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Sansui G-7500 no power/sound with burnt resistor on F2890 board

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dsmart705

Well-Known Member
Hello,
I have a G7500 that is in need of help. The unit will not power up or produce any sound (meter lights still turns on and the tuner appears to be working) so I pulled off the cover and after blowing out 40 years of dust I gave it a quick visual inspection. There is alot of brown goo stuff around the cluster of caps near the relay. It almost looks like the glue has corroded some of the resistors/caps and wiring in that area. Im not sure if that was what did my amp in or its just superficial but my intent is to clean it all off. The resistor that is burnt and R36 and is charred beyond being able to tell what colour bands it has but it looks to start with orange and end with gold. R78 which is one of the 5w.33ohm resistors is open circuit but the other 3 are testing good. The board itself appears to be unharmed. I cant see anything else that looks bad but havent metered out everything on that board yet.

from my research repairing this particular board may be over my head but I would like to give it a try. Ive tried to track down the parts I need but have not come up with anything solid. I was hoping for some guidance on the repair or if someone feel like a challenge I could always send out the board. Thanks

David
 
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If you have an open emitter resistor, then you probably have bad output transistors, as well as bad parts on the amp board like resistors and driver or other transistors in that channel.
 
dsmart705 said:
Hello,
I have a G7500 that is in need of help. The unit will not power up or produce any sound (meter lights still turns on and the tuner appears to be working) so I pulled off the cover and after blowing out 40 years of dust I gave it a quick visual inspection. There is alot of brown goo stuff around the cluster of caps near the relay. It almost looks like the glue has corroded some of the resistors/caps and wiring in that area. Im not sure if that was what did my amp in or its just superficial but my intent is to clean it all off. The resistor that is burnt and R36 and is charred beyond being able to tell what colour bands it has but it looks to start with orange and end with gold. R78 which is one of the 5w.33ohm resistors is open circuit but the other 3 are testing good. The board itself appears to be unharmed. I cant see anything else that looks bad but havent metered out everything on that board yet.

from my research repairing this particular board may be over my head but I would like to give it a try. Ive tried to track down the parts I need but have not come up with anything solid. I was hoping for some guidance on the repair or if someone feel like a challenge I could always send out the board. Thanks

David
Click to expand...
The schematic and service manual are at HiFi Engine.
 
Thanks for the replies. I did have a look at the schematic so im starting to build a parts list. I cant seem to find any reference to the type of resistor R36 is. Its not mentioned in the parts list. Is there a standard "replace while in their list" regardless of what my specific issue it. From what ive been reading it seems like people are doing the

1. Bias trimmers - VR01, VR02, VR03, VR04 replacement type ?
2. Output transistors - TR601, T602, TR603, TR604 (confused with these as there are some on the F2980 board and the same ones listed for the F2981 boards) replacement type?
3. Emitter resistors....that's an easy one
4. Capacitors (dont really want to shotgun replace those things if its not neccessary to get it running)

My amp has all the fuses intact, I have read through alot of threads on the matter but cant really come up with a definite parts list for the stuff I need. My experience with the components is fairly limited so excuse my ignorance. I have an interest in this sort of thing and want to "get my feet wet" and gain some experience which is why this project is happening.
 
Sansui doesn't list all the resistors in the parts list, but does show them on the schematic. R36 is a 150 ohm 1/4 watt resistor
I would suspect TR08, TR10, TR16, TR18, and the 2 outputs TR601, and TR602.
Also check R44 which is a 220 ohm 1/2W resistor, R38 10 ohm 1/2W resistor and the trim pot VR04 to make sure they aren't open. You should also check the Bias diode STV3 which is mounted usually on the heat sink, or on one of the driver heatsinks. Those diodes can short or open. (they are 3 diodes in series internal)
 
Tom B said:
Sansui doesn't list all the resistors in the parts list, but does show them on the schematic. R36 is a 150 ohm 1/4 watt resistor
I would suspect TR08, TR10, TR16, TR18, and the 2 outputs TR601, and TR602.
Also check R44 which is a 220 ohm 1/2W resistor, R38 10 ohm 1/2W resistor and the trim pot VR04 to make sure they aren't open. You should also check the Bias diode STV3 which is mounted usually on the heat sink, or on one of the driver heatsinks. Those diodes can short or open. (they are 3 diodes in series internal)
Click to expand...


Thanks so much for the guidance,I know im in over my head with this but honestly its not worth it to me to pay $400/500 to have this thing rebuilt..there is nobody in my area who is still doing this type of thing and the shipping alone would be a killer. You were right about R44 and R38....they were both open. I also found R54 was open. so far my list is

All 4 emitter resistors, TR601/602 output transistors for both channels, R36/38/44/54, All 4 trimpots (no sure what part to get yet), TR08/10/16/18.

Part numbers ive seen so far taken from actual component are. Hopefully they are all available or there are compatible replacements.

Emitter resistors x 4 - 0.33K Ohm 5w

R36 - 150 ohm 1/4w
R38 - 10 ohm 1/2w
R44 - 220 ohm 1/2w
R54 - 220 ohm 1/2w

Left Channel
TR601 - C2581N 02Y
TR602 - A1106 020
Right Channel
TR601 - A1106N 8DY
TR602 - C2581N 8NY

TR08 - A939 V
TR10 - C2071 V
TR16 - NEC D381 2M7YC
TR18 - NEC B536 2M8IC

VR01 - 8D5d 100ohm B
VR02 - 8D5d 100ohm B
VR03 - 8D5d 100ohm B
VR04 - 8D5d 100ohm B
 
Well I went ahead and ordered everything (doubles of some just incase). Managed to find all the parts using the help of others who have gone down the same path. Also ordered some of the capacitors that are in the brown goo area and may have been damaged. I will report back once I have everything replaced and will no doubt have more questions about setting the trimmer/bias pots and how to setup the amp. Thanks for all the help
 
dsmart705 said:
Well I went ahead and ordered everything (doubles of some just incase). Managed to find all the parts using the help of others who have gone down the same path. Also ordered some of the capacitors that are in the brown goo area and may have been damaged. I will report back once I have everything replaced and will no doubt have more questions about setting the trimmer/bias pots and how to setup the amp. Thanks for all the help
Click to expand...

