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Toshiba SC-335 Sporatic DC at output

rob29

rob29

Active Member
I recently picked up a full Toshiba 335 set. Preamp works beautifully but the amp has been slightly scetchy since I brought it home. I was originally getting a full 35VDC at speaker terminals but after some proding I found that there was a bare wire that was rubbing the power supply cap pins.

I kept testing and even dared to hook up some speakers to it and found that I was getting a sporatic pop from them every few minutes. Figured it was a bad connection so I deoxited all the terminals and switches and did a complete recap excluding large 10,000uF power capacitors. Power circuit is at a steady 35V throughout.

After this I am getting a sporatic DC in the Left channel that jumps to around 1V and Right chan is staying at a steady 70mV. It seems that I am getting some sort of leakage DC that is going through the entire left channel. Strange thing is that after unit has been on for about 10 minutes with 2 dummy loads, the DC goes down to 30mV in left and stays at 70mV in right.

So far I have deoxited, recapped, resoldered connections, doubled output ground wires, and still have been unable to get this amp to the way I want. Does anyone have some troubleshooting advice on where to look or a schematic or service manual available? There is a pot on each channel that is labeled Idle and a + and - idle terminal as well. Is there any general value to adjust this too?

Thank you all for your help.
 
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Hi Rob29,

This is not an uncommon symptom.

It might be wise to either fuse your speaker lines until this issue is resolved. If there is a speaker relay in the Toshiba, it's likely to save your speakers in a large DC offset event. ....Protection circuits are relatively quick in disconnecting the load and saving your woofers.

If you hear a frying bacon type of sound, I would get the unit looked at before re-connecting to speakers. High frequency static type of noises might not trigger the speaker protection circuit and all that high frequency could find it's way to your tweeters.

For the low frequency pop and high frequency static possibilities..... .....a good guess would be the input stage on the amplifier, usually a differential pair (Transistors) of some flavor.
 
I have serched for some dry joints and actually resoldered the majority of the joints on the amp and still the same result.

I suspected the input transistors on the left channel as I was getting some eradict readings off of them. Would they possibly cause the problem that I am having? I actually see a few different transistors, some that are regular sized half cylidrical and other chip ones. Should I just switch out both?

Thank you for the schematic, it's close enough to trouble shoot.

Could the problem possibly be one of the 10,000uF supply capacitors? I have a ac voltage trickle across the one that feeds the channels, but wouldn't that affect both channels? These are the only capacitors that I have not changed but for $20 each and a 1/2 drive each way to local electronics supply store, dont want to change them if they aren't the problem.
 
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The most probable cause is mismatched transistors in the differential input stage.

They are the two small (2SA841BL) transistors, identified as TR1, 3 in the Right channel, and TR2, 4 in the Left channel.

If your DVM has a hFE scale, remove those (4) transistors and measure their hFE. Ideally, those transistors should be matched within a few %. I have seen that a mismatch as low as 30% can cause a lot of trouble in an amplifier like this. Try to make the best match you can with the actual transistors, or replace them with matched sets.

I am sure the power supply is not in cause, since the other channel works perfectly.

Concerning the Bias adjustment:

The 3 diodes in the bias stage are NOT thermally fixed to the power transistors, nor to the driver transistors. They are close (~ 1cm) to the heat sink but thermal tracking is far from perfect in this amp (could be improved). I recommand that you set the bias to the minimum acceptable, say 10mV from + and - idle test points, after ~15 minutes of heating to normal listening level (top cover in place). Before making this adjustment, make sure the bias is below 10mV at cold start.

By curiosity, how much did you pay for the system?
 
rob29 said:
Thank you for the schematic, it's close enough to trouble shoot.
Click to expand...

