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SX-303 Problem

DAY4201

DAY4201

Active Member
I recently picked up a SX-303 from GW. It has a variety of problems. I have just begun testing and found R166 which is a 3 pin NOBLE RGC 22 resistor rated at .22 ohms 2 watts (,22ohmsKX2) to be faulty (I assume). From pin 1 to 2 it measures .42, pin 2 to 3 measures 3.9 ohms. This seems way out of spec but I want to ask. Also it seems a replacement is very hard to find. I did find one on ebay from Hong Kong. I am not sure it will be the real thing even though NOBLE is apparently a Chinese company. Any help would be greatly appreciated.
 
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merlynski said:
Found a Datasheet . . . too big to upload, here is the link: https://www.handsontec.com/pdf_files/noble-rgc-2.pdf

Looks like they have it in stock, 0.85 USD: https://handsontec.com/index.php/product-category/passive-components/resistors/page/2/
That is a 5W device. The link on their picture is broken, but you can still add it to your cart. Looks like they are in Mayalasia, shipping was $7.50 to me. What a cool resistor:cool:

No Afilliation
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Thanks for the sites. I was able to check the photo out. Looks basically the same. No dimensions but I assume I can get it to fit. I am wondering though about using a 5W rated resistor versus the 2W. Seems like a lot of power could get past it to other components before it failed. It is a pretty interesting resistor. I just wish they were easier to obtain. But that is the price for trying to fix this "vintage" receiver. That resistor with shipping will cost more than I paid for the receiver. It is an interesting project to see if I can get it working again. I think it sat in a chicken coup for a few years or more and was rained on a number of times. But it does basically work at least through head phones. Rock on!:D
 
redk9258 said:
It may be hard for your meter to measure .22 Ohms accurately.
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Thanks for your comment. You raised an interesting question. My measurements were from my Brymen BM869s. I decided to see what kind of measurements my EEVBlog BM235 would get. I used the same probe master probes all the readings. The BM235 seemed to have more trouble giving an accurate reading but I wonder if that has to do with it being a 6000 count versus the BM869s at 500,000. I also tested an ANENG AN8009 (9999 count) which measured very close to the BM869s. I was sort of impressed by that but it is clearly no real substitute for the BM235. Just a fun unscientific comparison. ;)
 
DAY4201 said:
Thanks for the sites. I was able to check the photo out. Looks basically the same. No dimensions but I assume I can get it to fit. I am wondering though about using a 5W rated resistor versus the 2W. Seems like a lot of power could get past it to other components before it failed. It is a pretty interesting resistor. I just wish they were easier to obtain. But that is the price for trying to fix this "vintage" receiver. That resistor with shipping will cost more than I paid for the receiver. It is an interesting project to see if I can get it working again. I think it sat in a chicken coup for a few years or more and was rained on a number of times. But it does basically work at least through head phones. Rock on!:D
Click to expand...
The Power rating on a resistor indicates how much energy can be dissipated continuously (as heat) by the device before it is damaged, not how much will be passed on to other circuits. A 5 watt device of the same resistance will function exactly the same in respect to the rest of the circuit, it just handles an 'overload' (if any) better. The biggest difference will be size, but should otherwise work. Looking at the datasheet shows the device internally has two 2W 0.22Ω resistors with a common lead in one 3 lead package package, so the 5W rating on the website listing may be slightly incorrect. If you cannot aquire an exact replacement two 0.22Ω 2W readily available resistors could be used instead.
One way to get a better reading of a small resistance is to touch the probes together and measure the lead resistance, then measure the resistor, then subtract the lead resistance from the total to get the actual resistor measurement.
 
merlynski said:
The Power rating on a resistor indicates how much energy can be dissipated continuously (as heat) by the device before it is damaged, not how much will be passed on to other circuits. A 5 watt device of the same resistance will function exactly the same in respect to the rest of the circuit, it just handles an 'overload' (if any) better. The biggest difference will be size, but should otherwise work. Looking at the datasheet shows the device internally has two 2W 0.22Ω resistors with a common lead in one 3 lead package package, so the 5W rating on the website listing may be slightly incorrect. If you cannot aquire an exact replacement two 0.22Ω 2W readily available resistors could be used instead.
One way to get a better reading of a small resistance is to touch the probes together and measure the lead resistance, then measure the resistor, then subtract the lead resistance from the total to get the actual resistor measurement.
Click to expand...

Thanks for the detailed explanation of the wattage rating of the resistor and how it works in the circuit. I ask because I don't know so I appreciate a knowledgeable explanation. I have thought about the option of creating the resistor package from 2 separate resistors but after looking on Mouser and Digi-Key I was not able to find any that were "fire proof" cement filled and all that. Maybe I wasn't using the right parameters. Given your explanation it appears that the 5W one is a better option. I did check the lead resistance which is .06 ohms which would change my results some but not significantly in this case. Good suggestion though. I will remember that for future measurements. Thanks again for your comments!

