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 Post subject: Re: Sencore CR70 Repair help
PostPosted: Nov Sun 16, 2014 6:20 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 10921
Location: Powell River BC Canada
did notice the leg of the resistor at cr5, next to the 4pin connector, gets very hot, enough to discolor the plastic connector next to it.

If this is the resistor in your photo,a few things check:

Polarity of C7 & C8, D5 & D7 and make sure the two regulators are correct.
( I've got bitten in the past mixing up a 79L15 with a A 78L15)

My CR70 has a run number sticker on the inside lid flap. Perhaps users could get
together and identify just what run numbers are involved with changes.

I did pay full dollar for my CR70, however in negotiating he final price,they threw in
extra adapters to sweeten the pot.

Sencore did have a "made right" guarantee, and when the TV business was roaring,they had
their people on the road with tech schools everywhere and any issues with faulty runs
would have been known.

The Sencore sales folks also worked their accounts and kept in touch regularly with news
and deals for TV shops.


l
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Sncore warranty.jpg
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 Post subject: Re: Sencore CR70 Repair help
PostPosted: Nov Mon 17, 2014 12:47 am 
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Joined: Nov Tue 04, 2014 11:15 pm
Posts: 10
Location: Cavendish P.E.I.
Thanks, I double checked the orientation of these components and they all check. Wanted to double check the voltages, I am basically getting 22.5V throughout this whole circuit. Straight off the windings of the transformer right to all the legs of IC3 with the exception of pin 5 and 8. I am not seeing 15V on anything yet.
radiotechnician wrote:
did notice the leg of the resistor at cr5, next to the 4pin connector, gets very hot, enough to discolor the plastic connector next to it.

If this is the resistor in your photo,a few things check:

Polarity of C7 & C8, D5 & D7 and make sure the two regulators are correct.
( I've got bitten in the past mixing up a 79L15 with a A 78L15)

My CR70 has a run number sticker on the inside lid flap. Perhaps users could get
together and identify just what run numbers are involved with changes.

I did pay full dollar for my CR70, however in negotiating he final price,they threw in
extra adapters to sweeten the pot.

Sencore did have a "made right" guarantee, and when the TV business was roaring,they had
their people on the road with tech schools everywhere and any issues with faulty runs
would have been known.

The Sencore sales folks also worked their accounts and kept in touch regularly with news
and deals for TV shops.


l
Attachment:
Sncore warranty.jpg


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 Post subject: Re: Sencore CR70 Repair help
PostPosted: Nov Wed 19, 2014 12:18 am 
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Joined: May Mon 18, 2009 5:55 pm
Posts: 3008
Location: Erie, PA
Sorry it took a while to get back to this, I needed a chance to get my CR70 taken apart. It is run number 44, most of the IC's date late 1993.

The mod with CR5 and CR7 with the resistor in the one photo is from the factory. The resistor is (of all values) 41 ohms. There is no evidence of charring or any damage to the resistor in my CR70, nor any apparent heat discoloration to the connector near CR5. R38 and 39 in my CR70 are 2.2k, and there are several diodes added to the back side of the board, all factory. It sounds like the OP's CR70 is nearly identical to mine, but something was making the OP's 15V supply get very hot- I would suspect one or both of those "Surge" branded Korean made 220 uF electrolytics to be a good culprit.

-Mark-


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 Post subject: Re: Sencore CR70 Repair help
PostPosted: Nov Wed 19, 2014 3:17 am 
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Joined: Nov Tue 04, 2014 11:15 pm
Posts: 10
Location: Cavendish P.E.I.
Thanks for taking the time to look at this. I checked this one and it's run number 33. They do sound identical. On the solder side, 2 zener diodes and one general purpose? I went ahead and replaced all those old caps. I checked the burnt up resistor on the the diode mod and as of now, it reads 33k ohm. No way to tell what it was originally supposed to be, or if it's still correct, but other than that, they sound identical. Still not sure why I'm getting 22.5 all over the place though.

