SX-880 basket case, with lots of issues. Wiil need some advice

[Uncle Bambi]

I picked up a SX-880 today, for a price too low to refuse. The problem is that someone else has been at it - he told me he was going to repair it but decided he didn't want to spend the money on a "low-end" receiver. Before he decided to sell it, he removed a couple of parts and took some of the larger diameter wires for another project. So, here's what he pointed out and I visually observed later on:

1) volume pot is physically broken. Looking at it from the front, the right most pin on there rear row has been snapped off. Part # ACV-161... I'm casting about for a replacement as we speak, and have found this helpful archived post:
http://www.audiokarma.org/forums/showthread.php?t=205716 COOL!

2) Transistors Q21 and Q22 on the GWK-119 board are missing. They are Pioneer part# 2SK34. I found some old posts talking about replacing these, but would appreciate updated replacement info if anyone has it. I did find the seemingly original part here:http://www.acme-sales.net/portal.php?country=USA And - MTF specifies Mouser part 526-NTE458 in an old thread - still a viable alternative?

3) Test point pins 53 & 54 are clipped flush on the component side, but still have their runs intact on the foil side and go to the tuner board.

4) Test point pin 15 is missing - it is an empty hole. Unfortunately, this pin is located on the fold in the service manual, so I cannot see where it might go.

5) The wires are missing on Test point pins 6, 9, 11, 13, and 17. I'll have to look at the schematic to see where these go exactly, but it looks like for the most part they go to the main filter caps.

Issues #1 & 2 are the big ones right now, so if anyone has a spare pot or knows the current replacement for these two transistors, I'd love to get some input/advice.

Also, I'd love to know where pin 15 needs to go - if anywhere. I can most likely figure out issue #5 on my own.


I may have bitten of WAY more than I can chew here, but it was near-free and I am not emotionally attached to it - so if I blow it up I won't lose too much sleep. But, I am really hopeful that I can get it going.... with the help of AK. :thmbsp:

 

[Uncle Bambi]

OK - talking to myself, here....

I found a volume pot cheap, so that's covered. Can anyone address the 2SK34 replacement issue?

 

[markthefixer]

EchoWars current source is the best bet in the power supply. that is the function they are doing.

gotta save the remaining few for use in audio circuits.

these are 50v jfets, and are no longer made.

the current source was last seen in Overundr1's SX-1980 rescue thread


and what's this about? 8:03 pm YESTERDAY to 1:01 pm TODAY???????????
That's what, 17 hours?

OK - talking to myself, here....

If you hadn't noticed lately, sometimes it's a WEEK before I am able to get back in.

 

[Uncle Bambi]

EchoWars current source is the best bet in the power supply. that is the function they are doing.

gotta save the remaining few for use in audio circuits.

these are 50v jfets, and are no longer made.

the current source was last seen in Overundr1's SX-1980 rescue thread


and what's this about? 8:03 pm YESTERDAY to 1:01 pm TODAY???????????
That's what, 17 hours?



If you hadn't noticed lately, sometimes it's a WEEK before I am able to get back in.

It's about me updating the thread to let anyone interested know that I found a volume pot.

I tossed in the request for confirmation of the missing parts just for good measure, as I had seen you discuss both NTE458 and NTE459 as possible replacements in some older posts. I happen to have a local place that can get me either of those, so I was curious about if they were acceptable replacements.

Sorry you took it as a sleight.

 

[Uncle Bambi]

4) Test point pin 15 is missing - it is an empty hole. Unfortunately, this pin is located on the fold in the service manual, so I cannot see where it might go.

5) The wires are missing on Test point pins 6, 9, 11, 13, and 17. I'll have to look at the schematic to see where these go exactly, but it looks like for the most part they go to the main filter caps.

Thinking out loud here, just to report some progress.

Looking at the schematics:

Test pins/points 6 and 13 look like they should be wired on the grounding lug/pin cluster to the right side of the transformer.

Test pins/points 9 & 15 look like they should be wired on the "+" lug of filter cap C2.

Test pins/points 11 & 17 look like they should be wired on the "-" lug of filter cap C3.

As always - if anyone sees anything boneheaded, feel free to take a whack at me.

 

[Uncle Bambi]

Regarding the 2SK34 jfets - as I mentioned, I have a local supplier that can get the NTE 458's for me, and I also just found a fellow that has a couple of spare 2SK30A-Y in his parts bin.

Can someone tell me which of these (or neither?) will best perform the proper power supply function? If either of them will work, I'm in business. This is the last major hurdle to overcome, before I draw up my order of battle.

EDIT: Looks like the 2SK30A-Y will work for my purposes, per post #8 in This Thread. Cool.

I've got the volume pot issue licked, and I'm pretty sure I've doped out how to re-connect the missing wires.

