My God, this amp is driving me insane!
I replaced ALL the burned resistors, all the GE diodes, and all the driver transistors:
H710-709: 2SC3421 as suggested
H711-712: 2SA1220 (I couldn't find the 2SA1358)
H705-706: BC639
H707-708: BC640
And you know what else? But
, obviusualy.
That was the R744 that smoked bad and arced a little bit.
Now it's not my fault! I've checked every passive component and replaced all the transistors. They also measure good, but I suspect on H712. Someone has something to say?
I replaced ALL the burned resistors, all the GE diodes, and all the driver transistors:
H710-709: 2SC3421 as suggested
H711-712: 2SA1220 (I couldn't find the 2SA1358)
H705-706: BC639
H707-708: BC640
And you know what else? But
, obviusualy.That was the R744 that smoked bad and arced a little bit.
Now it's not my fault! I've checked every passive component and replaced all the transistors. They also measure good, but I suspect on H712. Someone has something to say?
Hi Giaime,
Okay, R744 is the emitter resistor for H712. If you used 2SA1220, then you should use 2SC2690 for the NPN (2SC3421). Now, is it possible that H712 is inserted wrong in the PCB? You have different case styles all around now, watch your pinout.
I noticed that you have replaced H705 - H708. Why? Were the originals blown? Again we have different pinouts for the transistors.
So, grab a cup of coffee. Relax. MEASURE the leads on your replacement transistors to correctly identify the base, emitter and collector on each. Draw a picture of each number on paper with the leads identified. I would, so I'm not joking. When done that, MEASURE the leads on the originals, draw pictures.
When you are done that, relax and look at the pictures you drew. Install one type of transistor at a time. Check your work. Replace the smoked resistors. Look for solder bridges and cracked traces. I hope you are cleaning the flux off the PCB as you work. Please use a variac to do the first power up. Or, just hold your breath and switch on.
-Chris
Okay, R744 is the emitter resistor for H712. If you used 2SA1220, then you should use 2SC2690 for the NPN (2SC3421). Now, is it possible that H712 is inserted wrong in the PCB? You have different case styles all around now, watch your pinout.
I noticed that you have replaced H705 - H708. Why? Were the originals blown? Again we have different pinouts for the transistors.
So, grab a cup of coffee. Relax. MEASURE the leads on your replacement transistors to correctly identify the base, emitter and collector on each. Draw a picture of each number on paper with the leads identified. I would, so I'm not joking. When done that, MEASURE the leads on the originals, draw pictures.
When you are done that, relax and look at the pictures you drew. Install one type of transistor at a time. Check your work. Replace the smoked resistors. Look for solder bridges and cracked traces. I hope you are cleaning the flux off the PCB as you work. Please use a variac to do the first power up. Or, just hold your breath and switch on.
-Chris
Thank you anathec for your interest.
Yes, the pinouts are different for the main driver transistors: but I found a way to mount them with the correct pinout. Should I worry because the PNP one is not the right complementary one of the NPN? The guy in the shop gave these to me, and I explained my problem, but he said they would be right.
About that little ones, yes, one was shot (open B-C either ways), but the pinout is the same for the NPN, reversed for the PNP, but I double checked it and it looks right....
What I noticed is that the transistor near the burning resistor also arced a little bit, and I found a burn stain onto a near resistor, but it hasn't been damaged. There's only 60V in that amp, is it possible that arcing?
However, I'm gonna get it fixed by a qualified technician, because I'm really screwed up (and that's not about lost time, it's about money, I'm a poor student).
Maybe today I will look... Your suggestion is that I mounted wrong one of the transistors, and/or the usual bad soldering and/or broken traces on the PCB...? I will check...
Yes, the pinouts are different for the main driver transistors: but I found a way to mount them with the correct pinout. Should I worry because the PNP one is not the right complementary one of the NPN? The guy in the shop gave these to me, and I explained my problem, but he said they would be right.
About that little ones, yes, one was shot (open B-C either ways), but the pinout is the same for the NPN, reversed for the PNP, but I double checked it and it looks right....
What I noticed is that the transistor near the burning resistor also arced a little bit, and I found a burn stain onto a near resistor, but it hasn't been damaged. There's only 60V in that amp, is it possible that arcing?
