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Old 10th Feb 2018, 5:35 pm   #1
Jim - G4MEZ
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Default Leak Stereo 30 Plus - Setting Quiescent Currents

I'm fixing one of the above - the later silicon version with the daughter boards - for a friend of mine...

The original fault was a blown (gently) 1.6A DC fuse... Basic checks showed no really obvious issues and I powered up with a 500mA fuse and ammeters monitoring the output stage quiescent currents...

Basically, they were a bit high but not outrageously so. The amp works fine...

I set the quiescent currents as per the manual (30mA with no signal) and kept an eye on it, rechecking when the amp was nice and warm and had been playing at moderate volume for a while.

The right channel set fine with plenty of reserve travel on the setting pot (P5) but the left channel will only just set to 30mA with the pot on the end-stop...

I'm not happy with this and given the original fault was a popped fuse - perhaps from a bit of a runaway - and would like to see P5 set more or less mid-travel like the other channel.

Any suggestions for the most likely cause of this given that the amp works well and is clean with no distortion or any other obvious issues?

Thanks, any thoughts much appreciated...
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Old 13th Feb 2018, 4:33 pm   #2
SteveCG
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Default Re: Leak Stereo 30 Plus - Setting Quiescent Currents

The amp uses edge connectors. No doubt you cleaned them and their sockets to avoid bad contacts on re-assembly (and possibly an original bad contact that caused the fuse to blow) ...
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Old 14th Feb 2018, 12:56 pm   #3
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Default Re: Leak Stereo 30 Plus - Setting Quiescent Currents

Check grid leak resistor values. If they have gone high this could encourage thermal runaway. Also worth checking them against the EL34 datasheet recommendation; it is not unknown for valve amp designers to use the 'cathode bias' limit as the value for 'fixed bias', as this makes the driver design easier and reduces distortion until the valves get a bit gassy.
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Old 14th Feb 2018, 1:01 pm   #4
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Default Re: Leak Stereo 30 Plus - Setting Quiescent Currents

?

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Old 14th Feb 2018, 3:01 pm   #5
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Default Re: Leak Stereo 30 Plus - Setting Quiescent Currents

My own experience was with its bigger brother, the Stereo 70.

I too found it difficult to achieve the correct bias with pot-travel left.

I had to replace 'short-circuit' output devices in one channel (amplifier/set purchased by me as a duffer), which is why mine blew fuses, everything else seemed ok, aside from the PCB edge connectors & chassis-mount sockets, which were cleaned.

Long story shortened, the 2x diodes connected in series used as thermal monitoring of the output stage? One was leaky ...

12 years later, it's been a great amp with it's life-long partner, the Stereofetic & huge Leak floorstanders, of which, the wife does not approve

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Old 14th Feb 2018, 9:19 pm   #6
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Default Re: Leak Stereo 30 Plus - Setting Quiescent Currents

To expand on my previous post:
The largest value of grid leak resistor depends on the bias method. Typically, cathode bias can have up to twice the value of fixed bias. This is because cathode bias is self-adjusting - if the valve draws too much current then the bias is increased to help limit the rise. The grid leak resistor helps deal with any grid current, which if due to gas in the valve will tend to vary the bias in the way to increase the anode current.

The output stage grid leak resistor is often of a similar value to the previous stage anode load resistor, and thus loads the previous stage. Assuming this is a triode, extra load will reduce gain and increase distortion. Hence there is a short-term advantage to having a high value grid resistor, but the long-term problem is that as soon as an output valve goes gassy it is more likely to go into thermal runaway. What is sometimes seen is a resistor value which is the maximum allowed for cathode bias, yet used with fixed bias - in either case better design would be to use a value somewhat smaller than what is recommended in the datasheet.
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Old 14th Feb 2018, 9:43 pm   #7
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Default Re: Leak Stereo 30 Plus - Setting Quiescent Currents

From one Dave to another (G8HQP), there seems to be some confusion here as the Stereo 30 plus is a solid state amp, so no valves present.

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Old 15th Feb 2018, 8:15 am   #8
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Default Re: Leak Stereo 30 Plus - Setting Quiescent Currents

Hi Jim.

I would swap the driver pcbs over to narrow the problem down. This would then lead you to either a problem on that pcb, or the output transistors and the 2, diodes next to the heat sink.

Are all the transistors original?

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Old 16th Feb 2018, 12:15 pm   #9
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Default Re: Leak Stereo 30 Plus - Setting Quiescent Currents

Quote:
Originally Posted by teetoon
From one Dave to another (G8HQP), there seems to be some confusion here as the Stereo 30 plus is a solid state amp, so no valves present.
Ooops! I just assumed it was one of the 5-20 lookalikes produced by Leak. Thanks for putting me straight. My comments about grid leak resistors can be ignored, in the context of this thread!
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Old 16th Feb 2018, 3:09 pm   #10
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Default Re: Leak Stereo 30 Plus - Setting Quiescent Currents

Hi Jim,

Just which end of the pot are running up against?

