Clarke and Smith 8812 FM problem

[qualityten]

I recently bought a Clarke and Smith 88/12 tuner amplifier, mainly to satisfy my curiosity about them.

I am impressed by the build quality and layout. This one is in untouched condition and the amplifier and LW and MW radio all work pretty well.

However, there is no squeak from FM. Looking at the circuit online, I see that mine is the newer version with a Mullard LP1179 tuner section, replacing the older version which has a single ECC85 valve for RF amplification. I recall, when working on a Bush VHF 70, that the ECC85 often needs replacing in FM circuits and I wondered if the absence of a signal is caused by a faulty BC187 which seems to replace the ECC85. I say 'seems' because I don't understand the FM circuit.

I have downloaded the resources here: http://portabletubes.co.uk/boats/clarke.htm

Before I suspect the whole LP1179 unit, would it be worth simply replacing the BC187 transistor?

[Herald1360]

LP1179 uses lokfit transistors. That BC187 isn't one and may be a substitute. It's also a nominally audio device (BC not BF). Lokfits are known now to have odd failure modes and are almost as notorious as AF11x types.

Maybe worth a look inside to see if there are more of the little devils.

[ronbryan]

The BC187 is shown on the C&S schematic available on the portable tubes site to be an additional IF buffer amplifier stage, external to the LP1179. I would suggest you measure the voltages around that stage and see if it has any problems.

Ron

[qualityten]

Thank you Chris and Ron for your advice.

Ron, because of my very limited knowledge of FM circuits and transistors, what sort of voltages should I be looking for. The BC187 datasheet mentions a Uce of -25V. Should I be looking for this across the emitter and collector? What other voltages should I be looking for?

Apologies for my ignorance, and thanks to you both for your guidance.
David

[Nickthedentist]

My knowledge is limited too, but...

Here's the snippet of the diagram from Terry Martini-Yates' website:

The +ve supply line is derived from the 6.3V~ winding via a half-wave rectifier and a simple CRC filter with electrolytics rated at 10V max.

Please tell us what the voltages (w.r.t. chassis) on the three legs of the BC187 are.

And the supply rail voltage, measured at the "top" end of the 6K8 and 1K2 resistors or pin 7 of the module? Mullard specify 6.8V in their datasheet.

Nick.

[qualityten]

Thanks for further steering Nick. There is definitely something wrong with the FM stage. Looking at the power supply, I replaced the two 220uF 10V capacitors. After this, when I switched the FM stage in, the 4k7 resistor in the main power section started overheating and melting the insulation on the leads! There was also loud hum which had not been present before. I replaced the filter caps C1 and C2, which quietened the hum, but the resistor still overheated.

Having replaced C1 and C2, I felt confident enough to measure the voltages around the BC187 transistor. The supply rail measured -6.5V, the base -5.85V, the emitter -5.4V and the collector -2.7V.

I'm puzzled by the negative voltages. Should they be negative? The two capacitors feeding the supply are positive to ground, something I've only encountered the bias voltage supplies for valves.

[ronbryan]

The power supply derives +6.5V from the heater supply as Nick has said. The two 220uF capacitors smoothing that +6.5V supply should have their negative side connected to ground, which is the same point electrically as the metal can and pin 4 of the LP1179 FM tuner. You should check that is how they are connected.

If you have the negative (black) lead of your voltmeter connected to ground and the positive (red) lead of your voltmeter to pin 7 of the tuner, with FM selected, you should read +6.5V.

If you connect the red meter lead to the transistor electrodes in turn, the emitter of the BC187, a pnp transistor, should have the most positive voltage (i.e. +5.8V), the base should be 0.7V less positive than the emitter (i.e. +5.1V and the collector should be around 3V less positive than the emitter (i.e. +2.8V).

Ron

[qualityten]

Thanks Ron, for your checks and the attachment on the FM module.

I am even more puzzled now, because it seems that the power supply to my module is connected to the negative supply (pin 4) and this is allowed for in the document you attach. But is this simply an option? My pin 7 has never been soldered.

This means that my unit was intentionally wired to provide the negative supply and that the replacement orientation of the caps was correct when I did it.

However, by changing the polarity of two 200uf 10V capacitors as you said and changing the polarity of the diode AO202 I do get positive voltages to the transistor of emitter 3.75V, base 5.05V and collector 3V. But of course this is still connected to pin 4 which is described as negative supply on the attachment from Ron.

I'll connect the positive supply to pin 7 tonight and see what happens.

[qualityten]

P.s. the numbering on the FM module is smudged. It could be LP1179, but I can't confirm this visually. except that it has the same number of pins as the LP1179. Is it possible that Clarke and Smith used other modules here too?

I grabbed a free moment and connected the positive supply to pin 7, but something is grounding the supply after the CRC filter and so no voltage is reaching the FM unit.

