among others, are in my collection.
Thanks for your information about these beautiful audio legends.
Recently I bought another one that was not working, but it was a really steal, complete with wood cabinet. It did not arrived yet, the owner said that it was blowing fuses after Turning on, so I might look at the outputs also.
I love Marantz, the 510M I also had, comparing it is an inferior amp, the fun is very noisy, also as for the audio quality comparing with the 250 it is not as good for my ears.
I have also the 1300DC masterpiece, that is a 300DC and a 3650 preamp on a single chassis. This amp is awesome. 2385, 2270, 2330, 4400, among others, are in my collection.
Thanks for your information about these beautiful audio legends.
Hi All,
I'm strictly a hobbyist and found that these Marantz amplifiers can be refurbished/repaired if you do your homework. With the Marantz 1200, the homework is completing all the service bulletins and making sure to match any transistors you might replace. The relay-rectifier board will have lifted traces, the relay may need replacement and the electrolytic capacitors most likely need to be replaced. I also learned that the transistors should not be changed via the shotgun method, if needed, match the differential amp pair, the driver and output transistors.
There was a guy who created a very comprehensive website on The Marantz amps, but it's no longer around. The US based tribe over on that other website keeps their knowledge pretty close to their chest, maybe to drum up the work?
I really like sound of the Marantz 1200 I've got. It might be because I cut my teeth on it. The last 5 or 6 years I've spent on tube amps has caused me to spend the majority of my listening time with tubes, but all this talk is making me get off the internet and go listen to some music played on the Marantz 1200.
I'm strictly a hobbyist and found that these Marantz amplifiers can be refurbished/repaired if you do your homework. With the Marantz 1200, the homework is completing all the service bulletins and making sure to match any transistors you might replace. The relay-rectifier board will have lifted traces, the relay may need replacement and the electrolytic capacitors most likely need to be replaced. I also learned that the transistors should not be changed via the shotgun method, if needed, match the differential amp pair, the driver and output transistors.
There was a guy who created a very comprehensive website on The Marantz amps, but it's no longer around. The US based tribe over on that other website keeps their knowledge pretty close to their chest, maybe to drum up the work?
I really like sound of the Marantz 1200 I've got. It might be because I cut my teeth on it. The last 5 or 6 years I've spent on tube amps has caused me to spend the majority of my listening time with tubes, but all this talk is making me get off the internet and go listen to some music played on the Marantz 1200.
Hi murillollirum,
I have a 250 to rebuild as well. Struck by "technician" which caused heavy damage.
Okay, so what may have happened is that the amplifier broke into oscillation. Also, if an output is blown, change the entire set for one channel, and the driver transistors. In this case, Marantz had recommended devices with higher ratings than the originals.
In your case, you probably used outputs that were too fast compared to the original parts. I would use something like MJ15003 and MJ15004. MJ15024 and complement are probably too fast. MJ2119x devices would be okay too.
For working on audio power amplifiers (and power supplies or servo amplifiers) you absolutely do need an oscilloscope (a real one, not a sound card tester), an audio oscillator with low distortion (sound card probably okay for this) and a distortion analyser.
To set bias, use a 1KHz ~ 2KHz signal at about 1 watt (must be loaded!) into a 4R to 8R load. 4R is better, and a standard value. Start with the bias turned to minimum. Connect the 'scope to the output of the THD meter so you can see the residuals (what's left after the fundamental is removed). Slowly increase the bias current and watch the crossover spikes. You will see them decrease and then disappear. If you have been monitoring the bias current, remove the signal and load and read the current. It should be pretty low if you matched the transistors. You can increase that to anywhere from 10 mA to 30 mA per transistor. If you are already there or higher, something is wrong. Not drastically wrong, but enough to reduce the sound quality. DO the same thing for both channels.
If you figure out the supply voltages, you can figure out the bias current for a 10 watt increase in power draw. Divide the result by the number of output transistors. this is approximate, but close enough. Compare that to the method I suggested to you. The nice thing about doing it my way is that you know exactly when you have enough bias current. No guessing, no hoping. Comparing it with the manual is a sanity check.