However, I'm gonna get it fixed by a qualified technician, because I'm really screwed up (and that's not about lost time, it's about money, I'm a poor student).
Maybe today I will look... Your suggestion is that I mounted wrong one of the transistors, and/or the usual bad soldering and/or broken traces on the PCB...? I will check...
Hi Giaime,
Well, don't take it where you got the transistors! They sound like they are a TV repair shop. In any case, if they don't know that much .. hmmmm.
Normally, you want to use proper complimentary pairs. You also want to make sure the parts aren't rejects or fakes. The pinout on the BC's would be expected to be the same. I haven't checked to see if they are compliments or not, but they should be.
Recheck the resistors and pinouts, or have a friend do it. Sometimes a simple mistake needs a second pair of eyes to find it. You do have time, and coffee!
-Chris
Well, don't take it where you got the transistors! They sound like they are a TV repair shop. In any case, if they don't know that much .. hmmmm.
Normally, you want to use proper complimentary pairs. You also want to make sure the parts aren't rejects or fakes. The pinout on the BC's would be expected to be the same. I haven't checked to see if they are compliments or not, but they should be.
Recheck the resistors and pinouts, or have a friend do it. Sometimes a simple mistake needs a second pair of eyes to find it. You do have time, and coffee!
-Chris
Thank you anatech, is thanks to people like you that many people here gain experience and help.
Yes, I will receck the pinouts....
The shop wasn't a TV repair shop, here in Italy there aren't TV shops everywhere like in the USA, and that was the only electronic components trader for all the zone of Venice. They are very kind and helpful, and they DO have the knowledge...
One of the salesman had this same amp, never got a problem so never opened it, sold in the late '80s.... Maybe it is the same amp? I don't think they sold so much in the poor '70s Italy...
The first hi-fi here has been the NAD3020, which I proudly own.
Yes, I will receck the pinouts....
The shop wasn't a TV repair shop, here in Italy there aren't TV shops everywhere like in the USA, and that was the only electronic components trader for all the zone of Venice. They are very kind and helpful, and they DO have the knowledge...
One of the salesman had this same amp, never got a problem so never opened it, sold in the late '80s.... Maybe it is the same amp? I don't think they sold so much in the poor '70s Italy...
The first hi-fi here has been the NAD3020, which I proudly own.
Hi Giaime,
I used to be a warranty depot for Marantz in the good old days. We didn't need to repair many. The worst ones to repair were "struck by technician". If you get stuck, of course ask as you've been doing. Also, go back to the schematic and keep as close to original as possible. I think having a friend look at it as well will be very helpful. Possibly a good time too.
I wouldn't call these type amps great by Marantz standards, but they were much better than what was out at the time. If you see a 300DC, grab it! The 170DC is different, but still very good. The 1180DC integrated is another really good one.
-Chris
I used to be a warranty depot for Marantz in the good old days. We didn't need to repair many. The worst ones to repair were "struck by technician". If you get stuck, of course ask as you've been doing. Also, go back to the schematic and keep as close to original as possible. I think having a friend look at it as well will be very helpful. Possibly a good time too.
I wouldn't call these type amps great by Marantz standards, but they were much better than what was out at the time. If you see a 300DC, grab it! The 170DC is different, but still very good. The 1180DC integrated is another really good one.
-Chris
Excuse me, but I wasn't seeking for answers in that other thread, I was just giving an example.
That technician's got a cold. The shop is closed and I will drop it to him probably after christmas, because I'm going on holiday. And I've had an intense period at school, so I haven't looked at it.
What you can do is to suggest me, if you wish, some possible sostitutes for the drivers, possibly with the same pinout of the originals, possibly modern and easy obtainable. If you know some numbers, I will be grateful!
I can hear you saying: "this guy is too convinced in the transistors being frigged. It might be a passive component..."
I've already checked every resistor and capacitor in that board, and I assume that the fault lies on that board, oterwise it would be absursd (but still possible... )
Maybe I'll trash the whole power amp board and buy a power opamp and make a gainclone with a Marantz preamp...
Sounds like the "last opportunity" idea.
That technician's got a cold. The shop is closed and I will drop it to him probably after christmas, because I'm going on holiday. And I've had an intense period at school, so I haven't looked at it.
