9090 adjustment problems

drbacklash

drbacklash

Active Member
I picked up my first 9090 this past weekend and put it on the bench to check it out. It came out of protection fine and didn't sound too bad through my bench speakers. (This is where I should have stopped but you know I didn't or I wouldn't have the title above).

I have set the bias and offset on several Sansui receivers so I'm not a total rookie at this (this is my qualifier). The DC offset was too high on one channel so I adjusted both to specs (0V + or - 10mV). Since the offset was high, I had reason to suspect the bias was probably off too (oh, those damnable assumptions). The SM is for both the 8080 and the 9090 and of course the illustration for the bias adjustment is for the 8080 F-2546 board which is a little different from the 9090 board. For this adjustment, you have to pull the fuse from F07, adjust, replace the fuse, pull F08, adjust and replace that fuse. The problem is identifying F07 and F08 (the illustration is for the 8080, remember?) and the fuse number is actually covered by the fuse holders on F-2546. But by using the CB layout for F-2546 (that matches my board) on page 10 of the SM, I was able to identify the correct fuses (I think). They are the two middle fuses out of four located under the F-2436 Driver Board.

Pulled one of the fuses (yes, with the power off) and connected my DMM clips according to the instructions. Set the DMM to 200mV (target is 50mV) and turned it on. OL. Up the DMM to 4V. OL. I finally get a reading of about 65V (did a bell go off? Nope). Turned the pot to see if it would adjust down to 50mV (mistake). Tried the other Fuse with the same result. By now, the adjusting pots are no where close to where they were. Then I notice that the target is not mV but mA. The instructions list the L-CH adjustment as 50mV + or - 1mA. A typo that was not repeated for the R-CH but I missed it the first 100 times I read it.

Here's where it gets dicey. Turned the receiver off and as I was removing the DMM leads, the positive lead hit the metal case while the negative was still connected to one of the fuse holders. Sparks flew, loud noises were heard and I began to speak in unknown tongues. Apparently, my 9090, maybe not yours, holds over 55V on F-07 and F-08 even after you turn the unit off?

Replaced all the fuses and turned it on. Now it won't come out of protection unless I remove the driver board. I pulled all 8 output transistors and they survived (thank you deity of my choice). I figured the bias pots were way off and that was the problem so I set the DMM for 200mA and started over. I couldn't get any readings from either fuse although I still get voltage readings from both.

Do I:
1. most likely have a faulty DMM?
2. focus on the other two fuses assuming the SM is wrong?
3. have a problem with the driver due to the sparky thingy?
4. look for another hobby? (Be kind).
5. leave the fuses in place rather than removing them?

I'm at a loss here and sure could use some guidance (and a bottle of single malt whiskey).

Thanks.
 
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I've done that working on my 7070. Check the fuse in the multimeter. I blew the fuse in my mm while doing bias adjustments, and just kept messing with stuff trying to get a reading. I would have saved myself a lot of aggravation if I checked mm first.
 
If it's not bias or offset, Would this cause a driver transistor to go? Maybe one of the fusibles?
Also, if you were on a low current scale, you probably blew the fuse in the meter with the arcy sparky dance.
 
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"Here's where it gets dicey. Turned the receiver off and as I was removing the DMM leads, the positive lead hit the metal case while the negative was still connected to one of the fuse holders. Sparks flew, loud noises were heard and I began to speak in unknown tongues. Apparently, my 9090, maybe not yours, holds over 55V on F-07 and F-08 even after you turn the unit off?"

When you were disconnecting the DMM, was it set to ampheres or voltage ? On my meter, I have to move the black lead to the ampherage port when measuring current. If yours was on amperage, then your meter provided a current path from the case (ground) to one of the fuse holder blades. If the power transitor side (pins 31 and 32) of F2546, then that would have shorted the collectors to ground of the darlington configured final output transistors.

You pulled the final outputs and tested them (TR 701-708) individually ? It sounds likely that one or more are shorted now, which is why the unit stays (properly) in protection mode. You say you pull the driver board (F2546 or F2436) and the unit comes out protection.

Most bias POTS I have used turn full CCW to decrease current, but it depends on how its wired in and made. Maybe someone who has made some notes on this will know, then turn yours all the way that direction.