There are a couple of things you could check.
First, do check the DC voltage at the output, before the output coupling capacitor. It should be roughly half of the supply voltage. If the "half-voltage" is off, do check other voltages in the circuit as well.
I enclose a picture of the simulation of the circuit with a number of voltages and currents.

Do measure the voltage drop on those two combined biasing diodes, D1R, D2R, at 5mA current, for both channels and compare them.

The other thing that the simulation has shown that the existence of R49, 22kOhm (R22 in the simulation), is questionable, with that resistor in position, the value of P5 (R5 in the simulation), that 200 Ohm trimpot is nearly zero for the 30mA idle current.
Having simulated quite a number of audio amps with quasi-complemeter final stages this is the first amp where I see such additional biasing, such a resistor. You could try removing it.

If you feel uncomfortable removing that resistor then my bet would be on the different forward bias voltage on those two diodes.

Regards, Peter
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Old 16th Feb 2018, 5:48 pm   #11
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Angry Re: Leak Stereo 30 Plus - Setting Quiescent Currents

Iím not sure why you would need to modify the circuit at all. The Leak stereo 30 plus, was a very popular amp and has a proven reliability and performance record.


It should only require basic fault finding to get this back up & running as per the original design.

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Old 16th Feb 2018, 7:04 pm   #12
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Default Re: Leak Stereo 30 Plus - Setting Quiescent Currents

Thanks all Much appreciated... I shall get back to it this weekend and report back...

Yes, edge connectors cleaned as one of the first things I did...

Peter, the pot is very almost fully clockwise as you look down on it...

That simulation is interesting and gives me a good few things to check!
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Old 18th Feb 2018, 7:48 am   #13
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Default Re: Leak Stereo 30 Plus - Setting Quiescent Currents

Had another look at the amp yesterday... The driver boards are faultless. Swapping them proves the problem is with the output transistors and/or their temperature compensation diodes...

I checked both pairs of output transistors and diodes for leakage and can find none at all. One diode on the problematic channel has a slightly higher forward resistance but that's all...

Again, long soak testing at moderate volumes and power outputs reveal no problems...

I note that the setting of the quiescent currents are very temperature dependant... Checking this morning in a very cold workshop the current is a lot less that it was yesterday afternoon in rather warmer ambient temperatures...


Mr. Leak does not say at what temperature the currents should be set. Any thoughts? 20 degrees perhaps?

Won't get a lot done today; I'm off running a half marathon later...
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Old 18th Feb 2018, 10:22 am   #14
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Default Re: Leak Stereo 30 Plus - Setting Quiescent Currents

Hi Jim.

Did you test those two diodes to see if the volt drop across the junctions are about the same? I would replace both as a first attempt to clear the problem.

A pair of 1N4148’s should be fine.


Let’s hope you have some energy left after that marathon - good luck!


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Last edited by thermionic; 18th Feb 2018 at 10:24 am. Reason: Additional info
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Old 20th Feb 2018, 5:39 pm   #15
Jim - G4MEZ
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Default Re: Leak Stereo 30 Plus - Setting Quiescent Currents

Quote:
Originally Posted by thermionic View Post
Did you test those two diodes to see if the volt drop across the junctions are about the same? I would replace both as a first attempt to clear the problem.

A pair of 1N4148’s should be fine.
Good point Simon, Thanks for that I just have and they're different... So they're up for replacement...

Good to know a pair of 1N4148s will do as trying to source a set of good BA164s might be a bit tricky now...

The Half Marathon went very well... Just a few seconds over two hours and then I cycled home!
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Old 21st Feb 2018, 8:01 am   #16
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Default Re: Leak Stereo 30 Plus - Setting Quiescent Currents

Well, youíre a better man than me Jim. Just thinking about all that exercise makes me exhausted!

Let us all know how the diode replacement goes.


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Old 4th Apr 2018, 6:52 am   #17
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Default Re: Leak Stereo 30 Plus - Setting Quiescent Currents

Reading back-posts as I'm new to this excellent site. I've a couple of S70's of which one of them drew me here- the problem was different but from my early efforts, the IQs were challenging. Those trim-pots can be reversed, so as the less 'flaky' track is used. Just unsolder and rotate. Two of the legs are shorted. I've searched for replacments but no luck to date. This has settled my IQ's, which as you say, drift daily and in some cases, alarmingly- due to high-resistance on 50-year old presets!

As a side issue- I found BC377's for pre-amp trannies a good substitute.
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