2270 BIAS will not Adjust below 45 mV...

[Whaleman]

Thanks for the advise Rob, swapping sides would just move the problem to the other side--I think.

As a last resort, I took out the bias diode at H760 and strung 2 4148 DIODES in series, it only made the bias got up from 100 to 120 mV at junctions 754 and 760.

I do have the SM and the schematic, kind of worthless really. Nothing said about troubleshooting high bias. it would seem like the problem is somewhere else than where I am looking.

 

[Robisme]

Do the schematics have the voltages listed at certain junctions? Thats what helped me with the 4300 board.

Rob

 

[Whaleman]

Unfortunately the schematic does not have voltages.

But here is what I found:

R783 355 ohms should be 1,500
(R782 does measure 1,502)


Other resistors with with questionable values:

R771 4.76 K 27K
R772 6.1 K 27K

R775 14.8K 47K

R758 78.1K 220K
R759 70.2K 220K

Can someone confirm that I am not dealing with documentation discrepancy, this is an early unit.

 

[EchoWars]

Does the color-code on the resistors agree with the expected (not measured) value?

 

[Robisme]

Did you pull them out of the circuit to test them? You have to pull at least one leg. Do they have the color stripes on them? You can tell value from them.
To be that far off in value could be a problem. I mean will be a problem.

Rob

 

[Whaleman]

Thanks folks, all these readings are in circuit, so I will have to lift one side to confirm an accurate reading.

I list the actual reading, the colors and the specified value per the color. 758 and 759 should be the same and are not. 783 seems partially burnt as EchoWars had predicted inititially as a potential problem.

"...the money is on opened or partially burnt resistors"

R758 - 78.1K
(schematic = 220K)
red, red, yellow, gold = 220,000

R759 - 70.2K
(schematic = 220K)
brown, black, yellow, gold = 100,000


R771 - 4.76K
(schematic = 27,000)
red, violet, orange, gold = 27,000

R772 - 6.1K
(schematic = 27,000)
red, violet, orange, gold = 27,000


R775 - 14.8K
(schematic = 47,000)
yellow, violet, orange, gold = 47,000


R782 - 1,502
(schematic = 1,500)
brown, green, red, gold = 1,500

R783 - 355
(schematic = 1,500)
brown, green, red, gold = 1,500

.

 

[Whaleman]

Well, new resistors went in and fixed the problem. I had to go back and do the other side as the channel did not play the same, so I replaced the same parts on the other side, good or not, including the input pair transistors. What a payoff, the unit is rock solid and a true keeper. The sound is much improved with zero static and no issues. Here are some photos of the finished boards, following the unit as I received and then the finished product. I restrung the tuning mechanism on top of that and it ligns up smack on.

Note my input pair rig, I've not added shrink tubing to my tool box yet.

From basket case:

To shining beauty:

THANKS AUDIOKARMA folks for your help!
To:

 

[Robisme]

Glad you got it running. Nice work. Looks great.
When I first the pics, I thought, gee, why doesn't he clean it.
Then I saw the afters.:thmbsp:

Rob

 

[avionic]

Well, new resistors went in and fixed the problem. I had to go back and do the other side as the channel did not play the same, so I replaced the same parts on the other side, good or not, including the input pair transistors. What a payoff, the unit is rock solid and a true keeper. The sound is much improved with zero static and no issues. Here are some photos of the finished boards, following the unit as I received and then the finished product. I restrung the tuning mechanism on top of that and it ligns up smack on.

Note my input pair rig, I've not added shrink tubing to my tool box yet.

From basket case:

To shining beauty:

THANKS AUDIOKARMA folks for your help!
To:

And to think you were just about ready to bail on the project....Always remember "Failure is not a option"....Outstanding job grasshopper!.....:thmbsp:

 

[Whaleman]

I am finally done restoring this unit! After over a year. I just completed a 100% recap (minus 1 FM board underside), FM alignment, stereo separation adjustment. The unit rocks! No more squeal from the speakers, FM stereo is immediate after turn on. Here are final pictures, compare them to the before shots. All the electrolytics are new.

 

[signal-dude]

Excellent!!!!!