 

[Uncle Bambi]

I'm beginning to put the wires and pin 15 missing situation right. Since pin 15 was missing and is needed to make a connection back to the filter caps, I took pin 14 as a donor and moved it over to the empty pin 15 hole. since pin 14 is just a test point, it will not be necessary to replace it.

Replacing the wires is pretty straightforward...... but, I didn't want to just "J" hook the end of the wire around the pin and add a glob of solder. I came up with a pretty nifty solution by turning to one of my daughter's hobbies - beading.

Some of you may already know of this, and I'm sure the more skilled veterans among you have a better solution. But here's what I came up with; a pair of wire-bending pliers that are used to make earrings and such. The jaws are graduated round cylinders, so I used them to make a nice tight coil out of the wire that slipped quite snugly over the pin. Then I soldered it to make it permanent.

It worked out very, very well. Here's what the pliers look like:

Just out of curiosity, and I hate to be OCD.... but does anyone have any idea what color the wire pairs going to pins 9/15 and 11/17 should be? I suspect maybe red for 9/15 and orange for 11/17, going by what's on the other side of the binding post. The schematics don't say what the voltage values should be (that I can find). I've got black, red, yellow, and green on-hand.

 

[Uncle Bambi]

I finished re-rigging the missing wires and installed the new volume pot that arrived in today's mail. All I need now is the replacements for the 2SK34's, and I think I'm in business.

Fuses have been checked, and ohm good outside of their holders. I've given it a good visual going over, and nothing else jumps out at me.

Now........ is there anything special I should do when I attempt a power-up for the first time? I'm planning on plugging it into a DBT with no load to begin with, and see what happens. Any other tips, tricks, gotchas, or words of wisdom/warning?

 

[Watthour]

That's a creative use for the pliers, considering a one-sized wire-wrap hand tool is about fifty bones.

Where are you installing the FETs? If they are in the muting circuit, you should probably try to use an original, although I've recently used a pair of Fairchild BF-256B with good success in that application in a SX-780 - Sharp cutoff and no noise. If it is in the power supply/protection driver, just go with generics. I've used the 50V NTE-459s with success. The 2SK30A is also 50V, whereas the Fairchild FETs are 30V. NTE458 may have too low a cutoff GS voltage, but experimentation in the circuit may have to be done to condemn it as a replacement. I know it didn't function well in the muting circuit.

Watch the lead arrangement, since they are inconsistent from type to type.

As for start-up on something like that, I'm pretty much a chicken, and start with a lower wattage (25W) in a DBT, then "graduate" to 40W, 60W, 75W, and 100W looking for magic smoke all the way. The protection relay probably won't even fire on the lower wattage "throttles" but if there is a serious problem you should limit damage. You might want to start with no speakers, then if you have some handy, connect 8-12 ohm wirewounds. I use adjustable brown devils set a little high for testing and exercising the amps, then switch to true 8-ohm speakers for some audio testing. Long-term load testing almost necessitates resistors unless you have a soundproof room.

Good luck in the start-up. Check offset early and often once you get it powered.

 

[Uncle Bambi]

Thanks for the extremely informative post!!

That's a creative use for the pliers, considering a one-sized wire-wrap hand tool is about fifty bones.

Thanks, it worked out very well. Here are a few pics of how it looks - I just made the little wire coils, slipped them over the pins, and soldered them in place. Not factory, and certainly not as neat as wire wrapping. But the connections look very good, are solid, and are a damn sight better than the "J" hook and solder blob method.

Where are you installing the FETs? If they are in the muting circuit, you should probably try to use an original, although I've recently used a pair of Fairchild BF-256B with good success in that application in a SX-780 - Sharp cutoff and no noise. If it is in the power supply/protection driver, just go with generics. I've used the 50V NTE-459s with success. The 2SK30A is also 50V, whereas the Fairchild FETs are 30V. NTE458 may have too low a cutoff GS voltage, but experimentation in the circuit may have to be done to condemn it as a replacement. I know it didn't function well in the muting circuit.

Watch the lead arrangement, since they are inconsistent from type to type.

They are in the power supply. I have my choice of either the NTE458's or the 2SK30A. According to some old posts that I dug up, the NTE458 has the same lead arrangement as the missing 2SK34. Not sure about the 2SK30, but I'll make sure to verify before I put them in. Thanks for the cautionary note.

As for start-up on something like that, I'm pretty much a chicken, and start with a lower wattage (25W) in a DBT, then "graduate" to 40W, 60W, 75W, and 100W looking for magic smoke all the way. The protection relay probably won't even fire on the lower wattage "throttles" but if there is a serious problem you should limit damage. You might want to start with no speakers, then if you have some handy, connect 8-12 ohm wirewounds. I use adjustable brown devils set a little high for testing and exercising the amps, then switch to true 8-ohm speakers for some audio testing. Long-term load testing almost necessitates resistors unless you have a soundproof room.

Good luck in the start-up. Check offset early and often once you get it powered.