[ronbryan]

It is likely that the LP1179 is being used with pin 7 (+ve supply) soldered to the can and grounded, and pin 4 (negative supply and 'ground' ) connected to the negative supply. In this configuration it is normal for a 0.01uF decoupling capacitor to be connected between pin 4 and the can to prevent instability.

Can you confirm that this is how it is?

Can you check to see if the metal can is connected to the main chassis metalwork, with an ohm meter.

If the above is how the module is connected, do not as proposed in post #8 reverse the diode to change the polarity of the supply to positive, as a negative supply is what is needed. Ensure that the 220uF caps have their positive ends connected to chassis/0V.

The confusion about polarities has been caused by either your unit not being wired to the diagram, or a drafting error in the preparation of the diagram.

With regard to your last post (#9), I thought I had until this evening to reply!

Ron

[ronbryan]

Hopefully, this quick sketch shows how the power and LP1179 connections are on your Dynatron unit.

Ron

[ms660]

No wonder it doesn't work.

Lawrence.

[ronbryan]

Thanks Lawrence, I'll correct the polarity of the BC187 buffer circuit shortly.

In fact I've managed to attach the corrected sketch to this post.

Ron

[qualityten]

Thanks again for your kind replies Ron and to Lawrence for looking over your shoulder. I've not been able to get back to the 8812 tonight, but will have some time to do so tomorrow. I'm pretty sure that the LP1179 is as you describe in post #10. Hopefully working with the correct schematic will help to resolve my question about why the FM section is drawing so much current.

[qualityten]

Ron, referring back to your post #10:
pin 7 (+ve supply) is soldered to the can and grounded
pin 4 (negative supply and 'ground' ) is now connected to the negative supply. There is no decoupling capacitor and pin 4 is connected to an extra pin 8.

The metal can of the LP1179 is connected to the metal chassis.

I restored the polarity of the capacitors and diode so that there is a negative voltage to pin 4, but there are some differences from your circuit drawing. The main difference is that my unit has the transistor and resistors wired as per the original circuit with a positive supply to pin 7. The 120 ohm resistor and 470uF capacitor after the CRC power supply is absent.

I have drawn and attached the circuit as it is on my unit. Would this work with a negative supply?

One other detail, you'll see that there is a 0.1uF Hunts capacitor between the base of the transistor and ground. It measures over 200nF, so I have replaced it since taking the attached picture, but haven't energised the circuit yet.

[ronbryan]

From the chassis photo it looks like the buffer stage is a pnp grounded base amplifier, with the 500pF cap from the tuner connected to the BC187 emitter with 1k2 to -6.5V, the base biassed with a 33k to 0V and 6k8 to -6.5V and decoupled to 0V with 0.1uF, and the 10.7MHz output (white wire) taken from the collector with 4k7 to 0V (chassis).

Are the lead colours of the BC187 blue=collector, green=base and yellow=emitter? It is not really clear from the photo.

So if my analysis of the chassis photo is correct, your circuit should be corrected to show the 500pF going to the emitter, and the upper base bias resistor 6k8 not 5k6. The grounded base circuit as I described here should work, provided the transistor has survived the polarity reversal. Can you measure the transistor voltages again?

Ron

[qualityten]

Just before reading your latest post, Ron, I thought I might look more closely at the transistor and decided to remove it to test it. I found that it is a BC109, i.e. an NPN transistor, rather than the PNP BC187 on the circuit diagram. Sorry I should have looked at this originally, rather than blindly following the circuit diagram. Does this affect the advice you've just given?

To answer your question about lead colours, yes it is yellow - emitter, green - base, blue - collector, though of course for a BC109.

Update: I fitted a replacement BC109C and got the following voltages: emitter -6V, base -5.5V and collector -2V. That's without moving the 500pF capacitor to the emitter. Still no FM audio though.

Thank you for taking the time to work this through for me.

[ronbryan]

If the 500pF capacitor from the tuner output is wired to the emitter, as it appears to be on the chassis photo, and with the base decoupled to chassis with 0.1uF, then it is still a grounded base amplifier. With a BC109 (a highly unlikely RF amplifier transistor) with a negative voltage on the emitter, a more positive voltage on the base and ground on the collector load resistor it will work as a grounded base amplifier.

With regard to my previous post (#16), the polarities were wrong for a pnp transistor - apologies for the (recurring) error.

Can you measure the transistor voltages with respect to chassis?

Ron

[qualityten]

With a new BC109 in position, the transistor voltages to the chassis are as reported above in my update to post #17: emitter -6V, base -5.5V and collector -2V.

Should I leave the new BC109 there, or replace it with a BC187? Perhaps leave it there, if the voltages look right, and move on to finding why the LP1179 is not working?

[ronbryan]

The voltages look fine. By all means investigate the tuner and leave the BC109 in.

I don't understand your comment in the post #17 update about moving the 500pF to the emitter. It appears already connected to the emitter (yellow) and 1k2 in the chassis photo as far as I can see.

Ron