Nice job Whaleman!!! I was wondering what happened to you and this fine specimen. Now I know. So whats next, replace all the carbon resistors on the tone amp boards with Metal Film? Put in Polystyrenes also? Get rid of all the ceramics? Hint,hint,heh,heh........

 

[Whaleman]

Nice job Whaleman!!! I was wondering what happened to you and this fine specimen. Now I know. So whats next, replace all the carbon resistors on the tone amp boards with Metal Film? Put in Polystyrenes also? Get rid of all the ceramics? Hint,hint,heh,heh........

I am all for that, give me some guidance and I am up for it. When you say the tone amp boards, you mean the one with the tone pots and the one connected to it that I just recapped?

Carbon resistors are the tan ones, ceramics are the discs, and the polystyrenes would replace the green pillow looking caps?

I notice I still have some squeal at two of the setting of the treble knob, so not done yet :tears:

 

[signal-dude]

Whaleman said;
"I am all for that, give me some guidance and I am up for it. When you say the tone amp boards, you mean the one with the tone pots and the one connected to it that I just recapped?"

Yep, those are the boards.

"Carbon resistors are the tan ones, ceramics are the discs, and the polystyrenes would replace the green pillow looking caps?"

Carbon resistors are the tan ones alright. Basically all of the resistors on the two boards. I used the YAGEO MF's from Digikey, one percenters.

Ceramics are the flat disc shaped jobs that are usually brown. These replace with the polystyrenes also from Digikey. The green pillow jobs are film type and are good quality, no need to change them if in good physical shape.
Although I put in some of the infamous Blackgates here of .47 Uf size and it sounds darn good. Try them.

"I notice I still have some squeal at two of the setting of the treble knob, so not done yet :tears:"

No need for tears, this can be fixed. One of my 2270's that I got had a similiar squeal and it turned out to be that one of the red wire jumpers on the tone amp board was real close to one of the transistors. Moved the wire slightly and the squeal or oscillation was gone. Provided some positive feedback that caused it to go into oscillation.

In my totally modded tone amp board I moved the jumpers to the underside of the board and it works fine. Looks better too. This might be the source of the squeal or it could also be that the carbon resistors have changed value all over the place and the tone amps are on the hairy edge of oscillation.

I had a gain imbalance on my tone amp board that the resistor upgrade fixed in a rather spectacular way. The gain of the two channels are now .4 db different, basically nothing, and the whole stage got much quieter. Very big deal..

I left the original transistors in there as they are very good and are working fine.

I think I still have the resistor values around here somewhere as the MF's are not exactly the same value for some of the resistors on the two boards.

Now, the big question you might have is: Is all this changing of parts going to kill the three martini Marantz sound? Nope, not in the least. Clearer and cleaner would be my description.

 

[Whaleman]

Whaleman said;
"I am all for that, give me some guidance and I am up for it. When you say the tone amp boards, you mean the one with the tone pots and the one connected to it that I just recapped?"

Yep, those are the boards.

"Carbon resistors are the tan ones, ceramics are the discs, and the polystyrenes would replace the green pillow looking caps?"

Carbon resistors are the tan ones alright. Basically all of the resistors on the two boards. I used the YAGEO MF's from Digikey, one percenters.

Ceramics are the flat disc shaped jobs that are usually brown. These replace with the polystyrenes also from Digikey. The green pillow jobs are film type and are good quality, no need to change them if in good physical shape.
Although I put in some of the infamous Blackgates here of .47 Uf size and it sounds darn good. Try them.

"I notice I still have some squeal at two of the setting of the treble knob, so not done yet :tears:"

No need for tears, this can be fixed. One of my 2270's that I got had a similiar squeal and it turned out to be that one of the red wire jumpers on the tone amp board was real close to one of the transistors. Moved the wire slightly and the squeal or oscillation was gone. Provided some positive feedback that caused it to go into oscillation.

In my totally modded tone amp board I moved the jumpers to the underside of the board and it works fine. Looks better too. This might be the source of the squeal or it could also be that the carbon resistors have changed value all over the place and the tone amps are on the hairy edge of oscillation.

I had a gain imbalance on my tone amp board that the resistor upgrade fixed in a rather spectacular way. The gain of the two channels are now .4 db different, basically nothing, and the whole stage got much quieter. Very big deal..

