View Full Version : Sansui G-7500 swap meet find, flame out!!!


Plomaman27
09-22-2011, 06:58 PM
Hi all...HELP SOS HELP!
I bought a nice looking Sansui G-7500 receiver from a vendor at a swap meet for $60 and thought I made a good buy even though the owner said it had a blown fuse.... Im a decent tech so no problem, probably a shorted amp or filter cap or even a brideg diode.

Well...(the best laid plans as they say:) I verified that the main fuse was blown so I replaced it with a 7A 250V slow blow fuse. (There was a piece of clear cellophane tape with the size fuse to use for the selected input voltage 120v) All's good right?....you'd think....that is until I turned the unit on. In a matter of a few seconds, the driver board started to smoke and a small flame came out of a resistor lead hole where a now half-burned-away resistor is. I turned the switch off as fast as I could but the damage was fast.... It doesnt look that bad after I cleaned the board with alchohol. there's a single burn hole and loosened pad with a short piece of the resistor lead still connected. The transistor TR19 fried too and a few resistors in close proximity to the toasted one got hot enough where you cant read the color codes.....although they still ohm good. I think I can make all the necessary connections without having to solder to the burnt pad when I replace the burned components. A yellow DC supply wire also turned black at the entry point to the board near the fried components.

IMHO, this series of events was caused by one or more shorted ouput amps. Obviously, I need to ohm out the power transistors on both outputs but im not exactly sure how to go about doing that without removing the back mounting plate. Problem is there's a number of short wires attached to the components on the back plate that go to various parts of the unit. Is it possible to test the power amps without remoivng the back of the unit?

Heres a list of the burned components that fried on power up:

1. TR19 (transistor) according to the schematic, this is part of an audio protector circuit that activates to cutoff damaging audio signals from reaching the speakers when one of the power amps draws too much current (as when shorted).

2. Base bias resistors to TR19.

3. Yellow wire to node 41 turned black right where it solders into the board.

ANY HELP OR TIPS WOULD BE GREATLY APPRECIATED...

kevzep
09-22-2011, 08:06 PM
Nice amp, and certainly it will be easy enough to sort it out......They really kick ass when they are running right, you will not be disappointed...:thmbsp:

First thing though, its futile replacing a mains fuse which has blown, its blown for a reason, and most often because there is a short circuit in the main DC secondary supply, and as you said, most likely the output transistors....

I would imagine if you measured between either the main - or the + DC supply, you will find it shorted to ground.....

Putting that fuse in and powering it up has probably taken you back a few steps.....Fires are bad.....:thumbsdn:

Ive never worked on a G-7500, but I know the circuit well, its very similar to the G-7700 which I do have a lot of hands on experience with.

Okay, so you have established the supply is shorted to ground?
Well yes take the output transistors out, you will have to anyway....
Check the emitter resistors, for some reason with this circuit, I have seen many emitter go open and take out the output section.....
Note where they go as far as polarity goes.....Im not sure if they are like the 7700 where there is actually a circuit board or not, but if not, take a note...
In fact you can use your camera and take lots of pictures so you can see where and how things hook up.....

Also check the driver transistors, and all other transistors through the driver stage back as far as TR05 on the left channel and TR06 on the right channel....

Be careful with the STV-3H diodes, if you break a wire off them, you have a paper weight.....you cannot get these anymore...they are for thermal regulation of the bias current.....VERY Important...
You will see them attached to the heatsink somewhere, and they usually have white flexible insulation on the long legs back to the driver board......

Testing transistors, use the DMM on diode test, and measure the forward voltages....Google testing transistors and you will find something.....
Its easier than me typing out a tutorial here......But the most reliable way is to use the Diode method....

The driver transistors are TR15,16,17,18......the transistor which activates the protection is TR19,20.......but I have never seen one of these fail.....
So pull the drivers, they are in the line of fire when the outputs fail.....

Let us know how you get on......

nosirrah
09-22-2011, 10:09 PM
What he ^ said, and welcome to AK!

Fhamre
09-22-2011, 11:26 PM
I just finished a G-7500 but I am no expert. You are getting expert advice.
My two bits. Build a dim bulb tester and never start up a new to you or sick amp/receiver without it. Cuts down on the flame size.

anderslober
09-22-2011, 11:49 PM
Great unit........I totally agree with Fhamre.....build a DBT. I learned this the hard way.

Kevzep should be able to help you....along with a few others. Please post some pictures of the unit and the problem...........

Welcome to AK.

Plomaman27
09-23-2011, 12:43 AM
Thanks kevzep! My thoughts exactly, I should have NEVER put in a good fuse and powered up without checking the power amps for shorts! Arrrgh... in the electron world haste surely makes waste. Luckily though, it looks like the flame damage was contained to one pad and through-hole. not anything that I cant work around.

