Leak Stereo 30 Plus

[Sideband]

Originally Posted by PJL

Can you confirm that you put the working amp board in both sockets and in each case the amplifier worked? If that is the case then the output transistors and associated bias diodes in both channels must be OK.

In that case there is no point in measuring the output transistors, we know they are OK as it works on both channels with the good driver board.

[Pcspike]

Originally Posted by PJL

I suggest you do some resistance checks before trying to power up again. Set your DMM for resistance and check the probe leads are inserted in the correct +/- sockets. Putting the -ve on the base measure the resistance from base to collector then base to emitter. Repeat with the +ve probe on the base. Repeat for all 4 transistors on the PCB and the two power transistors. Then repeat the same tests on the good board.

Faulty T6 Base- bc=?ohms, be=?ohms: Base+ bc=?ohms, be=?ohms
Good T6 Base- bc=?ohms, be=?ohms: Base+ bc=?ohms, be=?ohms
Faulty T7 Base- bc=?ohms, be=?ohms: Base+ bc=?ohms, be=?ohms
Good T7 Base- bc=?ohms, be=?ohms: Base+ bc=?ohms, be=?ohms
And so on...

Post the results here.

A bit laborious but risk free, whilst using the lamp limiter can easily cause component damage as well as it being difficult to take voltage readings on the PCB when installed in the socket.

Faulty T6 Base- bc= 36.28 k ohms, be= 52.40 k ohms: Base+ bc=36.44 k ohms, be=496 k ohms
Good T6 Base- bc= 36.45 k ohms, be= 50.15 k ohms: Base+ bc= 36.75 k ohms, be= 486.5 k ohms
Faulty T7 Base- bc= 2.536 m ohms, be= 1.083 k ohms: Base+ bc= 348.6 k ohms, be= 1.083 k ohms
Good T7 Base- bc= 42.5 m ohms, be= 1.196 k ohms: Base+ bc= 313k k ohms, be=1.196 k ohms
Faulty T8 Base- bc=5.63 k ohms, be=488 k ohms: Base+ bc=4.908 k ohms, be=438.7 k ohms
Good T8 Base- bc=5.12 k ohms, be=471 k ohms: Base+ bc=4.600 k ohms, be=429.4 k ohms
Faulty T9 Base- bc=351.5 k ohms, be=412 k ohms: Base+ bc=2.536 k ohms, be=7.18 m ohms
Good T9 Base- bc=316 k ohms, be=457.5 k ohms: Base+ bc=8.1 m ohms, be=9.22 m ohms

Output Transistors
Faulty Board Right Channel

Faulty - bc=5.718 k ohms, be=261.6 ohms: Base+ bc=3.883 k ohms, be=261.6 ohms
Faulty - bc=39.93 k ohms, be=270 ohms: Base+ bc=3.898 k ohms, be=270 ohms

Good Board Left Channel

Good - bc=5.581 k ohms, be=257.5 ohms: Base+ bc=3.920 k ohms, be=257 ohms
Good - bc=40.24 k ohms, be=256.5 ohms: Base+ bc=3.899 k ohms, be=256 ohms

[PJL]

The results for T9 look different enough to be of concern, can you double check the Base+ measurements. If they are accurate then extract T9 from the faulty board and do the same bc/be resistance measurements out of circuit.

[Silicon]

On a previous post you mentioned that you had adjusted the pre-set bias resistor P5.

'I found (on one attempt) that with the resistance turned up on p5 the fuse held but when I turned it down slightly it blew.'

The fuse could be blowing due to an incorrectly set Quiescent current.

Correct me if I am wrong, IF you were to put a link wire across the pre-set P5, it would reduce the Quiescent current to a low value and possibly enable the amplifier to work.
A faulty or intermittently faulty pre-set would be bypassed and therefore have no effect on the circuit.

There will be some distortion but it might prove a point.

Can you measure that minimum Quiescent current?

How did you measure the Quiescent current previously?

I believe you need to set P5 to its minimum resistance first before you power up the amplifier.

Did you follow the procedure on the Leak instruction manual?

[Silicon]

Carefully check the contacts in the PCB socket for the faulty channel.
Corrosion or loose connections on these will create high resistance with the 'contact fingers' on the PCB.

Positions C and J are two of the most important connections.

Some components may test OK on a digital multimeter but then break down when subjected to the full working voltage in the amplifier.

[Pcspike]

Originally Posted by PJL

The results for T9 look different enough to be of concern, can you double check the Base+ measurements. If they are accurate then extract T9 from the faulty board and do the same bc/be resistance measurements out of circuit.

Hi,
T9 out of circuit
- B-C = 445.1K
- B-E = 459.4K
+ B-C = OL
+ B-E = OL

[Pcspike]

Originally Posted by Silicon

On a previous post you mentioned that you had adjusted the pre-set bias resistor P5.

The fuse could be blowing due to an incorrectly set Quiescent current.

Correct me if I am wrong, IF you were to put a link wire across the pre-set P5, it would reduce the Quiescent current to a low value and possibly enable the amplifier to work.
A faulty or intermittently faulty pre-set would be bypassed and therefore have no effect on the circuit.

There will be some distortion but it might prove a point.

Can you measure that minimum Quiescent current?

How did you measure the Quiescent current previously?