I left the original transistors in there as they are very good and are working fine.

I think I still have the resistor values around here somewhere as the MF's are not exactly the same value for some of the resistors on the two boards.

Now, the big question you might have is: Is all this changing of parts going to kill the three martini Marantz sound? Nope, not in the least. Clearer and cleaner would be my description.

Thanks for the info, I tried moving those red jumper wires every which way I could, as soon as I lifted my finger far enough away from the board, the squeal would start again. Get my hand close enough, it would stop.

Do you think if I move the red jumpers to the underside it will cure the ill? It seems to be concentrated on the right side of the board towards the phono amp board.

I am hoping it is not drifted resistors, but I do want to cure this.

If you have a list of Digikey part no., that would be great.

Thanks.

 

[signal-dude]

Whaleman said;

"Thanks for the info, I tried moving those red jumper wires every which way I could, as soon as I lifted my finger far enough away from the board, the squeal would start again. Get my hand close enough, it would stop."

That sounds just like the problem I had.

"Do you think if I move the red jumpers to the underside it will cure the ill? It seems to be concentrated on the right side of the board towards the phono amp board."

The red jumpers are the 27 volt DC feed to each side of the tone amp. There is apparently a feedback capacitance going on here. Moving the jumpers to the bottom should help a lot.

"I am hoping it is not drifted resistors, but I do want to cure this."

One other thing to do is change out C405, C406 to 470 Uf 35 volt electrolytic caps and also C409 to the largest cap you can jam in there physically that will fit. Height is a concern here. 470 uF will also do nicely. These stiffen up the power supply a bit and might help with the oscillation.

"If you have a list of Digikey part no., that would be great."

Have to check my notebook to see if I still have the order list.But most of the values were very close with just a few that didn't match up. For ex., 2.2 meg ohm is not a MF value but 1 meg is. So I had to wire up 2 one meg with a 200K to get the required value.

 

[Whaleman]

One other thing to do is change out C405, C406 to 470 Uf 35 volt electrolytic caps and also C409 to the largest cap you can jam in there physically that will fit. Height is a concern here. 470 uF will also do nicely. These stiffen up the power supply a bit and might help with the oscillation.

Thanks again for the info. Now C405 and C406 are 10 uF caps, so just want to confirm you say change them to 470 uF? and C409 uF that is 100 uF, change it to anything higher even up to 470uF?

 

[Whaleman]

I went ahead and moved the jumpers to the underside of the board and replaced C405 and C406 with 330 uF 63 V high quality Nichicon caps, and C409 got a 470 uF 50 V cap of the same.

I think the sqeal may be less but it is there only with the loudness on and the treble know at 3 o'clock or lesss. Here is the latest, compare to the above picture.

 

[signal-dude]

Jeez, this thing just won't give up. OK, you replaced the 3 caps with larger values and the squeal seems to be less. But you say it goes away if you rotate the treble past 3 o'clock?

If you place your hand near the tone amp does it go away? Or the board with the pots? Try to figure out where the sweet spot is that makes the squeal go away and move around the wire nest to stabilize the amp.

My gut feeling is that resistor value creep is causing an instability that occurs with a small capacitance with some nearby positive feedback. Positive feedback creates an oscillator.

Also stand up the transistors and caps so they are straight on the board. I do this without thinking when I see a circuit
board so this might help.

I only saw this problem on one out of four 2270's that I have but it might be more common than we think.

If this does not help than a trip to a professional that specializes in vintage gear might be a good thing to pursue.

 

[jstang]

I would buy a cheap hand held battery operated O-scrope, about 160$. With that and a schematic you should find the issue in an hour. Then you will have scope for the next time....

jk

 

[Whaleman]

I do have a Textronics scope here, weighs like 100 lbs. Don't know if I want to even bother learning it, it is tube operated.

The squeal only occurs with the treble between say 11 am and 3 pm and the loudness on. Then you have to rotate the volume to about 10-10:15 am and also between 11-11:15 am. Anywhere else, no squeal. Turn the loudness off and it happens much higher in the volume.

If you get your hand near the right half of the board, the squeal stop but it is not so specific that you can point to an area. Moving the related wires seems to make no difference at all. Neither standing up the components.

The scope is probably the way to go next.