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

[Skywatcher]

Hmm, well yeah that does make sense. I actually just compared the delay to my AU-7900 which is around 3 seconds. I think I will switch those 2 resistors around again and try it. I can't see a factory update having 2 resistors switched around, especially when they are so different in resistance but i could see the schematic being wrong. Maybe the board is marked incorrectly but it would have affected more than just my receiver. Makes it extra hard to figure this stuff out when you cant trust the schematic or the board......

The fact that you saw it means you look at details. That's good. You trust what is already on the board (factory), and where it is at on the board regardless of what is written. I always take lots pf photos before doing anything. It has saved my butt many times. Sansui isn't the only brand that has it's share of "mistakes" in documentation whether it be in a schematic, or even parts list(s). Typos happen. Having experience is where you learn of these mistakes. Many of the good folks here have that very experience. Always ask before making any "changes".

 

[Skywatcher]

I switched around those 2 resistors and now the relay delay is more like 6-7 seconds. It also seems to be a softer start if that makes sense. The relay doesnt click as abruptly and the green light kind of fades on. II guess going from a 150K to a 1.8K really speeds up the protection circuit!

If that is while it is on a DBT, the "soft" kick would be normal since the voltages are lower. I'd ask someone with more experience on this particular model though, just to be sure.

 

[Skywatcher]

I switched around those 2 resistors and now the relay delay is more like 6-7 seconds. It also seems to be a softer start if that makes sense. The relay doesnt click as abruptly and the green light kind of fades on. II guess going from a 150K to a 1.8K really speeds up the protection circuit!

Resistance is very important, especially in a delay circuit.

 

[Hyperion]

You should note that swapping those two resistors around (so that R612 is 1.8KΩ) will very likely apply far too much voltage to C622, 220µF, 10V - as it is being fed from a +33V regulator. I am guessing the original RC combination is intended to allow the regulator(s) to come up and stabilise before speaker relay activation. The values seem to me to be logically positioned according to the schematic especially w.r.t. voltages - and 'adjacent components transposed' would be a common and difficult to spot original assembly error.

Of course you may have increased the voltage of the replacement C622 (to 35V ?), in which case that part of my caution won't be an issue. Overall, I think speculative changes to resistor values in protection circuits is not a good idea.

 

[dsmart705]

I wasnt really tying to speculate but maybe put too much faith in the schematic....also a lack on experience with the nuance of vintage gear. I did replace C622 earlier but used the same value as a replacement. I will check that cap to make sure it has not failed due to my mistake. I only cycled the power afew times with the resistor switched around so it didnt have much exposure to the higher voltage but still I will confirm its good before moving forward. Its not really a "soft start", just not as aggressive as when I had the resistors switched around. It takes 7 seconds to come on after I switch the power on and the green light fades in. Not sure if that's normal?

 

[dsmart705]

Hoping somebody with a G7500 could confirm the correct resistance for resistor R616 and R617. The schematic lists them both at 3.3k Ohms but both of my original resistors are around 1.8k Ohms. Could this be another case of incorrect information on the schematic?

 

[kevzep]

Hoping somebody with a G7500 could confirm the correct resistance for resistor R616 and R617. The schematic lists them both at 3.3k Ohms but both of my original resistors are around 1.8k Ohms. Could this be another case of incorrect information on the schematic?

If the original resistors were installed at the factory, then just leave them, there were many non documented changes to those boards, you cannot believe the schematic.
The only questions are, "does it work" or "does it not work" and did it leave the factory like that. If the circuit is working, leave it!!
Do not obsess over what the schematic says, there are so many inconsistencies its not worth it

 

[dsmart705]

Roger that, ill put those ones back and leave it alone. I guess if working on these things were straightforward and easy everyone would be doing it! Thanks for getting back to me about this kevzep

 

[kevzep]

Roger that, ill put those ones back and leave it alone. I guess if working on these things were straightforward and easy everyone would be doing it! Thanks for getting back to me about this kevzep

Yeah, look honestly, these particular amps are a minefield of "gotcha's" you need to be very careful, I seem to be able to always get them working, and the way I do it is to be minimally invasive. Do NOT change anything other than E-Caps unless you really have to.
Check for components damaged by the corrosive glue, remove corrosive glue.
Hopefully it is the one which has the standard relay and not the stupid miniature Fujitsu thing they used sometimes, but replace that too.
Replace the trimmers.
Nothing else to do....
Light the fuse, dial in the Bias and DC Offset job done.