Current that stays low, then jumps to a higher level is a classic sign of oscillation in the output stage (or entire amplifier).
-Chris
Edit: I have the 300DC and 3650 in my main system. I also have an SC-9 preamp and a number of Marantz receivers as well. It's great stuff. A 1300 would be pretty cool to have. My normal bench amp is an 1180DC. My 2120 tuner was replace with a Revox B-126 (?), I also have a 150, but hope to get a 2130 some day. I have a worn out 5030B cassette deck. CD players are all Denon, the main one is a DCD-S10 that has been improved.
On the bench is a DCD-1560 (improved) and it's playing right now. It's all lovely stuff. In my office there is a Luxman M-02, C-02, T-03 and another Denon DVD player used for CDs only. It's a 5900 I think. Then there are the tube units and an excellent Cyrus system (2 x Mono-X and Pre-X). I've even got other stuff that is great, like the Yamaha C-50 on the bench and a CR-3020 waiting for me to pay attention to it.
I have a 250 to rebuild as well. Struck by "technician" which caused heavy damage.
Okay, so what may have happened is that the amplifier broke into oscillation. Also, if an output is blown, change the entire set for one channel, and the driver transistors. In this case, Marantz had recommended devices with higher ratings than the originals.
In your case, you probably used outputs that were too fast compared to the original parts. I would use something like MJ15003 and MJ15004. MJ15024 and complement are probably too fast. MJ2119x devices would be okay too.
For working on audio power amplifiers (and power supplies or servo amplifiers) you absolutely do need an oscilloscope (a real one, not a sound card tester), an audio oscillator with low distortion (sound card probably okay for this) and a distortion analyser.
To set bias, use a 1KHz ~ 2KHz signal at about 1 watt (must be loaded!) into a 4R to 8R load. 4R is better, and a standard value. Start with the bias turned to minimum. Connect the 'scope to the output of the THD meter so you can see the residuals (what's left after the fundamental is removed). Slowly increase the bias current and watch the crossover spikes. You will see them decrease and then disappear. If you have been monitoring the bias current, remove the signal and load and read the current. It should be pretty low if you matched the transistors. You can increase that to anywhere from 10 mA to 30 mA per transistor. If you are already there or higher, something is wrong. Not drastically wrong, but enough to reduce the sound quality. DO the same thing for both channels.
If you figure out the supply voltages, you can figure out the bias current for a 10 watt increase in power draw. Divide the result by the number of output transistors. this is approximate, but close enough. Compare that to the method I suggested to you. The nice thing about doing it my way is that you know exactly when you have enough bias current. No guessing, no hoping. Comparing it with the manual is a sanity check.
Current that stays low, then jumps to a higher level is a classic sign of oscillation in the output stage (or entire amplifier).
-Chris
Edit: I have the 300DC and 3650 in my main system. I also have an SC-9 preamp and a number of Marantz receivers as well. It's great stuff. A 1300 would be pretty cool to have. My normal bench amp is an 1180DC. My 2120 tuner was replace with a Revox B-126 (?), I also have a 150, but hope to get a 2130 some day. I have a worn out 5030B cassette deck. CD players are all Denon, the main one is a DCD-S10 that has been improved.
On the bench is a DCD-1560 (improved) and it's playing right now. It's all lovely stuff. In my office there is a Luxman M-02, C-02, T-03 and another Denon DVD player used for CDs only. It's a 5900 I think. Then there are the tube units and an excellent Cyrus system (2 x Mono-X and Pre-X). I've even got other stuff that is great, like the Yamaha C-50 on the bench and a CR-3020 waiting for me to pay attention to it.So we both have a lot of animals! My next dream it's to form the Marantz tube setup, but with these eBay prices it's a long time project. The Marantz 9, 7T and 10b are really icons of the golden age of audio. I wanna thank you so much for your generous and thoughtful information about these bias procedure, it's really a clever one and I think it works well. the scope I already have one, the THD analyzer I am looking for still. I noticed in some Marantz units a complete failure with the bias.pots, they just do not work, after changing to a multi turn one it's easier to adjust. But recently the 2330 really do not change anything when I adjust the bias pot, also there is a very noisy sine wave at the scope when I feed it with 1khz signal. I will try to post here the scope photo to see if you can help me trace this problem. This only happens with the left channel. Thx Chris for your invaluable help here.in this forum. People like you are is extinction nowadays.
