Any analogue scope with at least 10Mhz bandwidth and a sensitivity of 1 to 5 mv/div should be useful (which covers just about any traditional scope made in last 50 years).
Only silence and yet the VU meter registers... that points to some form of HF instability. If I remember correctly we proved the VU meter OK by disconnecting its input and it then showed zero.
He did say the scope was old, but probably not that old
.You are correct, the needles were zeroed in when C635 and C636 were removed. I also checked vu-meters with ohmmeter and they both read the same.
If I can now restore C506 hack , I can take pictures of them with and without that cap.
I am off now to the bookstore to lookup Oscilloscope for dummies
.And I wish all of you Happy New Year
Marko
There's probably quite a bit on-line on using scopes.
The oscilloscope is really just a voltmeter along the vertical axis, and the horizontal axis which "measures" time.
One very important point to be aware of is this. 99.9% of scopes will (should) have a mains grounded case and that means the "outer" or shield of the scope probe leads is connected to mains ground.
And if the amplifier is grounded... well you have to remember that you must securely clip the scope lead ground to "ground" in the amp. If the croc clip flirts off (and they do) and if it touches anything with voltage on it then you have a dead short to ground via the scope lead.
Also beware any uninsulated ground parts on the probe... its so easy to let it touch something "live" accidently.
2 seconds with a scope will show what is or is not going on with that amplifier stage.
The oscilloscope is really just a voltmeter along the vertical axis, and the horizontal axis which "measures" time.
One very important point to be aware of is this. 99.9% of scopes will (should) have a mains grounded case and that means the "outer" or shield of the scope probe leads is connected to mains ground.
And if the amplifier is grounded... well you have to remember that you must securely clip the scope lead ground to "ground" in the amp. If the croc clip flirts off (and they do
) and if it touches anything with voltage on it then you have a dead short to ground via the scope lead. Also beware any uninsulated ground parts on the probe... its so easy to let it touch something "live" accidently.
2 seconds with a scope will show what is or is not going on with that amplifier stage.
This is typical of the problem a scope is used for,
http://www.diyaudio.com/forums/anal...u-have-checked-see-its-stable-havent-you.html
http://www.diyaudio.com/forums/anal...u-have-checked-see-its-stable-havent-you.html
Elna's RLB series has 0.5uA leakage, it's 6x lower (vs 3uA) than regular aluminum electrolytics but not good enough compared to tantalums which is 0.05uA. Put back the 3.3uF tantalums and don't worry about the change of "sonics". People worry to much of the sound and not think of the implications when changing parts. Yours is no exception. It is a simple problem with a simple solution. Once you start hacking the insides of the amp, you tend to break things.
Did you have this problem before you recapped and change all the transistors in the tone control? Did you use reputable source for the transistors or are these counterfeit? Counterfeit transistors always have lower gain and if used on differential circuits, you will end up with higher DC offset. Did I mention that they are also noisier, In other words useless. Got to love Chinese counterfeiters
From experience, it's better to use Panasonic stacked metal film V series (Digi-Key - P4675-ND (Manufacturer - ECQ-V1H105JL)) when replacing electrolytic capacitors in the 1uF to 0.1uF range. The size for a 1uF V series is a little bit bigger than normal size aluminum electrolytics and you have no leakage to worry about.
Problem if you put high leakage capacitors in volume and tone control circuit is once they get noisy they tend to stay that way after correcting the problem, the damage has been done on that section of the pot. You can take apart the pot and move the wiper to a different section to fix it.
Polarized aluminum electrolytic caps also get leaky when they are connected backwards even if you put it the right way after, damage is usually permanent on the cap. After doing a recapping project, confirm with a voltmeter the caps are getting the right DC bias. Take a picture and make notes before recapping. Digital cameras are cheap. The silkscreen marking on PCBs are not always correct.
Taicon, ah yes Rotel usually make their stuff in Taiwan that's why it has Taicon caps. Junk caps.
Enough of the rant. You choose your own path, I'm just showing you mine.
