SX-838 troubleshooting assistance please

[markthefixer]

<snip>
I hope Mark comes by and adds his thoughts unless you have a 13v Zener you are on hold for parts.

It IS the -13v, but NOT what you expect.

At this point I am laying odds that it is the PHONO/equalizer amp!!!

here's why:
power supply pin 9 -13v

goes to:
1. Power amp board pin 12, into a 20k resistor as part of the offset adjust voltage reference circuit. So if this resistor is shorted to ground, only a small amount of current will flow.

2. Protection board pin 2
2.a. into a 5.6k resistor biasing the lower half of the DC detector circuit. So if this resistor is shorted to ground, only a small amount of current will flow.
2.b. into a 47k resistor biasing q6 off for the on/off circuit. So if this resistor is shorted to ground, only a small amount of current will flow.

3. Phono / equalizer amp board pin 2.
3.a. into a pair of 6.8k ohm resistors for second stage bias. again, low current
3.b. DIRECTLY into a 220uf 16v capacitor - Nothing in the way of large currents flowing. - this MIGHT be shorting/leaking and pulling too much current.... BUT we have a BETTER suspect..
3.c. Q5 and Q6 DIRECTLY!! Nothing in the way of large currents flowing here .... While they themselves are blameless, the input transistors are 2sa725's and they are DIRECTLY coupled to Q5 & Q6. Nothing in the way of large currents flowing.

and WHAT do we know about 2sa725's??? They get noisy and short out...

so... got some EXTRA ksa992's??? put them into Q1 and Q2 of the phono amp board. Later or if you have them, put in ksc1845's into q3, q4, q5, q6 to get a real improvement in the phono amp, along with the caps.

 

[goat67]

Thanks Mark

Rereading this thread I did not ever see a check of the Power supply voltages?

I thought something was off with the 22 volts also.

I do not know what parts we have available.

If we need to order again can we do anything to isolate this problem?
I know we can but would you recommend doing it?

I was thinking disconnecting pins 2 and 6 on the phono board AWF-011.

What is your thoughts on this?

Mouser Part Numbers for Transistors
512-KSA992FTA
512-KSC1845EBU

 

[Uncle Bambi]

.....so... got some EXTRA ksa992's??? put them into Q1 and Q2 of the phono amp board. Later or if you have them, put in ksc1845's into q3, q4, q5, q6 to get a real improvement in the phono amp, along with the caps.

:yes:

Mark, if I've learned nothing else, I've learned to heed the advice I'm given here. I also, by nature, like to hedge my bets. So...... In the previous Mouser order I ordered everything you've listed so far in this thread, and I ordered extra transistors in case I screwed one up.

So, I have on hand:

Quantity 3 512-ksa992fbu (these were replacements for the 726 transistors on the amplifier - can they be used for the 725's?)
Quantity 4 512-ksc1845ebu

Plus, all the caps for the phono amp. If you can verify that the transistors I have are good replacements, I'll try to do this today and report back.

Question 1:
Phono amp is same as equalizer amp, no? Board AWF-011-0? If so, I re-capped it yesterday - so that should rule out C-18, the 220uf 16v capacitor you mentioned.

Question 2:
On the Power Supply board:
Q1 2SD313-E 512-KSC2073TU
Q2 2SD313-E 512-KSC2073TU
R3 RT5B 221K 220 1W resistor

I notice that these get pretty warm very quickly, and the board around them is noticeably darker brown. Problem? I ordered the transistors and have some of these on hand, too.

 

[goat67]

:yes:

Mark, if I've learned nothing else, I've learned to heed the advice I'm given here. I also, by nature, like to hedge my bets. So...... In the previous Mouser order I ordered everything you've listed so far in this thread, and I ordered extra transistors in case I screwed one up.

So, I have on hand:

Quantity 3 512-ksa992fbu (these were replacements for the 726 transistors on the amplifier - can they be used for the 725's?)
Quantity 4 512-ksc1845ebu

Plus, all the caps for the phono amp. If you can verify that the transistors I have are good replacements, I'll try to do this today and report back.

Question 1:
Phono amp is same as equalizer amp, no? Board AWF-011-0? If so, I re-capped it yesterday - so that should rule out C-18, the 220uf 16v capacitor you mentioned.

Question 2:
On the Power Supply board:
Q1 2SD313-E 512-KSC2073TU
Q2 2SD313-E 512-KSC2073TU
R3 RT5B 221K 220 1W resistor

I notice that these get pretty warm very quickly, and the board around them is noticeably darker brown. Problem? I ordered the transistors and have some of these on hand, too.

The transistors you listed are just what you need for the Equalizer amp (phono board)

On Question 2
Lets get one thing working before doing anything else. The PS board does run warm as you have seen.

On the amp board you did replace all the transistors Mark asked you to right?

