9090DB, Went from Playing Normally to Quiet
- Thread starter BearsFan
- Start date
That says 9090, not 9090db. They are 2 different receivers. The service manual for the 9090db also includes the 8080db. You want the 9090db service manual which includes the schematics.
Yes, that section of the schematic is for the 8080db/890db that does use 7a fuses. Look at the section (different page) for the 9090db/990db, it will say 10a.. The service manual is for all of them and has 2 separate diagrams. Plus on some board layouts, the parts list will designate which parts are different. Look at the parts list for the power supply board. They both use the same board, but some components are of different values, and some aren't used on the 8080db that are on the 9090db.
BearsFan
Active Member
Good Morning - still waiting on the DBT, delivery expected tomorrow so I can put in the fuses and let her rip. . Just for grins I took readings across both big caps, in circuit with f1 and f2 fuses pulled
. Using a Fluke 116 on the ohms scale, both caps acted the same, resistance climbs to about 1 meg then drops as low as 400 ohms, meter briefly displayed OL, then settled to steady 2k ohms. On the volts scale I watched both caps drop from about .1vdc to zero over the course of a couple minutes.
I also measured from the positive cap side of both F1 and F2 and got a steady 2 meg of resistance from both caps. Reading the big caps, hot to chassis ground, saw the slow increase in voltage on both caps, more than likely influenced from the meter battery.
There's been no power applied since Saturday so no apparent charge in the caps.
. Using a Fluke 116 on the ohms scale, both caps acted the same, resistance climbs to about 1 meg then drops as low as 400 ohms, meter briefly displayed OL, then settled to steady 2k ohms. On the volts scale I watched both caps drop from about .1vdc to zero over the course of a couple minutes.
I also measured from the positive cap side of both F1 and F2 and got a steady 2 meg of resistance from both caps. Reading the big caps, hot to chassis ground, saw the slow increase in voltage on both caps, more than likely influenced from the meter battery.
There's been no power applied since Saturday so no apparent charge in the caps.
Well, at least not under load they shouldn't be blowing the fuses. FYI, electrolytic caps will "charge themselves" up to a point. Like a lead-acid battery, the electrolyte is "live". Larger caps can charge themselves enough to kill a meter, or blow an expensive Fluke fuse. I always keep ground to chassis wires on the large caps without bleed resistors while I am working on a unit to keep them discharged. Your 9090's bleed resistors should be enough to keep them discharged once you discharge them the first time.
BearsFan
Active Member
Gotcha, I just wanted to get a feel for circuit resistance to see if there were any obvious shorts. As soon as i get the tester, I'll run the fuse test. While I have you, is there any value in doing this in steps, i.e.,
A. Install fuses in F1 and F2, open in F4 and F5, driver board removed, power up.
B. If ok, install F4 and F5 and power up
C. If ok, install driver board and power up.
I'm thinking these steps will help to isolate the problem area. Thoughts?
A. Install fuses in F1 and F2, open in F4 and F5, driver board removed, power up.
B. If ok, install F4 and F5 and power up
C. If ok, install driver board and power up.
I'm thinking these steps will help to isolate the problem area. Thoughts?
Yes, do it in steps to isolate the problem. You can leave F04/05 in, but remove the driver board. Without the board in, F04/05 have nowhere to go. ;-) Be sure to align the pins on the driver board. Being one off will burn things instantly with or without a DBT. If the fuses 01/02 blow without the driver board, then the problem is in the power supply area. If so, I am thinking the problem may be a shorted transistor, or even a cap in the power supply.
Sounds good, actually it looks like the dbt is coming today, so I'll get on it as soon as I can. Did you say 100w bulb should suffice? ( Yes incandescent).
100w minimum. 150w would be better for this beast. ;-) 100w will be good for the 5050 with room to spare. Here, I measured 114vac with a 100w, and 117vac with a 150w, and only 109vac with a 60w "rough service" bulb (at 121vac line). Some things don't like anything less than 114vac, but the 9090db will still work on a 100w. It will, however, be slower at coming out of protection than with a 150w.
Good thing Obama didn't have his way or we wouldn't have any testers!
Actually, it was during Dubya's watch when "real" light bulbs were banned. Funny though, the term was "incandescent", then as soon as Obama came along, his "buddy" Jeff Immelt, the (at the time) CEO of GE, was given the green light to produce incandescent bulbs, but in the halogen flavor. Hmmm..... Just a money-buddy game...
OK, I have 4 packs of fast blow AGC 10 fuses and a 150 'incandescent light bulb, just waiting on the tester. I'll test the tester with a known good 5050 and a 75 w bulb then on with the fireworks!
LOL! You can still use the 150w with the 5050 but it won't glow as bright if there is a bad short, but will still be bright enough to take notice. Try the 75w on the 5050 and watch it's behavior. Then wait until the caps drain off (about 10 minutes) then try the 150w to get the "feel" for the difference. Check your line voltage before the bulb, and after to see what the voltage drop is. Some units don't "like" voltage below a certain point, and will give a "false alarm" (bulb will stay bright) when there is actually nothing wrong. An old Fisher 400T (solid state) did that to me on a 60w bulb and I thought I had a problem It would even go dim (normal) occasionally. Well a few days later I tried it on a 100w and all was fine. So I figured 114vac was just enough to work whereas 109v was "tickling the dragon" and it awoke now and then. lol!