I can remember what the original issue was, so I will read back over the thread and see if I can spot anything and report back..

 

[dsmart705]

Just going back to double/triple check the resistors I did change and I found another anomaly. R43 and R44 are listed as 220ohm 1/2W resistors in the BOM. The schematic lists R43 as 82ohm 1/2 resistor which the original one confirms. R44 however is not listed in the schematic with an ohm value. Mine is OL so it needs to be changed but the colour bands are very hard to make out due to charring and age. Another fellow mentioned earlier in this thread that it was a 220ohm 1/2w resistor but from what I can tell no possible combination of the colours,from what I can see, would ever make it be so. Also the fact that R43 was different than it's BOM spec makes me wary. Hoping someone could confirm the correct resistance for R44. I currently have a 220ohm resistor installed.

The unit powers up and runs. I hooked up some speakers and it works. There is some cut-in/out and scratchiness when switching buttons and my bluetooth adapter hooked up to aux I only get sound from the left channel but I think all the switches and pots need to be cleaned so im not to worried about that. Using the tuner I get sound from both channels and its sounds pretty clean even in my basement and without an antenna. The unit was sitting for years after it failed and the gentleman who owned it since new said he never pulled the cover off which was pretty apparent with the amount of dust. It did look like someone along the line may have though as afew transistor were soldered alittle funny and there was a sticker by the main fuse with its rating as opposed to the silkscreen.

Is there a trick to cleaning to pots and buttons on this unit or is it just patience, care and elbow grease?

 

[Skywatcher]

Just going back to double/triple check the resistors I did change and I found another anomaly. R43 and R44 are listed as 220ohm 1/2W resistors in the BOM. The schematic lists R43 as 82ohm 1/2 resistor which the original one confirms. R44 however is not listed in the schematic with an ohm value. Mine is OL so it needs to be changed but the colour bands are very hard to make out due to charring and age. Another fellow mentioned earlier in this thread that it was a 220ohm 1/2w resistor but from what I can tell no possible combination of the colours,from what I can see, would ever make it be so. Also the fact that R43 was different than it's BOM spec makes me wary. Hoping someone could confirm the correct resistance for R44. I currently have a 220ohm resistor installed.

The unit powers up and runs. I hooked up some speakers and it works. There is some cut-in/out and scratchiness when switching buttons and my bluetooth adapter hooked up to aux I only get sound from the left channel but I think all the switches and pots need to be cleaned so im not to worried about that. Using the tuner I get sound from both channels and its sounds pretty clean even in my basement and without an antenna. The unit was sitting for years after it failed and the gentleman who owned it since new said he never pulled the cover off which was pretty apparent with the amount of dust. It did look like someone along the line may have though as afew transistor were soldered alittle funny and there was a sticker by the main fuse with its rating as opposed to the silkscreen.

Is there a trick to cleaning to pots and buttons on this unit or is it just patience, care and elbow grease?

The schematics will only have the value on one channel of the amp, preamp, or any board with 2 channel symmetry, and the other channel won't for the same component. Left channel marked, right channel not marked. R43 - Left channel, R44 - Right channel (not marked) You better figure out why the resistor is open before just throwing one in and watching the smoke fly. There is a reason it toasted.

 

[kevzep]

Hoping someone could confirm the correct resistance for R44. I currently have a 220ohm resistor installed.

Whats in R43, as this resistor is the same value as R44.

 

[kevzep]

As an FYI the R43 and R44 on a G7700 is 22Ω 1/2 watt....

 

[dsmart705]

Im not totally sure why this amp fried and neither was the previous owner but there were many problem that I believe i've already sorted out. Really good tip on reading the schematic. I didnt even think they they would be listed like that for left and right channel but that is very helpful information. The case of these 2 resistors is tricky as the colour bands do not look the same but one is pretty charred so the heat might have altered the way the colours look. Im going to order 2 new 82ohm 1/2w resistors......hopefully that will be the last piece to the puzzle.