What you can do is to suggest me, if you wish, some possible sostitutes for the drivers, possibly with the same pinout of the originals, possibly modern and easy obtainable. If you know some numbers, I will be grateful!
I can hear you saying: "this guy is too convinced in the transistors being frigged. It might be a passive component..."
I've already checked every resistor and capacitor in that board, and I assume that the fault lies on that board, oterwise it would be absursd (but still possible...
)Maybe I'll trash the whole power amp board and buy a power opamp and make a gainclone with a Marantz preamp...
Sounds like the "last opportunity" idea.
Your biggest mistake is to plug the repaired amplifier directly to mains without using a variac to analyze its power-up behavior or even a simple 40..100W light bulb in series with mains power to limit power consumption and allow you to do some measurements before components start burning
I don't have a variac [I'm poor
], so I use a regulated and current limited external supply [LM317/LM337 home made] to apply power to the circuit progressively and analyze its behavior. Sometimes things start to go wrong with just +-5V rails [unexpectedly high current consumption], but not wrong enough to smoke or arc components...
I don't have a variac [I'm poor
], so I use a regulated and current limited external supply [LM317/LM337 home made] to apply power to the circuit progressively and analyze its behavior. Sometimes things start to go wrong with just +-5V rails [unexpectedly high current consumption], but not wrong enough to smoke or arc components...
Hi Giaime,
Try 2N3440 (NPN) and 2N5415 (PNP). These are the same case style and should have the same pin out. These are higher voltage and I think good for around 1A max. Marantz used these in the model 500 power amp.
I was just wondering how you were making out when I saw your other comment. Let me know how you are doing.
-Chris
Try 2N3440 (NPN) and 2N5415 (PNP). These are the same case style and should have the same pin out. These are higher voltage and I think good for around 1A max. Marantz used these in the model 500 power amp.
I was just wondering how you were making out when I saw your other comment. Let me know how you are doing.
-Chris
Hello! I'm back from my Internet holidays...
News on this amp that has at least became world-famous...
I finally approached the unit with a more relaxated feel and...
I replaced the outputs (they were all gone...) with 2N3442.
The drivers were replaced with the only PNP-NPN couple that I was able to find... BC639/BC640.
The unit turned on without smoke (wow! for the first time...) but has become very hot at extremely low volumes, and didn't get cooler when I shut off the audio. I measured only 0.2mV on R748, the one that is meant to measure about 7.5mV.
The reason is (I think) that the BC639/640 have too much gain. The original transistors (only one survived!) measured 50 on the hfe test on my cheap DMM, and these 200. Too much!
I think that the only solution is to replace them with the aforementioned 2SC3421/2SA1358 which measured (at least the NPN did) about 50 on the hfe test. I haven't been able to find the PNP yet. I am planning to order them.
But another question. How you measure the voltage on that R748? Near there are the connections for the +60V (mine measured 70v, another mistery...), and the risk to make a big short with the DMM tips is very large. I was thinking of momentary solder small wires to the resistor and then measure on them, far away from the unit. How do you would do it? How you do these measures when you bias an amp?
News on this amp that has at least became world-famous...
I finally approached the unit with a more relaxated feel and...
I replaced the outputs (they were all gone...) with 2N3442.
The drivers were replaced with the only PNP-NPN couple that I was able to find... BC639/BC640.
The unit turned on without smoke (wow! for the first time...) but has become very hot at extremely low volumes, and didn't get cooler when I shut off the audio. I measured only 0.2mV on R748, the one that is meant to measure about 7.5mV.
The reason is (I think) that the BC639/640 have too much gain. The original transistors (only one survived!) measured 50 on the hfe test on my cheap DMM, and these 200. Too much!
I think that the only solution is to replace them with the aforementioned 2SC3421/2SA1358 which measured (at least the NPN did) about 50 on the hfe test. I haven't been able to find the PNP yet. I am planning to order them.
But another question. How you measure the voltage on that R748? Near there are the connections for the +60V (mine measured 70v, another mistery...), and the risk to make a big short with the DMM tips is very large. I was thinking of momentary solder small wires to the resistor and then measure on them, far away from the unit. How do you would do it? How you do these measures when you bias an amp?
