Thanks! Those two slip-on TO-126 heat sinks are per markthefixer and are on the new Q6 & Q7. The KSA1381ESTU and KSC2690AYS are all plastic so easily accept the isolated heat sinks.
SX-850 In The House! Recap/Restoration Ahead!!
- Thread starter MarkyM
- Start date
Lots of progress today!
The Tone Board was first up and in order to clean the lever switches and pots properly, the board must be removed from it's module and the 2 pots have to be removed from the board.
No wires need to be removed as the whole assembly flips up for service.
Note that for cleaning, I use DeoxIT D5 for any metal-to-metal contacts such as switches (a drop of D100L if the contacts are really bad/tarnished) and ONLY DeoxIT F5 Faderlube in pots.
Speaking of DeoxIT, can we start a petition to send signatures to Caig in protest to that stupid yellow hinged straw that is way too big and folds down too easily? What's wrong with a standard red straw with a retainer clip??
The contacts for the lever switches are at the bottom as you can see here, just above board level and some D5 is sprayed into that slot right near the bottom. There is no way to get to the contacts from the front.
Here's the Tone Board all done and replaced in it's module but it's not going back yet because there are 2 push-button switch boards located below it:
I cleaned these push-button switches using EchoWars method:
With a switch board flipped up, it is super-easy to spray D5 into the "Spray Here SX-1250" location and the one opposite that, plus you can work the switches with the board at different angle to spread the DeoxIT. I don't think it is necessary to disassemble the switches unless they are really bad.
Again, no wires have to be disconnected for any of this. There is enough wire length to work on these boards pretty comfortably.
Here’s the input selector switch board located below the Flat Amp. Hint. If you hold down one of these interlocked selector switches, the other ones can be worked without latching.
Here's the Flat Amp before restoration and the pots (Volume and Balance) removed for cleaning. Note the 4 dreaded blue tantalum caps on this board:
Here's the Flat Amp after restoration:
My Flat Amp does have the 2SC1885 transistors so I left those alone:
And finally, the EQ Amp (phono preamp) after restoration:
Next up, the main filter caps. But I think I'm going to put it on the DBT first to check for shorts and then measure some voltages if it comes up OK.
--Mark--
The Tone Board was first up and in order to clean the lever switches and pots properly, the board must be removed from it's module and the 2 pots have to be removed from the board.
No wires need to be removed as the whole assembly flips up for service.
Note that for cleaning, I use DeoxIT D5 for any metal-to-metal contacts such as switches (a drop of D100L if the contacts are really bad/tarnished) and ONLY DeoxIT F5 Faderlube in pots.
Speaking of DeoxIT, can we start a petition to send signatures to Caig in protest to that stupid yellow hinged straw that is way too big and folds down too easily? What's wrong with a standard red straw with a retainer clip??
The contacts for the lever switches are at the bottom as you can see here, just above board level and some D5 is sprayed into that slot right near the bottom. There is no way to get to the contacts from the front.
Here's the Tone Board all done and replaced in it's module but it's not going back yet because there are 2 push-button switch boards located below it:
I cleaned these push-button switches using EchoWars method:
With a switch board flipped up, it is super-easy to spray D5 into the "Spray Here SX-1250" location and the one opposite that, plus you can work the switches with the board at different angle to spread the DeoxIT. I don't think it is necessary to disassemble the switches unless they are really bad.
Again, no wires have to be disconnected for any of this. There is enough wire length to work on these boards pretty comfortably.
Here’s the input selector switch board located below the Flat Amp. Hint. If you hold down one of these interlocked selector switches, the other ones can be worked without latching.
Here's the Flat Amp before restoration and the pots (Volume and Balance) removed for cleaning. Note the 4 dreaded blue tantalum caps on this board:
Here's the Flat Amp after restoration:
My Flat Amp does have the 2SC1885 transistors so I left those alone:
And finally, the EQ Amp (phono preamp) after restoration:
Next up, the main filter caps. But I think I'm going to put it on the DBT first to check for shorts and then measure some voltages if it comes up OK.
--Mark--
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Well, it passed the DBT but the protection relay never pulled in. I found out that can happen if Q5 is an NPN transistor instead of a PNP 
I had mistakenly put an 1845 in there instead of a 992....oops!
