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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Thu 22, 2016 6:42 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
CaveRat wrote:
Set R765 to minimum (lowest bias), then power this up through a variac with output transistors disconnected and check E/B voltages supplied to them. (Measure E-B, not to ground) Raise the AC voltage slowly while monitoring the E-B voltages. It should stay under 1 volt as you raise voltage to about 75 VAC line voltage in. If they go over one volt STOP, you have other problems and installing the outputs would likely free the smoke monster. Go no higher in this config. Also keep an eye out for any transistors heating up. That should not happen unless something else is bad. Don't connect speakers; not necessary for this stage of testing. There is also a large cap between the bootstrap and speaker terminal not shown on the schematic. Make sure it hasn't shorted. (Usually they don't but if it did it would take out the speaker and probably the outputs again.)

If you get the variac up to about 75 VAC in without the E-B voltage going over 1 volt, measure the supply voltage from ground to J755 point. Then measure the bootstrap at J758. The bootstrap should be about half the B+ supply voltage. (You may need to tweak the centering pot R760 to do this)

If all checks out, install the outputs and repeat the variac test. Measure all transistor voltages as you bring it up watching for anything out of normal. By the time you get to 75 VAC you should be able to get audio through it. reconnect speakers and test that. It may be a bit distorted, but both channels should be similar. If so bring it p to 120 VAC and monitor for anything out of the ordinary. Gradually bring bias current up to specs and you should be good.


Hi CaveRat,

I'm finally approaching the point of being able to apply this crucial bit of your excellent advice.
Pretty much everything you suggested is replaced, apart from the heat-sink diode. The Variac is here, so nothing to hold me back really.
Am I a bit nervous? You bet.

Regards,

Cris


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Fri 23, 2016 7:14 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
cri_sta wrote:
CaveRat wrote:
Set R765 to minimum (lowest bias), then power this up through a variac with output transistors disconnected and check E/B voltages supplied to them. (Measure E-B, not to ground) Raise the AC voltage slowly while monitoring the E-B voltages. It should stay under 1 volt as you raise voltage to about 75 VAC line voltage in. If they go over one volt STOP, you have other problems and installing the outputs would likely free the smoke monster. Go no higher in this config. Also keep an eye out for any transistors heating up. That should not happen unless something else is bad. Don't connect speakers; not necessary for this stage of testing. There is also a large cap between the bootstrap and speaker terminal not shown on the schematic. Make sure it hasn't shorted. (Usually they don't but if it did it would take out the speaker and probably the outputs again.)

If you get the variac up to about 75 VAC in without the E-B voltage going over 1 volt, measure the supply voltage from ground to J755 point. Then measure the bootstrap at J758. The bootstrap should be about half the B+ supply voltage. (You may need to tweak the centering pot R760 to do this)


Hi CaveRat,

Had a cautious test today: volume to minimum, R765 trimmer to minimum, variac from zero and slowly increasing.
At 75 VAC the output transistors E/B voltages were
H001: 3.35 VDC (measured between J757 and J756)
H002: 15.11 VDC (between J762 and J761)
Attachment:
1.jpg
1.jpg [ 16.53 KiB | Viewed 631 times ]


Also, transistor H752 (one of the three TO-39 types on the board and the only one without a heat sink) is heating up pretty fast to the point of uncomfortable to touch.

Not very good yet, is it?

Attachment:
P750 board layout color L.jpg
P750 board layout color L.jpg [ 115.65 KiB | Viewed 631 times ]


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Fri 23, 2016 7:48 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 383
Location: Fayette County, Pa
Not Good.

What is the voltage across R 763? It looks like the driver is drawing excessive current. What is voltage on J754 and J758 as you raise the line voltage? Does 758 stay about half that of 754 give or take 1 V?


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Sat 24, 2016 6:06 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
Hi CaveRat,

Got new data, but first wanting to make sure I got a few things right:

1.All readings are done with TO-39 output transistors removed
Attachment:
1.jpg
1.jpg [ 32.86 KiB | Viewed 611 times ]

2. I hope "voltage on J754 and J758" means voltage measured between junctions and ground
4. Still unclear what "bootstrap at J758" or "B+ supply voltage" refer to. Really sorry, I've tried to look it up but no result.
2. I'm bringing the Variac to a max of 75VAC, which is about 1/3 of a 230VAC environment. This is what we need, right? Thought I'd ask since I understand you're in a 110VAC and you must find the British arrangement rather unusual.
....................................................................
Now, readings with Variac to 75VAC:

voltage across R 763 -5.26VDC
voltage J754 to ground-17.7VDC
voltage J758 to ground -15.1VDC

voltage J755 to ground -20.3VDC

Merry Christmas,

Cris


Last edited by cri_sta on Dec Sat 24, 2016 6:26 pm, edited 1 time in total.