Here is a list, NOT EXHAUSTIVE, of the corresponding parts numbers between the two versions.

part number on the SC330 > part number on the SC335:

VR1, VR2 > not present, there is a 20dB attenuator instead
R7, R8 > R9, R10
R9, R10 > R11, R12
R11, R12 > R13, R14
R13, R14 > R15, R16
R15, R16 > R17, R18
R17, R18 > R19, R20
R19, R20 > R21, R22
R21, R22 > R23, R24
R23, R24 > R25, R26
R25, R26 > R27, R28
R47 > R39
C26 > C24
R49, R50, R51, R52 > Absent, replace by a short
TR11, TR12 > 2SD718-O
TR13, TR14 > 2SB688-O
F1, F2 > 4A rating

Some measurements from my good working unit:

Cathode of D1: +19.65V
Main supply voltages: ±40.5V (I have not monitored the line voltage)
From Idle - test point to Idle + test point: 10mV
Base of TR13 to Base of TR11: 1.13V
Base of TR9 to Base of TR7: 2.24V
From Emitter of TR5 to Base of TR5: 0.65V
Emitters of TR1,2,3,4: 0.57V

Offset voltage ~ -34mV, could be corrected if R9, R10 is replaced by ~1.1k or 1.2k

If you need new transistors for the differential pair, the following would be better replacement (less noise, but the original is pretty quiet too). They have the same pin-out as the original:
2SA1316BL, 2SA970BL, 2SA991E, 2SA1038E, 2SA1039E, 2SA992E

If you find (likely) other differences in parts numbers, or values, please let me know. I had to close the amp.

Good luck!
 
I am getting about 172mV on right and 115mV to 200mV on left between + and - idle points on right chan, and sporatic on left. I will de looking into getting a matching pair but am yet to get an hfe reader (gives me an excuse to go buy a mastercraft multimeter thats on sale at Canadian Tire!!!)

The idle is quite high and I change it only slightly with the idle pot that is on either chan. Could the culprits be the resistors or the pair of input transistors?

And I paid $30 CDN for the set which I think is fairly decent in price.

Cathode of D1: +19.65V
Main supply voltages: ±40.5V (I have not monitored the line voltage) ----- 35.3V
From Idle - test point to Idle + test point: 10mV ----- lot more(look above)
Base of TR13 to Base of TR11: 1.13V -------- 1.35V
Base of TR9 to Base of TR7: 2.24V -------- 2.38V
Base TR8 to base TR10 ------ 2.36V
From Emitter of TR5 to Base of TR5: 0.65V --------- .60V also same on TR6
Emitters of TR1,2,3,4: 0.57V ------------ right - .58, left- tr4 jumps from .4 to almost 1V, tr2 jumps from .6 to .9V

My readings are on the right.
 
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I use 2 Mastercraft multimeters. The one I prefer is the model 52-0052-2. I paid 30$ on sale, reg price 60 or 70, I don't remember. But sometimes he is on sale at 20$.

Your idle is too high, but there is no relation with the differential stage.

I suppose that the heat sink is hot. If so, replacing two of the three diodes with new diodes (1N4148 or 1N914) with long legs, glued to the heat sink, will reduce the bias current and improve the stability of this bias, and the reliability of the output stages.

Measure the resistance of the trim pot, at minimum. If the minimum resistance is not close to 0 Ohm, you will have too much voltage across the trim pot. The current in the bias stage (D4, D6, D8, VR4) is ~ 6mA. If VR4 minimum is say 20 Ohms, you have ~ 120mV minimum across the trim pot. You see the problem.

Are you sure the output transistors are good? Did you try to connect a cheap speaker on it? You may have a high bias current even if the output transistors are bad (if the collector junction is open).
 
That is the multimeter I have been looking to get. Its 60$ regular but it was on sale for 20 2 weeks ago then went up to 30. The heatsink runs extremely hot, I used heat transfer pads from plasma televisions underneath and left the top open, and it was still quite uncomfortable to the touch. The output transmitters seem to be ok to me as I measured the resistance across the components and compared to the other and they matched eachother.