All is well now. Relay pulls in after a few seconds.
Voltage readings: (note that this is an "S" model with the voltage selector. It is set to 120V and my line voltage right now is 117v)
AWR-101 Power Supply Board Voltage Readings:
Pin 2, 3 = +5.7 v DC (+5.4vdc)
Pin 4, 5 = + 13.75v DC (+13.9vdc)
Pin 6, 7 = +50.6 v DC (+51.5vdc)
Pin 8, 9, 10 = +36.14 v DC (+36.5vdc)
Pin 11 = +27.85 v DC (+28.0vdc)
Pin 13 = -13.04 v DC (-13.5vdc)
Pin 14 = -50.76 v DC (-51.5vdc)
Pin 15 = -18.16 v DC (-19.0vdc)
AWM-062 Protection Board Voltage Readings:
Pin 2 = -13.04 v DC (-13vdc)
Pin 10, 11 = +36.14 v DC (+36.5vdc)
Pin 12 = 7.02v AC (7.5vac)
AWH-059 Power Amp Board Voltage Readings:
pins 7, 8, 22, 23 = +50.6 v DC (+51.5vdc regulated)
pins 4, 5 19, 20 = +36.16 v DC (+36.5vdc regulated)
pins 1, 2 ,16, 17 = -50.78 v DC (-51.5vdc regulated)
pins 14, 29 = +41.8 (+45vdc unregulated)
pins 15, 30 = -41.8 vDC (-45vdc unregulated)
First audio test: No right channel audio on any source. Pulled the Pre-Out/Main-In jumpers and same from the Pre Out jacks. Flipping the Mono switch on provides the L to both channels.
So it looks like I have a preamp problem before the mono switch.
Since the mono (Mode) switch is before any active electronics on the Flat Amp board, the problem has to be passive ie. a broken wire on the output of the Switch Board (AWS-094) or in the switch matrix on the Flat Amp board.
I had mistakenly put an 1845 in there instead of a 992....oops!All is well now. Relay pulls in after a few seconds.
Voltage readings: (note that this is an "S" model with the voltage selector. It is set to 120V and my line voltage right now is 117v)
AWR-101 Power Supply Board Voltage Readings:
Pin 2, 3 = +5.7 v DC (+5.4vdc)
Pin 4, 5 = + 13.75v DC (+13.9vdc)
Pin 6, 7 = +50.6 v DC (+51.5vdc)
Pin 8, 9, 10 = +36.14 v DC (+36.5vdc)
Pin 11 = +27.85 v DC (+28.0vdc)
Pin 13 = -13.04 v DC (-13.5vdc)
Pin 14 = -50.76 v DC (-51.5vdc)
Pin 15 = -18.16 v DC (-19.0vdc)
AWM-062 Protection Board Voltage Readings:
Pin 2 = -13.04 v DC (-13vdc)
Pin 10, 11 = +36.14 v DC (+36.5vdc)
Pin 12 = 7.02v AC (7.5vac)
AWH-059 Power Amp Board Voltage Readings:
pins 7, 8, 22, 23 = +50.6 v DC (+51.5vdc regulated)
pins 4, 5 19, 20 = +36.16 v DC (+36.5vdc regulated)
pins 1, 2 ,16, 17 = -50.78 v DC (-51.5vdc regulated)
pins 14, 29 = +41.8 (+45vdc unregulated)
pins 15, 30 = -41.8 vDC (-45vdc unregulated)
First audio test: No right channel audio on any source. Pulled the Pre-Out/Main-In jumpers and same from the Pre Out jacks. Flipping the Mono switch on provides the L to both channels.
So it looks like I have a preamp problem before the mono switch.
Since the mono (Mode) switch is before any active electronics on the Flat Amp board, the problem has to be passive ie. a broken wire on the output of the Switch Board (AWS-094) or in the switch matrix on the Flat Amp board.
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Yep, it was the wire to Pin 44 on the Switch Board (the right channel output to the Flat Amp) that was broken off. Gotta love solid wire wire-wrap connections
The relay contacts are clean and working fine, the new Nichicon "Gold Tune" 22000uF/50V main filter caps are in and that pretty much wraps it up for the physical restoration except for the front panel and lighting!