Hi Giaime,
Firstly, the AC power may be jumpered for the wrong voltage. This would give excess voltages (ie 100V tap in a 120V land). Correct this first.
To measure bias, bring the test points out on leads attached to something so you don't get a short. I do this when I have to let an amp settle. Then I can leave the top cover on and get a real reading or readings. The reason you may have had a low reading when you measured bias is a poor contact to the test point. Check for oscillation while you are at it.
You are almost there! -Chris
Firstly, the AC power may be jumpered for the wrong voltage. This would give excess voltages (ie 100V tap in a 120V land). Correct this first.
To measure bias, bring the test points out on leads attached to something so you don't get a short. I do this when I have to let an amp settle. Then I can leave the top cover on and get a real reading or readings. The reason you may have had a low reading when you measured bias is a poor contact to the test point. Check for oscillation while you are at it.
You are almost there! -Chris
Thanks! but in my country there is 220VAC, and the primaries are connected right, since it does have multiple connections, but they are 100-120-230V. I suppose that the 220/230V difference would made the supply lower, instead it is higher.
Is this a sign of a failing rectifier? or it's just amp mistery? Bah, I don't care about it so much.... (clever note: if the rectifier was falling, I should hear hum, but I never heard it in the short time it has been on, about 30 minutes 'cause it was getting hot)
What do you think about that hfe difference for the drivers? Should I pursuit units that have the same hfe ratings of the original? I'm not an expert in solid state power amplification... and this design is a little complicated.
You know what? some times ago I repaired a Sansui A-40, with an usual AB complementary stage (not this quasi-comp of the Marantz). The drivers were burned, and I replaced them with the aformentioned BC639/BC640 (don't blame on me: this is all I can found near my town: they are cheap) and I didn't care about differences in gains. The amp worked from the first time and biased right, and it never get hot. It has been very cold during my usual heavy test (max volume, bass all the way up, heavy techno music on 4ohm, where behind the unit it says "never plug in a speaker under 8ohm). Maybe the originals had the same gain of the replacements...
Is this a sign of a failing rectifier? or it's just amp mistery? Bah, I don't care about it so much.... (clever note: if the rectifier was falling, I should hear hum, but I never heard it in the short time it has been on, about 30 minutes 'cause it was getting hot)
What do you think about that hfe difference for the drivers? Should I pursuit units that have the same hfe ratings of the original? I'm not an expert in solid state power amplification... and this design is a little complicated.
You know what? some times ago I repaired a Sansui A-40, with an usual AB complementary stage (not this quasi-comp of the Marantz). The drivers were burned, and I replaced them with the aformentioned BC639/BC640 (don't blame on me: this is all I can found near my town: they are cheap) and I didn't care about differences in gains. The amp worked from the first time and biased right, and it never get hot. It has been very cold during my usual heavy test (max volume, bass all the way up, heavy techno music on 4ohm, where behind the unit it says "never plug in a speaker under 8ohm). Maybe the originals had the same gain of the replacements...
Hi Giaime,
As for the supply voltage, I would still check the taps. I am fairly sure the tap is set low. A partially shorted primary could do this, rectifier failure will lower the voltage. Another thing to check is the meter accuracy on that range.
I haven't checked the specs on your replacement transistors, but do that now. If your supplies measure +- 70V, you would like a 160V rating on those drivers. Also check the current and dissipation. The gain-bandwith product on your replacements may cause the circuit to become unstable and oscillate. Check with a 'scope.
Deep breath, take your time.
-Chris
As for the supply voltage, I would still check the taps. I am fairly sure the tap is set low. A partially shorted primary could do this, rectifier failure will lower the voltage. Another thing to check is the meter accuracy on that range.
I haven't checked the specs on your replacement transistors, but do that now. If your supplies measure +- 70V, you would like a 160V rating on those drivers. Also check the current and dissipation. The gain-bandwith product on your replacements may cause the circuit to become unstable and oscillate. Check with a 'scope.
Deep breath, take your time.
-Chris
Sorry I've missed it: the supplies are not +-70V, it's just a single supply at +70V. Quite strange, like the rest of the amp.
