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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Tue 03, 2017 6:47 pm 
Member

Joined: Feb Sat 12, 2011 2:29 pm
Posts: 365
Location: Fayette County, Pa
You may be seeing the cumulative effect of some transistor substitutions. Often though with transistors claimed to be replacements the response curves will differ slightly between them. Plus you need to ask yourself which channel really has the response the amp was designed to have? Is it possible the "good" channel actually has too much gain? I have seen that happen too, example, a leaky bypass cap in the preamp may be causing a stage to amplify too much. That you would need to troubleshoot if it is a problem. As long as you can adjust the controls to where you get good sound I would be inclined to leave it alone, from what you describe they are pretty close. When you start trying to isolate problems like that you will need a scope and accurate generator to be precise.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Wed 04, 2017 2:58 pm 
Member

Joined: Oct Sat 29, 2016 12:53 pm
Posts: 56
Location: London, UK
Hi CaveRat,
CaveRat wrote:
You may be seeing the cumulative effect of some transistor substitutions. Often though with transistors claimed to be replacements the response curves will differ slightly between them.


Thought it would be the case.

CaveRat wrote:
Plus you need to ask yourself which channel really has the response the amp was designed to have? Is it possible the "good" channel actually has too much gain?


For what I've noticed, that's a notoriously difficult question to answer. We're drifting into the uncertain territory of subjectivity, only wish mr Saul Marantz was available for a reliable first hand statement.

CaveRat wrote:
As long as you can adjust the controls to where you get good sound I would be inclined to leave it alone, ....


So am I, especially since it seems to be very happy running with this configuration. It has been playing continuously at moderate volume (9 o'clock) since yesterday. The heat sinks are virtually cold, the sound couldn't be nicer. Again, that's according to my ear.

CaveRat wrote:
When you start trying to isolate problems like that you will need a scope and accurate generator to be precise.

As yet, I've neither a scope nor (more importantly perhaps) knowledge of using one. Having said that, I've got curiosity about this field of action so I'll start looking into it. There seem to be various software designed to partially substitute the scopes, the likes of VISUAL ANALYSER 2014.
Would you recommend that?

Regards,

Cris


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Thu 05, 2017 3:26 am 
Member

Joined: Feb Sat 12, 2011 2:29 pm
Posts: 365
Location: Fayette County, Pa
I don't recommend any software solution for a scope. The limitations of the computer is what is the problem. Unless you actually have a board installed in your computer to serve as the interface you are limited by the off the shelf components in the computer. I always recommend using a piece of equipment designed to do what it does, not a computer or software simulation to approximate it.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Thu 05, 2017 6:20 pm 
Member

Joined: Oct Sat 29, 2016 12:53 pm
Posts: 56
Location: London, UK
Hi CaveRat,

Confused again here: On the Right P750 I've already replaced:
H751- (2SC945 QR) with KSC945 CYTA,
H753- (2SC371 Y) again with KSC945 CYTA
and
H754- (2SA562 Y) with KSA733 CGBU

Everything seems to be working fine, but now I've moved to the Left in an attempt to do the same. At closer look I've noticed KSA733 is in actual fact an EBC, which seems to contradict the board's layout which requires ECB for H754
Attachment:
KSA733.jpg
KSA733.jpg [ 9.71 KiB | Viewed 208 times ]

Attachment:
H753_ 4.jpg
H753_ 4.jpg [ 2.78 KiB | Viewed 208 times ]


Moreover: as you can see, H753 itself it's a bit of a puzzle on the layout, with its BCEC.
Doesn't look quite right to me.
BTW, wasn't it a convention that the no 1 pin (first one on the right) is always the emitter? Unlike here. KSC945 CYTA, the one I've already used with (apparently) good results is in actual fact an ECB.

My brain is in a bit of a melting stage now while trying to make sense of this.
I would have thought that using the wrong transistor on the R_P750 would be instantly noticeable somehow.
How can this be?

Regards,

Cris


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Fri 06, 2017 5:38 pm 
Member

Joined: Feb Sat 12, 2011 2:29 pm
Posts: 365
Location: Fayette County, Pa
That has always been an issue with transistors. If you sub one you need to verify the type has the same lead configuration otherwise you can get yourself in trouble. US transistors generally used the EBC arrangement; (2Nxxx) Japanese went with the ECB 2SAxxx, 2SBxxx,2SCxxx, etc.) At least that was how it started back in the 70s. Now it's a mish-mosh of types and configurations which is why you need to check the data sheets before subbing anything.

Usually the emitter is on the left. But not always. There are a few oddball types that put it on the right, even a few that have it in the middle. Use the data sheet to resolve it for sure.