I'm going to do power amp offset and bias adjustments next, then the LED lighting (although all of the original bulbs are working) and do a nice cleaning of the front panel and reinstall it. finally a complete tuner alignment including optimizing the ceramic IF filters to reduce the hiss on stations transmitting the HD digital sidebands.
The relay contacts are clean and working fine, the new Nichicon "Gold Tune" 22000uF/50V main filter caps are in and that pretty much wraps it up for the physical restoration except for the front panel and lighting!
I'm going to do power amp offset and bias adjustments next, then the LED lighting (although all of the original bulbs are working) and do a nice cleaning of the front panel and reinstall it. finally a complete tuner alignment including optimizing the ceramic IF filters to reduce the hiss on stations transmitting the HD digital sidebands.
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Looking great, especially the cleaning.
Something that has not made it to your attention is the bypassing of the two big electrolytics
with Film Caps. EchoWars did that routinely with 2.2uf 250v non-polarized film caps.
Literally ON the capacitor terminals. All I see on yours is the bleeder resistors.
EchoWars used Digikey P10983-ND @ $1.20 ea Panasonic film 2.2uf 250v since at least 2007, and probably earlier.
Made audible differences as many posts will attest.
Something that has not made it to your attention is the bypassing of the two big electrolytics
with Film Caps. EchoWars did that routinely with 2.2uf 250v non-polarized film caps.
Literally ON the capacitor terminals. All I see on yours is the bleeder resistors.
EchoWars used Digikey P10983-ND @ $1.20 ea Panasonic film 2.2uf 250v since at least 2007, and probably earlier.
Made audible differences as many posts will attest.
I found a strange problem. If I have a pair of headphones plugged-in and turn off the Speaker switches, there is a loud hum/buzz in the headphones. If either or both of the Speaker switches are on, the headphone feed is perfectly quiet.
The other controls including Volume have no effect on this...except Tone Defeat. When in the Defeat position the hum/buzz is reduced but is still there. Simply turning on one of the Speaker switches cures the problem.
Also, the selector switch light (AUX for example) gets brighter when the Speaker switches are both off and the hum/buzz is present.
Any ideas?
The other controls including Volume have no effect on this...except Tone Defeat. When in the Defeat position the hum/buzz is reduced but is still there. Simply turning on one of the Speaker switches cures the problem.
Also, the selector switch light (AUX for example) gets brighter when the Speaker switches are both off and the hum/buzz is present.
Any ideas?
It sounds like a headphone jack grounding problem. Try cleaning the headphone socket. Also, make sure the jack and nut and tone control nuts are tight against the chassis.
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Thanks, but cleaning the socket and re-soldering it to the board made no difference. This only happens when both Speaker switches are off. Very strange.
skysailor64
Active Member
I noticed the same thing on the SX-950 I recently recapped. Need to keep one set of speakers turn on to eliminate the buzz. Be interested if this is normal or not. Nice job you are doing
edit - you could connect a wire to the phono ground screw on the back, and run it to the headphone barrel as a speaker lights off test.
The AWS-099 board, headphone jack ground is pin 10 - you didn't talk about checking that.
the speaker bulbs are fed by the 5.4v DC.
here is the important part:
the speaker bulbs are turned on by a separate GROUND through pin 8 to the chassis.VISUALLY verify both.
Check the pin 1 ground to chassis on the AWR-101 power supply. By that I mean VISUALLY identify.
this is the ground reference for the 5.4v and 13.5v supplies. The POWER RETURN for the 13.5 & 5.4v currents is pins 20 & 21.
If pin 1 floats, all bets are off. It may find a ground through the 5.4v regulator , speaker lamp(s) and pin 8 ground.
I believe that the speaker lamp(s) provide a (circuitous 5.4v dc) ground for the FLOATING pin 1.
and the floating pin 1 crud gets back into the 14.6v reference for the positive regulator.
Awr-101 grounds: pin 1, pin 12, (both reference grounds) pin 20 & pin 21 are current return grounds.
ALL must be connected to the main chassis ground. Visually check all the wires.
The service manual does NOT address the complex web of grounds in the unit.
