Hacker Sovereign 11

[kirstyd]

The radio works well enough but there seems to be some distortion at very low volume [i use this radio in my caravan and tend to listen at low vol while in bed ] The batteries are new and i have ruled out the speaker by connecting another. Any ideas guys ?

[buggies]

Not particular to this radio - but distortion at low volume can be caused by crossover distortion in the output pair. The quiescent current may need adjusting (at the expense of battery consumption). Might be worth checking anyway...

[Nuvistor]

The speaker voice coil binding can sound very similar to cross over distortion, worth checking both.

[mhennessy]

Originally Posted by Nuvistor

The speaker voice coil binding can sound very similar to cross over distortion, worth checking both.

Sadly, that's very common with that loudspeaker. I reckon that just about every one of those black+silver Celestion speakers I've seen suffered this to some extent.

It can be fixed with a hammer and a block of wood. The operation is a lot more delicate than it sounds, so it's not something I'd advise a beginner to do, but I've done loads with a 100% success rate. The basic idea is to feed in a pure tone from a signal generator that is at the resonant frequency of the drive unit - somewhere in the 115Hz region - and then pull on the magnet to work out which direction it needs to move to clear the distortion. Having done that, you now know how to distort the chassis to make that happen. Sounds crude, but it works.

[mickash]

You could also try gently wedging a piece of cotton wool or foam rubber between the speaker chassis and paper cone. A bit of trial and error while listening will help to find the correct position and how much pressure is needed.

[poppydog]

I'm sure I read that he ruled out the speaker by trying another??

[Radiocruncher]

If there is noise present at minimum volume then there is an issue with the amp. It could be a number of things starting with capacitors, lockfit transistors and finally the resistors. Do you have an amp from a similar set to substitute and compare?

Regards

Graham

[Nuvistor]

Originally Posted by poppydog

I'm sure I read that he ruled out the speaker by trying another??

Oops missed that, with Mark’s experience with these sets I wonder if the radio has both faults, duff speaker and amp.
Yes I am a pessimist

[crackle]

Is it the same on all wave bands and all stations. Some stations on MW sound distorted on certain radios anyway, something to do with the modern compression used in the transmitted signal.

Mike

[Bob_Moss]

Also, get some Deoxit on the connector to the power board, and exercise the skeleton preset, both can cause noise.

[Nickthedentist]

Originally Posted by buggies

Not particular to this radio - but distortion at low volume can be caused by crossover distortion in the output pair. The quiescent current may need adjusting (at the expense of battery consumption). Might be worth checking anyway...

I would try this. Download the manual and follow the instructions: https://www.service-data.com/product...29/3493/m12629

N.

[ajgriff]

Depending upon one's definition of 'distortion' this does sound a bit like it might be a noisy lockfit transistor on the amplifier board. T1 (BC108) is the most likely candidate I think.

Alan

[alanworland]

My amplifier board has a noisy transistor, replacement cured it.
Lovely sounding set.

Alan

[Herald1360]

BC108 isn't Lokfit, BC148 is.

[mhennessy]

Yes, the service data specifies a BC108, but BC148s were fitted. I don't recall seeing an example with BC108s as yet. My earliest RP25 is pretty early, but not the earliest by any means.

However, I do replace them with BC108. I reckon that's suitably sympathetic. But of course, you have to replace both T1 and T2 for that to look right. Ideally, include a short length of green PVC sleeving on the base of each, as Hacker would have originally done.

Regarding the OP's problem, it would be interesting to verify how the speaker was eliminated - just in case the test speaker was suffering from the same issue. Much as it seems unlikely, those black and silver Celestion speakers are virtually 100% unreliable in my experience (around a dozen, probably more), and the earlier all silver versions aren't much better. Too small a gap. The 5 by 7 inch Peerless units used in the Sovereign III "family" can also suffer the same problem. Whereas the various Elac units used by Hacker have a near-100% record of reliability - I only know of 1 failure (an open-circuit voice coil).

I concur with the advice to exercise the pre-sets. The quiescent current - measured at the test link - should be 3mA. I can't think of many cases where it's been low enough to cause conspicuous crossover distortion - if anything, they tend to drift the other way. Check the mid-point at the same time (8.9V) - being a bit away from that shouldn't cause distortion at low volumes, but it doesn't hurt to get it right. Obviously, the Lockfit problems are well documented - the LF rumble they produce is quite distinctive. The capacitors usually fitted to these are extremely reliable (especially by the time of the black/silver Celestion speakers), and I've not had too many problems with the resistors used - but the type used in the Hunter are another matter

All that said, I'm guessing the perhaps the OP doesn't have another equivalent set to swap the amp PCBs? That seems incredible, but I guess it can happen... This sounds like the perfect excuse to acquire another Hacker

[julie_m]

BC548 is the same silicon [as BC108 / BC148] in a TO92 encapsulation with the base in the middle (emitter at bottom when flat side is to the right, like a mirror-image of a capital D), so an easy substitution. BC549 is an even lower-noise, direct substitution.

[ajgriff]

Have always thought of the BC148 as a lockfit BC108. Sorry to cause confusion.

Alan

[julie_m]

That's my understanding too: BC107-109 = TO-18 metal can, BC147-149 = Lockfit encapsulation, BC547-549 = TO-92 plastic encapsulation; higher numbers = lower noise but also lower collector voltage; optional suffix A, B or C indicates min. hFE = 50, 100 or 150 respectively.

[Argus25]

Fixing a distortion problem like this is super easy with the right rest gear, a scope on the audio amp's output and a small signal generator, the latter is cheap, many small pcb's available that make sine waves. Then you could see right away what kind of distortion you were dealing with, asymmetrical clipping or crossover distortion, and also see the noise, if it is significant.

As noted the crossover distortion is caused by insufficient bias to the base-emitter junctions of the output transistor pair. Also if one output transistor has died/sick the waveform asymmetry can sound quite bad.

However without that test gear there are some other things you can do:

If you measure the emitter or collector currents of both the output transistors with a meter, they should be about the same. If there is a big difference, one is faulty.( I haven't got the Hacker circuit on hand, but if the output transistors have small value emitter stabilization resistors, its easy just to compare the voltage across those,so as to to compare their emitter currents, but I can't recall if the Hacker has these)

If you can get your hands on a crystal earpiece, put a 0.1uF cap in series with it and use it as an "audio probe" to listen to the audio in different parts of the amplifier circuit, you might find the signal sounds good on the input of a transistor (base circuit) but distorted on its output (collector) indicating that the bias on the transistor is incorrect or the transistor is sick or its associated components are faulty.

If there is noise, a common place is that it comes from the carbon composition resistors, but usually only the ones which pass a moderate current. Typically these are the collector load resistors (not the base bias or emitter resistors). So be on the look out for that.

Also leaky capacitors feeding a base circuit can upset the bias of that stage, and any others direct coupled to that stage. Low value capacitors will limit the bass response.

It is all very easy with the right test gear, but without it you have to improvise and draw indirect conclusions from other more minimalistic tests with the meter & earpiece. Mostly this is why people who repair a lot of radios have a scope and generator at hand.

[kirstyd]

Thanks guys for all of your interesting posts .After reading all of them this is what i have done .First of all i do have another hacker radio [hunter ]so i plugged the sovereign into the hunters amp .result problem gone and a vast improvement in sound quality so just out of curiosity i did the reverse [hunter through sov amp .result problem still there .next i connected a suitable good speaker to the sovereign no improvement .Next i played the hunter through the Sovereigns speaker sounds fine and the distortion is gone but the Fane speaker sound much better than the one in the Sovereign.All of that seems to narrow the problem down to the amp would you agree?