Did you have this problem before you recapped and change all the transistors in the tone control? Did you use reputable source for the transistors or are these counterfeit? Counterfeit transistors always have lower gain and if used on differential circuits, you will end up with higher DC offset. Did I mention that they are also noisier, In other words useless. Got to love Chinese counterfeiters
From experience, it's better to use Panasonic stacked metal film V series (Digi-Key - P4675-ND (Manufacturer - ECQ-V1H105JL)) when replacing electrolytic capacitors in the 1uF to 0.1uF range. The size for a 1uF V series is a little bit bigger than normal size aluminum electrolytics and you have no leakage to worry about.
Problem if you put high leakage capacitors in volume and tone control circuit is once they get noisy they tend to stay that way after correcting the problem, the damage has been done on that section of the pot. You can take apart the pot and move the wiper to a different section to fix it.
Polarized aluminum electrolytic caps also get leaky when they are connected backwards even if you put it the right way after, damage is usually permanent on the cap. After doing a recapping project, confirm with a voltmeter the caps are getting the right DC bias. Take a picture and make notes before recapping. Digital cameras are cheap. The silkscreen marking on PCBs are not always correct.
Taicon, ah yes Rotel usually make their stuff in Taiwan that's why it has Taicon caps. Junk caps.
Enough of the rant. You choose your own path, I'm just showing you mine.
There's probably quite a bit on-line on using scopes.
The oscilloscope is really just a voltmeter along the vertical axis, and the horizontal axis which "measures" time.
One very important point to be aware of is this. 99.9% of scopes will (should) have a mains grounded case and that means the "outer" or shield of the scope probe leads is connected to mains ground.
And if the amplifier is grounded... well you have to remember that you must securely clip the scope lead ground to "ground" in the amp. If the croc clip flirts off (and they do
Also beware any uninsulated ground parts on the probe... its so easy to let it touch something "live" accidently.
2 seconds with a scope will show what is or is not going on with that amplifier stage.
Do I really want to use the scope after knowing what can potentially go wrong?
Yes, I do
. I will be careful and not connect anything untill checked here first.One question comes to mind after I read your post Swapping Op-Amps... you have checked to see it's stable haven't you? test tone, do we need it for our scope test and what frequency if yes?
Also, that C506 we used for grounding the input stage, shall I restitute it to proper position or do I leave as it is for future scope tests.
Thank You
Marko
Elna's RLB series has 0.5uA leakage, it's 6x lower (vs 3uA) than regular aluminum electrolytics but not good enough compared to tantalums which is 0.05uA. Put back the 3.3uF tantalums and don't worry about the change of "sonics". People worry to much of the sound and not think of the implications when changing parts. Yours is no exception. It is a simple problem with a simple solution. Once you start hacking the insides of the amp, you tend to break things.
Did you have this problem before you recapped and change all the transistors in the tone control? Did you use reputable source for the transistors or are these counterfeit? Counterfeit transistors always have lower gain and if used on differential circuits, you will end up with higher DC offset. Did I mention that they are also noisier, In other words useless. Got to love Chinese counterfeiters
From experience, it's better to use Panasonic stacked metal film V series (Digi-Key - P4675-ND (Manufacturer - ECQ-V1H105JL)) when replacing electrolytic capacitors in the 1uF to 0.1uF range. The size for a 1uF V series is a little bit bigger than normal size aluminum electrolytics and you have no leakage to worry about.
Problem if you put high leakage capacitors in volume and tone control circuit is once they get noisy they tend to stay that way after correcting the problem, the damage has been done on that section of the pot. You can take apart the pot and move the wiper to a different section to fix it.
Polarized aluminum electrolytic caps also get leaky when they are connected backwards even if you put it the right way after, damage is usually permanent on the cap. After doing a recapping project, confirm with a voltmeter the caps are getting the right DC bias. Take a picture and make notes before recapping. Digital cameras are cheap. The silkscreen marking on PCBs are not always correct.
Taicon, ah yes Rotel usually make their stuff in Taiwan that's why it has Taicon caps. Junk caps.
Enough of the rant. You choose your own path, I'm just showing you mine.