Now I need to get back to a Sansui G- 4700

 

[Uncle Bambi]

i want you to verify that the transistors you installed are correct please list what part numbers you used

q1 =
q2 =
q3 =
q4 =
q5 =
q6 =
q7 =

q1 = 512-ksc2310ybu
q2 = 512-ksc2310ybu
q3 = 512-ksc945clta
q4 = 512-ksc945clta
q5 = 512-ksa733clta
q6 = 512-ksc945clta
q7 = 512-ksc2383ybu

 

[Uncle Bambi]

The transistors you listed are just what you need for the Equalizer amp (phono board)

OK - replaced the 725's on the phono (but not the 1313's), and the unit ran for 32 minutes before going into protect. Voltage on pin 8/9 is still -8v, before and after protect mode.

On the amp board you did replace all the transistors Mark asked you to right?

Yes - six, total:
Q1 through 4, with the KSA992FBU
Q5 & Q6, with KSC3503ESTU

It was the first thing I did, and it changed the symptoms from a few seconds then popping/crackling and into protect, to what I have now - 30 or so minutes of good operation, then in/out of protect.

After that, I changed Q1 through Q7 on the protect board, and then re-capped the amp, power, protect, and phono boards. Symptoms have stayed the same.

On Question 2
Lets get one thing working before doing anything else. The PS board does run warm as you have seen.

I just thought I'd mention it, since the symptoms suggests something thermal in nature?

EDIT: I re-checked pin 10 on the PS - yes, 13/14v.

Also - not a muti-voltage unit.

 

[goat67]

OK - replaced the 725's on the phono (but not the 1313's), and the unit ran for 32 minutes before going into protect. Voltage on pin 8/9 is still -8v, before and after protect mode.



Yes - six, total:
Q1 through 4, with the KSA992FBU
Q5 & Q6, with KSC3503ESTU

It was the first thing I did, and it changed the symptoms from a few seconds then popping/crackling and into protect, to what I have now - 30 or so minutes of good operation, then in/out of protect.

After that, I changed Q1 through Q7 on the protect board, and then re-capped the amp, power, protect, and phono boards. Symptoms have stayed the same.


I just thought I'd mention it, since the symptoms suggests something thermal in nature?

EDIT: I re-checked pin 10 on the PS - yes, 13/14v.

Also - not a muti-voltage unit.

What speakers are you using to test this?

Did replacing the transistors on the Eq Bd change the voltages?

Take the following voltage readings from the Power supply board

Pin 8 -13v

Pin 10 22v

Get me these readings and we will go from there.

 

[Uncle Bambi]

What speakers are you using to test this?

Did replacing the transistors on the Eq Bd change the voltages?

Take the following voltage readings from the Power supply board

Pin 8 -13v

Pin 10 22v

Get me these readings and we will go from there.

A pair or KLH satellite speakers, rated at 8 ohms impedance. I measured them at closer to 10 ohms. I also used a different small speaker on one channel only, rated (and measured) at 4 ohms, with the same results.

Changing the transistors did not change the voltages, nor did it change the symptoms.

Turned it on and measured from a cold start:

Pin 8 -8.2v

Pin 10 13.5v

Unit plays for about 30 minutes or so, and then goes into protect.

Could the other transistors on the phono board (ksc1845, Q3 - Q6) be a possible source of the fault? I can change them next - I have them in-hand. What about the big caps on the sub chassis... C1 - C3? I wouldn't think problems with the filter caps would present like this, but?

Also - I haven't changed this yet:
D8= 13V 1/2W Zener Diode. Part# 512-1N5243BTR $0.04ea

 

[goat67]

Go ahead and replace the other transistors on the Eq Amp Board.

Measure Voltages on PS board

Pin 8 -13.0v
Pin 10 22.0 v

If not correct shut it off.

Do you have the 13 volt Zener diode?

 

[markthefixer]

OK, at this point, after the 30 minutes has passed and the unit starts going into protect, COOL DOWN the 13v zener diode (unless you have the replacement - and if you do, just replace it) using canned air spray (cheaper, gentler) or freeze spray.

The -13v is pretty much independent of anything else on the power supply, so it should not be dropping, especially with new caps on the phono/equalizer board.

The -13v regulator circuit isn't just the 13v zener, it's the 2.2k ohm 1 watt resistor as well.

IF you haven't ordered the zener yet, maybe we should also get the resistor.

594-5073NW2K200J 2.2k 1 watt $0.16 ea

resistors fail usually to higher resistance, which would drop the voltage on this zener based regulator.

IF there is enough physical separation between the resistor r2 and the zener diode d8, you could try cooling them separately to see which is affected by the cooling.

 

[Uncle Bambi]

R2 = 594-5073NW2K200J 2.2k 1 watt metal oxide resistor
D8 = 512-1N5243BTR 13V 1/2W Zener Diode

I do not have these parts on hand, and will order them from Mouser.... but first, a question....

Radio Shack has a 12v 1W zener, and a 2.2k ohm 1/2W carbon resistor. Can I use these temporarily just see if we're onto something, or will I cause damage?

Also, I can pull the resistor and measure resistance. Is there any way to test a zener with a multimeter to see the voltage value?

 

[goat67]

Yes you can get the zener from rat schack it is a volt low so look to get -12 volts instead of -13v

As far as the resistor lift one leg and measure it.

If rat schack has 4.4K 1/2 watt use two of them and wire them in parallel

Resistors in parallel if same value half so any two that will get you close to 2.2K
will work.