I found that both of the driver xsistors on the heat sinks are shorted. Also TR601 power amp is shorted out as I was able to get my DMM probles to touch the three parallel leads. I havent checked TR602 yet because I cant get to it.

Why do both A and B outputs have the same designators for the power amps ie. TR601 and TR602?

The resistor that took the brunt of the amperage was a 1500 ohm 1/4 watter...it partially vaporized! TR19 didnt fare much better with the base and center leads burned loose from the xsistor.

Would you know the best way to access the power amps? It looks like I'll have to remove the back plate of the receiver to do that.. correct?

Thanks for the tip on checking the xsistors in the driver back to TR05 and TR06...very helpful! I'll check the outputs of the power supply (+63v and -63v) for shorts to ground. Will update with photos as I go along with the repair.

kevzep
09-23-2011, 02:31 AM
Thanks kevzep! My thoughts exactly, I should have NEVER put in a good fuse and powered up without checking the power amps for shorts! Arrrgh... in the electron world haste surely makes waste. Luckily though, it looks like the flame damage was contained to one pad and through-hole. not anything that I cant work around.

Good to hear you realized the old proverb, and as a couple of other members suggest, it might be a good idea to build a Dim Bulb Tester.....
I use a variac and ammeter......but with the DBT, you will certainly light the bulb instead of incinerating the amplifier.....

I found that both of the driver xsistors on the heat sinks are shorted. Also TR601 power amp is shorted out as I was able to get my DMM probles to touch the three parallel leads. I havent checked TR602 yet because I cant get to it.

As we suspected.........TR601 is the NPN output transistor 602 is the PNP output transistor, they appear to be mounted on a board F-2981.....They are both for the left channel output.
I would imagine like other Sansui's of similar nature, this F-2981 board will be able to be removed so you you can then desolder the transistors.....

Why do both A and B outputs have the same designators for the power amps ie. TR601 and TR602?

The A and B outputs are driven from the same amplifier, this is how it works, just gives you the option of being able to have another set of speakers in a different area, or add a second set to run at the same time as the speakers on the A output......most manufactures will do this unless its a quadrophonic amplifier where 4 amplifier channels are needed...

The resistor that took the brunt of the amperage was a 1500 ohm 1/4 watter...it partially vaporized! TR19 didnt fare much better with the base and center leads burned loose from the xsistor.

Sounds like a bit of a mess, but not to worry......can be sorted....

Would you know the best way to access the power amps? It looks like I'll have to remove the back plate of the receiver to do that.. correct?
I havent actually pulled one apart before, but Im sure someone who has will chime in, It is possible you may have to pull the back off, Im not sure, perhaps if you post some pics we might be able to see what needs to happen........

Thanks for the tip on checking the xsistors in the driver back to TR05 and TR06...very helpful! I'll check the outputs of the power supply (+63v and -63v) for shorts to ground. Will update with photos as I go along with the repair.

Until you take those shorted transistors out, there will be a dead short in the supply.......

Keep us posted.....and let us know how you are getting on.......

We will have to make some substitutes for the blown transistors because the ones in there are no longer manufactured, so as soon as we know what we need then we can find some to do the job.......

jancumps
09-23-2011, 02:40 AM
At least you can now see where the problem is located :)

kevzep
09-23-2011, 02:47 AM
At least you can now see where the problem is located :)

Follow the trail of destruction huh? :D

Plomaman27
09-23-2011, 11:31 AM
Yeah! I guess you could say that I was "lucky" that the resistor flamed out...I dont know about that one....but it sure made it easy to see what circuit was involved!

Seriously, what an awesome forum Ive found here... Im definately in the right place to find extremely helpful information from folks that have "been there and done that!"

I look forward to keeping you posted:)

Plomaman27
09-24-2011, 01:00 AM
Hello....Would anyone know how a typical Sansui PA blows?... does it open or short?.... and does it typically go emitter>collector...or base>emitter? Thanks

Plomaman27
09-24-2011, 01:18 AM
Great unit........I totally agree with Fhamre.....build a DBT. I learned this the hard way.

Kevzep should be able to help you....along with a few others. Please post some pictures of the unit and the problem...........

Welcome to AK.

kevzep, could you turn me on to some of your knowledge on the DBT? (how to build one and how its used to protect circuits)

Thanks for the help:D

kevzep
09-24-2011, 01:43 AM
kevzep, could you turn me on to some of your knowledge on the DBT? (how to build one and how its used to protect circuits)

Thanks for the help:D

Check your other thread, just posted in there.......

Plomaman27
09-24-2011, 02:37 PM
300395 Well here goes! All I have to do is order the parts.

Here are some pics of the repair that Im about to begin. Flame appeared on the driver board between two heat sink mounted drivers.

