Leak Stereo 30 Plus

[Chris55000]

I've had a look at the OP's pictures and that capacitor C37 between P5 and R54 appears to me to be coded orange–black–brown (my colour vision certainly isn't defective,
despite the parlous state of the rest of it!), which makes it 300p, however all the available diagrams mark this as 200p, so that could be a camera abberation making red look orange to me!) and that value was quite commonly offered in that particular range (I think they were Erie Manufacture).

I've got a Leak Stereo 30 book and one of these amplifiers in Wharfedale disguise with piano–key switches, so if anybody needs anything, please let me know!

As other Members have commented, it's very unlikely that C37 needs replacement, but you can get a high quality silver–mica replacement here :–

https://www.ebay.co.uk/itm/120797119...mis&media=COPY

Chris Williams

[Sideband]

Originally Posted by Chris55000

I've had a look at the OP's pictures and that capacitor C37 between P5 and R54 appears to me to be coded orange–black–brown (my colour vision certainly isn't defective,
despite the parlous state of the rest of it!), which makes it 300p,

It certainly looks like 300pF. The orange is very similar to the shade of orange on the 33k resistor at the top of the PCB. Nearest modern equivalent would be 330pF. The extra 30pF can be ignored.

[snowman_al]

To save confusion, I think you will find that the resistor at the top is actually a 22KΩ, not 33KΩ... It's R49.

Interesting to observe that we all see red in different ways, perhaps due to the setting of our computers and screens as well as our eyesight?

I'm sure the OP will be able to see what the leading colour is and perhaps confirm.

[Radio Wrangler]

The colour sensitivity of most digital cameras extends a bit more into the IR region than does our natural vision, so colour rendition can look normal most of the time, while still giving the off surprise. The classic example of this is in photographs of bluebells which exhibit quite a significant difference from real life, going somewhat purple-y. The camera is responding to some colour components we can't see and they get assigned as red.

In photographic circles, it's quite popular to take a digital camera once it's no longer seen as any significant value, and strip off the RGB colour separation filter from the sensor. This gives a B&W camera with useful IR sensitivity, so fit an IR pass filter (on a lens that's known to pass IR), and get on with taking creepy ghostly photos.

David

[PJL]

Originally Posted by Pcspike

I have checked every component on it

What checks did you do? How did you test the transistors? Did you check the output transistors?

It's quite possible for a fault to make the quiescent current enough to blow the fuse but for it to be lower when unstable without the capacitor.

[Pcspike]

Originally Posted by Sideband

Some capacitor colour codes seem indecipherable unless you have a data sheet for the particular capacitor. It's also sometimes difficult to know which end to read the colours from. In these cases it helps if you have a test meter capable of reading low value capacitors...say from 10pF up to several hundred uF. You can also buy Chinese component testers that seem quite popular and not expensive. Check out eBay.

Be aware of different ways of writing the same value vis: 103, 10k, 10,000pF, 10nF .01uF, brown, black, orange or perhaps you'll see 102, 1000pF, 1k, 1nF, .001uF, brown, black, red. Then perhaps you might see 152, 1500pF, 1k5, 1n5, .0015uF, brown, green, red... It takes time to get used to all these variations!

102 for instance translates into 1 , 0 and two zero's. 103 translates into 1, 0, and three zero's. Lower case k is simply kilo or 1000 and lower case n is nanofarad.

Some very low value capacitors like 15pF might be marked n15. It pays to try and swat up on these variants.....these are only the most common.....

Note that three numbers can also be found on surface mounted resistors. 105 for instance would be 1 megohm (1 0 00000). On a capacitor it would be 1uF.

Seems a bit mind-boggling at first glance (for a carpenter) like most things it takes time and practice. I found this when learning about transistors, had to go over and over it until it sank in. Regarding these types of capacitors I couldn’t see any markings to suggest polarity, presumably then, they go in any way round? I’m actually copying and pasting the info that you are providing for reference.