I have a couple comments.
1. If the fuses blew, there's a reason - find out this reason before changing them.
2. If this were my receiver to fix, and it came to be like that, the first thing I'd do is test the output transistors in the channel which blew fuses. It's really simple on a 9090 to remove the outer heatskink, pull out the transistors one by one and check them with the diode test function of a DMM.
3. If transistors are shorted, you have to go to the driver board, and check all the transistors for that respective channel, change any bad/shorted ones.
4. Measure resistors and replace those which are out of tolerance. Replace all fusistors as a matter of course, and any bad capacitors.
5. Clean the card connector with deoxit.
6. Install fuses, power up unit, set bias and offset following factory service manual procedures
7. Connect to speakers and enjoy some music.
The only way you will have blown fuses if the transistors are still fine, is if you have something way off causing the bias to skyrocket, OR, sustained overload, or maybe a shorted cap or wire. Since you weren't blasting it when it failed, I suspect there's been a semi catastrophic failure. Don't worry though, this isn't that hard to fix. A tech could get it back out there in no time. If any of the above seems too daunting, do consider taking it to a tech, since this is quite a valuable receiver and not the best one for learning.
1. If the fuses blew, there's a reason - find out this reason before changing them.
2. If this were my receiver to fix, and it came to be like that, the first thing I'd do is test the output transistors in the channel which blew fuses. It's really simple on a 9090 to remove the outer heatskink, pull out the transistors one by one and check them with the diode test function of a DMM.
3. If transistors are shorted, you have to go to the driver board, and check all the transistors for that respective channel, change any bad/shorted ones.
4. Measure resistors and replace those which are out of tolerance. Replace all fusistors as a matter of course, and any bad capacitors.
5. Clean the card connector with deoxit.
6. Install fuses, power up unit, set bias and offset following factory service manual procedures
7. Connect to speakers and enjoy some music.
The only way you will have blown fuses if the transistors are still fine, is if you have something way off causing the bias to skyrocket, OR, sustained overload, or maybe a shorted cap or wire. Since you weren't blasting it when it failed, I suspect there's been a semi catastrophic failure. Don't worry though, this isn't that hard to fix. A tech could get it back out there in no time. If any of the above seems too daunting, do consider taking it to a tech, since this is quite a valuable receiver and not the best one for learning.
Welcome to the party! ;-) Yes, it is very possible that one, or more output transistors have shorted. Or could be a combination of things. I don't know if you saw the pics of the driver board, but I don't like the looks of it. However it appears all the fusible resistors have been changed out with metal oxide, though a clearer photo would help confirm it.. The VD1212 are still there though. And like you mentioned, all 8 output transistors need to be checked for shorts since they are connected directly to the +61v and -61v rails. The DBT will help in that matter too. If the bulb stays bright *without* the driver board in, then we take the next step to finding the cause of that symptom. The output transistors are certainly part of that step. Although, like you said, high BIAS could have played a role and will be addressed in another step. BIAS runaway could have easily toasted and output or two, if that did indeed happen. The root cause is what we're seeking. Once the "symptoms" have been identified, we'll move to finding the cause, and ultimately the cure. One step at a time. You don't rebuild a house on a shaky foundation. The power supply is the foundation.
Welcome to the party! ;-) Yes, it is very possible that one, or more output transistors have shorted. Or could be a combination of things. I don't know if you saw the pics of the driver board, but I don't like the looks of it. However it appears all the fusible resistors have been changed out with metal oxide, though a clearer photo would help confirm it.. The VD1212 are still there though. And like you mentioned, all 8 output transistors need to be checked for shorts since they are connected directly to the +61v and -61v rails. The DBT will help in that matter too. If the bulb stays bright *without* the driver board in, then we take the next step to finding the cause of that symptom. The output transistors are certainly part of that step. Although, like you said, high BIAS could have played a role and will be addressed in another step. BIAS runaway could have easily toasted and output or two, if that did indeed happen. The root cause is what we're seeking. Once the "symptoms" have been identified, we'll move to finding the cause, and ultimately the cure. One step at a time. You don't rebuild a house on a shaky foundation. The power supply is the foundation.
Oh yeah, I forgot about the stupid fusing the positive rail but not the negative rail thing. I'm putting my $$ on one or more blown PNP output transistors. If the NPN ones were bad, then F04 or F05 would have probably gone first. Also may want to make sure the rectifiers haven't shorted with a quick in circuit test, and that the filter caps are okay. Frankly I wouldn't apply power to find this stuff out though - you can simply measure continuity across the filter capacitors and see if you have a short. A shorted transistor will read as a short here, and avoid stressing anything with power.
The filter capacitors were already tested. Although it is possible SS-5 or SS-5R may have failed, it is extremely rare. They are as tough as a tank. Nothing is ruled out at this point though. Heck, a speaker shorting could have also contributed, or even a mica insulator developing a crack. At this point, we just don't know. There is nothing wrong with using the DBT for a quick test as long as you have your hand on the switch and cut the power within a couple of seconds if the bulb stays bright. That's what it's for.
