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Old 15th Sep 2020, 10:09 pm   #1
samjmann
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Default Teleton GA-203 Output stage fault.

This little integrated amp came my way, it's in really good condition. Initially there was no Rh channel output at all. This was down to a 330K base bias resistor being open circuit in the tone network. Both channels then worked (I was working it with a pair of headphones). However there was an annoying 'tearing paper noise on the LH channel, this was down to noisy 22k resistor in the second transistor biasing in the tone network. All sounded well on headphones... till I tried on speakers!

Now there is heavy distortion on the LHC output into a 8ohm speaker. RHC is perfect. So I thought it must be C619 1000uf 35v. No... No amount of heating makes any difference to the output stage. All the capacitors are original except for the two speaker coupling caps that I changed together. The output stage is straightforward by today's standards. Every DC voltage is the same on the left/right on each transistor. My money is on C617 being low in value.

At very low volume there is no distortion into the speaker. Heavy bass makes the distortion much worse.

What's the thoughts of anyone here?

I'm not not keen on blanket mass re-capping, but sometimes it's the only way!

Regards, SJM
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Old 16th Sep 2020, 1:35 pm   #2
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Default Re: Teleton GA-203 Output stage fault.

Hi SJM,

C617 seems like a good candidate, especially that there is little distortion at low level!
Do check all the electrolytic caps for ESR.
Alternately, you could just replace them, as they are old enough!

Regards, Peter
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Old 16th Sep 2020, 6:30 pm   #3
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Default Re: Teleton GA-203 Output stage fault.

Early plastic output transistors often go faulty
Please check base emitter base collector
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Old 16th Sep 2020, 7:37 pm   #4
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Default Re: Teleton GA-203 Output stage fault.

With a scope you can check that the positive end of C617 tracks the amp's output voltage. It is a bootstrap for the load resistor of the voltage gain stage.

If C617 goes open circuit, then the positive end will only have a third of the swing of the output voltage.

Loss of bootstrap will kill your open loop gain and will limit the output swing ability especially positive going, and it won't affect quiescent DC votages and currents.

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Old 17th Sep 2020, 9:27 pm   #5
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Default Re: Teleton GA-203 Output stage fault.

Thanks everyone for reading and contributing.

The DC conditions appear to be spot-on. Normally electrolytics will respond to heat, but maybe in this case as they are so old that they are totally shot!

RW: How can you work out that on if C617 goes o/c you would only have one third of the output voltage on it's +ve leg? I'm not doubting your judgement at all, but would like to know the theory behind it!

Some time ago, I did have the same bootstrap arrangement fail on a Quad 303.
This was highly sensitive to heat, but at low temperatures the amp would not work at all.

Will look at the amp over the weekend and report back. Virtual 12hr days on the 'day-job' preclude doing much through the week...

Regards, SJM.
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Old 19th Sep 2020, 7:24 pm   #6
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Default Re: Teleton GA-203 Output stage fault.

Had a chance today to fit the suspect parts, all to no avail! The bootstrap cap proved OK as did every other electrolytic cap in the LH output stage.

So I thought I'd better have a look with a scope. The positive half wave was clipped, much worse on a speaker than running into headphones. The top 2SD315 had 0.955v across base emitter (forward bias). Obviously too high. The RH channel had 0.55v. It would appear that the transistor had gone resistive!
The mid point voltage was spot on compared to the other channel.

I moved the suspect 2SD315 to the other channel and the fault 'moved', so that must be it!

I can't believe what's being asked for this transistor - £15.00 on E-bay from a variety of sellers in the UK. I think this transistor may be used in the Sansui AU-101 which has magical sound quality according to some sources

Regards, SJM
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Old 19th Sep 2020, 7:40 pm   #7
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Default Re: Teleton GA-203 Output stage fault.

They are nothing special. Won't a 2n3055 substitute? Replace both then set up.
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Old 19th Sep 2020, 8:06 pm   #8
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Default Re: Teleton GA-203 Output stage fault.

Unfortunately they're TO66 cans. 2N6294 or 6295 maybe?
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Old 20th Sep 2020, 12:37 am   #9
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Default Re: Teleton GA-203 Output stage fault.