Have placed an order for the cap from the link that snowman_ul provided, and yes have one of those testers from China, for some reason never thought to use it on this cap I tried it today and the cap reads ok, so that blows my theory out of the water

I notice that your location is Croydon, that’s quite close to Redhill isn’t it? Used to travel down there from North Wales to spend my school summer holidays in the 1950's (opps given my age away there) with my uncle and aunt. Happy days.

[Pcspike]

Originally Posted by snowman_al

I assume you have a meter and can take resistance readings.
As you have C37 'free' (out of circuit) measure the resistance across it. It should be infinite and show O.L on the meter. Any resistance reading, low ohms to 100s of K ohms would be a fail.

As the whole unit has 48 volts on it, from the circuit diagram, so a 63 volt or 100 volt ceramic cap will be fine. (Example https://www.cricklewoodelectronics.c...Capacitor.html ) You will not get an original type today.

Perhaps it is time to build a 'dim bulb tester' / lamp limiter and take some voltage readings round the faulty side.

Going to make this my next job. I think I have some of the old bulbs somewhere. Will any wattage do?

[Pcspike]

Originally Posted by snowman_al

I'm sure the OP will be able to see what the leading colour is and perhaps confirm.

I would say red, black, brown, white, think it has faded over time. Looking at the other amp board this cap has a 200k stamped on it.

[snowman_al]

Originally Posted by Pcspike

Going to make this my next job. I think I have some of the old bulbs somewhere. Will any wattage do?

40 or 60 watt should be okay for simple testing. Just note they have to be filament (incandescent) or halogen types. Not LED or fluorescents.

[Pcspike]

Originally Posted by snowman_al

40 or 60 watt should be okay for simple testing. Just note they have to be filament (incandescent) or halogen types. Not LED or fluorescents.

Thanks

[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.

[Pcspike]

Originally Posted by PJL

What checks did you do? How did you test the transistors? Did you check the output transistors?

It's quite possible for a fault to make the quiescent current enough to blow the fuse but for it to be lower when unstable without the capacitor.

Hi, I removed the transistors and checked them using my multimeter and a transistor tester, I measured the resistors in circuit. I also checked the P5 pot. I have changed the electrolytic capacitors but not the other capacitors. I’m considering rechecking all the resistors, do you think I should remove them to test? As mention previously, the amp works on both channels using the working amp 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.

Thanks. Will give it a try tomorrow.

[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.

[Pcspike]

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.

Hi, yes, I can confirm that this to be correct.

[Silicon]

Have you measured the quiescent current of the output transistors on each channel?

The pre-set potentiometer on the power amplifier PCB sets the quiescent current through the output transistors.
If you swap boards, you cannot assume that the previous setting will be good for the other output channel transistors.

You need to adjust the pre-set for minimum quiescent current before you turn on the power.

There is a removable link for the insertion of an ammeter to measure the quiescent current through the output transistors. If the test leads fall off during the procedure, some components can be damaged.

I soldered a 1 Ohm resistor across the link terminals and left it there.
I then measure the voltage across it to calculate the quiescent current.

When I have finished adjusting the quiescent current, I solder a small link wire across it to act as a bypass.

[Pcspike]

Yes I did set it a few months back on the good board. Obviously, not done it on the faulty one (yet).

Re the 1 ohm resistor across the link before testing, I'm sure it's a good idea, excuse my ignorance, but what does that achieve?
Thanks

[PJL]

I did not realise you had already tested the transistors out of circuit on a transistor tester. The fuse has also protected the output transistors so far so ideally we don't want to put those at risk. To blow a 1.6A fuse implies there is a serious fault that is not going to be due to an incorrect bias setting.

The resistance checks on the transistors may still reveal a problem so still worthwhile.

[Pcspike]

Originally Posted by PJL

I did not realise you had already tested the transistors out of circuit on a transistor tester. The fuse has also protected the output transistors so far so ideally we don't want to put those at risk. To blow a 1.6A fuse implies there is a serious fault that is not going to be due to an incorrect bias setting.

The resistance checks on the transistors may still reveal a problem so still worthwhile.

With the boards in place I have measured the output transistors. As for the transistors on the amplifier boards do the boards need to be plugged in when measuring or can it be do when unplugged?

[PJL]

Unplugged