I have been trying to see if any components have gone out of spec but have been unable to pinpoint the problem, which leads me to believe that there are a few issues and it looks like the amp hasn't been ever serviced.

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.

EDIT:
Trim Pots seem ok, got Right down to 2.5ohm, left to 1ohm. I have 4 extra diodes for the bias that i will use silicon thermal paste to glue to heatsink. The only problem that I have found is that when I am using the diode test fucnction on multimeter, the main power transistors D718s seem to have an extremely slow response(?) rate across collector to emmiter. It start at 300ohm and rises to about 1050ohm in about 10 seconds. Are these normally this slow?

EDIT 2: I changed the diodes near the heatsinks and it seems to have ehlped a little in left chan, still no change in right one. I also measured 195mV across the .47ohm resistors in right channel, and about 170-195mV in left chan. Is this normal or still way to high?

I was going to look at the small ceramic filter caps next. I am a little unsure of values though, what are 56P, 10P, and 5P? PicoFarads? Also 104K = .1uF correct?
 
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I think a problematic component finally went. Now everythime I try to turn out the main fuse blows. Nothing looks to be shorting but i will have to start measuring. It is almost a paused blow taking almost a second to go. Any ideas where to look?

Another question that I have is that I have some fast-switching diodes that have a better raiting then the recitifier chip in the amp, Higher voltage/ current rating, and much faster switch time. Would it make any difference if I were to switch this out?
 
rob29 said:
the main power transistors D718s seem to have an extremely slow response(?) rate across collector to emmiter. It start at 300ohm and rises to about 1050ohm in about 10 seconds. Are these normally this slow?
Click to expand...

Across collector to emitter, resistance should be infinite. It is best to remove the transistor from the circuit to take this measurement. It looks to me that there is a large leakage current in this transistor.


rob29 said:
I also measured 195mV across the .47ohm resistors in right channel, and about 170-195mV in left chan. Is this normal or still way to high?
Click to expand...

Too high, much much too high. The current may comes from the leakage.

rob29 said:
I was going to look at the small ceramic filter caps next. I am a little unsure of values though, what are 56P, 10P, and 5P? PicoFarads? Also 104K = .1uF correct?
Click to expand...

Yes, P is picoFarad, and 104 means 10 x 10 000 pF = .1uF
K here is a tolerance factor (10%)
 
Oups, remove the power transistors, all four. But prior to removing them, mark their actual position. It may help to locate further damage upstream.
 
for the resistance check of the transistors, I was using the diode feature on the meter because of the infinite resistance that I had read from them. The blowing of the fuse started after I replaced the 4 bias diodes so I thought that some leads were shorting on the heatsink but that wasnt the case. I might double check them later on tonight and still try to find the fault that is causeing the DC.

I also just took apart the transformer and removed the leads for the front lights because I found they were just causeing problems and I only had 1, which happened to be broken. The problem that I had was that the transformer would hum until I would shift the blue wires that were connected to the lights for the front panel.
 
So I just came to the conclusion that I should no longer be soldering after 2am. Last night when I put in the new diodes I seem to have shorted the legs onto the heatsink and now the D718 power transistors are shot. I am in the process of taking them out now and will be also taking out the B688 transistors and this amp will be going into the pile in the corner of the room until I get replacement transistors for it.

:banana:

Should I just get the same D718 and B688 transistors or are there other replacements? This is why I have such a love for digikey :D

EDIT:
Are these good replacements?:
D718 >> FJA4313 ( http://www.fairchildsemi.com/ds/FJ/FJA4313.pdf )
B688 >> FJA4213 ( http://www.fairchildsemi.com/ds/FJ/FJA4213.pdf )

Rob
 
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Hi rob29, after these output transistors are sorted out, if you are left with the original static problem, it is almost uselass to make measurements with transistors that are noisy. The "noisiness" is likely to be intermittent and thermally activated. Sure you will see unsteady voltages when measuring, but it will be part of the signal and appear everywhere in the amp's signal path.