Good luck... my rebuild on my working unit went well. I assembled a cap list on the thread.

http://audiokarma.org/forums/index.php?threads/sansui-g7500.835199/

I love my G-7500.
 
dsmart705 said:
Well I went ahead and ordered everything (doubles of some just incase). Managed to find all the parts using the help of others who have gone down the same path. Also ordered some of the capacitors that are in the brown goo area and may have been damaged. I will report back once I have everything replaced and will no doubt have more questions about setting the trimmer/bias pots and how to setup the amp. Thanks for all the help
Click to expand...
Not sure if you know this but ebay is not a great source for transistors, especially outputs and drivers. There are many counterfeits being sold. It is usually better to get your transistors from a place like mouser. That being said, those original outputs and drivers are not being made anymore, so you probably need to get on-semi replacements.
 
So I went ahead and changed all the parts that I suspected were faulty and some preventative maintenance ones. Powered it up on the dim bulb and it seemed good. The bulb light up and then dimmed down to almost nothing. After 10/15 seconds the protection relay clicked and power light came on!! I was quite pleased with myself and tested the power 3 or 4 times to make sure it wasn’t beginners luck. Anyways I powered it down and went upstairs for a minute. Came back downstairs and was going to attached the base plate so I could rest it down to set the bias and offset but decided to test it one more time. This time the relay did not click and no power light. The dim bulb still acts the same and there is no heat/smoke or burnt parts. Anyways I’ve given up for the day but thought I’d post up an update. The bournes trim pots were all set in the middle (around 105ohm) which was the same settting as the old ones I removed. Hope I didn’t fry something else but based on the dim bulb everything seemed good.
 
Two of those trimmers are for DC offset VR01/VR02, nothing to do with bias, those need to be set somewhere in the middle.
BUT, the other two VR03 and VR04, must be set to minimum. These DC high speed amplifiers are very touchy with regards to bias and its easy to overdo it and then they'll run away on you and then its all over.
NEVER set bias trimmers to 50% and turn the unit on. ALWAYS start from minimum.
Did you monitor the bias when you powered it up?
Did you monitor the output transistor heat when you powered it up, where they getting hot?
 
When I initially turned it on I was touching the heat sink on the output transistor (all new 200w replacements) and the transistors i replaced tr16/18 tr17/15 tr06/08 tr05/07 and nothing was creating any heat. I did not monitor the bias when I switched it on but it was never on for more than 20/30 seconds at anytime. I only turned it on to check for shorts with the dim bulb tester (100 watt bulb). I measured all the original trimmers before I removed them from the board and they were all around 100ohms. The new ones measured the same so I figured it was close enough to switch on momentarily. I’m very new at this so I should have done more research before powering it up but I have so little time to work on it that I was rushing to test it. Hopefully nothing was damaged but like I said there was no sign of anything catastrophic occurring in the short time the power was on
 
Where the bias was before is meaningless as you are fitting new transistors, correct protocol is to start from scratch.
Its never a good idea to rush things, especially if you are new to this and not experienced.

You might have got away with it, but take more care and ask questions.
 
Those are wise words, in the back of my mind I knew I should wait until I had more time. If the relay is not coming on now is it fair to say that something is fried or could the bias just need to be adjusted down to bring it out of protection?
 
dsmart705 said:
Those are wise words, in the back of my mind I knew I should wait until I had more time. If the relay is not coming on now is it fair to say that something is fried or could the bias just need to be adjusted down to bring it out of protection?
Click to expand...
Turn VR03 and VR04 all the way ANTICLOCKWISE and try it.
 
I adjusted vr03/04 down to zero and switched on the amp (still on dbt) it came out of protection. Let it sit for 30 seconds and checked for heat anywhere which was good. Dialed in 10mv on each channel and let it sit for a minute. Checked again and the bias had risen to 12mv on each side so I dialed it back down to 10mv. At this point I noticed tr602/603/604 getting warm as well as the 2 resistors r605 and r606 which are in the same area. I turned the amp off and let it sit for as long as it took me to write this message (5 minutes). Just turned the amp back on and the relay won’t click out of protection. Checked the bias again and it’s at 6mv on each channel but it seems to slowly rise.....not sure how high it will go. Also not sure how hot those parts should get but they are pretty warm. The 2 resistors are hot enough to be uncomfortable to touch and the transistors are not far behind.

It seems like if it sits for awhile it will come out of protection but after it’s been switched on for afew minutes it will not come out of protection if the power is cycled.
 
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Do not keep adjusting the bias as it sits there idling.

Turn it on, let it sit for 5mins, then adjust to the factory setting, and then leave it, you're done. Do not keep adjusting it. The bias changes with the STV diode thermally tracking the heat in the output transistors, its supposed to do that.

You need to set the DC offset, if its in protection, you need to measure the output of the amplifier before the relay as the speaker terminals are disconnected via the relay.
 
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Ok ill do the bias procedure again and also set the DC offset. Is it normal for those other parts to get hot when the unit is just idling?
 
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