Sansui 881, relay click but no sound

[Chris1227]

Hello all, just bought a Sansui 881 as-is off ebay for a good deal, so here I am with my first post. The unit turns on and relay clicks shortly after, but I am getting no sound (tried speakers A,B,C and FM,AM,Aux inputs). So I sat down with the service manual and started looking around. I see that fuses F07, F08, F09, F10 are all blown on the power supply circuit board. Aside from that everything looks as it should. I'm usually pretty good at troubleshooting things, but I know squat about vintage audio electronics. So I'm hoping you all can give me some direction as to what may be blowing those fuses. Thanks!

 

[baeron]

Hello all, just bought a Sansui 881 as-is off ebay for a good deal, so here I am with my first post. The unit turns on and relay clicks shortly after, but I am getting no sound (tried speakers A,B,C and FM,AM,Aux inputs). So I sat down with the service manual and started looking around. I see that fuses F07, F08, F09, F10 are all blown on the power supply circuit board. Aside from that everything looks as it should. I'm usually pretty good at troubleshooting things, but I know squat about vintage audio electronics. So I'm hoping you all can give me some direction as to what may be blowing those fuses. Thanks!

First thing I would do is check with seller.
How was it sold in the ad.

Don't want to just replace them fuses and try again. They blew for a reason

May want to build DBT.
Dim Blub Tester.
Wait for the more knowledgeable ones to help out

 

[tnsilver]

I see that fuses F07, F08, F09, F10 are all blown on the power supply circuit board

This is bad as in catastrophic and you need to be careful! I'll go through the steps to solving this, but you're likely going to have to do extra research so I'll try to provide some links to help you. I'm assuming you're in the US or Canada on a 120V/60Hz grid?

The protection relay clicks b/c the amp circuit related fuses are all blown. There's no DC component in the output to protect from, since there's no AC audio output either. Right now it's a dud, but if you replace the fuses they'll instantly blow out again. The problem likely stems from a fault on the driver amp, but it's likely escalated to the output transistors. You'll need to work it out backwards from there.

• Like mentioned earlier, assemble a DBT (70W to 100W bulb) and never operate the unit w/o it, until all issues are resolved. Don't be tempted to power it up too soon.
  
• Make sure the main AC power fuse (F-701 on the back) isn't blown. Replace with a 117V/5A slow blow fuse if needed.
  
• Pull the heat sink and the output transistors (TR-701...TR-704) out the back (careful with the mica sheets) and verify none are blown. Research transistors testing with a DMM or a components tester if you need to, and buy replacements (2SC1115 => MJ21194G) only from a reputable distributor - never from eBay. Note there's the mica sheets that insulate the transistors from the chassis and the thermal grease in between - to account for.
  
• Verify the 4 emitter resistors on the power supply pcb (F-2347) still read 0.33Ω
• Visually verify no charred components, lifted traces or structural cracks exist on F-2347. Cracks in traces can get hair line miniature, so use appropriate lighting and magnification and a DMM continuity test when in doubt. Make sure there's no bulged or leaky electrolytic capacitors and no faded color or burnt resistors on the pcb. Test diodes and verify all transistors for shorts. Needless to say - replace suspects. I recommend this thread as an excellent info source for Sansui substitute transistors.
• Replace the blown fuses F7...F10 with 117V/5A fast acting fuses.
  
• Remove the metal bracket around the driver amp (F-2436) and pull the suspect PCB out of the socket.
• At this stage you'll need to fix or rebuild the driver amp. I'll just opt for a rebuild:
  • You can start by researching one of the many 'rebuild 8080/9090 F-2436 driver amp' threads around here. You could use KSC1845 sub for the TR01...TR04 differentials. The driver/power transistors (2SD381 => MJE15032 and 2SB536 => MJE15033G) and temp/co transistors (2SC984) in the bias circuit are the same as the X0X0 units. Pay attention to those 2SC984's and take extra care to verify they aren't shot and that they closely compare to each other hfe wise. Note they are clamped to the power transistors heat sinks and that the stock devices can is insulated from the legs. Other modern subs may have plastic bodies, other older variants have conducting cans. Watch out for those!
    
  • You could also use the service manual trouble shoot section 2-1 (p-9), which basically calls for replacing nearly all transistors and diodes on that PCB. I had a driver-amp transistors pinout gotcha thread posted if you want to research it.
    
