SX-850 AM TUNER PROBLEM

DAY4201

DAY4201

Active Member
Over the last weekend I finished recapping and replacing all transistors and diodes (on the Power AMP (AWH-059). I also replaced the output transistors. New mica and grease. Previously, I had recapped all the other boards and replaced the main filter caps. Some of this is documented in another thread “KR-500 restoration Issues”. I know that may seem weird, but it just worked out that way.

The only board I have not restored is the Tuner BD. I have known for a while that the AM tuner is not working well. FM works fine. I can pull in a few stereo stations without an antenna from my basement workshop. All other modes work well also.

There is very faint distorted sound from a strong local AM station if the volume is turned all the way up. When the unit is turned on with AM selected the Signal meter moves to a position between 1 & 2 and stays there.

According to the SM (pg 13) the AM tuner is “controlled” (maybe not a good choice of words but I don’t know how else to specify it) by IC HA1138. Given FM works I assume there is the proper level of voltage coming to the Tuner Bd (maybe the wrong assumption).

So, I measured the voltages on the 14 pin HA1138. Results provided. The only pin that was way out (at least according to the schematic) was Pin 12. Of course, I am wondering if the schematic is in error for that pin but I don’t know.

So, I hope to receive some guidance in resolving this issue before attempting to recap this board.THANKS!!

SX850 AM 1.jpg
 
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Looks like a pin 12 typo on the schematic to me, that is a +B connection, should be +13vdc +/- 1vdc. R62 and C65 provide power supply decoupling and will drop the dcv, but only marginally, since R62 is 47Ω.
 
merlynski said:
Looks like a pin 12 typo on the schematic to me, that is a +B connection, should be +13vdc +/- 1vdc. R62 and C65 provide power supply decoupling and will drop the dcv, but only marginally, since R62 is 47Ω.
Click to expand...
Thanks for taking a look at this and confirming my suspicion that the schematic was in error for that pin. I am going to proceed with restoring that board. I am not necessarily hopeful that it will fix the AM problem but it needs to be done.

Yesterday I decided I needed to give the switches on that board another shot of D5, in this case Faderlube F5. I was able to remove the top of the AM switch (easily removed by removing 2 plastic clips) and apply the F5. I have very small nozzle adapter so it is easy to get into tight spaces. There is a cover (it is just the internal part of the push button) inside the switch that slides along with the pushing of the switch. Keeping light pressure on it, I applied the F5 and a very small piece of what I am sure is a part of one of the very delicate foil pieces that slides along the stationary rails came out.

So, it appears to me that the switch is in need of repair or replacement. I have some experience with this type of switch (ALPS I assume). I can honestly say my experience opening those up to clean or replace those "sliders" is mixed. While I can't clearly say that is the problem I am inclined to believe it is.

I see from the SM for that switch assembly (AWS-094) is one part number. I was hoping each switch had a separate part number but they don't. I am curious if I can replace the AM switch with Phono 2. They appear to have the same pin placement on the foil side from the SM but I need to look more closely at that board. Unfortunately, that board is a pain to remove and I am afraid of damaging it more. I really don't care that much about AM but I do care about AUX and Phono 1.

I have the possibility to replace that board with a "parted out" one from epay. At about $45 it is not a very attractive alternative.

Anyhow, think I will worry about that after I restore the board. Thanks again for your help!
 
You are welcome!
I have never disassembled one of those switches myself, but reading about them on AK indicates the sliding contacts are very fragile and easily deformed.
Since the sections interact, releasing the depressed one when another is pressed, it gets a single part number because it is a 'single' mechanical assembly with multiple 'internal' parts not designated as separate components. Pioneer probably did not manufacture the switch itself. :dunno:
 
Well, its daja vu all over again. When I turned on the 850 today it was in FM mode as I left yesterday. Speakers attached. Very low volume. It started cracking and popping. I generally keep it on it side (transformer side down). I tried to move it some to see if I could what the problem was. More cracking and popping. So I turned it off and put it on the DBT. Turned it on. Bright light then very dim as usual.

But the relay did not click. So no sound in any mode. No more cracking and popping which makes sense as the relay is not coming out of protection now.

Just like the KR-5200 the lights come on. The tuner, at least in FM, seems to work or at least the tuning meter works and the stereo light comes on when set to a local station.

I tested all the voltages on the PS and Power amp. All were consistent with my previous tests when it worked. I removed Q7 on the protection bd and replaced it with a new but same KSC2690AYS that was there before. I checked the 220ohm 1watt resistor that connected to its emitter (good) and the 22ohm resistor connected to its base. Both good.

I did the diode test on the 2690 that I removed. It tested good. I pulled the relay to see if I could see any damage to it. None noted.

So I need your help again! I didn't have the time or inclination to take any more measurements today so I decided to beg for forgiveness and as the expert. Probably more interesting than that tuner anyhow which I never touched. Tomorrow is another day. I had hoped to make some progress on that tuner but ..... I will be very interested to see what the problem is if you will be so kind as to take this Problem on!
 
Staying in protection?

List ALL the protect board ww pin voltages ( 1 through 11, all inclusive) referenced to ground..
 