This is excellent advice - especially varying the wattage in the DBT. I hadn't thought of that. I was planning on checking the offset ASAP, after ascertaining that the whole thing was more or less stable. Not sure what 8-12 ohm wirewounds are, but I have some 8 ohm load resistors I can use for a dummy load before risking anything of value.

The 2SK30's will be on the way later this week, and I'll be sure to report back when it's time for the smoke test.

 

[Watthour]

Not sure what 8-12 ohm wirewounds are, but I have some 8 ohm load resistors I can use for a dummy load before risking anything of value.

I use adjustable tap, non-inductive, wirewound 200W Ohmite resistors for testing. They use an adjustable strap for setting the resistance, and when I'm feeling particularly sheepish I'll start out around 12+ ohms in case there is more bias current or a chance of running straight bus voltage to a speaker. Adjusting back to a true 8Ω for checkout comes after the initial proofing, and I can take them down to 4Ω for testing at that range. They actually get fairly warm after a period of robust use.

Your 8Ω loads should be fine as long as they are high enough wattage. I'm probably overly cautious but want to to reserve the smoke for the BBQ grill.

 

[Uncle Bambi]

Your 8Ω loads should be fine as long as they are high enough wattage. I'm probably overly cautious but want to to reserve the smoke for the BBQ grill.

I forget what they are rated for, but they are fairly big light blue resistors that I bought at RS a while ago. Somewhere in the 2-5w range, I think?

Anyway, the 2SK30's are en-route, and the local supply house says they can order the NTE458's if I need them. I'm going to go with the SK30's as there seems to be a lot of disparaging info about NTE parts in general.

I'm confused about lead arrangement - if I'm reading the datasheets right:

2SK30, flat side up - SGD from left (pin 1) to right (pin 3):
http://www.datasheetcatalog.org/datasheet/toshiba/1032.pdf

NTE458 seems the opposite, flat side up - DGS from left (pin 1) to right (pin 3):
http://www.datasheetcatalog.org/datasheet/nte/NTE458.pdf

Original 2SK34, flat side up - SGD from left (pin 1) to right (pin 3):
From the SM, page 31 - everything I find on the web is written in Japanese.

Can anyone confirm lead arrangements among the three? Looks like the 2SK30 is the safe bet, both because it is not NTE, and it is directly replaceable in the same orientation as the original - correct?

 

[Watthour]

2SK34 is S-G-D whereas the NTE459s I have are D-G-S and the NTE458 is listed the same. The 2K30 (2SK30ATM) is S-G-D according to the Toshiba spec sheet:

http://140.120.11.1/prophys/ael/File/Datasheet/k30e1.pdf

 

[Uncle Bambi]

2SK34 is S-G-D whereas the NTE459s I have are D-G-S and the NTE458 is listed the same. The 2K30 (2SK30ATM) is S-G-D according to the Toshiba spec sheet:

http://140.120.11.1/prophys/ael/File/Datasheet/k30e1.pdf

Thank You. :thmbsp:

 

[Overundr1]

Interesting replacement however not the easiest part to locate.
2sk30atm vgds -50v, gate 10ma, drain 100mw, SGD
2sk34 vgds -50v, gate 10ma, drain 150mw, SGD

I think EW would have jumped on this so there may be another parameter that makes it a poor substitute :headscrat
-Lee

 

[EchoWars]

If this is being used as a current source, all we're really concerned with is that it has an appropriate stand-off voltage, and that we are able to source the appropriate amount of current. As far as pinout, most small FET's have their source and drain interchangeable...it will not matter which is which. I try to match the data sheet, regardless.

Do NOT install these new devices until you measure Idss yourself. Your goal is about 2 to 3mA.

 

[Uncle Bambi]

Do NOT install these new devices until you measure Idss yourself. Your goal is about 2 to 3mA.

I have a copy of your document "JFET Testing" and it discusses measuring the voltage between the gate and the drain, and then measuring the voltage between the gate and the source. Is this how to measure Idss?

If not, can you or someone explain what this is, and how it is measured - hopefully with a DVM?

 

[EchoWars]

Idss = 'Current, from drain to source, with shorted gate'. The is the maximum current that the FET is capable of sourcing. Very simple to measure.

 

[Uncle Bambi]

Idss = 'Current, from drain to source, with shorted gate'. The is the maximum current that the FET is capable of sourcing. Very simple to measure.

Perfect. Crystal Clear.

THANKS!

 

[Uncle Bambi]

Do NOT install these new devices until you measure Idss yourself. Your goal is about 2 to 3mA.

I have three 2SK30's in hand, and using the diagram you supplied I measure 7.86mA with the test.

Black probe on the drain, red probe on the "+" terminal of the 9v battery, and the source and gate on the "-" side of the battery.

Are we looking for a max of at least 2 - 3mA? If so, is more is OK or will that be injurious to other components? If these are "good to go" I can fire this thing up during the weekend.