Toshiba SC-335 Sporatic DC at output

[Jon_Logan]

I read "moved bias" and "blows fuses". I was thinking that the outputs might be okay, but the were "on" with an open bias string. The outputs might have survived with correct fusing.

 

[rob29]

I am also agreeing with the thermal runaway theory for the shot output transistor. The diodes that were changed in were close enough to the spec of the originals and I also triple checked to make sure they were all properly placed in the circuit.

There were a few other problems that I encountered when measuring resistance of individual components. There was a 2.2K resistor that was measuring about 600ohm but I attributed it to being parrallel with another resistance. Since I am still nowhere near the amp, I am unable to tell you which resistor it is but I will probably take it out tonight to check out of circuit.

My thoughts on the problem of the circuit are that both channels are at an extremely high bias which lead me to believe that there is a problem prior to the channel circuits. I was thinking the problem might be in the rectifier circuit and that was why I asked about changing the diodes and the main power capacitors. Could one of the 10,000uF capacitors or the rectifier chip being leaky cause the high DC offset?

 

[rob29]

if the quiescent current is too high why not adjust it with the preset ?, why are you changing the bias diode ?

The bias is much to high that the 2 ajustment pots hardly make any difference in the output. The changing of the bias diodes was just because I had extra ones.

There is quite a bit of leakage current in some component and this was what was causing the high bias.

 

[rob29]

The replacement transistors should work.

At this point, I suggest to check the amp without drivers, and without power transistors. This will tell you if the differential input stage and the voltage gain stage are working properly. You should disconnect any load on the speaker side.

But, to perform this test, you must connect one resistor (10k) from the point where the base of TR7 is actually connected, to the point where the emitter of TR11 is actually connected. You must also connect a resistor of the same value (10k) from the point where the base of TR9 is actually connected to the point where the emitter of TR13 is actually connected.

Do the same for the other channel.

Why?, may you ask. To close the feedback loop.

You should be able to adjust the bias trim pot to have ~1.9V to 3.7V from the collector of TR5 to the anode of D3, AND nearly 0V at the speaker fuse terminal. Do you have this?

So which transistors to I need to remove to test? Only TR 11,13,7,9 in right and same in left correct?

 

[ecluser]

I was concerned about the common problem in both channels, but you measured good voltage at the cathode of the zener diode D1, and this kind of circuit has a very high common mode rejection ratio. This means that you may have +40V on one supply line and - 20V on the other, and the circuit will work.

In your post #23, you reported a 2.2k measuring 600 Ohm. Measure carefully the two resistors between the bias diodes and the + supply rail. They are R17, R18 (3.9k) and R19, R20 (2.7k). Lift one side of one resistor at the common point between the two resistors. By the way, the part numbers here are the "corrected part numbers" and not the one on the original (SC330) schematic.

If the resistors are significantly lower than the spec, there will be a current higher than 6mA in the bias diodes. The voltage across the string of diodes will be larger and consequently the voltage across the emitter resistors in the output stage will be higher. The dynamic resistance of small signal diodes is high, and the forward voltage drop in them is very dependent oo the forward current through them.

It is important that you realize that any excess voltage across the bias diodes string will reflect as an identical excess voltage at the emitter resistors, and excessive bias current in the output stage.

The main power supply is not the problem.

 

[ecluser]

So which transistors to I need to remove to test? Only TR 11,13,7,9 in right and same in left correct?

Yes

Sketch the schematic with the transistors removed, and the resistors to the output as I proposed. You will see.

Correction: I proposed to use 10k resistors, use 22k ~ 39k instead. This will simulate the base current in the drivers.

 

[ecluser]

solution 1, replace all the transistors (with new uns) and start again
solution 2, take it to someone who knows what their doing

Do you mean I don't know what I am doing?

Solution 1 is for those who dont understand how circuits work, and solution 2 is actually a defection.

 

[Jon_Logan]

If I could take a guess ecluser, I think Inductor was saying it might be better rob29 to take it to a shop. I don't think the comment was directed at you. Let's face it, it's hard to convey techniques and what to watch out for over a forum. There's just a lot of hard won knowledge that comes from a lot of pain when working on amps.

With amps, as you well know, they are within a flea turd of erupting if somebody make a small mistake. It would not be rob 29's fault if things went south in a hurry. It would be nobody's fault. It's just the nature of repair and particular experience levels. Really, it's not a knock on anyone, it's just the nature of the hobby/business.

rob29, you have to ask yourself "do I feel lucky?"... ...."well, do ya punk?". sorry a little fun. rob29, you might have to make a couple more orders from parts houses with their minimum orders and ashipping charges. Also, there may be some time and frustration involved. Are you in? There might be a quick conclusion, but prepare for the long haul.