Besides, the damage is already done.
BearsFan
Active Member
Hi "Max" , as Skywatcher mentioned, welcome to the party! I've been very conservative chasing this issue since it began. Just to quickly recap:
> Receiver was playing loud, not cranked, about 85-90 db on my decibel meter from 10 feet from the speakers, about "10 o'clock" on the volume knob.
> I went over to investigate and could hear a very faint tinny sound out of the right channel. Left channel was dead
> Put the receiver in mono to see if i could get anything out of the left side, no go.
> Popped the cover and found both F1 and F2 open, F3,4 and 5 were OK. There is no smell of burining or overheating inside.
> Removed the driver board and looked for obvious physical signs of component failure, none found.
> Put a meter across both big filter caps, both act the same, voltage climbs to about a volt and then drops to 0 volts. Ohmed out , the resistance bounces around then stabilizes at 2K ohms on both big caps.
> Did the same test ground to positive side of big caps, similar result.
> I have not ohmed out thedriver transistors; I could do a quick check in the morning and compare readings from all 4 transistors in circuit to see if there is an indication of a short. At a minimum I can compare the reading from the left and right pairs.
The plan is to try three steps suggested by Skywatcher to isolate power supply issues from driver / output stage: install fuses, power up with F4 and F5 out, F4 and F5 in, and Driver board in, nothing attached, volume minimum, switch in aux.
If, between the two of you the consensus is I should pull the output transistors and do a diode bias test on them first is the correct sequence, I can do that. I am assuming that the case is the collector and the two pins are emitter and base.
One last thought I have is based on what I discovered when I opened the unit up. As noted, the workmanship on the driver board was pretty piss poor and the dirt and surface corrosion should have been taken care of when the unit was refurbed IMO. The four pots at the top of the driver board were not replaced and they look 40 years old. Is there any logic to reaching out to the community and seeing who might have the time and expertise to rebuild these two boards? I have the technical ability to remove and replace them, set up biases, etc, and test and check the output transistors, install replacements, silicon the metal contact, etc.
It's just a thought. I don't have a thousand dollars to redo a recap, rebuild of the whole unit, but these two boards in particular look ratty and may be the rot cause for this issue. I'm getting ahead of myself but just thought i would put it out there. Thanks a bunch for the support! Skywalker, I will hold off on power up until I hear back from you tomorrow. In the meantime I am going to poke around the unit and take some further resistance readings from the driver board connector. Hopefully I can figure out the pinout between the board and the schematic, I don't see any pin number references on the board itself. Here are some updated, and hopefully better pictures of the driver board.
> Receiver was playing loud, not cranked, about 85-90 db on my decibel meter from 10 feet from the speakers, about "10 o'clock" on the volume knob.
> I went over to investigate and could hear a very faint tinny sound out of the right channel. Left channel was dead
> Put the receiver in mono to see if i could get anything out of the left side, no go.
> Popped the cover and found both F1 and F2 open, F3,4 and 5 were OK. There is no smell of burining or overheating inside.
> Removed the driver board and looked for obvious physical signs of component failure, none found.
> Put a meter across both big filter caps, both act the same, voltage climbs to about a volt and then drops to 0 volts. Ohmed out , the resistance bounces around then stabilizes at 2K ohms on both big caps.
> Did the same test ground to positive side of big caps, similar result.
> I have not ohmed out thedriver transistors; I could do a quick check in the morning and compare readings from all 4 transistors in circuit to see if there is an indication of a short. At a minimum I can compare the reading from the left and right pairs.
The plan is to try three steps suggested by Skywatcher to isolate power supply issues from driver / output stage: install fuses, power up with F4 and F5 out, F4 and F5 in, and Driver board in, nothing attached, volume minimum, switch in aux.
If, between the two of you the consensus is I should pull the output transistors and do a diode bias test on them first is the correct sequence, I can do that. I am assuming that the case is the collector and the two pins are emitter and base.
One last thought I have is based on what I discovered when I opened the unit up. As noted, the workmanship on the driver board was pretty piss poor and the dirt and surface corrosion should have been taken care of when the unit was refurbed IMO. The four pots at the top of the driver board were not replaced and they look 40 years old.
Is there any logic to reaching out to the community and seeing who might have the time and expertise to rebuild these two boards? I have the technical ability to remove and replace them, set up biases, etc, and test and check the output transistors, install replacements, silicon the metal contact, etc.It's just a thought. I don't have a thousand dollars to redo a recap, rebuild of the whole unit, but these two boards in particular look ratty and may be the rot cause for this issue. I'm getting ahead of myself but just thought i would put it out there. Thanks a bunch for the support! Skywalker, I will hold off on power up until I hear back from you tomorrow. In the meantime I am going to poke around the unit and take some further resistance readings from the driver board connector. Hopefully I can figure out the pinout between the board and the schematic, I don't see any pin number references on the board itself. Here are some updated, and hopefully better pictures of the driver board.
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