Hi hitachi-nut,
And sorry for not replying sooner. I assure you I did not remove tantalums for sonic benefits, they were my suspects knowing they can also leak when they go bad and since I changed all of the electrolytics...
I am willing to put them back, or rather buy new tantalums considering these are 30 years old. But maybe it would be wiser to do that after the arrival of the scope and determineing where the problem is.
I did have a problem with popping before recapping and transistor change. As for the scratchinness in volume pot, I dont know maybe it was there all along but I was not aware of it because I used a knob while rotating it instead touching it bare.
There is probably no such thing as a reputable source for transistors in Croatia, but I beleive it to be safer then e-bay transistors. I am enclosing a picture of new and old transistors, maybe you can help me determining whether they are fakes. I am willing to sacrifice one in scientific purposes if they need to be broken. Changing transistors was probably my biggest mistake, since I used SC2240GR with typical HFE of 250, while old transistors SC1327 had beetween 450 and 500. Only comfort to me is the old ones were poory matched between channels.
I am sorry I did not know about Panasonic stacked metal film V series before I bought ELNAs, if you beleive the replacement is worth it I can order them, there are only four caps like that in the circuit.
Confirming cap polarity with DMM is a good suggestion, and I can do that while I wait for scope.
I did not understand your post as ranting, I received several good advices for which I am grateful for. You obviously know your way around caps and if it turns out the problem is in one of the caps I installed wrong or I used wrong type I would like to hear your recommendation on substitute.
Thanks Marko
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You can leave the C506 as it is as we know the problem is present with it like that. It eliminates any (further) trouble from the input and pot side of things.
The transistors. In practice the gain of the transistors won't make any practical difference in the performance of the amp as long as they are reasonable. The gain of the amp as a whole (and the various stages) is determined entirely by the feedback components.
You could always fit the originals back as they appear undamaged but lets scope it all first and see what is going on. The ancient BC549C (different pin out) is a good low noise device in you can get reputable ones.
The transistors with "painted" fronts are likely genuine, I have seen and used plenty of official Philips (as an example) supplied parts marked like that.
The transistors. In practice the gain of the transistors won't make any practical difference in the performance of the amp as long as they are reasonable. The gain of the amp as a whole (and the various stages) is determined entirely by the feedback components.
You could always fit the originals back as they appear undamaged but lets scope it all first and see what is going on. The ancient BC549C (different pin out) is a good low noise device in you can get reputable ones.
The transistors with "painted" fronts are likely genuine, I have seen and used plenty of official Philips (as an example) supplied parts marked like that.
Marko,
The 2SC2240-GR you have are 0.0000001% original. The 2SC2240 has long been discontinued by Toshiba, so most of the parts out there (eBay) are counterfeit. This applies to a lot of Japanese parts so be careful where you buy them.
The physical dimensions of counterfeits are smaller than the originals. It's a general rule and it doesn't always apply. Measure with a caliper and compare. The Chinese counterfeiters has to skimp somewhere, in this case the epoxy material.
The other counterfeit test is with a curve tracer to verify gain and HFE linearity. A DMM HFE test is not sufficient. Most Japanese audio transistors have uniform HFE across the operating range as shown on their datasheet and a curve tracer will give you this information. Like I said before, counterfeits are always short in gain and this test will weed out most of them.
The final test is in-circuit where noise performance is measured. Noise difference can be 6dB (2x) or more. Bad news when you are dealing with phono preamps.
I routinely do all of the above plus more battery of tests to evaluate unknown parts.
People on this forum can only offer you so much, some are good advice some are just rubbish. It's up to you to analyze the information presented and pick one that is factual.
re:rant, yeah I could have used another word but that was the first thing that pop into my head when I finished the post
The 2SC2240-GR you have are 0.0000001% original. The 2SC2240 has long been discontinued by Toshiba, so most of the parts out there (eBay) are counterfeit. This applies to a lot of Japanese parts so be careful where you buy them.
The physical dimensions of counterfeits are smaller than the originals. It's a general rule and it doesn't always apply. Measure with a caliper and compare. The Chinese counterfeiters has to skimp somewhere, in this case the epoxy material.