As a counter-example, the spec-1 preamp has a plethora of grounds (mostly current return paths), a wire from EVERY board, all going to the SAME place. the ground ON the power supply regulator board.
A coax or shielded cable has a ground, but it should NEVER carry any current on it. One side is typically grounded, while the other side can be unconnected, unless the reference carries through the board with the signal.
The AWS-099 board, headphone jack ground is pin 10 - you didn't talk about checking that.
the speaker bulbs are fed by the 5.4v DC.
here is the important part:
the speaker bulbs are turned on by a separate GROUND through pin 8 to the chassis.VISUALLY verify both.
Check the pin 1 ground to chassis on the AWR-101 power supply. By that I mean VISUALLY identify.
this is the ground reference for the 5.4v and 13.5v supplies. The POWER RETURN for the 13.5 & 5.4v currents is pins 20 & 21.
If pin 1 floats, all bets are off. It may find a ground through the 5.4v regulator , speaker lamp(s) and pin 8 ground.
I believe that the speaker lamp(s) provide a (circuitous 5.4v dc) ground for the FLOATING pin 1.
and the floating pin 1 crud gets back into the 14.6v reference for the positive regulator.
Awr-101 grounds: pin 1, pin 12, (both reference grounds) pin 20 & pin 21 are current return grounds.
ALL must be connected to the main chassis ground. Visually check all the wires.
The service manual does NOT address the complex web of grounds in the unit.
As a counter-example, the spec-1 preamp has a plethora of grounds (mostly current return paths), a wire from EVERY board, all going to the SAME place. the ground ON the power supply regulator board.
A coax or shielded cable has a ground, but it should NEVER carry any current on it. One side is typically grounded, while the other side can be unconnected, unless the reference carries through the board with the signal.
Thank you for the detailed reply Mark!
I tried this...no change.
Black wire to chassis is present from Pin 10 and checked 0.53 Ohms.
Black ground wire on Pin 8 present and checked good to chassis ground pin.
Black wires from Pins 1, 12 & 20 verified and checked to chassis ground. Pin 21 is grounded through trace on the board from Pin 20 - brown wire from transformer connected to pin.
I tried this...no change.
Black wire to chassis is present from Pin 10 and checked 0.53 Ohms.
Black ground wire on Pin 8 present and checked good to chassis ground pin.
Black wires from Pins 1, 12 & 20 verified and checked to chassis ground. Pin 21 is grounded through trace on the board from Pin 20 - brown wire from transformer connected to pin.
Thanks!I noticed the same thing on the SX-950 I recently recapped. Need to keep one set of speakers turn on to eliminate the buzz. Be interested if this is normal or not. Nice job you are doing
I'm beginning to think this is an original design problem.
Can others here with access to an 850 or 950 check this on theirs? Or if you have fixed it, what did you do?
ClasicTecGuy
New Member
I don't have access to an 850 or 950, but I'm currently restoring a 980 and it does not have that problem.
I'm just about to wrap this project up but I thought some of you might be interested in what I do to improve the FM selectivity of these old FM tuners and reduce the hiss from the digital HD sidebands some (most now) FM stations transmit.
This is going to be a long post so my apologies in advance
Here's the problem...
Back in the day, the entire FM band looked like this group of 3 FM channels centered on the station here in my area on 88.9mHz. It is not running HD so has no digital HD sidebands.
In this spectrum analyzer screenshot from my SDRplay RSPdx DSP receiver, you can see that the occupied bandwidth of the station's transmission is well within the allotted 200kHz bandwidth of it's FM channel. You can also see it's neighbors on 1st adjacent channels of 88.7 and 89.1 mHz as well. None of these stations is running HD. I have a rooftop omnidirectional antenna so most channels on the lower-powered low-end of the FM band are occupied here.
That said, the audio modulation we're interested in detecting and recovering in the radio is contained within the center 150kHz (+/- 75 kHz) region. Filters in radios generally are wider than this by a bit to keep distortion low etc. The shaded area in the center shows the (pretty close to perfect) 192kHz digital FIR filter (+/-96kHz) this DSP receiver uses.The old analog ceramic IF filters have a gradual slope on each side instead of the virtually perfect vertical "slope" of the digital FIR filter.