Screenshot-20181021-174041 — imgbb.com
Screenshot-20181021-174041 — imgbb.com
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Hi murillollirum,
That's oscillation all right. You might be able to cure it by adjusting the value of the capacitor across the feedback resistor from the output. Tell me, what output transistors are you using, and drivers please? A shot of the output with no signal might be helpful too. Right now I know that the oscillation isn't high in frequency. So it probably involves the entire circuit.
Thanks for your comment. I was taught by folks a lot older than I was, so it is a cycle.
-Chris
That's oscillation all right. You might be able to cure it by adjusting the value of the capacitor across the feedback resistor from the output. Tell me, what output transistors are you using, and drivers please? A shot of the output with no signal might be helpful too. Right now I know that the oscillation isn't high in frequency. So it probably involves the entire circuit.
Thanks for your comment. I was taught by folks a lot older than I was, so it is a cycle.
-Chris
I still stand behind my claims that it is a flawed design.
I'm going to see what I can do about better SOA for several transistors beyond
the drivers and outputs.
Hi Pete,
That's okay if you think it isn't a good design. It would be in good company.
I don't think there are ratings issues beyond the drivers and output transistors. If I were doing that rebuild (and I am on mine), I would be using all modern devices that I matched. They would be selected with reasonable safety margins.
No matter what I may have forgotten, I'll know it inside out again once I'm done that rebuild. I've noticed all kinds of older Marantz products seeing the light of day in the last couple years. It would be nice if they were in good cosmetic condition, but most have issues after being called junk through the 80's and 90's. So each one we can bring back to life is a worthwhile effort.
-Chris
That's okay if you think it isn't a good design. It would be in good company.
I don't think there are ratings issues beyond the drivers and output transistors. If I were doing that rebuild (and I am on mine), I would be using all modern devices that I matched. They would be selected with reasonable safety margins.
No matter what I may have forgotten, I'll know it inside out again once I'm done that rebuild. I've noticed all kinds of older Marantz products seeing the light of day in the last couple years. It would be nice if they were in good cosmetic condition, but most have issues after being called junk through the 80's and 90's. So each one we can bring back to life is a worthwhile effort.
-Chris
not to thread crap but has anyone applied these fixes?
I found this here while searching for all I can on this amp.
Here is my rebuild thread on the other forum Marantz 250 rebuild | Audiokarma Home Audio Stereo Discussion Forums
It seems difficult to find a lot of details on fixing these amps.
Let me know if you see any obvious mistakes or flaws.
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.
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.
I found this here while searching for all I can on this amp.
Here is my rebuild thread on the other forum Marantz 250 rebuild | Audiokarma Home Audio Stereo Discussion Forums
It seems difficult to find a lot of details on fixing these amps.
Let me know if you see any obvious mistakes or flaws.
Hi rickl,
Great! Now why didn't Marantz put out a service bulletin detailing your fixes??
Oh, I know. Because there wasn't a problem to begin with! Honestly, when you begin to assume you know better than the engineers that designed the product you are treading on very, very thin ice. The engineers that designed these products were actually very good at what they did. They also did send out service notes on any and all problems - including engineering errors.
I guess I could read through the AK thread, but I often can't stomach what passes for knowledge there. Most of it is misguided, but the intent is good. However, if your amplifier wasn't stable, I would look at everything very carefully. These amplifiers are not unstable designs at all. They are if you don't service it properly - just like any other good design.
This is just more of the "Marantz is junk" stuff we saw through the 80's and even up to past 2000. Then all of a sudden, Marantz became okay and the prices went through the roof.