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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Sat 24, 2016 6:18 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
Progression of voltage at junctions
Attachment:
1.jpg
1.jpg [ 9.01 KiB | Viewed 610 times ]


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Sat 24, 2016 6:50 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 383
Location: Fayette County, Pa
You are drawing excessive driver current and have a severe imbalance What is the condition of H755? Is it wired correctly or shorted? What is voltage across R770? (Measure this across the resistor using an isolated DVM not to ground) Should be less than .5V. If it is, look for something pulling the bootstrap high.

If not, CAREFULLY tack solder a jumper between the base of H755 - H756 and bring up the variac slowly while watching voltage drop across resistors (not to ground) R770 and R 773 They should be 0 Volts as you go up to about 75 VAC line voltage (The jumper has cut off both drivers, assuming everything is working right.) If you see voltage you have a problem with the drivers either leaky or shorted. Replace them both (or check for miswired) and try again.

Bootstrap is the lead between each side of the amp. It is on J 758 Supply is the lead from power supply found on J 755. In this type of Amp, the bootstrap is approximately half of the supply voltage (give or take a couple volts or so) Your amp shows a supply of 66.5 V, bootstrap at 29V The difference between each half is to compensate for gain differences in the complementary transistors designed into this circuit. FWIW, when you encounter a wide discrepancy between half voltage at the mid point bootstrap and supply it tells me that a design issue required extreme measures to get things set right. (If one of my engineers would have handed me a design with this wide of discrepancy I would have sent him back to his drawing board to start over)


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Sat 24, 2016 7:03 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 383
Location: Fayette County, Pa
Another thing to check. Remove the jumper I mentioned earlier (if you installed it) to put things back to normal. measure the voltage from the junction point what looks me like H757 - H758 and the collector of H752. As you raise the variac it should go to about 2.1 Volts and stay there. If it goes higher check your polarity on M005 on the heatsink and H757. Make sure they are forward biased, not reversed. (I am not 100% certain on these part designations as my JPG schematic is a bit fuzzy but that is what it looks like to me)


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Wed 28, 2016 7:24 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
Hi CaveRat, quite busy with work these days (a bit out of sink with the rest of the world but that's the way it is).
Here's another schematic of the board, easier to read I hope.
Attachment:
p750.jpg
p750.jpg [ 31.54 KiB | Viewed 560 times ]

Attachment:
p750R.jpg
p750R.jpg [ 46.58 KiB | Viewed 560 times ]


The only measurement I could take without dismantling the board was the one across R770: it's 2.56VDC.
I intend to replace both drivers tomorrow and see if that makes any difference.
Now I know bootstrap, I know supply and relationship between them. Great, thanks.

Regarding polarity: H005 is the square diode attached to heat sink, which hasn't been touched so it's in the initial position.
H757 however, I've reconstructed out of new parts following this:
Attachment:
H757 replacement.jpg
H757 replacement.jpg [ 33.73 KiB | Viewed 560 times ]

Parts used were
-thermistor 150 OHM @25°C, Digikey part NTCLE100E3151JB0
and
-resistor 18 Ohm 1/2W, Digikey part CFM12JT18K0

For what I can gather, neither thermistor nor resistor have polarities. I've taken great care to place the fabrication with the same orientation as previous one.

Not sure where H757/H758 junction is.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Thu 29, 2016 3:07 am 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 383
Location: Fayette County, Pa
2.5 Volts across R770 is too high, looks like excessive driver current.

Try the jumper I mentioned earlier if you haven't and verify the voltage drops to 0 volts across that resistor. If not, you have driver issues, either transistors or something causing excessive current flow. (short on PCB?)


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Thu 29, 2016 6:08 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
Working on replacing drivers now.

BTW, while ordering the parts for transistors H753 and H754 I had to choose between 120 and 200 gain @ 1mA.
Didn't know which would be better so ordered both for both. Which ones would you recommend using?


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Thu 29, 2016 7:20 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
New H752 transistor in (not heating up any more which I take to be a good thing).

New measurements:

Output transistors
H001 (J756-J757) -3.2V
H002 (J761-J762) -9.6V

Across R763 -90 mV
Ground to J755 (supply) -21.9V
Ground to J758 (bootstrap) -18.2V
Ground to J754 -20.6V

All values without yet tweaking any of the pots .