MarkPalmer wrote:
Sorry it took a while to get back to this, I needed a chance to get my CR70 taken apart. It is run number 44, most of the IC's date late 1993.

The mod with CR5 and CR7 with the resistor in the one photo is from the factory. The resistor is (of all values) 41 ohms. There is no evidence of charring or any damage to the resistor in my CR70, nor any apparent heat discoloration to the connector near CR5. R38 and 39 in my CR70 are 2.2k, and there are several diodes added to the back side of the board, all factory. It sounds like the OP's CR70 is nearly identical to mine, but something was making the OP's 15V supply get very hot- I would suspect one or both of those "Surge" branded Korean made 220 uF electrolytics to be a good culprit.

-Mark-


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 Post subject: Re: Sencore CR70 Repair help
PostPosted: Feb Wed 13, 2019 10:38 pm 
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Joined: Feb Sat 02, 2019 2:39 pm
Posts: 1
Yeah, I know this is an old post and I am sorry for bringing it back from the dead - try not to hate me.... I have questions about replacement electrolytics for these things...

According o the schematic, the following values are required and the form factor is fairly evident from the figures in the manual (I am in El Salvador and my CR is oin USA, so I cant just look at it, but I want the parts there when I get home...):

MAIN BOARD
C1-5 ----> 10mfd@100V+/-10 AXIAL - easy to find
C6,7 ----> 200mfd@50V +80/-20 AXIAL - could not find, selected a 220mfd@50V +/-20 RADIAL as the closest I could find
C10, 20, 21 ---->2.2mfd @ 50V +/-20 RADIAL - easy to find
C14 ----> 10mfd @ 450V AXIAL no tolerance listed - found 10mfd/450V+/-20% AXIAL
C19 ----> 50mfd @ 15V +80/-20 RADIAL - found 56mfd @16V +/-20% RADIAL

SWITCH BOARD
C1 ----> 2200mfd @35V +/-10% AXIAL - found correct values but RADIAL
C2 ----> 47mfd @25V +50-4% RADIAL - found 60mfd @ 25V +/-20% AXIAL

CHASSIS MOUNT
C1 ----> 100mfd @ 450V +/-20% no indication whether it is axial or radial - got one each axial and radial of correct value and will whichever makes sense.

So, here's my question... From the schematic, there is no way to determine what specific type of electrolytic to get (other than form factor) - e.g. low ESR, etc. In several cases, it really didn't matter because the only devices I could find that were in range were the only device available. Can someone tell me if there is a specific type of capacitor that is preferred (or required) at each of the locations in this unit? Can someone also explain why there are those funky tolerances (e.g. +50/-4, +50/-20)? I assume this is because there is an optimal capacitance value under which it is absolutely not acceptable to go, but the consequences of higher capacitance in that location is not a big deal. Is it as simple as that?

I am a mechanical engineer by education and trade and the analogous (tolerancing) situation would be when tolerancing for pin/hole fits, so I get it that there are times when it is indicated, just not sure I have my head fully around it for capacitance. Taking SWITCH BOARD C2 as an example, the OEM tolerance is ~45 - 71mfd; the closest device with otherwise compatible numbers is as specified above (found 60mfd @ 25V +/-20%) and has an acceptable range of ~48 - 72mfd - pretty close. Obviously, I can measure the capacitance and find a capacitor that is in range. Is this standard practice?

Another question: once I recap this thing - IF I recap this thing, will it require recalibration to account for the new capacitance values which, necessarily, will be different from the original capacitors (due to the tolerances)? Or is the circuit simply not critical enough/sensitive to changes in capacitance to matter.

Oh, and there is no mention of polar vs bi-polar on the schematic/part list, I assume that if any of these were NP, it would be so noted? Or do I have to visually inspect the actual devices to make that determination?

As far as form factor, aside for the potential issue of having excessively long leads and possible packaging issues, I assume it makes no matter?

Sorry for the barrage of probably inane questions and thank you all very much in advance far any help on this.


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