On your meter in-line fuse, if it is blown, then the meter was in the current measure mode, meaning that the meter enabled you to short circuit one side of the fuse holder to the chasis. If you can recall which side of F07 or F08, then you can follow that side back to what it connects to (either the collectors of the output transistors or the diode rectifier D02/D03. Since F02 and F03 aren't blown, you must have touched the blade connected to the outputs.
 
A trick I have used with those 'suis is to take a blown pigtail fuse and put that in the fuse holder. The pigtails provide a nice non-slip spot to attach the meter leads.

If she comes out of protection with the driver board out measure all those %$#@@ fusistors.
 
I'm pretty certain that the DMM was in the mA mode when it shorted. I have to move the positive lead from the voltage setting to either mA or A in order for it to work. I never thought about a blown fuse on the DMM so that's something I can easily check. I like the idea of wiring an old fuse for this type of work. The clearances just arent enough for alligator clips and there's nothing for the grab-all clips to attach to. And who has two hands to hold two probes and another to adjust a pot (per the illustration)?

I did check the outputs and thought I had a bad one but the collector connection was loose and cleared up when I reconnected it. I will check them again when I pull them out to clean off the dryed-up insulation seal.
 
Somewhere in this forum is a picture of a test unit that I made. I used a blown fuse and soldered leads on to it and then put female connectors on the wires and use those for my bias adjustments. I have two meters so I made two and connect both meters so that I can run the unit for a good five minutes and let it stabilize then can make both adjustments without turning the unit on or off. when I searched it appears that the pictures are gone, so I will repost them here and copy the description of the build process.


I know how difficult the bias adjustment is on some of the Sansui receivers and amps where the fuse must be pulled and an ammeter connected to measure the current.

Here is a tool that I built to help in that adjustment, It requires a blown fuse, some wire and a couple of .093" series Molex single circuit connectors (female). Before inserting the connectors in the holder check the fit with your test leads and if needed crimp the connector closed a bit.

Solder the wires to the blown fuse, and crimp to the molex connector.
To use remove the fuse place the "tool" in the fuse holder, connect the ammeter to the connectors and power the unit on. The test leads fit all the way inside the molex insulators and prevents any shorting.

I have been using this for sometime and it works great.
 

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Great idea, CDFixer. I made one this afternoon before I read your post but it does not have the molex connector. I will add those after my next visit to Lowes. Your tool certainly reduces the possibility of an accidental short. I wish I had it two days ago
 
I'm still working on this 9090 that won't come out of protection mode. I soldered a wire to each end of a blown fuse to do the testing. I checked the protection fuses in each of the three DMMs I have (all good). I tested the new rig by setting the DMM on V (works fine). Then I move the red lead from Volts/Ohms to 200mA (then later to 10A) and nothing. No reading at all on any of the three DMMs. So I ask myself "How can a circuit have 50-60 Volts on it and no amps? The obvious answer is that I using the DMM incorrectly. The black wire is on com and the negative side of the fuse holder (per the instructions). The red lead is on 200mA (or 10A) DC current and the positive side of the fuse holder. What am I doing wrong here? Even if I had a shorted output transistor, I should be getting a reading, shouldn't I? Maybe I should just wet my finger and stick it in there and adjust the pot until I can't stand the pain anymore. Imprecise, yes, but I would get immediate feedback.
 
If you have a suspicion that you are learning electronics in general by working on your 9090, then I think you should take a step back.

Around 1976 I bought and worked through one of these Radio Shack 130 in 1 electronic project kits, then I wanted to change from ship fitter-welder to electronics, joined the AF in 1977 and was an instrumentation/autopilot technician (7 mos tech school).

http://cgi.ebay.com/Radio-Shack-Sci...|66:3|65:12|39:1|240:1318|301:0|293:1|294:100

I'm sure you are making light of your current situation-" I should be getting a reading, shouldn't I? Maybe I should just wet my finger and stick it in there and adjust the pot until I can't stand the pain anymore. Imprecise, yes, but I would get immediate feedback. "

The 9090 is a classic, kind of like a 1965 Mustang GT Fastback. Take your time, and work on a kit or something first.
 
Maybe you blew a fuse in your DMM? I keep an analog meter around for just such purposes as this, digitals tend to react a bit slower.
 
"If you have a suspicion that you are learning electronics in general by working on your 9090, then I think you should take a step back.

