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8080DB F2624 driver board frustration

crkeith65

crkeith65

Member
Let me set this post up right to begin with. I have owned this 8080DB since new. After letting it sit idle for many years because of driver board failure, I took it out to reclaim it's glory. The driver board was completely gone through along with re-capping the power supply board and 4 new output transistors I made a few repairs on the Dolby board just for the fun of it. All switches and pots were de-oxed and lubed. After weaning it off the dim bulb tester, offset and bias were set according to the manual. Everything was working fine for several days still on the work bench. The day that I was going to reassemble everything the FM became very erratic. ( separate post) I narrowed it down to a defective front end. While I had the tuner section off to remove the front end I powered the unit to check a few more things. One thing I found was a stuck contact in the relay. I removed the relay to free the contact then re-installed it. That is when the protection circuit started kicking in and out randomly. I thought that it was just because the tuner section was removed. I ordered a new relay and a salvaged front end and installed it today. Good news is that the tuner now works. Bad news is receiver stays in protection. This post is getting long so I will reply to give my latest symptoms.
 
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I have checked and re checked every component on the driver board, some in the circuit and some removed. I am getting very weird voltage reading everywhere I check. For example I read voltage on every lead of the outputs whether they are in circuit or not. BTW I checked the outputs and even installed a different set to rule that out. I read no bias current on either channel therefore it can't be adjusted and the dc voltage is rail voltage. If I remove the two fuses on the relay board the light turns green. Here is another weird observation: these are the voltage readings on the F2624 connector pins:
Pin 05 -8.1volts
Pin 06 +41 volts
Pins 07 through pin 14 -41 volts
Pin 15 -44 volts
Pin 16 0 volts

Pin 28 -7volts
Pin 27 +41 volts
Pins 26 through pin 19 -41 volts
Pin 18 -44 volts
Pin 17 0 volts

As you can see both channels are exactly the same
To say I'm frustrated is an understatement. time to turn it over to you guys. I think I've read every post related to this and have tried practically everything suggested. I do have a 9090DB that I can use for comparison if I have to but it is working perfectly I just don't want to take any chances with it.
Thanks for reading all this and I am looking forward to your responses. Ask me questions and I will respond in a timely manner, just not tonight I need a break!
 
I would go back to the relay and check it for shorts. You may have in the process of freeing the frozen contact, by accident bent the contact in a way that is shorting the wiper contact to the N/O contact. The N/O contact is actually connected to ground. If this kind of short is seen by the power amp output, it will look to shut the relay and turn the led red. Also make sure you don't have shorts where you resoldered the relay.
Remember, you didn't have protect problems until you worked on the relay.
 
crkeith65 said:
I have checked and re checked every component on the driver board, some in the circuit and some removed. I am getting very weird voltage reading everywhere I check. For example I read voltage on every lead of the outputs whether they are in circuit or not. BTW I checked the outputs and even installed a different set to rule that out. I read no bias current on either channel therefore it can't be adjusted and the dc voltage is rail voltage. If I remove the two fuses on the relay board the light turns green. Here is another weird observation: these are the voltage readings on the F2624 connector pins:
Pin 05 -8.1volts
Pin 06 +41 volts
Pins 07 through pin 14 -41 volts
Pin 15 -44 volts
Pin 16 0 volts

Pin 28 -7volts
Pin 27 +41 volts
Pins 26 through pin 19 -41 volts
Pin 18 -44 volts
Pin 17 0 volts

As you can see both channels are exactly the same
To say I'm frustrated is an understatement. time to turn it over to you guys. I think I've read every post related to this and have tried practically everything suggested. I do have a 9090DB that I can use for comparison if I have to but it is working perfectly I just don't want to take any chances with it.
Thanks for reading all this and I am looking forward to your responses. Ask me questions and I will respond in a timely manner, just not tonight I need a break!
Click to expand...

BTW the above voltages are on a 100w DBT. On full AC the voltages are 54 and -54 volts and -58 v on pins 15 and 18. Keep in mind that these readings are with the driver board installed and the protection fuses pulled which allows the light to go green. As far as the relay the problem started before I changed the relay. That is how I found the bad relay. A new one is now installed. I will take more voltage readings with the protection fuses installed and report back.
 
When you say the protection fuses are pulled, do you mean the ones on the protection board? I would do the checks with the fuses in place but leave it on the DBT. I would use a 60w bulb in this case. The voltage values will low but should still give you an idea. Start with DC offset. If it's high, you need to get that under control.

When it's in protection you can check DC offset on the Blue and Gray wires that come off the bottom of the power supply board. They carry the audio signal to the protection board.

- Pete
 
What is the voltage on pin 3 and pin 4 of the protect board? These points are coming from pins 37 and 38 on the main board that has the fuses on it with dim bulb tester used, and fuses in. The voltage on all points should be 0 volts + or - 0.3v. If there is any significant voltage present on either channel (pin 3 or pin 4) you have an issue with the driver or output stage for that channel. If you don't have any voltage present, the issue is probably in the protect circuit or wiring on the protect board.
 