This is the left speaker circuit.

*Both driver transistors are shorted out and will be replaced.
*TR19 (spkr protection transistor) fried as well as a 1500 ohm resistor and 27 ohm resistor (voltage divider) on its base.

*The large rectangular white PA emitter resistor is OPEN and will be replaced along with TR19 and 3 or 4 resistors.

*Will R&R TR601 and TR602 power amps mounted on their own PC board on the back panel. To access I just unscrew the back panel, raise it up, unscrew F-2981 PC board and remove the amps.

Plomaman27
09-24-2011, 04:49 PM
hmmmm... Not sure why pics aren't showing... Oh well! I can't wait to get this baby firing on all cylinders... My friend has a pair of magnepans I want to hook up to. Should sound excellent!!!

kevzep
09-24-2011, 06:33 PM
300395 Well here goes! All I have to do is order the parts.

Here are some pics of the repair that Im about to begin. Flame appeared on the driver board between two heat sink mounted drivers.

This is the left speaker circuit.

*Both driver transistors are shorted out and will be replaced.
*TR19 (spkr protection transistor) fried as well as a 1500 ohm resistor and 27 ohm resistor (voltage divider) on its base.

*The large rectangular white PA emitter resistor is OPEN and will be replaced along with TR19 and 3 or 4 resistors.

*Will R&R TR601 and TR602 power amps mounted on their own PC board on the back panel. To access I just unscrew the back panel, raise it up, unscrew F-2981 PC board and remove the amps.

Okay, we need a few more things.......
I would replace the Bias trimmers, look up "bournes multi turn trimmer" I get the 25turn trimmer personally, but you can go for 10turn or whatever, get the DC offset ones too, the trimmers are a weak point in an amplifier this old....and I suspect the bias trimmer may have caused this fiasco in the first place......

TR19, I was trying to find a sub for that, you need a TO-92 (PNP)-120V, hfe=500min, ft=100mhz.........you may have trouble finding the right pinout....if we are lucky someone might chime in with a sub for this...

Make sure the emitter resistor is a 5% tolerance...I would replace both of them, the other one might have been stressed and could be weak now....

Dont even think about firing this thing up without the DBT.........:yes:

Plomaman27
09-24-2011, 10:26 PM
Okay, we need a few more things.......
I would replace the Bias trimmers, look up "bournes multi turn trimmer" I get the 25turn trimmer personally, but you can go for 10turn or whatever, get the DC offset ones too, the trimmers are a weak point in an amplifier this old....and I suspect the bias trimmer may have caused this fiasco in the first place......

Are you referring to VR01 thru VR04? Two of each correct?

TR19, I was trying to find a sub for that, you need a TO-92 (PNP)-120V, hfe=500min, ft=100mhz.........you may have trouble finding the right pinout....if we are lucky someone might chime in with a sub for this...

I have an idea!...I'll try Mouser Electronics... will let you know if I find one there...


Make sure the emitter resistor is a 5% tolerance...I would replace both of them, the other one might have been stressed and could be weak now.......

Where would I find some of these kevzep? What are they called?


Dont even think about firing this thing up without the DBT.........:yes:...

Great advice. The DBT sounds so good compared to "hit or miss". The only time I like to smell smoke is when its coming from my beach fire!:D

kevzep
09-24-2011, 11:21 PM
Are you referring to VR01 thru VR04? Two of each correct?
VR 01/02 are the DC offset trimmers, might as well get those as well, these ones should be 25turn, makes it a lot easier to set the DC offset.....
VR 03/04 are the bias trimmers, and these can be 10 turn, I usually just get all 25turn so they are all the same....



I have an idea!...I'll try Mouser Electronics... will let you know if I find one there...

Yes you should find them there, or Onsemi, they may not be ECB pinout they might be EBC, so you might have to bend the legs around......
I found some at RS components 2SA970, but Im not sure what there agent is in USA...or if they have them available there......

Where would I find some of these kevzep? What are they called?

Wirewound ceramic resistors.... 0.33ohm 5 watt.....they will say 0.33ohmK ther K meaning 5%....


Great advice. The DBT sounds so good compared to "hit or miss". The only time I like to smell smoke is when its coming from my beach fire!:D

Yes, thats they way I like it too.....:thmbsp:

Plomaman27
09-25-2011, 02:09 PM
Hi kevzep, could you shed alittle light on the trimmers I need for this rcvr? I cant find them on my schematic parts list. What voltage, temp, ohms, power ratings should they be? I can easily see the package/shape/pinout but the rest I would be guessing on.

