Bush TR230 problems

[mikeald]

Hi all,
I am trying to restore a Bush TR230.
I am a novice at the hobby, but have previously fixed up a TR130.

I paid £3 for this radio, and it did not work at all when I received it.

The dial cord did not 'function' at all, so I successfully replaced that with braided fishing line after around 20 attempts
None of the wave change switches would stay down when pressed, so a bit of contact cleaner sorted that.
And finally I had to re tension the little strip of metal that acts as the on/off switch, as the radio would not even turn on.

After the above, I then had sound, allbeit of poor quality.
I then checked all of the voltages against the service data, and they were all within 0.1 of a volt (so I assume that is ok)?

As I had gone this far, I decided to replace the electrolytics.

I then checked the current draw, and it is pulling 22ma under no signal conditions and volume at minimum.

After replacing the electrolytics the sound quality was still poor. I.E there is lots of crackling and popping whilst tuning in. And when I do tune in to a station the crackling is still present and the sound becomes rather muffly.
This is the same on mw and lw, the 247m preset button is also crackly and muffly.

I am a bit stuck now, especially as this radio has a TAD 100 IC inside. I do not know what else I can check or test?

I can tell you that when I inject a signal at the volume centre tag, the audio comes through loud and clear. Does this definitely mean that the output stages are ok and the fault is prior?

Any advice welcomed please, I must be pretty close.

[Phil G4SPZ]

Hi Mike,

Looking at the Trader service sheet (3019) for this set, all the gain before and after the volume control is provided by the TAD100 I.C. You seem to have proved that the audio stages are functioning OK, so that leaves the passive components in the RF and IF stages, and the chip itself.

Start with the obvious. When you describe crackling and muffled sound when tuning, have you checked that there are no shorting vanes within the main tuning capacitor CV1 & CV2? Try applying a little switch cleaner to the brass sliding contact on the rotor shaft, if fitted. Have the waveband switch contacts been thoroughly cleaned?

Do any of the adjustable coils or trimmers show signs of having been twiddled? The realignment procedure requires a sweep generator plus an oscilloscope to align the IF stage... although if you have a basic signal generator, you could try simply injecting a modulated RF signal into the car aerial socket and see what it sounds like.

Check the wiring and connections to and from the ferrite rod antenna, and check that the coils are still in their original positions on the rod. Check with a meter that there is no DC potential across the volume control; if there is, suspect C14 which could be leaky.

[mikeald]

Hi Phil,

Thank you for the response.

I can confirm the following:

THe tuning cap is definitely free from shorting vanes, and yes the switches are certainly clean.

THe cans all seem intact.

I have a sig gen and have injected 470khz, this comes through loud and clear with no distortion, I then coupled a loop antennae to the set and set the radio to mw, I injected a signal at either end of the scale and the signals both come through loud and clear with no crackling or interference at all when the signal output is set to high.

But when I reduce the signal strength from the sig gen, I then start hearing the crackling/muffling/interference.

Ferrite rod connections are absolutely fine.

Please could you explain exactly where on the volume pot I place my probes to check for DC potential?

I believe the radio is 'tuning' in fine to the stations, as they do come through at a good volume, only with poor sound quality.

Thank you for the help

['LIVEWIRE?']

Measure the DC potential across the two outer tags of the volume control. This should be zero, as it should also be between the 'earthy' end of the V/C and the middle tag at any setting. If a DC voltage is present at these points replace the coupling capacitor(s), in this case, C14, as Phil says.

[mikeald]

Dc voltage across outer volume tags = 1.25v

I replaced c14 with a 47nf replacement, and no change.

After replacing the cap I then re measured the voltage across outer tags of volume control.

It is now 1.2v

THere is 0.1 v present between center tag and earth tag.

Could the problem possibly be the silver/clear capacitors C5, C3 and C1

I believe the output stages are probably ok, so I am assuming it must be something before the IC

[Herald1360]

The volume control is directly connected to TAD100 pin8 which the circuit shows as having 1.45V present, so the control has dc on it by design (hmmmmmmmph). There might be some mileage in replacing C16 1u electrolytic by a plastic film type to keep dc definitively off the output side.

Try going over the PCB with a magnifying glass under a bright light looking for any poor joints (or just reflow the lot).

Littlediode has some TAD100s but at £17+. (Where's that one I never used in the '70s hidden? )

[mikeald]

I have already re flowed the entire lot!

C16 has already been replace with a new electrolytic.

I can tell you that when tuned in to 247m (absolute radio), if I then tweak Ct5 (mw osc)and CT1 (aerial) then the sound quality gets much better and noticeably louder.

However, I can not get it perfect at all.

Correct me if I am wrong, but am I right in saying that unless the IF stages are correct, then no matter how much I adjust CT5 and CT1, then it will never be right?

I am starting to think this is possibly an alignment issue....

I might make a short youtube video later on....

[Phil G4SPZ]

Yes, I did ask whether a twiddler had been at work...

