HELP! Constant Annoying Audio Hiss On KB WP31 Radio

[camtechman]

After success in curing this fault on many other makes of transistor radios, usually by changing an OC70, 71 or OC75, I've now got round to sorting out the constant audio hiss on my KB WP31 radio, even when the vol control at zero.

Firstly, I replaced TX4: a Texas 2G317B with a Mullard OC70 (as this was stated as an option on diagram/parts list) but the hiss remained.

In order to isolate the RF from the equation, I then disconnected the input signal to the volume control but still the audio hiss remaind.

As this seemed to indicate that the problem was only in the AF stage, I then changed TX5: a Texas 2G374B with a Mullard OC75 (as listed) but the hiss remained and so I continued to individually change the remaining audio transistors TX6: 2G374B with an OC81D, TX7: 2G381A (OC81) & TX8: 2G339A (AC127).

However, none of these changes cured the audio hiss and I wonder if anyone could advise, or is the hiss part of the audio circuit design?

Full Circuit & Audio Only Circuit below:

[Peter.N.]

Sounds as though you have done it all. I can only suggest that you shunt the base of each transistor to earth via a largish capacitor, this should stop the audio from the previous stage from being fed in and should allow you to isolate where its coming from.

Peter

[Alistair D]

Hiss could be the result of high frequency oscillation. It would be worth bridging all of the decoupling caps on the supply, including the battery decoupling electrolytic.

Al

[camtechman]

Thanks guys, I'll give that a go & report back later.

Cheers

[geofy]

I would check out the telephone/mic amp which seems to be permanently connected via C22 and switch 1. Then R31 in the output feedback in case this is introducing noise back to the input. Then suspect a poor soldered joint or a noisy capacitor such as C17. Make sure the jack sockets are clean.

[camtechman]

Hi Geof,


I was going to say in my original post that I'd did that all that but, as it would have made the post too long I ommited it, but thanks for that suggestion.

[Andrew2]

I wonder if this is normal for this radio? There's a fair bit of audio gain (two amplifying stages and the output stage) after the v/c, so it could simply be the result of this.

[geofy]

You could try a bit of top cut negative feedback, it will reduce gain but improve the quality of sound, first try a capacitor of around 0.01uF across R27 to give more feedback at the top end where the hiss is.

Then try a small value capacitor across the base collector of the driver transistor, this could be just a few pF. If it works then it can be fitted to the track side.

Is is assuming resistors like R25 and R27 are ok and not noisy.

Good luck

[Chris Parry]

KB Calypso - ah yes, the marketeer's special! Nine transistors. (But no diodes.) These are a very nice set. When healthy, the audio is smooth and reasonably free of hiss, so I'd be looking for noisy carbon resistors at the front of the audio amp. Chris.

[Top Cap]

What is the mid-point voltage at the junction of the two emitter resistors? One would assume you adjust R31 for 4.5V here under no signal conditions? Definitely worth checking out the electrolytic caps around the output stages as a leaky one could inject noise. Sometimes freezer spray or the reverse (hair dryer) can show up a noisy component.
Les

[camtechman]

Hi All,

Sorry I've not been back to report my progress with this fault but until yesterday I hadn't had the time to deal with it.

I was able to get a few hours spare and set about trying to find where the fault was originating from and, at this stage, I owe an appology to Geofy - as I stated that I had ruled out the telephone/mic amp but I was wrong to do so.

This time, I FULLY isolated this circuit and the excessive hiss stopped and could listen to all stations on both LW & MW without the distracting noise.

I'm now about to finish my second cup of coffee and will set to finding out what, in this circuit, is causing this hiss.

I'll report back a soon as the fault is found.

Tony

[camtechman]

At last, the fault is cured.

Yes the hiss was caused by the telephone/mic amp, not the transistor but one of the passive components.

I patched up a replica circuit on a piece of vero-board, using new components & an OC44 (as per my T.I.T.S equiv book) and the high hiss stopped. Then I replaced the OC44 with the original 2G309 and the hiss didn't return.

I then fitted the new components with the original 2G309 onto the originally fitted tag board and the radio was still hiss free. Then, as a matter of elimination, I replaced each of the new passive components one by one with the old components and eventually, one of the carbon comp resistors (R33 100k) was the culprit.

Job done and the radio is now providing quality listening.

Thank all for your help & advice.

[Heatercathodeshort]

Carbon composition says it all. I recently binned all of mine. They were a waste of time when new and the years have finished them off. John.

[Philips210]

I have encountered this problem in the past and it took a long time to solve by trial and error. Funnily enough that was in a little early 1960's KB transistor radio. The resistor in question appeared normal and within tolerance. ISTR the resistor was of the PCB type having formed ends around the carbon rod. On inspection, there was a hairline crack in one end cap which may have been the cause of the fault.

I have also had intermittent crackles due to some internal failure within the resistor, again time consuming to locate.

I tend to agree with John, they're not very reliable, not to mention their tendency to increase in value. I guess the only redeeming feature is their ability to withstand higher voltage than carbon film types though that won't really matter in low voltage equipment.

Slightly OT but again regarding resistors, other than high voltage circuits, would there be any other reason to use composition as opposed to film in certain circuits?

Symon.

[Herald1360]

For HF applications where the helical nature of a film resistor is too inductive.

Can't think of any other reason.

Oh and one very specialised application, cheap cryogenic temperature sensors. Unfortunately the supply of suitable resistors has just about dried up for that.

[cmjones01]

Carbon composition resistors are also relatively good at handling brief bursts of power several times higher than their rating, because of the sheer mass of the resistive element. They don't tend to 'fuse' like film resistors or even some types of wirewound do under momentary overload conditions.

[Philips210]

Thanks for clarifying that point Chris.

Symon.

[crackle]

Hello Camtecman
Was the faulty R33 within tolerance?
Thanks
Mike

[Heatercathodeshort]

Manufacturers also loved them due to price. A typical half watt size was costed at less than 1d when purchased by the million.
These were the most common types widely used in television receivers. It was great to stab the AVO across anything over 100K to either read many megs or completely O/C. Never has anything looked so innocent yet so guilty! The typical 27K/47k 1 watt types usually crept down in value ending up in a burnt up and damaged printed circuit board. Thorn 850, GEC sound and Regentone video come to mind.
Oh well, they did keep the radio repair trade in business so maybe not so bad after all.

[bc312]

Just as an aside, would a metal film resistor substituted in an HF application have the same inductive issues as carbon film?
Thanks, Mike.