Later: I did find .22Ω 2W 5% tolerance metal film resistors on Mouser and Digi-Key made by Panasonic #ERX-2FJSR22E that look suitable if I have to go the route of placing them in series.
 
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I downloaded the schematic and looked for that resistor, found it in the 'protection' crowbar circuit. When there is a dc fault detected in the amp output D15 turns on, shorts out the power supply and dumps the charge in the filter capacitors through that resistor, and then pops the transformer primary side fuse. That resistor takes a LOT of abuse if the protection circuit is triggered repeatedly, and so do the power supply rectifiers. If someone puts a bigger fuse in the fuse holder the next fuse to go is the one inside the transformer, which converts it to a dead weight of copper and iron (sometimes repairable, more often not). So DO NOT put a bigger fuse in the fuse holder, and check it to see that no one before you did. The transformer you save will be your own. That amp needs to be on a dbt while trouble-shooting under power: Dim Bulb Tester diagram pictorial...
If you do not already have one, build one, it is cheap and easy.
 
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I checked the fuse which was the right size and not blown. I mentioned that the unit does turn on and is partially functional. At this point there is no AM or FM. When I hook up a source to the tape in I get volume from only one channel. I did get rather garbled but clear enough volume for both channels through the headphone jack. I removed the large filter caps C145 & C146 both at 4700uF 50V. They have low ESR but one of them is showing higher capacitance at around 5300uF. I can replace them with a Nichicon UVK1H472MRD‎ at the same voltage or go up to 63V. Not sure if that makes sense. AFter removing those caps I was able to get to Q12 2SD880. I tested it in-circuit which I realize is not the way to do it but to get it out I will have to disassemble the whole unit. Unfortunately, the plate on the bottom panel doesn't really allow for much access to the circuit board. Q12 which is an NPN did show 1.29V on the "emitter to base" test (- lead on the base, + on the emitter). It appears that a suitable replacement would be KSD880O (ON/Fairchild). I also pulled D15 which turned out to be a TRIAC. I had to look up how to test it once I figured out what it was. It tested OK. I have since continued to try to cut this unit up. I have deoxed all the switches, pots and speaker connections. Glue from the filter caps is a real mess and was very sloppily applied. There is also real evidence that the FM tuner area had something fairly thick and maybe paint spilled on it. I have been working on cleaning that up but I have my doubts that the FM tuner will ever be functional. However, it is a good learning experience. I have several much better units including a Sansui 7, Marantz 2226 and an SX-850 all functional but needing obvious work. Let me know if you have any thoughts on the replacement parts I am thinking about.Thanks again for your help! Oh yeah, I do have an use a Dim bulb tester.
 
One of the first things I would do is to check all the power supplies and verify they are in spec, if you have not done so already. While on the DBT the (unregulated) supplies will be 10-15% low, so after it passes the DBT test try it on full line voltage. AC power line voltages are a bit higher now than they were years ago, so the unregulated ±40vdc supplies may be a bit higher than listed on the schematic. This is also a good reason to go a bit higher on the voltage ratings of the filter caps, C145 & C146. The higher capacitance reading may indicate that cap is drying out. I like replacing old large electrolytics. Sometimes a larger capacitance and higher voltage modern cap will physically fit better than an exact value replacement. The only downside I see with greater capacitance is stress on D15 and R166 if/when the protection circuit is triggered. Regulartor transistor Q12 (NPN) diode test reading in circuit with - on the base and + on the emitter will reverse bias the junction so it only tells you it is not shorted, nothing else should be inferred. NPN: - + - will test a junction forward biased. When powered up. if the base measures +14vdc and the emitter measures +13.4vdc (referred to ground) that circuit is regulating properly, imho no need to replace.
 
Just sent in an order for replacement caps. I was not sure about increasing the capacitance (i.e., how much increase) but did increase the voltage when I could. As I was looking at mostly replacing large "filter caps" I became aware that I had to make a choice in some cases between through-hole "snap-in" or regular radial leads. The most of the larger ones that I have pulled I assume are "snap-ins') so that is what I went with mostly. I didn't buy the usual 10+ to get a better rate as I was not sure exactly if the snap-ins will actually fit plus they can be a little pricey. So I hedged and bought some of both. Not a big deal as I assume I can make both styles work. Should get them at some point on Friday so I can hopefully get to the testing you suggest over the weekend. As always thanks for your help!:)
 
Radial leads are what most of the old caps have, if they are soldered directly to a pcb. Sometimes finding a cap with the same lead spacing is the hardest part of the process. The physically bigger ones have clamps around the body, so finding a matching diameter becomes the hard part, or coming up with a shimming scheme to hold them in place. Most of the posters I have seen on AK increase capacitance 10% to 50% for power supply filter caps. Going too big will stress the rectifiers and transformer. If the unit has a toroidal transformer much bigger caps may draw an excessive current flow from the AC line at power on. If you look at some of the more powerful amps they have a soft-start circuit that limits the inrush current.
 
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