Hey, you're sayin about a scope? It is a god-bless that I can find a good soldering iron, where the hell can I find a scope (I'm so poor!)?
For the oscillation thing, I saw the ratings of the main power transistors. The originals where something like many MHz, these are for 200kHz, if I remember it right, so there shouldn't be problems. Maybe a little too low? Will I lose highs? Sorry, but the 50MHz-graded MJ15003 costs about two times my cheap 2N3442....
I will thank you forever! Let's hope we will meet again after the fix of this *******' amp!
Hey, you're sayin about a scope? It is a god-bless that I can find a good soldering iron, where the hell can I find a scope (I'm so poor!)?
For the oscillation thing, I saw the ratings of the main power transistors. The originals where something like many MHz, these are for 200kHz, if I remember it right, so there shouldn't be problems. Maybe a little too low? Will I lose highs? Sorry, but the 50MHz-graded MJ15003 costs about two times my cheap 2N3442....
I will thank you forever! Let's hope we will meet again after the fix of this *******' amp!
Hi Giaime,
'Sokay. I forgot it was cap coupled output. Your outputs are fine. I would use 100V devices giving some headroom.
Use a 0.01uF cap in series with a diode to meter positive, with another cap to your meter negative. You now have an RF probe. With no signal you should measure 0V connected to the speaker output. Remember to connect the meter neg to the chassis also.
-Chris
'Sokay. I forgot it was cap coupled output. Your outputs are fine. I would use 100V devices giving some headroom.
Use a 0.01uF cap in series with a diode to meter positive, with another cap to your meter negative. You now have an RF probe. With no signal you should measure 0V connected to the speaker output. Remember to connect the meter neg to the chassis also.
-Chris
Hello guys!
I still need help: I can't find anywhere the transistors someone suggested me for the drivers (2SC3421/2SA1358). Someone knows a replacement for these? I'm looking for about 100V 1A, but it is fundamental that the hfe is about 50.
Other suggestions seems about 2SC2690/2SA1220, or MJE243/MJE253, but what about the BD series? I think I can find them easier... a BD139/140 would be good I think, but the 80V max seems a bit too low since I have a 70V supply, or is good? And if I could find them, I must always measure them to know if they are about 50 on hfe...
That hfe problem is important. I don't have much knowledge, but I learned that going too high on hfe for the drivers (like 200 for the BC639/640) would make my outputs very hot.... Someone can explain me about that and suggest me a range of hfe for the transistors? The schematic can be found on previous posts...
I still need help: I can't find anywhere the transistors someone suggested me for the drivers (2SC3421/2SA1358). Someone knows a replacement for these? I'm looking for about 100V 1A, but it is fundamental that the hfe is about 50.
Other suggestions seems about 2SC2690/2SA1220, or MJE243/MJE253, but what about the BD series? I think I can find them easier... a BD139/140 would be good I think, but the 80V max seems a bit too low since I have a 70V supply, or is good? And if I could find them, I must always measure them to know if they are about 50 on hfe...
That hfe problem is important. I don't have much knowledge, but I learned that going too high on hfe for the drivers (like 200 for the BC639/640) would make my outputs very hot.... Someone can explain me about that and suggest me a range of hfe for the transistors? The schematic can be found on previous posts...
I don't know of any amp, least of all one made by Marantz, that is designed to be dependant on hfe of any transistor (to a point). I do believe you simply need to set the bias current (or at least measure it and determine what it is running for idle current at present).
For the sake of symmetry, I'd replace all the drivers with the BC639/640 's. And recheck your measurements. If the output devices are getting hot, then there is a lot of current flowing. No other way for this to be. There are four 'emitter' resistors (although in the lower half these are actually on the collectors, but no matter) and I'd suggest you calculate the current after measuring the voltage across each one.
Make sure that you are really seeing 0.2mV and not 0.2V across that resistor.
For the sake of symmetry, I'd replace all the drivers with the BC639/640 's. And recheck your measurements. If the output devices are getting hot, then there is a lot of current flowing. No other way for this to be. There are four 'emitter' resistors (although in the lower half these are actually on the collectors, but no matter) and I'd suggest you calculate the current after measuring the voltage across each one.
Make sure that you are really seeing 0.2mV and not 0.2V across that resistor.
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