Degenerating the diff pair by that amount will do horrible things to it's distortion performance. Increasing Cdom isn't going to help the performance either. Base stoppers have a very negative affect on operation as well. So what you have now isn't a Marantz 250 anymore. I have a Marantz 250 as well. But, mine is still a Marantz product.
So have you come by this knowledge in threads or by studying amplifiers? I would suggest you read books by Doug Self and Bob Cordell. Truth can be found within those pages.
Okay, so I read your thread (short one). You don't mention any of these "fixes", but you do admit to replacing the drivers, outputs and maybe other parts with totally different devices. Do you think that might require restabilizing the amplifier? I certainly do. I don't think you followed any procedure. Looks more like you followed off-line advice and once it was working you patted yourself on the back, fat, dumb and happy.
Have you tried a sweep up to 1 MHz to see if you have excessive HF droop, or maybe a peak in the response? How have you determined that your amplifier is properly compensated? If you are good at fixing tube amplifiers, you know how to do this.
-Chris
Great! Now why didn't Marantz put out a service bulletin detailing your fixes??
Oh, I know. Because there wasn't a problem to begin with! Honestly, when you begin to assume you know better than the engineers that designed the product you are treading on very, very thin ice. The engineers that designed these products were actually very good at what they did. They also did send out service notes on any and all problems - including engineering errors.
I guess I could read through the AK thread, but I often can't stomach what passes for knowledge there. Most of it is misguided, but the intent is good. However, if your amplifier wasn't stable, I would look at everything very carefully. These amplifiers are not unstable designs at all. They are if you don't service it properly - just like any other good design.
This is just more of the "Marantz is junk" stuff we saw through the 80's and even up to past 2000. Then all of a sudden, Marantz became okay and the prices went through the roof.
Degenerating the diff pair by that amount will do horrible things to it's distortion performance. Increasing Cdom isn't going to help the performance either. Base stoppers have a very negative affect on operation as well. So what you have now isn't a Marantz 250 anymore. I have a Marantz 250 as well. But, mine is still a Marantz product.
So have you come by this knowledge in threads or by studying amplifiers? I would suggest you read books by Doug Self and Bob Cordell. Truth can be found within those pages.
Okay, so I read your thread (short one). You don't mention any of these "fixes", but you do admit to replacing the drivers, outputs and maybe other parts with totally different devices. Do you think that might require restabilizing the amplifier? I certainly do. I don't think you followed any procedure. Looks more like you followed off-line advice and once it was working you patted yourself on the back, fat, dumb and happy.
Have you tried a sweep up to 1 MHz to see if you have excessive HF droop, or maybe a peak in the response? How have you determined that your amplifier is properly compensated? If you are good at fixing tube amplifiers, you know how to do this.
-Chris
I have to agree regarding AK. Mostly a bunch of hacks who call themselves "techs," their "owner" is a jerk. And I'm being nice...
Most of the time, the posts end without a fix/conclusion, or they take it onto private messages so that you'll be encouraged to have to call on the 5 or few Marantz experts techs, most of them on the opposite side of the USA, the other 5 who don't want to deal with the same old moans, same old groans.
Most of the time, the posts end without a fix/conclusion, or they take it onto private messages so that you'll be encouraged to have to call on the 5 or few Marantz experts techs, most of them on the opposite side of the USA, the other 5 who don't want to deal with the same old moans, same old groans.
Hi rickl,
Sorry for the terse response, but I get really grumpy when I see posts like yours along with the tone of what was written.
Your AK thread did not include any of the information you released here, so I'm curious why not? Why to post here with that when there isn't any backup information? It would have been in your favour if you had the source of your information.
-Chris
Sorry for the terse response, but I get really grumpy when I see posts like yours along with the tone of what was written.
Your AK thread did not include any of the information you released here, so I'm curious why not? Why to post here with that when there isn't any backup information? It would have been in your favour if you had the source of your information.
-Chris
I did provide a link to the thread with the updates. Sorry if it wasn't clear. It was on page 3
Marantz 250M Repair
I wasn't too excited about the updates and wondered what others thought.
Thanks again.