 

[ecluser]

I will be taking out the ossiciloscope soon and will be trying to see if there are any components that are leaky. This is quite a pain but that is what makes troubleshooting fun.

I will be here to guide Rob, and have fun with him, until this simple amp will be fully restored. At the end, he will be improved.

My program for the next days is to open my SC335 again, and revise the schematic. I will make the SC335 schematic from the SC330 schematic on eserviceinfo.com

I will take picture of the actual board, so you can appreciate the nature of the bad thermal tracking of the bias stage in this amp, and I will show with a picture how to improve this very important stage.

I will upload my SC335 schematic, and some pertinent informations, to eserviceinfo

I will help Rob to check the operation of the input stage and the gain stage alone.

Once we will be satisfied with those stages, we will add the driver stage and check for good operation.

Finally we will add the power output stage and this amp will sing again.

I don't see any technical limit to assistance via a forum like we have at AK, with the possibility of posting picture, schematics, etc.

What Rob needs is some guidance and encouragements, not discouragements.

 

[Jon_Logan]

......I don't know know if you think I am discouraging him.. .....I just want him to know the possibilities. I encourage him too. Some people get a bad taste if things don't go well, though.

 

[rob29]

Do you mean I don't know what I am doing?

Solution 1 is for those who dont understand how circuits work, and solution 2 is actually a defection.

I think that the post was more directed towards me ecluser. The reason as to why I am asking so many questions is that I just recently started repairing audio amps and want to learn as much as I can.

I do in fact work as an electronics technician and have a diploma for electronics so I would be the one fixing it :yes: . I do have an understanding of electronics and amplifier circuits and have always been a strong believer in not paying someelse to do anything when you can do it yourself.

As for the removing of the transistors, I will take them out tonight and will also start taking out some out of spec resistors to measure them out of circuit.

Any points that I shoudl measure for a comparison with yours ecluser?

EDIT:
Just read the last few posts. I am really not to concerned if things go south and I need to put the amp aside until I can completly rebuild it. I have the attitude where I do not look back and I have learned alot from my mistakes in the past. I have worked on electronics befroe and reversed the polarity of the Capacitors...boom they went and all I could do was laugh and then redo it. I am not going to get too frustrated with this amp, and if I do...I have another technics, pioneer, and Akai reciever I can work on, or a 42" Plasma that I recieved for free and can fix for $100. I believe that if I am able to troubleshoot a Plasma TV, an amp shouldnt be too much worse. I take this as a learning experience with a great outcome.

Be Happy.

 

[Jon_Logan]

rob29, I see that you are serious about taking this on. ....and you do have the experience level to do this. Amplifiers are like big op-amps. Treat them like that when doing things such as "closing the loop", determining gain, etc. The push-pull stage is kind of a bolt on but a lot of op amps have complementary output stages, like discrete power amps.

Tools of the trade:
VariAC
Switchable current limiter (lamp)
AC current meter

If the bias lets go, this setup will prevent collateral damage. You'll be able to bring stuff up slowly while watching the current draw. Sometimes full throttle with fingers crossed isn't the preferred method.

When the Lamp glows, sumpin ain't right.

 

[rob29]

rob29, I see that you are serious about taking this on. ....and you do have the experience level to do this. Amplifiers are like big op-amps. Treat them like that when doing things such as "closing the loop", determining gain, etc. The push-pull stage is kind of a bolt on but a lot of op amps have complementary output stages, like discrete power amps.

Tools of the trade:
VariAC
Switchable current limiter (lamp)
AC current meter

If the bias lets go, this setup will prevent collateral damage. You'll be able to bring stuff up slowly while watching the current draw. Sometimes full throttle with fingers crossed isn't the preferred method.

When the Lamp glows, sumpin ain't right.

Please don't remind me of opamps, I had an entire course on them with my professor continueing to go on about artificial ground every class. I will be making a variac I guess even though I do enjoy the finger crossed game. I do have the other tools needed and the knowledge to use them, I am just usually too lazy to take the osscilliscope out of the corner as its from the 80's and extremely heavy and ugly.

The main reason I actually bought the amp in the first place was because it was a package deal with the preamp and tuner and I really only wanted the preamp for a gainclone build that I may soon be starting. But after reading up on the amp I found that it can perform extremely well with a few modifications.