Not TO66 very close but not quite correct
The trio Ka 2000 used similar sized transistors Are they mounted on sockets ?
With the Trio units it was possible to fit TP31A with a bit of fiddling as the output devices became unavailable
I have repaired a few amps this way not the ideal solution but at least they worked for many years more
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Old 20th Sep 2020, 7:19 am   #10
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Default Re: Teleton GA-203 Output stage fault.

Easy once you know what a bootstrap is and what it is for.

Q603 is what is calles the VAS, voltage amplifier stage. Its job is to get the signal up to the full voltage swing needed at the output. (swing is a key word... we're talking AC components here. DC wise, the average voltage on its collector is about half of the supply voltage)

If you look at Q605, Q501, Q607, they are all emitter followers to the output and have only unity voltage gain. Q503 is a neat trick, it's an NPN power transistor where normally you'd have expected a PNP one as another emitter follower, but it's used in a tiny little loop with Q607 to boost the current Q607 emitter puts into the output. It's called a quasi-complementary output stage. Q607 and Q503 together fake a high pwer PNP transistor in an era when power PNP transistors were not very good and hard to make.

Back to the chase...

So the voltage gain of the power transistors and the driver transistors is close to unity. So every volt of swing going to the loudspeaker means you need a volt of swing out of the VAS transistor collector.

For best power output, you want the output to swing between the supply rails of 0v and I think it's 30 or 40v on the supply.

This is a single supply amplifier, so the output of the transistor stages hasa quiescent voltage of about half the supply, with no input, and so it has to have a whacking great DC blocking capacitor on the way to the loudspeaker, C619

So, if the output at the + end of C619 needs to swing close to the rails, then so does the collector of Q603 because it's all unity voltage gain from there onwards.

Q603 isn't handling the big currents, but it's only a small transistor and it's having to do the full voltage swing. It's probably the hardest working transistor in that circuit, and in the early days of transistor amplifiers they were one of the places where failures due to 'second breakdown mode' showed up.

So on to the collector circuit of Q603. It looks fairly complicated, but it breaks down into a few simple functions. Q603 is carrying a modest amount of DC quiescent current, so there will be a small voltage drop in R627//VR603 and the diodes D601. This voltage makes a difference in the base voltages of Q605 and Q607. This difference sets up their quiescent current in R629 and R631 and that sets the quiescent in the power transistors. So VR601 is the adjuster to set the output stage quiescent currents. The voltage drop in R627//VR603 and D601 will be a couple of volts or so.

Let's forget C617 for the moment.

Q603 can pull its collector voltage down low quite well when it has to. Qhen Q603 lets go, R621 and R625 try to pull Q603 collector voltage (and the bases of the drivers) up towards the power rail.

But together they add up to 4.8k Ohms, and as the drivers take some base current, it doesn't get as close to the rail as we'd like. It will fail by several volts, and that translates into a significant reduction in output power over what the voltage of the power supply would suggest ought to be possible.

So instead of having a single resistor at the top of 4.8k, it's been split in two, and the AC component of the output voltage of the amplifier has been AC coupled via C617 into the two resistors.

So when the output is made to swing positive, C617 drives that resistor junction more positive, keeping it always higher than the output. It will even go higher than the supply rail.

So Q603 thinks it's got a higher supply voltage than the rest of the amplifier, and over half the cycle, it has. This overcomes the voltage drop in the VAS collector load resistors caused by the driver base current and the amplifier can now swing a lot closer to the supply rail.

C617 is applying positive feedback, which may sound alarming, but the gain is a bit less than unity, so it's stable. R625 is the one being bootstrapped. As the Q603 collector voltage goes up, the drivers and output stage emitter follows it, and the -ve end of C617 goes up by only a little less than the same amount. C617 is big enough to drive the R625, R621 junction up by the same amount.

Consequently R625, AC-wise has almost the same voltage at both ends, and so it must be carrying a constant current throughout the signal waveform. Bootstrapping not only boosts the voltage Q603 can swing into the drivers, it also boosts the voltage gain of Q603. I told you that little transistor had to be a hard worker!

