Hacker Hunter background noise

[Roy London]

I need some help fixing my Hacker Radio

I have received a Hacker Hunter this morning.....It's not a surprise that a radio of this age has noise...and it still has a great sound, all things considered....

When switching on, before turning up the volume, there is a background noise, fairly low frequency, a bit like the sea, or distant traffic...

this doesn't change in level as you increase the volume

I can post a link to a youtube video demonstrating the noise if required......

Any help on which components may be relevant to this fault would be gratefully received....

I guess carbon resistors can generate noise,

I'm wondering how easy it is to remove the pcbs

It has two fresh ever ready batteries.

thank you

http://i142.photobucket.com/albums/r...o/100_3493.jpg

[paulsherwin]

There are a number of possible causes, but the most likely one is a bad audio transistor immediately after the volume control. I don't have a circuit to hand, but a large number of replacement transistors will usually work well in this position.

I've never dismantled one of these so can't help in that respect - shouldn't be difficult though.

Paul

[Jonster]

I seem to remember this happening on my Hunter, the problem was traced to the plug which connects the main radio part to the small amplifier PCB in the bottom of the set. In any case, the problem must be after the volume control if turning it down doesn't cure the problem. If it's not the connector, it could be a noisy resistor or a leaky capacitor.

What current does the radio draw when switched on and the volume down?

[Roy London]

Many thanks

I ought to hunt the schematic

I can now see that probably all/most of the existing transistors seem to be BF types....which I am assuming may be replacements...

I can see a pair of leads (blue and a black) plugged into the board below.... this board also seems to contain an old black glass transistor, maybe an oc71 or similar hidden at the back.... - one of the sockets is pretty wobbly

it looks and feels like the whole boards have been sprayed with switchcleaner etc , there is residue - collecting dust....

thanks for a good start, I'll work on those....

thank you

[VHFHerald]

I had this problem on my own Hunter, it turned out to be the OC71 on the audio board.

[Roy London]

Originally Posted by solartron

I had this problem on my own Hunter, it turned out to be the OC71 on the audio board.

Thank you.

I have some oc75 and some OC44 so maybe I could try a 75, I wonder if that would work out OK...probably best to source the 71 but might try a 75 in the meanwhile.....

[paulsherwin]

It is unlikely, though not impossible, that a bad silicon transistor would cause this symptom. I was assuming the radio used germanium types.

It seems unlikely (though again, not impossible) that the original transistors have *all* been changed. It should be easy to see if the solder joints have been remade as part of a repair or restoration.

I have tried a large number of Ge transistors to replace an OC71 audio preamp and all of them worked acceptably well, including an OC44. I haven't actually tried an OC75 but I'd expect it to work too.

Paul

[Roy London]

Thanks Paul

I got the lower board out and had a look, I guess this board is the output stage....

the two metal cased output transistors are housed in a brass dual sleeve which attaches to a heatsink...

On this board, there are other transistors I hadn't noticed before, apart from the OC71, which I replaced with a 75 and the same problem occurred so i have put the 71 back in.....

there are a BC195, BC148 (2 off) ... the circuit board has printed component layout for these transistors and this layout print is the same six sided shape so i think you are correct, these are probably not replacements, i think I was expecting metal cans.....rather than these plastic 6 siders....

the quality in there is nice but boy does this show up how bad my soldering is....I need a decent solder sucker....my desoldering braid has always seemed useless....and my fluke does not test transistors..seems I ought to get some bits ......

the way the volume does not affect the noise situation does also - as suggested - make me think that the fault tends to be towards the output, so the fault may possibly be on this board, I learn as I go....

thanks again

[chriswood1900]

It looks like you have a RP38a with amplifier A209 is similar to the A205 used in the RP25 would recommend a few quick checks before you go to far, do the tone controls vary the noise level, as they form part of the circuit T1 & T2 after the volume control. Next with volume at minimum measure the current drawn from the battery it should be around 18-22 ma, this will indicate if the radio and particularly the output stage is drawing to much power. The next area to check if the above does not show up anything is the settings of the presets RV3 & 4. A quick check on battery voltage to ensure it is around 18v or use a PSU. Short circuit the input on SK1 then measure the voltage at between R14 & 15 which is the + end of C10 it should be around 8.85 volts set by RV3 finally open the test link and measure the current drawn by the output transistors 3ma adjusted by RV4. Beware some copies of the manuals contain errors in circuit designations for RV3 and 4.
Finally I have usually found the noise on the versions like yours with silicon transistors to be a result of the resistors and capacitors around T1 & T2 R1,2,3,4,5 changing in value and C3.
Good luck it is a great radio when working well. Chris

[Roy London]

Thanks Chris

It is indeed an A209 board.