  • You will want to verify (or 'replace' if you're rebuilding) all the fusible resistors on that PCB. You could probably use some researching on the fusibles too but those are basically all the light blue resistors that are obviously not brownish or tan carbon resistors. You can replace them with metal oxide (MOX) or metal film modern resistors of the same wattage.
  • Replace suspect diodes or just shotgun all of them: 1N4739A-T for the Zeners, 1N4148 for the rectifiers,
  • Electrolytic capacitors replacement, probably long overdue anyways... I use Nichicon general purpose for all but C07/C08, for which I use MUSE series bi-polar capacitors.
  • You'll definitely want to replace the stock trimmers (VR01...VR04). Don't bother cleaning them up. It's not worth the trouble and they stock are too much of a risk. 3386H-1-472LF is the Mouser part number of a Bourns single turn 4.7KΩ/500mW trimmer that is a common replacement for the stock DC offset trimmers. 3386H-1-102LF would replace the 1KΩ bias trimmers. You could use multi-turn trimmers too. Leave the trimmers in the default middle position. This will be of benefit with the bias current and DC offset adjustments - next.
• Hopefully, by now, you'll have a fully functional rebuilt driver-amp but you're gonna have to be extra careful when you fire up the unit for the first time. Install the driver amp back on the power supply socket, make sure to align the driver amp traces with the pins in the socket, so they all make good contact. Don't apply too much pressure and support the power supply PCB from bellow (block of wood) if you have to in order to sink the driver amp all the way in. Make sure nothing on the driver amp shorts with components on the power supply PCB.
  
• You're going to have to follow the bias current adjustment procedure as described in section 3-1 (p-4) in the SM. Note figure 3-2 may be deceiving as it illustrates the procedure on the wrong fuse. When performing you must remove the fuse and measure current in series (probably using a spent fuse tool for the job) on the vacant fuse holders. I've posted a thread about biasing a 9090 unit a few years back with pictures. It's a good thread - but notice you're aiming for 30mA with the 881 as opposed to the 50mA of the 9090.
  
• Note that overshooting the bias adjustment may result in semi-conductor damage even on DBT. That's why we've left the trimmers in the middle default position. In case the DBT lights up brightly (after the initial quick bright & dim as a result of the filter caps charging), you will need to reduce bias current by turning the trimmer (gently) to the other side. Whenever the DBT lights up - shut the unit down and baby step the trimmers until the DBT stays dim. Remember that VR-03 and VR-04 mirror each other, so once you've got the direction for reducing bias current with one trimmer, the other trimmer is going the other way to achieve the same result on the other channel. Always remember there's two channels and only one DBT that doesn't care which of the two draws current. It can get tricky for the novice.
  
• My thread also demonstrates the DC offset adjustment procedure for a 9090, which is basically identical to the 881 procedure in the same section as the biasing. The one caveat to remember is about the default DC trimmers position in the middle. In case there's too much DC, the protection circuit kicks in and won't allow the relay to engage. You'd still have bias current flowing, so don't let it throw you off. The protection circuit isn't triggered by current, but by a threshold of DC component in the AC output. You also won't have any output in the speaker terminals if the protection circuit kicks in - no matter which channel is at fault. Fortunately, it's easier and less dangerous to find the trimmers directionality to control DC offset. Once again, when you figure one out, the other trimmer goes the other way for the same effect on it's respective channel.
• A little more about that... The biasing and DC offset procedures are interlaced. You set one, you upset the other. It's a balancing act that typically needs to be done a few times. The DC offset reading may appear to fluctuate (depending on the filtering ability of the unit's capacitors) a few mV. It's important not to get anal about it. It needs to be close to 0 and it doesn't matter if it's close negative or close positive. The allowed range is 0 to 10mV which is quite broad.
  
• Finally... Remember! While you're on DBT your bias current readings are a few mA lower than when directly plugged to the wall. If you set it at exactly 30mA on DBT, it will get a little higher when you're off DBT. I usually set bias 3 mA lower than needed on DBT, b/c I really don't want to mess with the sensitive trimmers with raw utility voltage. Also, bias current is time/temperature dependent so if you don't time your measurements - you'll never get it right. I take bias current measurements after exactly 4 minutes from ambient temperature and I let the unit cool for 20 minutes before measuring the other channel.

Alright, I've blabbered enough and it came out too long - but if this is your first time you're gonna need all the help you can get and it's still probably not enough. Best of luck with it. Be careful. Baby steps with the trimmers!

 

[Chris1227]

tnsilver, thanks for the comprehensive reply, got me off to a good start. I pulled the output transistors and they all check out. The 4 emitter resistors are reading 0.00 ohm on the power supply board. So I will look for replacements for those resistors and buy some new fuses. Could you recommend a rebuild kit for the driver amp board? Would it be worth it to buy a complete rebuild kit and repair board by board? I see there are some kits on ebay, but not sure if they can be trusted. Thanks!

 

[Chris1227]

assemble a DBT

Got my DBT built and working. Just to see, I plugged the receiver into it and powered it on and the bulb was very dim. I did notice something and it might be a dumb question, but in back of volt selector, on PU01, there are 2 white wires jumped together with solder between their posts on the top right. Might be a dumb question but I couldn't make sense of it from the schematic.