If C617 is good, the junction of those resistors will show as many AC volts as the output is giving. If C617 is open circuit, R625 and R621 become a potentiometer reducing the swing at their junction, and the AC voltage at their junction will be reduced to 1.5/(1.5+3.3) of the output swing which is about a third.

Without the bootstrap working, the amplifier will not be able to swing its output anything like as close to the supply rail as it used to, and so you get bad distortion when it tries.


Bootstrapping is a neat trick, but it has its costs. If an amplifier is driven into clipping, the charge on the bootstrapping capacitor can be changed and it will take time to recover. So that little clip can get extended into a longer period of disrupted operation and becomes a lot more audible. Small amplifiers that have to be efficient in power useage, and parts count, use bootstrapping in most cases. Posh power amps usually avoid it and have to make other arrangements.

David
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Old 23rd Sep 2020, 5:59 pm   #11
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Default Re: Teleton GA-203 Output stage fault.

Thank you David for such a detailed reply, I'll copy and paste this and keep it with the scan of the circuit diagram.

Maybe this could be made into a 'sticky' in the audio section? Audio output stages can be tricky even for the experienced, detail such as this you don't come across that often and is invaluable in learning the circuit operation.

SJM
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Old 23rd Sep 2020, 7:41 pm   #12
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Default Re: Teleton GA-203 Output stage fault.

A lot of people are scared of transistor power amp stages. This comes from their propensity for sending folk round in circles blowing up and replacing the same devices in a circle.

Once you know how they work you can break out of the loop of confusion. They aren't actually hard to understand, just different and a bit more involved.

Learn to see the basic structure input stage, VAS, bias generator, and emitter follower driver/outputs. Spot the feedback path. Understand that the whole thing has so much gain the DC condition of the output will ram itself against one rail or the other unless the DC feedback is working - so you can't try them without DC feedback.

If you have an amp you know once worked, pull all the semiconductors, test them and replace the duds, so then you can fit a known good set all at once before you try it out.

If you're developing a new amplifier and the circuit is unproven, you have to build it in stages, using current limitd power supplies and making temporary feedback arrangements to control the DC conditions. Tiy have to do the maths for the stability analysis as well. You can't simply nick a bit from here, a bit from there. Go down this route and you really learn what's going on.

Self's books are rather good.

David
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Old 27th Sep 2020, 7:49 pm   #13
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Default Re: Teleton GA-203 Output stage fault.

Fitted the replacement 2SD315 today. Success..! Undistorted positive half cycle at last. Next job, check the quiescent current. Nothing but the circuit diagram with no voltages on it. Looking at other 15watt amps, they have a standing current current through the output transistors of around 20mA.

Setting the amp to this the output transistors get barely warm, even when driven quite hard when on for an hour or so.

When I first switched on - before adjustment, there was nothing across R637 at all!! Despite this it didn't sound distorted even a very low level.

I can't believe how punchy this little amp sounds.

Does 20mA seem reasonable for quiescent current on this type of circuit?

The replacement 2SD315 had slightly closer mounting centres for the T066 case. (See photo) I had to file the heatsink mounting hole at one end to get it to fit.
The insulating washer is from the original transistor, the new transistor is the one in the photo. Two versions of T066?

SJM.
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Old 27th Sep 2020, 10:24 pm   #14
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Default Re: Teleton GA-203 Output stage fault.

Sounds good.

They don't make it easy, do they?

David
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Old 9th Mar 2021, 10:26 pm   #15
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Default Re: Teleton GA-203 Output stage fault.

Teleton re-visited...

After repairing this and setting up all was well. In fact it was nearly 6 months ago. However it came back with the output transistor I had replaced - failed again. This time the transistor was leaky collector emitter, as opposed to resistive base-emitter. I'm not sure if the new part was genuine, the Sanyo logo did not look quite right somehow.

The original 2SD315 (TO-66) is rated at 4A Ic, and 60v C-E. These are quite modest by todays standards. Would a MJE15030 (TO-220) act as a replacement? For the time being I fitted a TIP41A as it was all I could at home and the amp is running. All the drivers and emitters resistors appear to be OK.

Any thoughts and advice on replacements would be welcomed.
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