A frustrated Motorola

[Herald1360]

Originally Posted by Newshound68

Techman, when you say "take a signal out from the volume control", how is that done exactly? Connect a wire from the middle tag of the volume pot to an amplifier? Just a single wire?

The low level audio signal from the detector goes to the "top" of the pot, the "bottom" is connected to the common signal line (ground, earth, 0V, whatever). A varying level (volume) is tapped off by the moving contact as it travels fromthe bottom (no output) to the top (max output). This level is then amplified by the following audio stages until it's big enough to drive the loudspeaker.

No, not just one wire, a connection to the bottom of the pot for signal return is also needed. A bit like the chassis/body connection on a car. If connecting to another separate piece of equipment it may also be safer to put a capacitor about 1uF in series with the signal lead in case there is any unexpected dc difference between the radio and the other kit.

['LIVEWIRE?']

With the readings provide by the OP, it's fairly obvious that the audio amplifier, especially the output stage, is faulty. The voltage at the emitters of TX7 & 8 should be about half supply, i.e. between about 6 & 7v. As this is a DC coupled amplifier, the problem could be anywhere, but one suspect might be a short circuit speaker coupling capacitor(C29) This is easily checked by connecting a multimeter, set to a low ohms range across the capacitor (with no power applied to the radio.) Other possibilities are leaks or shorts in any of the preceding AF Transistors.

[Radio Tech]

Hello Newshound68

I have some of those power transistors if you would like them

Regards

Ken

['LIVEWIRE?']

Ken, do you mean that you actually have some of those Motorola Transistors? I thought they became obsolete many years ago!

[Newshound68]

Chaps,

This is all really helpful - thank you!

A couple of follow-ups, please give as much feedback as possible:

1. Re TX measurements I posted. Just to be ABSOLUTELY CLEAR I put the black ground lead to the radio case and the red +ve to the bottom solder point On the PCB of the three nodes on the transistor in each case.

BUT I was only pumping 12v via my bench supply because it's a car radio ergo, wouldn't 13.8v break it

But I can retest at that level of people urge me to ...


2. I have secured a nice old-school RF signal generator off eBay. No idea how to test it though when I pick it up. Help?!

3. Would buying a 2nd hand Scope and patiently testing EVERY and ALL 150 components in circuit be the perfect solution?

4. Re comments on feeding to an amp. I only really have a Marshall guitar amp, which was pretty pricey. Could I use that? Or buy and construct one of those Maplin/ Volleman diy amps with a speaker?

5. Do I need a 100mhz scope for radio work? Or will say a cheaper 60mhz Hameg scope be ok for most things?

Thanks in advance

[Newshound68]

Yes please Ken!

Mod edit: email removed, please use the PM system.

[Newshound68]

Interesting. C29 is the one cap which responds when I play an audio signal through it. It allows audio through to speakers.
Could this actually mean it's the secret villain? That it's open?
I hadn't thought about whipping C29 out (I've replaced C20, C19 & C24 although they tested ok out of circuit.

[Newshound68]

I just tested C29 in circuit. It goes up and up to infinity on the ohms scale (like the new and good caps I have tested) so I guess it's ok after all

[crackle]

In answer to your questions;
1. There is no need to re-measure the voltages on all the transistors. But it would help if you could re-measure the voltages on TX1 as I believe you may have written down the measurement for the base incorrectly. You put 10.87 in your results table, it should be more like 0.87v.

2. If you are thinking of entering the world of vintage radio repair then the signal generator will be very useful, but for this repair I suspect the fault lies in the last 3 transistors TX6, 7, & 8 so a signal generator is probably not required. (unless the RF coils and trimmer capacitors have been fiddled with and the set needs a full RF & IF re-alignment)

3. A scope is rather more complicated to use and will not help with this fault, I suggest you wait before getting a scope till you absolutely need one to trace a fault.

4. If the Marshall amp has a line input then it would be ideal, but it would be advisable to use a capacitor in the feed FROM the volume control. As already said the other connection for this test would go to the outer connector of your jack plug on the amp to the chassis / ground of the radio.
If you are serious about your hobby then there is a Velleman Signal tracer kit available which I found useful, for both injecting a signal as you have tried already with your Ipad or monitoring for a signal like the test you need to do now as it also contains a small amp.

5. A 100MHz scope would be nice, but for domestic radio work you can often get away with a 20MHz scope. (the IF frequency of FM radios is 10.7MHz) It depends how serious and advanced in your hobby you are and how much money you have. Again I would wait to see how you progress with your hobby. (unless there is a real bargain coming your way)

My suggestion now is re-check the voltage on TX1 base, then do the test with taking a feed from the volume control to an external amplifier, you may be surprised to find the front half (RF) of the radio works.

Mike

['LIVEWIRE?']

In response to Newshound's post #25 above, all 12 volt Car radios will work OK between ca. 11 and 14.5v. This is because, although the nominal voltage is 12 (6 x 2v cells), each cell of a car battery will measure up to approx. 2.4v when fully charged, i.e. the battery voltage, off load, can be as high as 14.4v, although 13.8 is often quoted as the 'nominal' voltage. As to testing that signal generator set it to produce a modulated AM signal of about 100uv or so and inject a 465-470kHz (The Intermediate Frequency or I.F.) to the base of TX3. This should produce an audio out put in the speaker, though only after the fault in the output stage of the radio has been repaired.