I am really in no rush to complete the amp as I have another setup that I am using but there was a noticeable increase in quality when I hooked this amp up to a set of speakers on the right channel which had the lower DC. I just like electronics and enjoy working on them.

On a side note I will be dedicating the majority of my evening to myself tommarrow and will be trying to get as much completed on the amp as I possibly can.

 

[ecluser]

I will be making a variac I guess

You can built a very effective 500W variac, at low cost, with a light dimmer coupled to an AC outlet. Total cost is less than 10$.

The power supply in vintage audio gears is filtered by large capacitors and present a capacitive loading on the variac. This is no problem for a light dimmer.

 

[rob29]

I measured from collector to anode and got the steady voltage that you described in the left channel but still sporatic in right. I also have quite a sporatic DC in right (12-90mv) and still about 12-1200mV in left channel.

*20 minutes go by with screen open*

After I started poking around near power supply I got a 0v across one of the coupling capacitors in the rectifer circuit. Though that it was shorted and took it out of circuit which cause left channel to drop to a sporatic 30-40mV which I think is quite an accomplishment. Took out all of them and now I am getting about 30-40mV in left, and 40-120mV in right. Quite a drop in both but still unable to change them using bias adjustments.

Put in a set of matching transistors for input stage and now left at 51mV and right at 42mV. Sporatic DC is gone. But Bias adjustment still not changeing the DC at output.

EDIT:
I have put in an order for new coupling capacitors, some more ceramic filter caps, and the main power transistors. I should be getting these within the next 2 days and will put everything together and this amp should be singing again.

EDIT #2:
I decided that since the bias pots were hardly making a difference then there must have been a resistor that went out of spec. The resistors that were beside both pots, R41 and R42 were both brown-black-brown which is 100ohm. I pulled the first out and measured 1400ohms across it, second one almost 2200ohms. Replaced with 1/4W ones but I just thought about it and I will put in some slightly larger ones. Too bad I didnt get it before my order .

 

[ecluser]

The resistors that were beside both pots, R41 and R42 were both brown-black-brown which is 100ohm. I pulled the first out and measured 1400ohms across it, second one almost 2200ohms. Replaced with 1/4W ones but I just thought about it and I will put in some slightly larger ones. Too bad I didnt get it before my order .

This may be the root of the problem. Those resistors are there to limit the total resistance in series with the string of diodes, and limit the bias voltage to the drivers within safe limits.

Yesterday I completed the schematic revision I started before, and I discovered those resistors. As you know, they are not present in the SC-330 schematic you are working with...

If you can adjust the bias voltage to the bases of the drivers to 1.8V ~ 2.2V with the new 100 Ohm bypass resistor, you can safely install the new output and driver transistors.

For the offset, you will need to trim the value of resistors R9 and R10

If the original offset is negative, which I think it is, you will need to reduce the resistor. I prefer to keep the original resistor and bypass this resistor with another resistor soldered under the board. This gives a more precise adjustment (with 5% tolerance resistors). In my unit, the original offset was -37mV and -30mV. With bypass resistors of 3.6k and 3.9k respectively, I reduced the offset to 0±1mV

If the offset is positive, you will need to replace R9 anr R10 by larger resistors.

You can replace those resistors by a 2.5k multiturns trim pot, with the screw on the top, preferably. This will give you a very precise adjustment.

The important point to remember is that the way to adjust the offset in this amp is by triming R9 and R10.

 

[ecluser]

The Toshiba SC-335 Power Amplifier Schematic Diagram is now available on eserviceinfo:

http://www.eserviceinfo.com/download.php?fileid=41555

 

[rob29]

Here is the original position of the bias diodes:

And the new position of them:

Thank you for the schematic, it will be extremely helpful in troubleshooting. I also recieved my package from digikey so I am going to start work on it.

 

[rob29]

So I tested the HFE of the main driver transistors. My PNP transistors have a gain of 44,48, and 50. My NPN transistors have gains of 66,66, and 67. Will this cause a problem with the push-pull configuration of the amp or should I be ok with using two 66 and the 48,58?

 

[Jon_Logan]

So I tested the HFE of the main driver transistors. My PNP transistors have a gain of 44,48, and 50. My NPN transistors have gains of 66,66, and 67. Will this cause a problem with the push-pull configuration of the amp or should I be ok with using two 66 and the 48,58?

No problem there. Matching more important in the differential stages for like pairs. There is no need to match complementary pairs.

If there are paralleled outputs, it is good to beta (Hfe) "match" the NPN's to each other and "match PNP's to each other.