 

[kevzep]

Im not totally sure why this amp fried and neither was the previous owner but there were many problem that I believe i've already sorted out. Really good tip on reading the schematic. I didnt even think they they would be listed like that for left and right channel but that is very helpful information. The case of these 2 resistors is tricky as the colour bands do not look the same but one is pretty charred so the heat might have altered the way the colours look. Im going to order 2 new 82ohm 1/2w resistors......hopefully that will be the last piece to the puzzle.

Did you look at what R44 was??

 

[dsmart705]

The original R44 resistor measures 82 ohms which is the same as the schematic shows. I guess R43 is supposed to be the same but as it was open and the colours were damaged I put a 220 ohm resistor in its place. This was done on my first repair attempt when I started this thread long before I was informed by you guys to not trust the information provided by Sansui. Also before I knew that they only labelled one channel in the schematic with values. Its been a steep learning curve but so far I’m enjoying the challenge. In the end having a working G7500 would just be the cherry on top

 

[Skywatcher]

The original R44 resistor measures 82 ohms which is the same as the schematic shows. I guess R43 is supposed to be the same but as it was open and the colours were damaged I put a 220 ohm resistor in its place. This was done on my first repair attempt when I started this thread long before I was informed by you guys to not trust the information provided by Sansui. Also before I knew that they only labelled one channel in the schematic with values. Its been a steep learning curve but so far I’m enjoying the challenge. In the end having a working G7500 would just be the cherry on top

Are the first 3 bands on that resistor Grey-Red-Black? The 4th band is probably gold (5% tolerance). Get a color chart if you don't have one.

 

[kevzep]

The original R44 resistor measures 82 ohms which is the same as the schematic shows. I guess R43 is supposed to be the same but as it was open and the colours were damaged I put a 220 ohm resistor in its place. This was done on my first repair attempt when I started this thread long before I was informed by you guys to not trust the information provided by Sansui. Also before I knew that they only labelled one channel in the schematic with values. Its been a steep learning curve but so far I’m enjoying the challenge. In the end having a working G7500 would just be the cherry on top

Okay well that agrees with the schematic I have here showing R43/44 as being 82Ω. So slip an 82Ω in there and you'll be good, you could use 100Ω it wont make much difference, they are there for current limiting.

 

[Skywatcher]

Im not totally sure why this amp fried and neither was the previous owner but there were many problem that I believe i've already sorted out. Really good tip on reading the schematic. I didnt even think they they would be listed like that for left and right channel but that is very helpful information. The case of these 2 resistors is tricky as the colour bands do not look the same but one is pretty charred so the heat might have altered the way the colours look. Im going to order 2 new 82ohm 1/2w resistors......hopefully that will be the last piece to the puzzle.

The color bands fade (change color) with heat so that would be "normal". As kevzep pointed out, it's 82 Ohm (as if there was any doubt with Kev ). Since you mention this unit "fried" before, it would seem the root cause wasn't addressed in any previous repairs. It would be prudent to check every component in that "chain" to find out why it had drawn enough excessive current to toast the resistor in the first place. It could be a resistor somewhere has drifted high, or a diode out of spec, a ceramic cap, bad solder joint, a "rogue" transistor, or even a problem with the input pair of FETs or it's circuit, or a combination of things. If you don't find the root cause, it is inevitable that it will become "fried" again at some point in time.

 

[dsmart705]

I received the unit from a co-worker who purchased it new back in the 70's, it stopped working for him afew years ago and has sat ever since I managed to convince him to release it to my care. The problems I found were a burnt emitter resistor on the right channel R54, R43, R35 were also burnt. I didnt test all the output and driver transistors but instead just went ahead and changed them all (TR602/603/604) (TR1/2/3/4/5/6/7/8) (TR15/16/17/18) (All 4 output transistors). Changed out all the trimmers and most of the caps on the main power board other than the big filter caps. I did change some other resistors but have stopped doing that on Kevzep's advice. Today I hit all the pots and switches with deoxit and put it through the paces. All the scratchiness and channel related issues are gone and im happy to say I feel its fixed and partially restored. I still have to replace R43 with the correct resistor but felt the 220ohm one would be alright in there for testing purposes. Thanks again for all the help, another G7500 will live on. I havent heard too many high wattage Sansui's but this thing has a really "big" sound to it...its more upfront than my au-7900 (although that one is all original so it might benefit from some love). Thanks again to everyone for all the help with this.....now I need to find another broken Sansui!