Should I replace the big caps in the power supply? Ive read some posts that suggest "if it aint broke dont fix it":)

kevzep
09-25-2011, 04:04 PM
Hi kevzep, could you shed alittle light on the trimmers I need for this rcvr? I cant find them on my schematic parts list. What voltage, temp, ohms, power ratings should they be? I can easily see the package/shape/pinout but the rest I would be guessing on.

Should I replace the big caps in the power supply? Ive read some posts that suggest "if it aint broke dont fix it":)

Heres the sort of thing you are after, I am assuming the driver board is not mounted vertically, if it is you might have to search out on which mounts differently....
http://nz.mouser.com/ProductDetail/Bourns/3266P-1-201LF/?qs=sGAEpiMZZMtPBa8RTRoEHlH0POhnqxZj
This is a 200ohm one, I think you need 100ohm.....but just search Bournes+multi turn+trimmer+ 100ohm and you should find it....you are after the 500mW ones.....

Ive just done a Big Cap job on my G22000 (as well as a complete re-cap), I think it is a good idea, but in your case I would get the amp working and then look to doing a full re-cap....
It is worth it....
No you dont need to fix something that aint broke, but then you car isnt broken when you get it serviced either......
The caps are over 30 years old and past their serviceable life......

Plomaman27
09-25-2011, 07:57 PM
Wow...a G-22000? Must be nice! Kind of like having a vintage Ferrarri in the garage!

Plomaman27
09-25-2011, 08:18 PM
Kevzep, for your info my driver/PS board mounts horizontally. Whats involved in a complete recap?

kevzep
09-25-2011, 09:34 PM
Wow...a G-22000? Must be nice! Kind of like having a vintage Ferrarri in the garage!

It is indeed, as soon as I get this small protection issue sorted, it will be unleashed in full force......:thmbsp:

Kevzep, for your info my driver/PS board mounts horizontally. Whats involved in a complete recap?

A complete recap is to replace every electrolytic capacitor in the unit.....

The best way to do it, is to physically make a list from the unit itself, the service manuals dont always have a complete parts list and of course during the course of the production of the amplifier they would have and did make amendments to the circuit which the manual wont show.

I get a big black marker pen and color the tops black and as I go through writing them down on a list.....
Then you do a big order and get to work on it......

Many of the caps will be smaller than the originals, and with the main caps, I tend to try and get something close to the original size to fit into the clamps....
You will have to go up in microfarads to achieve this.....for example in my AU-999 I went from 4700uf to 10,000uf.....on the G22000 I went from 15,000uf to 22,000uf......
You have to careful not to increase the capacitance too much because the inrush current may take out the bridge rectifier.....
Going up in capacitance will improve the power-supply performance....

It takes a bit of searching and fiddling around to find everything you need, but its worth it....
I use Panasonic FC and FM caps mainly due to availability, others seem to like some the Nichcon brand.........

Plomaman27
01-14-2012, 05:34 PM
Hello!

Its been about 3 months and I finally got my G-7500 repaired to the point for a power up. I took the very wise advice and built a DBT this time around... (one smoke check is enough)....it saved me a lot of rework!:yes:

When I powered up, the 200W bulb glowed super bright. Short! Now Im troubleshooting the power supply:scratch2:

I disconnected the two yellow leads (41V and 41V) from the xformer secondary that supplies the bridge rectifier and the bulb doesnt lite up anymore.

But first I think I should explain what Ive done so far....

I replaced anything that was shorted, discolored and/or showed signs of heating including:

On PS-Driver board:
TR19 over voltage protection xsistor TO-92 (fried)
R55 2.2K
R57 1.5K (toast)
R67 27
R69 150
R71 and 72 ...0.33 ohm ceramics (R71 open)
TR15 and TR17 drivers
R51 220 ohms NIR resistor


On Left Channel Output Amplifier PCB
STV-3H thermistor diode (found lead was broken and epoxied on)
TR601 and TR602 final output amps (both shorted)

Now that the yellow Xfrmr leads are off, the unit powers up, the front panel bulbs light up and the DBT is dark.

I removed the two bridge rectifiers and they seem to check out ok with a diode checker... they measure about 450mV forward and open in reverse.

Does anyone have any suggestions as to what might cause a direct short on the power supply?

KingBubba
01-17-2012, 09:41 AM
The STV-3H is going to be a problem if it is dead. They are absolutely unobtanium. There are a few places that have them but they are pricey. If the epoxied repair was done well you will be alright, but if it is broken and you can't re-repair it, it will cost you, if you can find one. IIRC, avionic knows where some are for sale.

Your shorted outputs are most likely going to be where the issue ends up. You will need to replace the driver transistor, as I believe you already found out. Your problems may have started there in the first place. I know I have read somewhere on AK which replacement transistors you should use, but off the top of my head, I can't remember. Do a search, using the original designations as the search heading. My guess is they are the OnSemis that end in the nineties.