 

[tnsilver]

Could you recommend a rebuild kit for the driver amp board?

No way! I told you, stay away from eBay. Those kits are ripoffs for the the lazy and the uninformed. The 881 driver amp kit that I see on eBay doesn't even include resistors or power transistors. I'm going to break it down to you with a list of parts I would order from Mouser if it was my own unit. The following package is sold on eBay for $19.99:

C01/C02 0.47μF/63V/5% MKS2C034701C00JSSD Wima film capacitor ($0.79) total $1.58
C06/07 100μF/6.3V UES0J101MPM Nichicon MUSE series audio bi-polar capacitors ($0.53) total $1.06
C11/12 100μF/50V UKL1H101MPD Nichicon standard audio capacitor ($0.64) total $1.25
VR01/02 4.7KΩ 10% 0.5W 3386H-1-472LF Bourns trimmers ($1.56) total $3.12
VR03/04 1KΩ 10% 0.5W 3386H-1-102LF Bourns trimmers ($1.60) total $3.20
D01..D08 100V/200mA rectifier Diode 1N4148 Fairchild ($0.10) total $0.80
TR01...TR04 Differential pairs BJT NPN KSC1845FTA ON Semi/Fairchild ($0.27) total $1.08

Total: $12.09

Those are basically the same parts and obviously cost $8 less. With Mouser's shipping fee you're still better off by a couple of bucks if you order directly, but that not the issue. The real deal is getting four matched differential transistors, so don't lazy up or skimp and buy a 20 KSC1845FTA reel and match four of them by HFE.

Now, let's throw in the best fusible replacements. I'd go with KOA Speers:

R29/R30 Metal Film Resistors 'fusible' 100Ω/1%/100PPM MF1/4DCT52R1000F2 KOA Speer ($0.23) total $0.46
R43/R44 Metal Film Resistors 'fusible' 10Ω/1%/100PPM MF1/4DCT52R10R0F2 KOA Speer ($0.23) total $0.46
R47...R50 Metal Film Resistors 'fusible' 4.7Ω/1%/100PPM MF1/4DC4R70F KOA Speer ($0.10) total $0.80

Total so far: $13.81

You're still doing more than $6 better than the eBay kit. Finally, let's throw in the power transistors:


TR11/TR12 BJT 8A 250V 50W NPN MJE15032G ON Semi ($1.59) total $3.18
TR13/TR14 BJT 8A 250V 50W PNP MJE15033G ON Semi ($1.59) total $3.18

Final Total: $20.17

So basically, it costs 18 cents more than the kit to get the whole package from Mouser.

BTW: The white leads jumper on the voltage selector looks normal to me. It's odd the emitter resistors are all shot. Are you sure about it? How did you test the output transistors?

 

[Chris1227]

It's odd the emitter resistors are all shot

Thanks for the breakdown, I'll get all that ordered. About the emitter resistors, after I made that post I saw that the plug in the middle of the board wasn't seated properly. Plugged it in all the way and ended up getting .3 ohms on my DMM. Took some fiddling but I eventually got all of them to test properly.
While I'm at it should I buy replacement caps for the Power Supply Circuit?

 

[tnsilver]

What about the output transistors? I'm only asking b/c if you're wrong, you'll fry components on the driver amp after you've rebuilt it.

 

[Chris1227]

What about the output transistors?

I did my research on those and since I'm more so a visual learner, I found youtube video from D-Lab Electronics where he showed how to test those. They all checked out, reading similar voltages on base to emitter, and base to collector. Then all tested O.L. on the emitter to collector readings.

 

[playittwice]

Good luck with your amp, Chris. Patience is a virtue when troubleshooting, unfortunately I’ve been there as well.

You’ve got a great instructor/teacher onboard! (@tnsilver).

 

[EastPoint]

@tnsilver, are you sure there are fusible resistors on the 881 driver board? There sure weren’t any one mine.

 

[tnsilver]

Not at all. I just call them 'fusibles' with quotes b/c of their role on the X0X0 F-2436 driver amp. It's easier to ID them this way. The actual fuse component in the design had long proven to be moot. Those are just the equivalents for the 180Ω,150Ω and 4.7Ω 250mW resistors (maybe some others too) on the later runs.

 

[EastPoint]

What I’m saying is that my 881 had regular carbon film resistors on that board, so it didn’t need new ones.

 

[LIN0]

I have an 881 too with same symptom. Lights all work but no sound & same fuses blown. Looks like the OP only had to reseat the driver amp to get his 881 working?

 

[LIN0]

yep! Just reseating the driver board and making sure all components on every board inside the 881 is cobweb free and not touching its neighbors along with replacing the fuses brought this wonderful sounding amp back to life! Thanks AK! I let this thing play radio all night while I slept and I think I slept better...like its Therapeutic!