Your picture showing the E and C on the same point is wrong. They are not interchangable (misprint on drawing?) Go by the schematic and connect accordingly, the emitters are on the bootstrap; the extra C on H753 is wrong. That is based on the fact that those transistors are intended to turn on if bias current goes too high. The E-B voltage polarity is opposite since the bootstrap is reference for this purpose. Thus one transistor is a PNP, the other an NPN with both emitters at the same potential. As bias increases the base voltage is pulled positive and negative on the respective transistors turning them on. This lowers the difference between the voltages on the bases of the drivers effectively reducing the bias on them.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Fri 06, 2017 6:07 pm 
Member

Joined: Oct Sat 29, 2016 12:53 pm
Posts: 56
Location: London, UK
Hi CaveRat,

It's sorted, all revolves around two things:
1. H753 being initially wrongly represented on the layout
Wrong orientation
Attachment:
H753_ 4.jpg
H753_ 4.jpg [ 2.78 KiB | Viewed 200 times ]

Correct orientation
Attachment:
H753_ 1.jpg
H753_ 1.jpg [ 2.86 KiB | Viewed 200 times ]


Therefore, the requirement for H753 becomes clearly ECB

2. Although the technical sheet only shows one option (EBC), there are in actual fact two kinds of KSC945. One of them being KSC945C, where the "C" signals that the collector has been moved to the center pin.
Attachment:
KSC945.jpg
KSC945.jpg [ 17.31 KiB | Viewed 200 times ]

As it happens, I've installed the "C" version myself, no wonder the thing works as it should. A rare case of two wrongs making a right, if I may say.
For H751, I've used the KSC2383 on the other channel as well so now the sound is balanced and well coloured on both. It seems that the weakness on the Right was somehow due to using the KSC945C for H751. It was doing the job, only not quite as it should have.

Very pleased with the way things are now, going to push the unit a bit with extended tests. It seems to be holding very well so far after playing non stop over night, no heating at all.

Regards,

Cris


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Sun 08, 2017 3:47 pm 
Member

Joined: Oct Sat 29, 2016 12:53 pm
Posts: 56
Location: London, UK
Hi CaveRat,

It's an absolute joy listening to the sound of it now (full-bodied, warm yet very precise... I could go on a tangent here).
There's no distortion at high volume or any sign of transistor heating, although it's been playing continuously since Friday, day and night.

Don't think I've mentioned: I did the bias and DC current adjustments quite a few times (surprising how far off the values are thrown by replacing one part or another), as well as putting in output transistors with fresh mica insulators and new trimmer pots etc.

Speaking of TO-92 transistors, hope I'm not wrong concluding that EBC is mostly the "western" or more contemporary version, while for Japanese 70's or 80's gear the ECB configuration is to be expected.

CaveRat wrote:
Your picture showing the E and C on the same point is wrong. They are not interchangeable (misprint on drawing?) Go by the schematic and connect accordingly, the emitters are on the bootstrap; the extra C on H753 is wrong. That is based on the fact that those transistors are intended to turn on if bias current goes too high. The E-B voltage polarity is opposite since the bootstrap is reference for this purpose. Thus one transistor is a PNP, the other an NPN with both emitters at the same potential. As bias increases the base voltage is pulled positive and negative on the respective transistors turning them on. This lowers the difference between the voltages on the bases of the drivers effectively reducing the bias on them.


Well CaveRat, what can I say: myself I'm obviously only aping things here, but I know wisdom when I see it. What an impressive example of deep and wide knowledge this is, thanks for sharing.

Regards,

Cris


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Mon 09, 2017 2:36 pm 
Member

Joined: Oct Sat 29, 2016 12:53 pm
Posts: 56
Location: London, UK
Hi CaveRat,

I think the next step forward for me would be to try improving on the machine's radio performance.
The sensitivity, both on AM and FM isn't great and I wonder if there's anything that can be done.
If you're kind enough to continue helping me with advice, I guess it may be a good idea to open a new thread, making it easier to follow.

Speaking of following, I notice there's been a healthy number of viewings to our conversation so far, which must mean there are people out there that find the whole matter attractive.
In fairness, they may find some bits are perhaps more interesting than other: your knowledge possibly more than my ignorance.

Regards,

Cris


Last edited by cri_sta on Jan Wed 11, 2017 5:31 am, edited 1 time in total.

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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Mon 09, 2017 3:52 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 18885
Location: Utica, NY 13502 (USA)
cri_sta wrote:
I think the next step forward for me would be to try improving on the machine's radio performance... ...I guess it may be a good idea to open a new thread, making it easier to follow.
The protocol on this forum is to have only one thread for a particular piece of equipment. That way, all information and comments about that equipment stays in one place. The title can be edited and modified slightly to indicate that the discussion has gone in a different direction. Members who are following the discussion can click on the little yellow icon immediately preceding the title to automatically go directly to the first new post in a multi-page discussion, thus bypassing all that has been previously read by that member.