[TrevorG3VLF]

As I said in #18, TX 6, a pnp transistor is wrong. If you get this right, then the output transistors may come up to reasonable voltages.

Correct what is wrong before playing elsewhere.

['LIVEWIRE?']

Re-reading the voltage measurements on TX6 in particular, I agree with Trevor, that this transistor is faulty, meaning that TX7 & 8 are 'off' i.e. not conducting due to lack of base bias. so replacing TX6 should cure the fault, though in this, as in all DC coupled amplifiers, TX4 & 5, and associated components should also be checked. Unless Ken (RadioTech)has some of the original Motorola Transistors, as he mentioned in post #23, then commercially available substitutes will have to be used.

[Newshound68]

Many thanks chaps

Mike - You are correct 're TX1 - I wrote down 10.8 and should have posted 0.8

I looked at the signal.generator kit from Maplin/Volleman. Good value at £7.99. It goes up to 1khz. There's also an Android app which is quite good.

I have got a lead now which has a guitar-style plug on one end and two audio clips on the other end to connect to the middle tap of the volume to test.
I will hold a 1uF capacitor to the hot middle tap and clip the earth to the case of the radio or the bottom of the volume?

As I mentioned earlier I have removed TX 1-5 and put them through the hFE on the MM. All give a reasonable reading and also test as they should base to e; base to c; b to c etc. Also they test as the correct type.

So it does point to TX 6, the "Corbyn" as I call it!

Ken - any news on those trannies?

Cheers all

[Herald1360]

ko

Originally Posted by 'LIVEWIRE?'

In response to Newshound's post #25 above, all 12 volt Car radios will work OK between ca. 11 and 14.5v. This is because, although the nominal voltage is 12 (6 x 2v cells), each cell of a car battery will measure up to approx. 2.4v when fully charged, i.e. the battery voltage, off load, can be as high as 14.4v, although 13.8 is often quoted as the 'nominal' voltage.

For a 6 cell lead acid battery, at around 20C temp, the voltage for a rapid full charge and that which a car charging system will apply is around 14.4V. 13.8V is the level for a float charge which will just keep a fully charged battery up to scratch indefinitely. 12V is a typical on load off charge voltage untill the battery starts to get near full discharge when the voltage will drop off rapidly.

[Newshound68]

Just thought I would upload some more pics of the Motorola 114.
I bent back the side panel to get to the output transistors and the Volume pot.
If you look closely TX6 is indeed broken - the collector snapped.
But I found it like that last week and re-soldered it. It has broken again.

Question: can you fix a transistor leg with solder?

Also, I would still like to do the Volume pot to amp test but I am a little worried so just wanted some final reassurance.
I still find a couple of things confusing about this, and as I have an expensive Marshall amp I don't want to damage it or even risk it. SO:

1. By connecting a 1uF capacitor, a modern one - surely that has to be an electrolytic, so what about the polarity issue?

2. Using the two photos I am posting, does anyone have the tech to actually repost an arrow showing which bit to put the +ve lead to the amp?

If not, I can repost with numbers annotated to the pics in a day or so
Sorry for these baby questions, but I'm hoping these questions could be helpful for people for years to come. Indeed Ive learned a great deal from posts of five years old or more ...

Thanks again in advance

[Newshound68]

PS. Can people also see the "nicotine" colored stains on the PCB in the output side of the PCB. I thought this might be capacitor leakage, but I can't find any obvious bloated caps., others may spot something of course ...

[TrevorG3VLF]

TX6
A broken collector lead will of course give no output current so the circuit will do its best to make the output correct. This seems to have killed the transistor since there is no way that more than about 1V should be between base and emitter.

You could remove and test the transistor if you wanted. If you wish to reconnect the emitter, then use a short lengh of fexible wire to allow some movement without too much stress.

[crackle]

Hi
I have annotated photo 4.
a b c are the tone control connections.
1 2 3 are the volume connections but there should also be a 4th one around the side.

I cant make it out as there seem to be wires missing from the volume connections, there should be a coloured wire coming off tag 2 on the volume connections.
I can see a bare wire connecting from 2 to c and what looks like C22 connecting between c and b, but it also looks like a bare wire is connecting b to a, which is not correct according to the circuit diagram.
I wonder if the volume pot does not have a "loudness" tap.
I cant make it out.

Mike

[crackle]

I have just noticed in photo 3 that it looks like the loudness tap on the volume pot has not been connected.
That's strange.

From the photos it looks like the red wire goes to R19 and the white wire goes to R16.
So in theory the point to connect to an external amp is; inner wire via 0.5uf capacitor (use foil type or a non electrolytic) to tag 2 and outer/shield to tag 3.

But the wiring of this radio is not the same as the service manual for the Motorola 114
Mike

['LIVEWIRE?']

Point 2* of the volume control (The wiper) should be connected to R19, which is on the PCB, and C 22, an 0.22uf disc ceramic, which is part of the tone control. R17 & C21 form the Loudness correction components, and, IIRC, are mounted on the PCB. (* Numbers as per Crackle's annotated photo in post #39. This is marked (18) on the Motorola Circuit Diagram, whilst point 1 in Crackle's photo is marked (17) on the circuit diagram.