I believe you need to set P5 to its minimum resistance first before you power up the amplifier.

Did you follow the procedure on the Leak instruction manual?

Hi.
I set the quiescent current on the good board as outlined in the service manual and adjusted it accordingly. I will try what you suggest regarding a link across P5, I take it that these would be the two outer pins? If this proves to be the fault is it possible to get a replacement that would physically fit the board?
It may be a little while before I can do this as I need to make a lamp limiter as well as other things that I need to get on with.

[Pcspike]

Originally Posted by Silicon

Some components may test OK on a digital multimeter but then break down when subjected to the full working voltage in the amplifier.

Thanks for that, and do I agree about the components checking ok but being faulty under load. I believe that the problem is related to the variable resistor P5, or damage caused because of it. If I replaced all the components on this board it would work and with this in mind given the little cost and effort, I've ordered a new set of resistors and see how I get on from there.

[Sideband]

Originally Posted by Pcspike

Hi,
T9 out of circuit
- B-C = 445.1K
- B-E = 459.4K
+ B-C = OL
+ B-E = OL

To get any meaningful results, you need to test transistor junctions with the diode mode on your meter. This wiLL bias the junctions and the readings you see on the meter will be the voltage across the junction, infinity (or should be) one way and around half a volt or so the other way.

[Pcspike]

Originally Posted by Sideband

To get any meaningful results, you need to test transistor junctions with the diode mode on your meter. This wiLL bias the junctions and the readings you see on the meter will be the voltage across the junction, infinity (or should be) one way and around half a volt or so the other way.

-b-c = 653
b-e = 658
+b-c = ol
b-e = ol

[Sideband]

Also check c-e which should read ol both ways. If it does than the transistor is probably OK. What it doesn't test of course is gain (unless you have an hfe function on your meter in which case you will also have a transistor test socket).

[PJL]

Originally Posted by PJL

Faulty T9 Base- bc=351.5 k ohms, be=412 k ohms: Base+ bc=2.536 k ohms, be=7.18 m ohms
Good T9 Base- bc=316 k ohms, be=457.5 k ohms: Base+ bc=8.1 m ohms, be=9.22 m ohms

It is these 2 figures that could be showing up an issue. It does not mean that T9 is faulty as it could be anything forming the b to c circuit. Out of circuit T9 looks OK so while it is still out of circuit do the same measurements but on the pads where the transistor should go. Use the resistance mode and not the diode mode.

[Pcspike]

Originally Posted by Sideband

Also check c-e which should read ol both ways. If it does than the transistor is probably OK. What it doesn't test of course is gain (unless you have an hfe function on your meter in which case you will also have a transistor test socket).

C-E ol both ways.
hFE 108
Vf 802 mV

[Pcspike]

Originally Posted by PJL

It is these 2 figures that could be showing up an issue. It does not mean that T9 is faulty as it could be anything forming the b to c circuit. Out of circuit T9 looks OK so while it is still out of circuit do the same measurements but on the pads where the transistor should go. Use the resistance mode and not the diode mode.

- b-c = 359.1 k
- b-e = .903 m
+b-c = ol
+b-e = ol

The measurement - b-c starts as a lower reading and builds up to .903m takaking about 30 seconds before it arrives at .903

[Pcspike]

Update
As pot P5 seemed to be cropping up pretty regular, I decided to measure it out of circuit, and on removal, one of the pins fell off at the fixing point. As I have a Leak stereo 70 handy, I checked the P5 on that to find that it was the same rating and decided to try it as a temporary replacement. As I was at it I also changed R54 and capacitor C3 only because they were directly connected to P5, though they did measure ok. I decided to try it without a dim bulb tester (because I’m waiting for some bits to make one) and found that it didn’t blow the fuse. I tried a CD and it played ok without any distortion, but it only lasts for about 15 seconds before just fading out. I tried this a few times and each time the same thing happens, it’s like the volume is being turned down.

[PJL]

Can you set the quiescent current now?

[Pcspike]

Originally Posted by PJL

Can you set the quiescent current now?

As soon as the sound goes I've switched off, not wanting to push my luck. I have set the wiper to the same position as the working card thinking that it will be close enough at this stage. I intend doing it when I get the dim bulb tester up and running. I do feel quite optermist now that progress has been made even though not quite there.

As regards the sound fading, is this characteristic to any particular component failing that you know of?

[Sideband]

...but if it's fading out on both channels, surely this indicates another problem? I'd check power supply first. Clip your meter, set to DC volts (use the 50V or 100V range....whatever you have on your meter) to the main smoothing capacitors and see what happens to the voltage when it fades out (if anything). What I'm getting at is that a fault on the pre-amp board on one channel should not affect the good working channel. If both channels fade out it has to be something common to both, like the power supply.

[Pcspike]

Originally Posted by Sideband

...but if it's fading out on both channels, surely this indicates another problem? I'd check power supply first. Clip your meter, set to DC volts (use the 50V or 100V range....whatever you have on your meter) to the main smoothing capacitors and see what happens to the voltage when it fades out (if anything). What I'm getting at is that a fault on the pre-amp board on one channel should not affect the good working channel. If both channels fade out it has to be something common to both, like the power supply.

Hi, I can confirm that I removed the good board before trying the modifications that I made to the faulty board so as to protect it.

[Sideband]

This is all getting very confusing.....!