Congratulations on being able to resolve the amplifier problems in your Marantz!

Dave


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Mon 09, 2017 10:13 pm 
Member

Joined: Feb Sat 12, 2011 2:29 pm
Posts: 365
Location: Fayette County, Pa
The first thing will be to determine whether the sensitivity is really bad or if that is the best the radio will do. The first thing would be to compare it with other radios in your area and how it fares against them. If it is relatively close I would not want to do anything. Tweaking on IF transformers will likely just get you in trouble!

Step one would be to make sure you have a good antenna, especially for FM. Often that is all you need!

If it really is much worse than average then you may need to first determine if it is a component issue or an alignment problem. AM alignment should be done using an RF generator tuned to the IF frequency (455kHz) of the receiver. While their is a procedure, most can adjust the AM IF frequency by just measuring the AVC voltage and adjusting to maximum. Then adjust the upper and lower ends of the band to track (Oscillator coil frequency and padding cap on the tuner.) That will get you reaonably close for AM.

FM is another matter. You will need an FM sweep generator for that, as well as a stereo MPX generator or equivalent to set the FM coils. Plus a scope to view the waveform. There is a procedure for the order to make those adjustments as well. You likely will need the service manual since I can't say off hand if any test points are available / or need utilized on that particular model. It i not just a matter of setting those coils to maximum; you also have to consider the bandwidth of each (10 kHz). The sweep generator will allow for that to be done.

Discriminator adjustment is also important since a problem there will skew the MPX carrier rendering the stereo inoperative.

You might try adjusting the 1st RF trimmers on both FM and AM without causing any problems. These are also the two most likely to be out of adjustment. Quick and dirty test is to simply set the radio to a weak station about 2/3 of the way to the top of the band and adjust for maximum signal strength. Sometimes that is all you need to boost the gain, and those generally won't get you in any trouble with bandwidth, especially on the FM side.

My recommendation regarding alignment would be that unless you have the proper equipment (scope and sweep generator) and are familiar with its use, see if there is a local service shop there who could do the setup for you.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Wed 11, 2017 7:02 pm 
Member

Joined: Oct Sat 29, 2016 12:53 pm
Posts: 56
Location: London, UK
Hi CaveRat,

Thanks for your thorough advice.
The intention is not to use the tuner primarily but simply to have it there to a reasonable degree, for variation.
FM reception in the area (central London) should be very good, with literally hundreds of stations cramming the ether. The antenna is a very basic indoor 70's cord, but still took me a bit to make sense of which way it must be hooked.
As I said, I don't have a scope yet and the technical knowledge for using one is certainly beyond me. Having my 2230 send to a service shop doesn't appeal to me either after all the obsessing with its entrails.

So I find that the middle of the road, intuitive and as low-tec as possible way it's what's left. A variation of your "quick and dirty test", basically. The resources available were
Attachment:
FM.jpg
FM.jpg [ 95.32 KiB | Viewed 114 times ]


and

Attachment:
P100 FM assembly layout.jpg
P100 FM assembly layout.jpg [ 106.28 KiB | Viewed 114 times ]


For what I can gather, with L104 one can align the stations in the proper place on the scale while C106 provides some fine tuning to that.
L101, L102 and L103 seem to work the antenna's sensitivity.
After fiddling with pots there, the reception and clarity is much improved so I'd call it a success. Limited one I'm sure you'll call it.
Or a total disaster, perhaps?

Regards,

Cris


Last edited by cri_sta on Jan Wed 11, 2017 7:18 pm, edited 1 time in total.

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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Wed 11, 2017 7:07 pm 
Member

Joined: Oct Sat 29, 2016 12:53 pm
Posts: 56
Location: London, UK
Dave Doughty wrote:
cri_sta wrote:

Congratulations on being able to resolve the amplifier problems in your Marantz!

Dave


Thanks Dave


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Thu 12, 2017 5:22 pm 
Member

Joined: Feb Sat 12, 2011 2:29 pm
Posts: 365
Location: Fayette County, Pa
The procedure you posted will allow you to peak the RF section. It is about what I outlined except that I didn't have component designations to add since I don't have the service manual for that model receiver. Sounds like it did peak up some, alignment may be all it needed there.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Mon 16, 2017 6:02 pm 
Member

Joined: Oct Sat 29, 2016 12:53 pm
Posts: 56
Location: London, UK
Thanks CaveRat,
It has certainly improved the performance and the main stations are loud and clear now.
On a related issue, I hear Norway took the other day its radio emission off the air, all being digitally transmitted now. Is this the sign of times to come I wonder? It'd be sad.
Regards,
Cris


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