I'm sure you are making light of your current situation-" I should be getting a reading, shouldn't I? Maybe I should just wet my finger and stick it in there and adjust the pot until I can't stand the pain anymore. Imprecise, yes, but I would get immediate feedback. "

The 9090 is a classic, kind of like a 1965 Mustang GT Fastback. Take your time, and work on a kit or something first."

Yes, I am making light of my current situation. Self-deprecating humor is a distinctive characteristic that separates homo sapiens from the other higher primates. If I did not have such a easy going sense of humor, I could probably take offense with some of the implications of taking a step back and working on a kit first. Insightful perhaps but not particularly helpful to the issue at hand.

I agree that the 1965 Mustang GT Fastback was a classic. So was the 1966 Mustang Convertible I restored from the ground up. Perhaps I should have taken a step back on that also and only worked on VW Beetles.

Merrylander, thanks for the feedback. I did check all the DMM fuses again and they were ok. I think having an analog meter is an excellent idea.
 
Hey, we have something in common then, I restored (7 years ago) a C code 66 Mustang convertible, vintage burgandy, black power top, automatic, ps, pb. We have two left (1966 fastback and 1970 Mach 1 R Code). This was in Santa Maria, Ca. Bought that car in Tehachipi, Ca.

I'm just trying to figure out your expience level, sounded like the 3 meters were giving you fits.

Anyway, hope you get that 9090 working. One of the few receivers I know of where you literally break into the collector current of the final darlington pair to meaure the 'EXACT in this case' bias current.

There should be a way to wire test leads off the emitter resistors if you have it opened up, and just work into the equivalent mV reading you would have across them (E=I*R).
 
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That is what Yamaha does although it tends to confuse some who wonder why they are measuring 'current' in 'millivolts'.

Whatever, this does not solve your problem. Perhaps with the power off and the fuses out, measure with the ohms scale the resistance of the load side of the fuseholder to ground. The only way you could be getting no current reading would be if there is an open circuit for the load.

Also, I note that you wrote the 10A (200mA) setting. AFAIK most meters have two current scales, the 10A scale measures up to 10A regardless of where the dial is set, there should be a seperate current scale, at least that is the way my four meters work.
 
I have a little more information now. Last evening I tested my DMM by connecting it to a 9V battery. I got a mA reading (don't remember what it was) with the meter. Ok, so I was right that the problem was not with the DMM or the way I was using it.

What I discovered that I thought was unusual was that one of the fuses (either F7 or F8) continues to hold current after I turn off the unit. I can have the dummy fuse in for a test, turn the unit off and get a 60V reading on the DMM. If I leave the probes on the fuse, the voltage will eventually drop to 0 after five minutes or so. This is how the first short happened when I turned the unit off and was disconnecting the alligator clips from the fuse holder. One side was still connected and the other touched the case when I removed it from the other side of the fuse holder.

Just to be sure that the output transistors were still ok, I pulled them all again and tested them. They needed new sink compound anyway. The transistors were all good.

Merrylander, good observation. I will test the circuit using the resistence setting to see what pops up.
 
According to the SMs I have, the 9090DB came with 4.7k bleed-down ressistors accross each of the 80 V 12000 uF main filter caps. The 9090 SM shows the only bleed down available is through the bias resistor network for the final stage, and the main caps are 71V 6800 uF types.

Here's a good link to checking output transistors.

A way ahead you can try is to pull them all, leave the driver board in, see if it comes out of protection, then check them with this procedure.

http://www.audiokarma.org/forums/showthread.php?t=43186
 
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Ok, its finally working now. The problem was bad fusible resistors on the driver board and a bad DMM. Got a new DMM, set the bias and readjusted the offset to specs. It sounds pretty good now but I have a problem with the lights-all of them except the power meter lights are inoperative. The bulbs are good and I get a voltage reading but apparently there is no current flow. Suspecting another bad fusible resistor, I found one on the ligt board and replaced it. Didn't solve the problem and besides it tested ok. What should I look at now? The stereo light is also out along with all the other indicator lights.
 
Right might also want to see if you have 6.5 VAC between pin 11 and ground (on 19) on the F2546 power supply board. The pin 11 source provides power for the 7 volt 0.1 A rated dial lamps (PL 701-705). If not, check that Fuse F05 has continuity. The other side of this fuse is the secondary 6.5 VAC winding off the main transformer.
 
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