Since FM channels are spaced 200kHz apart and they (still) try to keep strong stations in a market at least 2 channels (400kHz) apart, tuners and receivers back in the day used fairly wide filters. For example, 300kHz (+/- 150kHz) filters allowed excellent adjacent channel selectivity (400kHz) and kept distortion low. The tradeoff is gain/distortion vs. the narrowness of the passband.
But 300kHz is too wide nowadays for stations transmitting HD. Here's the spectrum of my local (very strong) station on 92.3mHz that is running HD:
You can see that the digital HD sidebands start at just over 100kHz away and bleed into the 1st adjacent channels! While the nicely perfect 192kHz digital filter here (shaded area) avoids the HD sidebands completely, our nice, wide analog 300kHz filter is too wide to avoid picking up "hiss" from the inner parts of these HD sidebands, especially since it's slopes are much more gradual.
Thanks to the nice folks over at the Groups.IO FM Tuners group, members there have come up with a way to test and sweep ceramic filters using a device called the NanoVNA (used by Ham radio operators), a filter test fixture and a PC app that comes with the NanoVNA:
An IF strip in a vintage receiver is typified by the SX-850 which has three 3-pin 290kHz ceramic filters, all the same, basically in series: (they did this to improve the ultimate rejection outside the passband).
Here's a sweep of one of the Pioneer's original filters:
The -3 bandwidth of this filter is 293.333kHz (+/- 146.6kHz) so too wide to avoid those pesky HD sidebands that start at only +/- 100kHz.
Another issue with these analog ceramic filters is that they are not always centered on frequency. Ideally you want them centered perfectly at 10.700mHz. (the IF frequency used in virtually all FM radios)
This one is quite high at 10.739 mHz. Since it is so wide, this worked OK and kept the receiver manufacturers from having to hand-select filters.
So what can we do to help reduce the hiss? Replace one or two of the original filters at the end of the chain with a narrower filter.
In the case of this SX-850, I'm going to use one or two180kHz filters to replace the last or middle and last filters. Also, since I am able to hand-pick the center frequency, I'm going to replace the first one or two filters as well with 280kHz filters. All three filters will me selected to have close to the same center frequency. An IF alignment of the tuner will be needed afterwards.
Here's the sweep of the 180kHz filter. It actually has a -3dB bandwidth of 195kHz and a center frequency much closer to 10.700mHz: (the center frequency accuracy is more important with narrower filters)
You can also see that the gain of this more modern filter is pretty close to the original filter even though it is narrower. This keeps the sensitivity of the tuner high.
Some experimentation is required to end up with the best compromise between distortion, gain (sensitivity) and selectivity (avoidance of the HD sidebands). An added benefit of the increased selectivity is being able to nicely separate 2 (weaker) stations on adjacent channels! (as long as they aren't running HD)
Murata was the most common manufacturer of these ceramic filters but, unfortunately, most have been discontinued :-( You can still find them new from some suppliers and used or new/old stock on ebay. They can be found in either the older style 3-pin form factor or in the more modern surface-mount package. Once again, the fine folks at the FM Tuners group have come up with an adapter board to allow SMT filters to be converted to 3-pin.
Here's my SX-850 with 2 SMT 280kHz filters on the adapter boards in the first two positions followed by a 3-pin 180kHz filter:
400Hz distortion is still really good at 0.12%so I may try using two of the narrower filters instead of just one.
I know this was a lot. I'd be happy to answer any questions!
--Mark--
This is going to be a long post so my apologies in advance
Here's the problem...
Back in the day, the entire FM band looked like this group of 3 FM channels centered on the station here in my area on 88.9mHz. It is not running HD so has no digital HD sidebands.
In this spectrum analyzer screenshot from my SDRplay RSPdx DSP receiver, you can see that the occupied bandwidth of the station's transmission is well within the allotted 200kHz bandwidth of it's FM channel. You can also see it's neighbors on 1st adjacent channels of 88.7 and 89.1 mHz as well. None of these stations is running HD. I have a rooftop omnidirectional antenna so most channels on the lower-powered low-end of the FM band are occupied here.