Pins 7 and 8 are the driver overload sense from the right amp. Your readings indicate -8.5v's. If that is truly volts and not millivolts then the right amp needs work.
Those pins 5, 6 for the left amp (Which looks good) and 7, and 8 for the right amp should be zero or very close to zero. (DC balance)
The negative reading from the right also indicates the issue is on the NPN (Positive side of the amp) side.

Can you adjust the idle on the right amp?
Take the amp loose and do a very thorough visual check on the entire board. Look for cold solder connections (Reflow anyway), loose solder pads, toasted or overheated components, wire wrap connections for a tight fit, and corrosion.
I would replace Q1 and Q2 (Replace with KSA992's) on the right amp and go from there. If that cures it then replace the same transistors on the good left amp. Those 2sa726 transistors are well known trouble makers.
If your in there replace the few caps that in circuit. I see a 330/10, C11 on the center rail that could cause trouble.
Note:
It's not railed completely to the negative side which means the drivers are probably okay. Something in the pre-stage is causing the imbalance. (Maybe?)
Its a simple amp with minimal components, That doesn't mean easy but it limits potential issues.

There are some voltage readings you could take. Mark is showing interest so maybe he has a quick voltage check procedure you could do?
 
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zebulon1 said:
Pins 7 and 8 are the driver overload sense from the right amp. Your readings indicate -8.5v's. If that is truly volts and not millivolts then the right amp needs work.
Those pins 5, 6 for the left amp (Which looks good) and 7, and 8 for the right amp should be zero or very close to zero. (DC balance)
The negative reading from the right also indicates the issue is on the NPN (Positive side of the amp) side.

Can you adjust the idle on the right amp?
Take the amp loose and do a very thorough visual check on the entire board. Look for cold solder connections (Reflow anyway), loose solder pads, toasted or overheated components, wire wrap connections for a tight fit, and corrosion.
I would replace Q1 and Q2 (Replace with KSA992's) on the right amp and go from there. If that cures it then replace the same transistors on the good left amp. Those 2sa726 transistors are well known trouble makers.
If your in there replace the few caps that in circuit. I see a 330/10, C11 on the center rail that could cause trouble.
Note:
It's not railed completely to the negative side which means the drivers are probably okay. Something in the pre-stage is causing the imbalance. (Maybe?)
Its a simple amp with minimal components, That doesn't mean easy but it limits potential issues.

There are some voltage readings you could take. Mark is showing interest so maybe he has a quick voltage check procedure you could do?
Click to expand...


Thanks for your reply and useful information. Just so you know I recently "restored" the power amp board. I didn't list the parts replaced as I didn't do anything unique and just used replacement parts others had used. I also replaced all the output transistors with the recommended replacements. A list of the replacements below.

At the time, I was able to set both the dc offset and the bias. The only issue remaining was the AM tuner which is why I started this thread. I now believe that problem is due to a broken switch which I describe above.

I used the unit everyday since restoring the power amp until yesterday when an obvious problem developed. I will remove the power amp board as suggested and check it over.

You asked about being able to set the idle on the right amp. I am able to do so but I have to turn the new pot (single turn) all the way clockwise to get a -20mv reading. Before this happened I was able to set it at 20mV. I can still set the left amp to 20mV without issue.

Capacitors:
C1 / 2.2 uF 25v Electro (CSSA) / 2.2 uF 63V WIMA
C2 / 2.2 uF 25v Electro (CSSA) / 2.2 uF 63V WIMA
C3 / 1 uF 25v Electro (CSSA) / 1 uF 63v WIMA
C4 / 1 uF 25v Electro (CSSA) / 1 uF 63v WIMA
C5 / 33 uF 16v Electro (CEA) / 33 uF 35v UKA
C6 /33 uF 16v Electro (CEA) / 33 uF 35v UKA
C11 / 330 uF 10v Electro (CEA) / 330 uF 25v UPW
C12 / 330 uF 10v Electro (CEA) / 330 uF 25v UPW

Transistors:
Q1 / 2SA726 (matched Q1-Q2) / KSA992FBU
Q2 / 2SA726 (matched Q1-Q2) / KSA992FBU
Q3 / 2SA726 (matched Q3-Q4) / KSA992FBU
Q4 / 2SA726 (matched Q3-Q4) / KSA992FBU
Q5 / 2SA818Y / KSA1381ESTU
Q6 / 2SA818Y / KSA1381ESTU
Q7 / 2SC1628Y / KSC3503ESTU
Q8 / 2SC1628Y / KSC3503ESTU
Q9 / 2SD381 / KSC2073TU
Q10 / 2SD381 / KSC2073TU
Q11 / 2SB536 / KSA940
Q12 / 2SB536 / KSA940
Diodes:
D1 / 1S2473 / 1N4148 / 200mA/100v
D2 / 1S2473 / 1N4148 / 200mA/100v
D3 / 1S2473 / 1N4148 / 200mA/100v
D4 / 1S2473 / 1N4148 / 200mA/100v
D7 / 1S1885 / 1A 100V / MUR220RLG / 2A 200V Ultra Fast Recovery
D8 / 1S1885 / 1A 100V / MUR220RLG / 2A 200V Ultra Fast Recovery
D9 / 1S1885 / 1A 100V / MUR220RLG / 2A 200V Ultra Fast Recovery
D10 / 1S1885 / 1A 100V / MUR220RLG / 2A 200V Ultra Fast Recovery