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Thu 29, 2016 7:28 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 383
Location: Fayette County, Pa
Gain is not the important factor here. Collector current and voltage are. With a supply of 66 Volts the minimum I would use would be a transistor rated at 120 Volt B-C. Current wise I hope you meant 1 Amp, not 1 ma..... 1 amp collector current would be sufficient. Regarding gain, these stages provide a current gain but their main purpose is to prevent loading of the first stage by the outputs.

Did you install the test jumper? Your voltages are still a bit off like the drivers are conducting too heavily.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Fri 30, 2016 2:56 am 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
To replace H751 (2SC945) and H753 (2SC371), I've got
-KSC2383YTA, Bipolar (BJT) Transistor NPN 160V 1A 100MHz 900mW Through Hole TO-92-3

To replace H754 (2SA562), I've got
-KSA733CYTA , Bipolar (BJT) Transistor PNP 50V 150mA 180MHz 250mW Through Hole TO-92-3
or
-KSA733CGBU , Bipolar (BJT) Transistor PNP 50V 150mA 180MHz 250mW Through Hole TO-92-3

Some people online say these are good equivalents for the originals. I certainly have no valid opinion on the matter. Do they seem ok to you?


I'll replace H755 and H756 sometime today as soon as I get a chance. If voltages still no good after that, I'll put the test jumper on.

Also, is there any test I could do on diodes H758 and H759, I think I read somewhere that one of them being toasted created similar problems.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Fri 30, 2016 6:04 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
Replaced drivers H755 and H756, also diodes H758 and H759.
Very different story altogether, values @75VAC with pots still not tweaked:

Output transistors
H001 (J756-J757) -140 mV
H002 (J761-J762) -79 mV

Across R763 -42 mV
Ground to J755 (supply) -23.05 V
Ground to J758 (bootstrap) -14.6 V
Ground to J754 -15.4 V

Kept it on just for a few minutes, but none of the drivers got hot. Could it be good news?


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Sat 31, 2016 5:11 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 383
Location: Fayette County, Pa
Those voltages are actually looking much better The bootstrap is somewhat unbalanced but may be OK considering you are operating at about 30% of rated supply. Try raising the variac up to about 120 VAC supply and see if the bootstrap comes closer to 1/2 of the supply. If it does then raise the supply on up to where the supply is about 50 VDC. The bootstrap should be about 23 Volts. If so things are looking good. Shut it down and reinstall the outputs and try again with everything as normal.

If the other channel is working, start raising the line voltage and compare channel 1 and 2 at the bootstrap. They should stay relatively close to each other between channels. If not try adjusting the bootstrap balance pot R760. Also monitor the voltage between J757 and J759. as you raise the line voltage. At no point should that exceed .2 Volts. If it does, check your bias control R765 on the channel you have been working and try to adjust it to lower the voltage. If you can, continue raising the line supply. If not STOP. You have another problem. Once you reach full supply voltage compare both channels at each connection ( J numbers). They should be about the same on each channel and things should be working.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Dec Sat 31, 2016 7:36 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
Results! Followed the instructions and things seem to come into line.

Attachment:
T.jpg
T.jpg [ 30.87 KiB | Viewed 502 times ]


Really pleased with the way it went, especially adjusting bootstrap balance with a 25 turn pot is a joy. Also, as you can see between J757 and J759 there's virtually no tension, even without adjusting pot R765.
Everything looks stable and there's no sign of overheating.

It may seem silly, but I don't want to push my luck with further testing (and possibly messing thing up) so I'll leave it for this year. I'm really glad to step into 2017 wrapped up in a glowing sense of achievement.

Many thanks again for your advice CaveRat, and a Happy New Year!


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Sun 01, 2017 5:27 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 383
Location: Fayette County, Pa
And a Happy New Year to you!

Voltages look good to me, give it a shot and see what happens. Just monitor for anything out of the ordinary as you would with any major rework.

One thing we have been negligent about mentioning are two electrolytic caps. C 753 and C 757. These seem to be OK from the voltages you got, but they could breakdown over time and cause problems (Not a common failure just something to be aware of) If you have a couple new ones and want to change them fine, but if not I wouldn't be afraid to go with the ones you have in there as they seem to be working.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Mon 02, 2017 4:03 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
Hi CaveRat,
Good start in the new year with everything working fine @240VAC. Adjustments of R765 and R760 brought values on Right P750 board into proper balance and very close to the ones of the other channel.