The best method (IMO) for troubleshooting noisey transistors is to use freeze-spray and thermally shock them. Use a piece of tubing to isolate where the cold spray is going (you want to shock these one at a time) and monitor the noise generation. Make sure you allow ample time for the transistor to return to it's operating temperature before moving on to the next transistor. It takes practice and patience to develop a feel for this technique. It's best done with a scope and a ACmVM, to prevent any damage to speakers and for the most sensitive monitoring.

Yes, soldeing at 2 a.m. is more risky.:D
 
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?
 
Thanks for the advice on the freeze spray and the transistor checking. I just got off work and will be heading out for a few hours but later on tonight I will start some measuring.

I also found out that it wasnt my fault that the transistor went because the heatsink is not connected to ground in anyway. The strange thing is that the transistor blew in the right channel and the left was the problematic one...

After I test and find some other faulty parts I will make a order through digikey and get a few things to make this amp run great (hopefully) but with help I think this goal is possible.

Lastly, I asked in a previous post about changing the diodes with higher rated fast switching diodes just because I have them on hand and I also am able to get my hands on a very nice aluminum heat sink that should work with the 4 chip diodes being perfectly attached. I may then upgrade the main capacitance to about 30,000uf or as much as I am able to fit in the mounting tabs.

And soldering after 2am will resume today I think muahaha. This is the only time I usually have as I work 2 jobs and have about 5 other projects going at the moment.
 
I don't think a change to fast switching diode is a significative upgrade in a 120 Hz power supply.

You want a better upgrade... move the fuse at the speaker output to the supply line. You will need two fuses per channel.
 
Hi rob29, I am wondering if the output failure had anything to do with the bias diode change. Could they be in backward? Also, with bias diodes it's very important that the part parameters are similar. If they are used for thermal tracking, you want them to have similar Vf (forward voltage) and temperature coefficient. That way, they will have a more correct Vf/deg value. When doing things with output sections, bias, etc, it's good to use a VariAC, in line lamp current limiter, and an AC current meter all to help catch things like 2a.m.soldering and other things. Power amps have fairly high rail voltages and low resistances. This is a recipe for disaster if somethings not quite right. Nobody will die, but these parts are closely connected and daisy-chain (expensive) failures can occur.

I'd recommend staying with the original bias diodes if they ain't broke. There's no need in "upgrading" bias diodes. The only upgrade you would consider for upgrading bias diodes is if they thermally track better. ......That is a pretty difficult thing to get right.
 
Jon_Logan said:
Hi rob29, I am wondering if the output failure had anything to do with the bias diode change. Could they be in backward?
Click to expand...

Rob reported a very high bias on post #8, prior to my suggestion to move the bias diodes closer to the heat sink. If one of the diodes was in backward, the base of driver TR7 would be high in the sky.


Jon_Logan said:
Also, with bias diodes it's very important that the part parameters are similar. If they are used for thermal tracking, you want them to have similar Vf (forward voltage) and temperature coefficient. That way, they will have a more correct Vf/deg value.
Click to expand...

The 1N4148 or 1N914 are near identical to the original bias diode (M8513A-R).
The original bias current in Rob's amp was too high. I had good reasons to think that the heat sinks were hot. This was for this exact reason that I suggested to move the bias diodes to the heat sink, because at the place they are on the printed circuit board, the bias diodes are really cold. As you know, the forward voltage drop of a cold P-N junction is HIGHER than it is in a hot junction. This agravates the problem. In short, there is a very bad thermal tracking between the bias diodes and the output transistors in this amp. This bad thermal tracking may lead to thermal failure of the output stage.

It is not the only probable cause. If there is a high leakage current in the output transistors, the bias current will be too large also. This transistor becomes out of control from the driver, and can go in thermal runaway.
 
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