Variable Resistors:
VR1 / 10K Trimmer (Offset) 3/8" 10K Trimmer 67PR10KLF 3/8" Sealed Multi Turn 10K Trimmer (TT)
VR2 / 10K Trimmer (Offset) 3/8" 10K Trimmer 67PR10KLF 3/8" Sealed Multi Turn 10K Trimmer (TT)
VR3 / 100 Ohm Trimmer (Bias) 3/8" 100 Trimmer 3386H-1-1010 3/8" Sealed Single Turn (100 Ohm) Bourns
VR4 / 100 Ohm Trimmer (Bias) 3/8" 100 Trimmer 3386H-1-1010 3/8" Sealed Single Turn (100 Ohm) Bourns

output transistor q13-q16 2sd371a 863-MJ21194G
output transistor q17-20 2sb531a 863-MJ21193G

Thanks again for your help!
 
Turn the Idle back to minimum.
The uniqueness of this amp is the two regulated voltages it uses for the pre stage. Why? I have no clue but it is something that should be checked. Pins 19 and 22.
While you have power on - check the base of Q6 and Q8. Remember, when probing, use an insulated tip. Only the very end is exposed.
Diode D4 should be checked for condition with power off.
Troubleshooting is a logical process and you won't get a lesson from me. My emotions keep haunting me. I'll check and prod but I'm always looking for something visual I did. So keep your eyes open!
 
OK. You don't have to worry about teaching me anything. I've already decided I am destine to be mostly ignorant regardless of how great the help I receive is. So, I mostly just follow instructions if they make some sense to me and hope to get the problem solved.

I would say emotions are what makes us human but that sounds really corny so I'll leave it at that. I'll hope to get back to the unit today and let you know what I find. Also, I use mini-grabbers (I think that is what they are called) when checking voltages. Thanks again!
 
Checked voltages on power amp pins 19 = 36 VDC and 20 = 50.1 VDC.
Q6: E = 48.2, C = -41, B = 47.7
Q8: E = -50.0, C = -43.6, B = -50.1
Diode 4 = .552 forward voltage drop. OL reversed. (measured in circuit, no power)

The transistor measurements are way off. Further inspection of the foil side of the board while still in place revealed that I had failed to cut one or possibly 2 of the leads on either Q6 and or Q8short enough so that when the unit was sitting on its side and moved around it was flexing enough to short the leads against the grounded back plate that supports it in place. Bang, crack, pop!

Unfortunately, as I am sure many know that board is one of the most hassle to remove to work on. However, it is now free. The two STV4H's have been removed (again) as have all of the output transistors and heat sink.

Given I really don't know how much damage was done to the components on that board I will replace all the transistors regardless and maybe the diodes (or at least test them). I will also test all of the output transistors.

As I don't know of any way to test that board without re-installing it along with the outputs and heat sink. If I should test and/or replace anything else I am open for advice. Thanks!
 
Well. Good news and some bad news.
I’ve done that. It explains the weird offset.
 
Yes, just another "learning" experience. Taking a day off from it today. Probably watch the Reds while I barbecue a chicken. Chicken should be good not so sure about the Reds. Never know what to expect from them.
 
images
 
Pulled and tested Power Amp transistors Q5-Q12. Q12 was shorted. The rest were all OK but replaced with new. Also, pull and replaced D10 even though it tested good. Pulled R40 (150ohms, .5W). It tested OK. Didn't have a replacement so I reinstalled it. Am pulling and testing every component in Q12's direct path.

Also, tested all the output transistors which all passed based the diode test. Have not found any obvious damage to the board including no burnt or darkened places or components.

I did discover that one of the output transistor holders had a broken screw insert. Could cause the screw to short against the heat sink. I covered part of the screw with heat shrink tubing which I assume will provide enough insulation to protect from shorting the screw.

Seems like a good fit and should not interfere with the output transistor being secured to the heat sink. Photos may help demonstrate the problem and my solution.

As usual I am open to suggestions as I work through the rest of the board and the repair of the broken output transistor connector.

850 broken .jpg
850 screw 2.jpg
 
Out of "Protection" and back in action after replacing most of the transistors on the Power Amp bd. Only one tested bad as noted above. Even got back AM but still think there is a problem with it. I am moving on to the tuner. Hopefully, once recapped both AM and FM will still work and sound better. My thanks to zebulon1 for helping get me back on track!
 
zebulon1 said:
images
Click to expand...
Just finished recapping the Tuner Bd. and noticed there is a transistor (Q10) 2SC1312-G on the Bd. I assume it needs to be replaced with the KSC1845 as recommended on the thread of "bad"/noisy transistors. I don't recall seeing any reference to replacing it as part of the replacement parts lists.

Just curious if there is a problem with replacing it and would I then have to replace any of the others? What do you think? Thanks!
 
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