The other counterfeit test is with a curve tracer to verify gain and HFE linearity. A DMM HFE test is not sufficient. Most Japanese audio transistors have uniform HFE across the operating range as shown on their datasheet and a curve tracer will give you this information. Like I said before, counterfeits are always short in gain and this test will weed out most of them.
The final test is in-circuit where noise performance is measured. Noise difference can be 6dB (2x) or more. Bad news when you are dealing with phono preamps.
I routinely do all of the above plus more battery of tests to evaluate unknown parts.
People on this forum can only offer you so much, some are good advice some are just rubbish. It's up to you to analyze the information presented and pick one that is factual.
re:rant, yeah I could have used another word but that was the first thing that pop into my head when I finished the post
Is the above post a dig at me ? I think it must be as myself and hitachi_nut are the only ones posting.
Here are some 15 to 20 year old parts from before the days of widespread counterfeiting and internet use. One is very definitely suspect... the MPSA56. As tested it's a good NPN device (the clue is in the NPN bit). Could it be a genuine marking mistake. I don't know. It was from a batch of 50 or so from a major well known UK distributer and a handful were like this. Many common devices are made under license so just because it isn't marked as the original manufacturer doesn't mean it isn't "genuine".
The painted ones such as the Philips were supplied by Philips themselves as part of official repair kits for Philips consumer products.
With the explosion of internet use, auction sites and so on, the last few years have seen an exponential growth in counterfeiting and these days the only sure way (as sure as can be) is to buy parts from major companies that guarantee traceability and that what is offered is actually what you get.
Lets hope the scope checks help reveal the Rotels problem.
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Hi hitachi_nut,
I said I did not buy transistors from ebay, but locally. However I can not call my local shop reputable, although one of its sources is dutch company Nedis (Groothandel en distributeur voor kabels, consumenten electronica en accessoires | NEDIS).
However, if transistors on eBay are mostly counterfeits, and the pictures of SC2240 transistors on eBay are totally different from my transistors I can only concur that mine are originals, and yes I measured them with caliper, dimensions check out OK.
Lets hope the scope checks help reveal the Rotels problem.
I agree
Maybe, and I don't want to jinx it, our O-day could be as early as tomorrow.
I agree
Maybe, and I don't want to jinx it, our O-day could be as early as tomorrow.
Lets hope so
Try and get some "proper" probes with it too as we are looking at tiny amplitude signals.
Thus all effects comes from the preamp section.
Replace all electrolytic caps in the signal pad against foil caps, at least C529/C530, C537/C538 and C505/C506 and introduce additional foil caps in series to R503/R504. Otherwise there are low voltages at the potentiometers through leaky character of electrolytic caps.
This I must often perform particularly by various Rotel amps and often also by the Linn Pretek/Intek.
remove also the high - and the low filter switches and do make a hardwiring (very important to avoid popping destroy of your loudspeaker voice coil while switching)
Check also the PCB aera, whether there is residual liquid from various old caps, especially ELNA's gold caps special made for audio like that one from this URL:
http://www.wollenweber-audio-modification.de/Bilder-Bauteile/Silmic-2 001w.jpg
One of the most unpleasant capacitors by service at legacy hifi components because of the unwanted oxidation of conductor tracks (PCB wires) and time-consuming PCB cleaning.
By my service work semiconductor errors by Rotel amp devices in general are extremly rare (except if there are corroded leads by leaking capacitors).
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Hi tiefbassuebertr,
Very interesting to hear your first hand experiences with Rotel amps. Do you have any particular foil cap recommendation? Aren't they too big to squeeze in because of dimensions and axial leads?
Can you please explain with a little more details what do you mean by "remove also the high - and the low filter switches and do make a hardwiring "
As you can see from the pictures in post #67, there was lots of dirt on the silkscreen, I presume from glue rather then leaking caps, mostly in amplifier area, less so in preamp. I managed to clear most of it, and I agree it was very time consuming job.
Thanks
Marko
Lets hope so
Try and get some "proper" probes with it too as we are looking at tiny amplitude signals.