Thanks for the response. With the protection board fuses in place the voltages are pretty much the same as with them out. There is zero (0) bias current and the offset is not adjustable, stays at rail voltage...-44v on DBT.
 
the +53V rail is sitting at -7/-8.1V so there is zero bias. The PSU board was recapped and worked for some days before problems arose. I recall that there a silkscreen print error on the PSU board in the polarity marking of at least one of the electrolytics but don't recall which one/s and the markings are hidden when the caps are installed. I can't post a picture right now to show the cap orientation in a working PSU board because the battery in my camera is flat...
 
I tried to match the caps up cap for cap, but of course I could have made a mistake. Yes it worked fine for a couple weeks while I checked other things. Today I did find two 100ohm resistors reading 178 ohms. Changed them with no help.
 
Did you replace all of the fusable resistors on the driver board?? They are quite notorious for big value swings. They are usually the 180 ohm, 150 ohm, 10 ohm and 4.7 ohm.
Not that I believe in changing out parts just to change them, but I have had a lot of issues with fusable resistors on the sansui 8080db, 9090 and 9090db.
 
voltage readings on the F2624 connector pin with all fuses in, and connected to DBT and power on
Pin 05 -8.1volts Should be +53v, may be +44 on DBT. If not or very low could be bad 1 ohm fusable on main PS board
Pin 06 +41 volts Should be +50v, may be +41 on DBT. If not or very low could be bad 7a fuse F04 on main PS board
Pins 07 through pin 14 -41 volts
Pin 10-11 (Last Check )Should be ~ 0 Volts. If high DC voltage here, then bad components on driver board or outputs.
Pin 13-14 Should be -50v, may be -41 on DBT. If not or very low could be bad 7a fuse F05 on main PS board
Pin 15 -44 volts Should be -53v, may be -44 on DBT. If not or very low could be bad 1 ohm fusable on main PS board
Pin 16 0 volts Not sure but probable -1.5v, may be 0v on DBT. Tied to Pin 17 only.

Pin 28 -7volts Should be +53v, may be +44 on DBT. If not or very low could be bad 1 ohm fusable on main PS board
Pin 27 +41 volts Should be +50v, may be +41 on DBT. If not or very low could be bad 7a fuse F04 on main PS board
Pins 26 through pin 19 -41 volts
Pin 22-23 (Last Check )Should be ~ 0 Volts. If high DC voltage here, then bad components on driver board or outputs.
Pin 20-19 Should be -50v, may be -41 on DBT. If not or very low could be bad 7a fuse F05 on main PS board
Pin 18 -44 volts Should be -53v, may be -44 on DBT. If not or very low could be bad 1 ohm fusable on main PS board
Pin 17 0 volts Not sure but probable -1.5v, may be 0v on DBT. Tied to Pin 17 only.

If all above is now OK -- If everything measures sort of OK to the above reading and the unit still goes into protect on DBT, verify that there isn't any DC voltage present on either of the fuses on the protect board. If there isn't any major DC on either fuse, then shut the unit, turn speaker selector switch to off, and measure the resistance from each one of the fuses on the protect board to ground. It should read 3.3k ohm or higher. If it shows anthing close to a short, then you have a problem on the protect board.

If the resistance value reads OK, then Set the DC balance pots to mid point, and the bias pots to mid position as well. Then power up the unit with using just headphones. See if protect disengages and you get a green light. Sometimes the decreased voltage from DBT will keep the protect engaged.
If you still have protect light problems after this, check the protect board for leaky or open electrolytics, leaky diodes, or bad resistors/transistors.

Tom
 
Tom B said:
Did you replace all of the fusable resistors on the driver board?? They are quite notorious for big value swings. They are usually the 180 ohm, 150 ohm, 10 ohm and 4.7 ohm.
Not that I believe in changing out parts just to change them, but I have had a lot of issues with fusable resistors on the sansui 8080db, 9090 and 9090db.
Click to expand...

Yes those and then some. Thanks for the correct voltage readings that you posted. That is the first time I've seen that posted. It will probably help me and a lot of other people. I did go ahead and remove the f2624 board from my 9090DB to try it and at least satisfy me that it is a bad card and not something entirely different. I will try all this in the morning.
 
Here is a game changer! I installed the F2624 board from my 9090DB and I get the exact same symptoms which to me rules out the driver board. Now I will concentrate on the power supply etc. Any suggestions! The 1 ohm resistors test OK.
 
crkeith65 said:
Here is a game changer! I installed the F2624 board from my 9090DB and I get the exact same symptoms which to me rules out the driver board. Now I will concentrate on the power supply etc. Any suggestions! The 1 ohm resistors test OK.
Click to expand...

How do the voltages match up to the above?
Do you have DC voltage on either of the 2 fuses on the protect board?
 
Thank You, Thank You, Thank You, Tom B. Thanks to your voltage recommendations, I traced the pin 5 and pin 28 voltages one the driver board back to Tr01 on the power supply board. Tr01 was open. I found a suitable replacement from some of my other junk and now I am setting offset and bias. Also thanks to LBPete and DougBrewster for your replies and your interest.
 
Yes, Happy endings where another vintage unit is brought back to life, are always great.
 
Tom B said:
Yes, Happy endings where another vintage unit is brought back to life, are always great.
Click to expand...
You just don't know how happy this happy ending is. I was losing sleep, having headaches,and stressing my very patient wife over a hobby that is usually so much fun and challenging. But the satisfaction makes up for all that. Also thanks to Doug Brewster for his instructions in a previous post on changing my pointer lamp. And thanks to LBPete for ALL his previous posts.
 
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