That said, the audio modulation we're interested in detecting and recovering in the radio is contained within the center 150kHz (+/- 75 kHz) region. Filters in radios generally are wider than this by a bit to keep distortion low etc. The shaded area in the center shows the (pretty close to perfect) 192kHz digital FIR filter (+/-96kHz) this DSP receiver uses.The old analog ceramic IF filters have a gradual slope on each side instead of the virtually perfect vertical "slope" of the digital FIR filter.
Since FM channels are spaced 200kHz apart and they (still) try to keep strong stations in a market at least 2 channels (400kHz) apart, tuners and receivers back in the day used fairly wide filters. For example, 300kHz (+/- 150kHz) filters allowed excellent adjacent channel selectivity (400kHz) and kept distortion low. The tradeoff is gain/distortion vs. the narrowness of the passband.
But 300kHz is too wide nowadays for stations transmitting HD. Here's the spectrum of my local (very strong) station on 92.3mHz that is running HD:
You can see that the digital HD sidebands start at just over 100kHz away and bleed into the 1st adjacent channels! While the nicely perfect 192kHz digital filter here (shaded area) avoids the HD sidebands completely, our nice, wide analog 300kHz filter is too wide to avoid picking up "hiss" from the inner parts of these HD sidebands, especially since it's slopes are much more gradual.
Thanks to the nice folks over at the Groups.IO FM Tuners group, members there have come up with a way to test and sweep ceramic filters using a device called the NanoVNA (used by Ham radio operators), a filter test fixture and a PC app that comes with the NanoVNA:
An IF strip in a vintage receiver is typified by the SX-850 which has three 3-pin 290kHz ceramic filters, all the same, basically in series: (they did this to improve the ultimate rejection outside the passband).
Here's a sweep of one of the Pioneer's original filters:
The -3 bandwidth of this filter is 293.333kHz (+/- 146.6kHz) so too wide to avoid those pesky HD sidebands that start at only +/- 100kHz.
Another issue with these analog ceramic filters is that they are not always centered on frequency. Ideally you want them centered perfectly at 10.700mHz. (the IF frequency used in virtually all FM radios)
This one is quite high at 10.739 mHz. Since it is so wide, this worked OK and kept the receiver manufacturers from having to hand-select filters.
So what can we do to help reduce the hiss? Replace one or two of the original filters at the end of the chain with a narrower filter.
In the case of this SX-850, I'm going to use one or two180kHz filters to replace the last or middle and last filters. Also, since I am able to hand-pick the center frequency, I'm going to replace the first one or two filters as well with 280kHz filters. All three filters will me selected to have close to the same center frequency. An IF alignment of the tuner will be needed afterwards.
Here's the sweep of the 180kHz filter. It actually has a -3dB bandwidth of 195kHz and a center frequency much closer to 10.700mHz: (the center frequency accuracy is more important with narrower filters)
You can also see that the gain of this more modern filter is pretty close to the original filter even though it is narrower. This keeps the sensitivity of the tuner high.
Some experimentation is required to end up with the best compromise between distortion, gain (sensitivity) and selectivity (avoidance of the HD sidebands). An added benefit of the increased selectivity is being able to nicely separate 2 (weaker) stations on adjacent channels! (as long as they aren't running HD)
Murata was the most common manufacturer of these ceramic filters but, unfortunately, most have been discontinued :-( You can still find them new from some suppliers and used or new/old stock on ebay. They can be found in either the older style 3-pin form factor or in the more modern surface-mount package. Once again, the fine folks at the FM Tuners group have come up with an adapter board to allow SMT filters to be converted to 3-pin.
Here's my SX-850 with 2 SMT 280kHz filters on the adapter boards in the first two positions followed by a 3-pin 180kHz filter:
400Hz distortion is still really good at 0.12%so I may try using two of the narrower filters instead of just one.
I know this was a lot. I'd be happy to answer any questions!
--Mark--
Last edited:
Doctorhugo
Los Angeles CA
Amazing, thanks for the detailed thread! I especially appreciate the post on cleaning/lubing the switches and pots, since I just bought some FaderLube to do my own 850 (I already have the D5).
BTW I don't have the buzz on the headphones that you describe.......
Enjoy!!
BTW I don't have the buzz on the headphones that you describe.......
Enjoy!!