The bass and treble controls do alter the noise accordingly.

The current from the battery measured on my Fluke between the end of red battery lead and it's tag at the ON switch reads 14.5ma.

The battery voltage between the ON switch and earth reads 19 volts (fresh batteries) with the set on.

I shorted the input on the plug cluster (on the A209 board) to the centre lug (earth) and measured the voltage between R14/15 and it reads 1.14 volts, adjusting RV3 makes very little difference, it went to 1.10 volts...

I took out one half of the TEST link and measured the current ...this appeared to be 6.4 ma. Adjustment of RV4 again did not appear to affect this value....

I may look again at RV3 and RV4, seems odd no changes seem to occur.....

but I may rebuild the area you highlight, perhaps if the components around that area are not all good the presets might not work anyway?

many thanks indeed to all....I am determined to get this radio working nicely....

[Roy London]

Correction, the voltage at the positive of C10 is 2.87 volts, this varies a tiny bit with RV3 adjustment....

[Alan Kewley]

I have had a similar problem with an old Japanese National. The noise level was so bad any connection with the I.F. Stage was deafening. Problem was R.F transistors. Not just one but two of the germanium PNPs.

[Roy London]

Thanks Alan

and thanks all

I've now ordered a service manual for the RP38a.

and will report back on progress as I go.

Cheers

Roy

[paulsherwin]

Originally Posted by Roy London

my fluke does not test transistors..seems I ought to get some bits ......

The very cheapest modern DMMs will have a basic transistor test function, so it's worth buying one just for this. I actually own one which cost £2 from a market stall and it tests transistors well enough to identify faulty ones.

You can also make a simple transistor tester with a couple of resistors, a transistor socket and a PP3. You will need a meter to indicate the current but you can use your Fluke for that, or use a VU meter from a scrap cassette deck.

Paul

[Roy London]

Thanks Paul

I must put my mind to that....hmmm....

a socket...a bit of veroboard, perhaps...a pp3, a pp3 connector, ... work out the value of the resistors needed and get re-acquainted with the pinouts etc of the transistors......

could be good to get the old melon working

[Roy London]

Further Symptom

I left the RP38a on Radio 4 for about an hour and the noise has died away to a much lower level..... still there but definitely not as heavy...

[Roy London]

Then returns after 40 minutes rest.

[Roy London]

Pleased to report that the noise seems to be conquered.

I changed some of the carbon resistors on the audio board, and measured them as they came out, they seemed generally to all be 50% out...ie a 1k would show 1k5...

I changed some capacitors as well.

Noise still there.

Today i received some BC148 in the post and swapped out the two BC148 from the audio board for these new ones.

The noise now appears to have completely gone.

Next I will work towards getting the incorrect voltages looking correct.

Thanks to all for your kindness in helping/suggesting, much appreciated.

[steve.seven]

Hi Roy,

Had a very similar problem with my Hacker RP25A, very noisey with volume at minimum setting, sometimes radio would not work at all, very intermittent.
Radio finally ceased to work, traced fault to open circuit base/emitter of first stage audio amp T1.(BC148).
No exact replacements to hand so used BC182 as temporary fix, nice to have it working again, these radios are real quality and a pleasure to listen too.
Audio board fitted was type A205.
Regards,
Steve.

[Roy London]

Thanks Steve

T1 and/or T2 were the problem - similar problem to yours - .

I'm glad this radio is now free from the fault,

the old Elac speaker sounds sweet and the radio tunes great.....some small tone mojo in Germanium I expect....

great little radios....I really like this Hacker