8080DB low output one channel

LBPete

Rolling Along
Subscriber
Hi,
I have an 8080DB that has very low output from one channel. When I got this receiver, it was blowing the main fuses on the power supply board. Traced that to two shorted output transistiors. Replaced them and got good sound from one channel and very very low sound from the other. Disassembled the front pannel and cleaned the daylights out of all the switches and pots with deoxit d5. Still one channel. While playing with the volume and balance, there was a pop and both channels started working.

Turn it on a few days later, only one channel. Repeated the cleaning and cycled all switches and pots till my wrists were sore. No change. The power meters aren't working either. I've also tried swapping the output transistors right to left but the problem stays on the same side.

I've read through a lot of the discussions in this forum on the Sansui one channel curse. Unfortunately the repair manual I have doesn't have a diagram for the dolby board or the push switch board. It also doen't show how the power meters interface with the output.

Overall this is a very clean unmolested receiver. I would appreciate any ideas the group may have.

Thanks,
LBPete
 
You need to look at the plug in amp board. With shorted outputs on this one you sould also look for shorts or opens in the driver section. The 8080DB is similar to a 9090DB so do a search here for the 9090DB.
 
I've looked through the posts for the 9090db. They are pretty general. I don't see anything visually wrong with the driver board. It looks clean, undamaged wiht nothing lookig burned or heated up. Could there be a fault with the driver board that is intermittent?
 
I've been using a CD player into the aux terminal. I'm not sure what there is to gain by pulling the jumpers and connecting another source. I have jumped a pair of test speakers right off the audio output from the power board so I know it's not in the protector relay circuit or speaker select rotory switch.
 
This would seperate the receiver into parts so you can divide and conquer the problem. Does it affect the amp, the pre amp or both?
 
Usally if the output transistors short the several of the resistors on the plugin driver board go bad. Test all the resistors located betweem the heat sinked transistors and the large connector, you will probably find some bad ones. Were the meters working before you had this problem? The meters are connected off of the outputs. I am out of town but will see if I can get a diagram to you of the protection board.
 
I don't think the protection board is the problem. I pulled the two speaker fuses on the protection board before the relay to isolate the board and then jumped test speakers directly to the audio output on the power supply board. Sill very low sound from one channel.
 
I only suspected a problem with the protection board due to the fact that the meters were not working. I think the low output is probably caused by the driver board. I might have that diagram with me if so I will email it to you. Check all those resistors, the driver board outputs are tied to the output transistors by 4.7 ohm resistors, these usally go bad when the outputs short. If they are of a high resistance you will have a low output.
 
another thought on the meters not working. Is the dolby switch pressed in? Far right push button, if it is and the dolby switch is in the off posistion the meters will not work.
 
I've tried the dolby switches in every position. In or out, no meters. Thanks for the help. I'll take a look at the resistors tomorrow. The factory diagram in the shop manual has the schematic for the protector board. It's the dolby board and the push switch board, I think numbers 2654 and 2655 that aren't in the shop manual.
 
I have yet to see a 9090 or 8080 or 7070 (all very very similar in design) blow only the outputs. There are a slew of fused resistors (mainly the 4.7-ohm base stoppers from the driver to the outputs) that almost always open up, and quite often a driver or diode for the current limiting gets shorted too...although if the relay is engaging you may not have shorted a driver or diode, since that would put DC on the output and the protection would kick in.

Your work be not finished... :scratch2:
 
This may not be relevant but - if the 8080 (like the 7070) has the driver and meter circuits on plug-in boards that sit vertically on the mother board (big main board in the middle of the unit at the bottom - power supply and output) then you may want to flip that baby over and inspect (with a magnifying glass) the circuit traces and circuit board panel) for signs of a crack on that board.

Your description of the problem is nearly identical to mine - one channel very weak; started working once then died again.

The one channel that was low was also very distorted.

I'll attach a pic of my board before repairs.

The problem is that if you stack anything heavy on the top of the unit it can push those two plug-in cards down into the mother board and stress it.

Paul
 
One other possibility, there are two versions of the dolby switch/push button assy. One has a Printed circurt board (early designs) connecting the two boards, the other has a solid ribbon wire. If yours has the printed circuit there is a high probability of having broken, especially if the two boards have been moved. This could cause both problems. (low output no meters). If you have the service manual for the 8080/9090DB the dolby board is on the same sheet as the tuner.
 
Thanks everyone for the information. This 8080DB must be a later one. It has the ribbon connector between the push switch board and the dolby board. The serial number is 8261000099.
I've been pretty busy at work so it may be a couple of days before I have a chance to chase down all the leads provided. I'll post the progress.
 
Actually - That is an early build of October 1976 and is the lowest production number on the Sansui list for 8080DB and is receiver #99 built on that day. Edit Note: There should be only 9 numbers in the serial check it again - I think maybe there is one zero too many in there.
 
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Update. Based on Morden 2004's suggestion, I checked the driver and power supply board carfully for cracks and I don't see any. Then moving on to Echo Wars resistor suggestion, all the 4.7 ohm resistors test ok but on carefull inspection, a 150 ohm resistor, # 43 looked a little burned and tested open. I've checked the other "fuse" resistors and they are all OK. I don't have a resistor of that value, so I'll have to pick up a couple.

As far as it being an early production 8080DB, it is in very good orginal condition. It's pretty clean and there is no evidence of any one messing with it. I'll take some pictures if any one is interested.

CDfixer, did you ever get the tuning meters you were looking for? I have a pair out of a 7070 if you are interested. They are just bare meters, no circuit board.

- Pete
 
LBPete:

I have registered your 8080DB in the database as : 826100099 - with a zero removed as suggested by RnR.

If this is not correct email me.

Paul
 
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