The silver/clear capacitors sound like polystyrene types and are always suspect. The fault does sound like low RF or IF gain or misalignment. Yes, the IF needs to be in correct alignment before anything else is tackled. You might be able to align the IF stage without the specified sweep generator and oscilloscope, but you could also make matters worse. But you've nothing to lose, so try just peaking up the four IF coils at 470kHz and see if it makes a dramatic difference.

Sorry I misled you about DC across the volume control; it is unusual, to say the least, to find it designed-in!

[mikeald]

Hi Phil - No probs, I appreciate your help.

THe polystyrene caps have been tested on my lcr meter and they are fine.

Perhaps someone has been tinkering with it, I really do not know.

But I find it hard to believe the IC is faulty. And there really is not anything else I can replace or check!?

I even removed each of the bypass caps earlier on and tested them and they are fine.

And the fact that all of the voltages are in accordance with the service data, makes me think that it can perhaps only be an alignment issue...

I have access to a sig gen, a scope and a multimeter at home.

Tonight after tea I will post a youtube video, and will post the link here.

It would be great if you guys could take a look and advise...

THanks for your time...

[mikeald]

here you go, please take a look and let me know what you all think:

https://www.youtube.com/watch?v=9zlu...ature=youtu.be

[Stylo N M]

Hi,

it's only a thought but have you checked the ceramic disc type capacitors or solid tubular type ones, there not above suspicion they have a nasty habbit of reading correctly on the multimetre, but cause all sorts of problems in circuit if there happens to be a faulty one.

paul.

[mikeald]

Yes, I have now checked every capacitor....

I might try peaking the IF to see what happens...

Dont know what else to try now!

[Phil G4SPZ]

From your video, the symptoms sound like IF instability - whistles whilst tuning, burbling distortion, and excessive gain. A couple of possibilities come to mind, but as I don't have the circuit in front of me at the moment, you'll need to look at any capacitors that decouple supplies to and within the IF stage. Look with the 'scope for oscillation at IF in the absence of incoming signals.

Also see if you can measure the AGC voltage and check that it rises in magnitude as a station is tuned in.

One more thing; the radio seems to go deaf above mid-way along the MW band. Can you check that the local oscillator is still running? Put the scope probe across the oscillator gang of the tuning capacitor.

[jonnybear]

You say you have checked every capacitor, have you done this by replacement or with a capacitance meter?. there is a lot of decoupling capacitors on that chip, ICs are less tolerant than transistors or valves and will go unstable very quickly. I have checked old capacitors before which read the correct capacity yet leak like a sieve.Have you checked the print for cracks?.
John

[mikeald]

Jonny - I have checked the caps on an lcr meter. I have not replaced them all.

I am hoping to retain as many of the original components as I can...

Phil - Ok, I will get hold of some caps and replace all the ones related to the IF stage.

With regards to using a scope etc, I am now in slightly unfamiliar territory.

Could you please explain in simple terms exactly where I place the scope probes to do as you say above:

"Look with the scope for oscillation at IF in the absence of incoming signals."

I assume there should be no oscillation?

Pins 10 and 14 on the chip seem to be for the IF?

I assume this needs to be done under no signal conditions? And what exactly should I be seeing on the scope?


THank you

[Phil G4SPZ]

This circuit uses a pair of IF transformers and a ceramic resonator, all of which are passive devices, as an external 'IF block filter'. All the gain is provided within the TAD100; the incoming signal is mixed down to 470kHz in the TAD100 then passed, from pin 14, to the input of the IF filter. The IF filter's output is passed back into the TAD100 via pin 10, from which point it is demodulated and amplified in the chip. There must be a lot of gain in this chip, and if there is any leakage of signal between the amplified IF output and the input to the IF filter, the stage will potentially behave as an oscillator at 470kHz. It is also possible that such leakage could arise from an internal fault in the chip.

I think the crucial capacitors are C14, C27, C17 and C12, and possibly C8 (all Trader sheet 3019 references). As John says, capacitors can't always be proven 100% by in-circuit tests, sometimes only substitution will prove the point one way or the other. Also check that the metal screening can of the IF block filter is securely bonded to chassis (0 volts).

Try clipping the 'scope probe to pin 10 or pin 11. With a strong signal input from your signal generator, you should be able to see the modulated 470kHz signal. As you steadily reduce the input level, the amplitude of the IF waveform should reduce steadily until with no signal input you should be seeing nothing other than a little random noise.

[mikeald]

THanks for the detailed and easy to understand reply Phil (well fairly easy)!

I will try to address the caps today.

And will get my scope out tonight, and will post another youtube video of my findings.

THank you for your time....

[JohnBHanson]

It might be worth a check of the AGC line as it looks like the sound is clear with a sig-gen. Another possibility is that the mixer (Usually the first transistor) is mall-functioning. I have had a problem with the mixer on a TR230 I repaired. This caused some very strange behaviour at the top end of the band.

[mikeald]

THe mixer is built into the IC on this set.....

[60 oldjohn]

The TAD100 IC is in my experience very reliable. I have tested some "Pulls" from none working sets and found them to work ok. As others have said replace the Electrolytic caps first I expect them to have a high ESR value.
John.