Testing a Zener requires a voltage source so no easy test with a DMM.

Still order the correct parts.

What we want to prove is that we get the -12 volts. also check the 22 volt pin.

Do not leave it on more than necessary.

 

[Uncle Bambi]

Go ahead and replace the other transistors on the Eq Amp Board.

Measure Voltages on PS board

Pin 8 -13.0v
Pin 10 22.0 v

If not correct shut it off.

Do you have the 13 volt Zener diode?

OK - so, while waiting for an answer on my resistor/diode question, I went ahead and replaced the four 1313/1845 transistors on the phono amp board.

Flipped it on, and measured voltages:

Pin 8 -12.8v
Pin 10 22.5v



I've been listening to the unit (FM only) for 2:45 and counting. :tresbon:

Crossing my fingers - will report back in a little while.

 

[goat67]

Great news
Now I would check the bias and dc offset adjust if needed.
Then you can start to recap the rest of the unit. Follow my suggested steps.

 

[Uncle Bambi]

Now I would check the bias and dc offset adjust if needed.Then you can start to recap the rest of the unit. Follow my suggested steps.

Listened for 4 solid hours without issue, before turning it off and going to bed. I now have my garage stereo.

Bias and dc offset adjust - Done. I got the 30mv pretty much dead-on, but adjusting the other to 0v was problematic. The best I could do was hovering at -.1 to +.1mv. The pots were just too sensitive to get it to zero out perfectly. I'm measuring .5mv at the speaker outs.

All that's left to re-cap is the control amp and the mic amp - I've done everything else...... including replacing 18 transistors on three different boards. I'm mildly surprised that I didn't make any mistakes.

BTW - that 5W 220 ohm resistor (r3) on the PS runs hot - too hot to touch. Should I be concerned about that?

 

[markthefixer]

<snip>

BTW - that 5W 220 ohm resistor (r3) on the PS runs hot - too hot to touch. Should I be concerned about that?

no, it's 5w and intended to get hot. otherwise Q1 would have to dissipate that heat instead....

 

[Uncle Bambi]

Ok - so, I will finish the re-cap and button this one up.

I'd like to take a minute and recognize all you fine folks that stepped up to help a new member with his problems. I am a member on a couple of other (non-stereo related) forums, and over there I'm one of the folks with a lot of the answers. So, I know what it's like to hand-hold, and patiently explain the same thing over and over to a seemingly constant stream of new members looking for guidance. It gets tiring, and it can get frustrating - especially when there are a high percentage of drive-by's...... folks that just pop in to tap your knowledge, and then are never heard from again.

I know I cannot contribute much in the way of knowledge or expertise - most of everything I know I learned over the last two weeks, because you people took the time to teach me. But, what I can offer is support..... so I just now subscribed to AK to show my appreciation for the wonderful help I've received here.

Thanks guys - you've got a pretty awesome forum, here. :thmbsp:

 

[markthefixer]

go check out bartertown!!!

 

[Uncle Bambi]

Ok - I have some ancillary questions regarding the re-cap and the front panel illumination.......

On the front panel:
I have a couple of bad lights. One is for the signal meter, and is apparently an 8v .3a fuse. Is that what it really is, just a fuse? Also, the FM light is burnt out - I think it is spec'ed as a "Pilot Lamp, 8v, 50ma". Can someone verify? I'm probably going to have to order from Mouser.

On the re-cap:
I made a mistake when cutting/pasting the BOM, and am one cap short for the Mic board AWM-064. C7 is a .47uf 50v cap, and I don't have one.... and neither does Radio Shack. What I do have are a 50v 2.2uf UKL type electrolytic cap, and a 25v .47uf polyester film cap. Are either one of these close enough, or do I need to order the right one from Mouser? How big a sin would it be to just leave the original cap be and forget about it?

 

[markthefixer]

Ok - I have some ancillary questions regarding the re-cap and the front panel illumination.......

On the front panel:
I have a couple of bad lights. One is for the signal meter, and is apparently an 8v .3a fuse. Is that what it really is, just a fuse? Also, the FM light is burnt out - I think it is spec'ed as a "Pilot Lamp, 8v, 50ma". Can someone verify? I'm probably going to have to order from Mouser.

On the re-cap:
I made a mistake when cutting/pasting the BOM, and am one cap short for the Mic board AWM-064. C7 is a .47uf 50v cap, and I don't have.... and neither does Radio Shack. What I do have are a 50v 2.2uf UKL type electrolytic cap, and a 25v .47uf polyester film cap. Are either one of these close enough, or do I need to order the right one from Mouser? How big a sin would it be to just leave the original cap be and forget about it?

unfortunately, that is a vulnerable cap, and a low noise to boot. eventually it will need to be changed out. and we can't go with a 25v cap. So order up a poly film 0.47 at 50v on some other order in the future...

2.2 uf will leave you with a lot of mic rumble coming through... the ukl part of the spec would be a match and the 50v part of the spec would be a match..

lamps:
http://www.dgwojo.com/index.php?p=1_2_Products



they are "fuse lamps" , and the stereo lamp is 8v 50ma and 4mm or so...