I did several sound tests, culminating with having it on with volume at 9 o'clock for about 10 hours. No heating at all on this side. Output transistors of the other channel only slightly warm. I suspect the new transistors installed (someone described them as a bit of an overkill for this very job), together with the high spec thermal grease may be doing a good job here.

The only concern I have is that the sound on this (R) channel seems to have lost some of its original "beefiness". The sound is clear but now quite feeble in comparison to the (L) unspoiled channel. Plainly speaking, it seems that the mid to low-mid range is weaker now.

CaveRat wrote:
One thing we have been negligent about mentioning are two electrolytic caps. C 753 and C 757. ... If you have a couple new ones and want to change them fine, but if not I wouldn't be afraid to go with the ones you have in there as they seem to be working.

C753 and C757, together with the rest of electrolytics on the board are brand new, having been replaced during the initial refurbishment. As a matter of fact, the current problem derives from a recap campaign going slightly wrong.
I still have some spare caps though so I could re-replace them as a matter of precaution if you think so.

Regards,
Cris


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Mon 02, 2017 5:45 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 383
Location: Fayette County, Pa
(Edit to add) I assume you tried switching the speakers to confirm its not a speaker issue.

To troubleshoot this you will need a scope and audio generator. Since you have the original channel for comparison it will be fairly easy to do a freq response curve evaluation between them. You will need to feed the same siganl into the AUX inputs of both channels. Make up a test cable to do this; tie two shielded cables together on one end and connect to the output of the AF generator. Keep the output level low and plug both cables into the L -R AUX input jacks. Set tone controls and balance control to midpoint. Use the scope to observe a 1 kHz sine wave at a level of 1 V P-P on each speaker. (You may need to adjust the balance control slightly to get the same amplitude on each channel due to slight differences in gain between them) Next adjust the frequency of the generator to the following and record the P-P amplitude of each on a chart. Do not change any amplifier settings as you change frequencies; This will compare frequency response curves between the channels. Take readings at the following frequencies:

20 Hz, 40Hz, 80 Hz, 150Hz, 300 Hz, 400 Hz, 600 Hz, 1kHz, 1.5 kHz, 2,kHz, 3 kHz, 5 kHz, 7 kHz, 10 kHz, 12 kHz, 15 kHz, 20 khz.

In a perfect world the levels will change and change the same on each channel; but don't expect to see that. What results will determine where possible response problems may occur.

Another test would involve the use of a signal tracer with a coupling capacitor on a high impedance input. This would allow you to hear the signal as it goes through the amplifier. Again, compare the good and bad channels at the same points in each. The critical test points would be the base of H 752 and the bootstrap point. The signal should be the same at each for each channel. (You will need to attenuate the bootstrap signal to prevent overdriving the signal tracer). The results will pin down where the signal loss is occurring.


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 Post subject: Re: Readjusting bias current and DC offset - Marantz 2230
PostPosted: Jan Tue 03, 2017 5:05 pm 
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Joined: Oct Sat 29, 2016 12:53 pm
Posts: 65
Location: London, UK
No scope yet so willy-nilly I'll have to use the lo-tech method: by ear. Temporary solution perhaps, but the only one available so the customer (me, that is) will have live with it for the time being.

What I noticed was that the sound wasn't too bad overall when balance was tilted about 1/3 towards R, to compensate for that side's "loss of beef".
At that stage, the R channel had many parts replaced compared to the L: most of them transistors, resistors and diodes on the P750 board, but the output transistors as well. Which got me thinking that there may be a chance to bring things into balance by also replacing the outputs on the L. Very easy job, couple of screws each and a bit of thermal paste so went for it. The result is a very convincing balance between channels; the oscilloscope may disagree to some extent, but my ears seem easier to deceive and therefore forgiving.
The only difference I find from previous status quo is that for a proper, "mature" sound I need now the Bass and Mid controls a bit beyond the 12 o'clock position I was used to.
Like this:
Attachment:
main.jpg
main.jpg [ 29.87 KiB | Viewed 447 times ]


On a positive note, the sound stays crisp, tight and lacking distortion even with the volume beyond 1 or 2 o'clock; it wasn't the case before all this, that would have been uncomfortable to listen to. Had it playing for about two hours now with volume at 1 o clock and the two heatsinks are pleasantly warm. The L one slightly more than the R, which makes me wonder whether the changing of driver transistors on the L P750 could be a good idea.


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