Hi Mooly,
unfortunately I don't know what kind of probes will come with the scope. If needed I will buy new set.
Thanks
Marko
Switches in serial to the signal pad causes mostly unwanted popping while switching - actually you must always turn the volume control knob in the "ZERO" position before swtich-on/switch off the individual function (most user's don't perform this). This is most pronounced by selecting various modes in the phono RIAA head amps, but also in the equalizer/filter aera in the line amp section.
A lot of speaker voice coils died through this unwanted popping. Thus I put these functions dead after asking the user, which funktions he need
You can not meet a certain visual appearance by large dimensions and axial leads - the appearance of this work is very very ugly. Nevertheless I do that mostly in this kind, if the user agrees and if no acrylic glass cover is in use.
Otherwise the problem is, that either the cut off frequency of the unwanted high pass (subsonic filter) function goes upwards to higher frequencies (up to 40 Hz, not so critical by small bookshelf loudspeakers - in opposite by large three-way loudspeakers) - or one is yet again forced to use polarized capacitors.
I use WIMA caps for this task - and in the meantime there are a lot of new low voltage MKS models on the marked with high capacity up to 100uF by small sizes. But this new low voltage models (high volume/capacitance ratio) I have never heard and compared to that one with larger sizes, but I am sure, sound is much more better than by polarized electrolytics in use at unpolarized condition.
WIMA Capacitors, Product Range
WIMA
Also this type is useful:
http://uk.rs-online.com/web/p/plastic-film/0335094/
http://de.rs-online.com/web/p/folien/0334732/
For most ultimate sonic quality I use MKP like this:
http://uk.rs-online.com/web/p/produ...031754620363030562673633D592677633D4E4F4E4526
But this is only useful, if you bypass the complete line pre amp of your integrated version - that means between the input selector and the power amp input is only the volume control resistor and the serial caps without any additional active line stages.
In this case you must also remove the NFB polarized cap from the power amp section and replace through a foil version (main problem in respect to the available space).
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Thank you tiefbassuebertr, I appreciate very much your detailed explanation. I know I already asked too much but if and when you have time could you answer just one more question. What do you think of a "compromise", instead of replacing but rather bypassing electrolytic cap with small value film cap. Does that bring out the best of both worlds, at least it is not space consuming and thus appearance is better?
Thanks
Marko
It is here
This is xmas after xmas for me, santa brought me this:
Oszilloskop GO10 Equipment Grundig Radio-Vertrieb made in Germany, which is always a good thing, but dangerously close to 50 years Mooly mentioned by the looks of it. I hope it will suffice, as well as the probes.
This the only link to specs I found online:
http://www.radiomuseum.org/r/grundig_oszilloskop_go10go_1.html
This is the moment when I listen carefully.
This is xmas after xmas for me
, santa brought me this:Oszilloskop GO10 Equipment Grundig Radio-Vertrieb made in Germany, which is always a good thing, but dangerously close to 50 years Mooly mentioned by the looks of it. I hope it will suffice, as well as the probes.
This the only link to specs I found online:
http://www.radiomuseum.org/r/grundig_oszilloskop_go10go_1.html
This is the moment when I listen carefully.
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Partly yes and partly no.
Definitly no for remove your described effect "Second symptom is scratchiness in left channel while turning volume pot up or down"
Which loudspeaker you have?
Now an "Off-Topic question to you: many years ago I have heard sometimes on Sunday night a jazz program on AM radio im my car while driving, station "HR-1" (Hrvatska Radiotelevizija) from your country (1134 KHz AM aera, HRT Zagreb). It was in the years between 1989 and 1998 (either between 11:00 PM until 12.00 PM or between 12:00 PM and 1:00 AM of follow day).
Reception of this radio station was only possible in the night because of the distance to the station.
I don't understand serbo-croatian language and thus I don't know the exactly name of this radio jazz programm. Perhaps you can ask about
HRT: HRT u?ivo
and sending the info to my e-mail address
kirschner-hifi@tiefbasswiedergabe.de
I think, you get also the information about the currently name and currently broadcasting time of this programm, so far it still exist.
Thank you very much
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