Hacker Sovereign 11

[ajgriff]

Am I right in thinking that the set worked well without the distortion after cleaning but before anything was adjusted?

Alan

[orbanp1]

Here is a simulation of a Hacker audio amp with the DC voltages.
It is the A205, that is the schematics I did find.
I did some simplification, omitted the Vbe generator for the final stage, just used a simple resistor and adjusted it for about 2.8mA idle current.
Also assumed 18V supply voltage (2x 9V?)
Also left out some of the tone control circuits.
Audio gain is about 60dB.
Hopefully it is similar to your amp and it helps you.

Regards, Peter

[DangerMan]

Back in post 2, George (buggies) suggested check of the quiescent current but I can't see that this has been done (.. or did I miss it?)

I'd agree it would be worth cutting the test link and measuring the current there.
If it is less than 4-5 mA - I'm not sure of the manufacturer's exact figure - and cannot be adjusted correctly using RV4 then it will quickly point to the faulty area.

Pete

[kirstyd]

to answer your question Alan .The set has always worked but there is sort sort of distortion at very low volume .I use it in the bedroom and consequently have it on quite low .When turned up to a reasonable level it seems to work fine

[kirstyd]

Originally Posted by mhennessy

But what does it actually sound like?

Q. Is there a low frequency rumbling noise that is there all the time, even when there is no audio?

Q. Or is there a "roughness" to the sound? Almost like a scratching?

Q. Is is present at medium and high volume too?

Q. Does it change as you alter the treble and bass controls?

yes mark theres a sort of rubbing sound but its not the speaker as i have tested it to destruction today on various bit of equipment

[ajgriff]

So, is the distortion still there after the work you have done thus far? If it is then checking the quiescent current as suggested above is the next step. The current should be 3mA as indicated previously by Mark. By the way, I hope you haven't literally destroyed the speaker!

Alan

[DangerMan]

Be sure to check the quiescent current with the volume at minimum so no signal output to the speaker.

[mhennessy]

Originally Posted by orbanp1

Here is a simulation of a Hacker audio amp with the DC voltages.
It is the A205, that is the schematics I did find.
I did some simplification, omitted the Vbe generator for the final stage, just used a simple resistor and adjusted it for about 2.8mA idle current.
Also assumed 18V supply voltage (2x 9V?)
Also left out some of the tone control circuits.
Audio gain is about 60dB.
Hopefully it is similar to your amp and it helps you.

Good work

The gain is on the high side because the bass control network was left unconnected - it is a loss network. If you set R8 to 0k (bass at min) and join R9 to the collector of Q5, you should find the gain is nearer to 45dB.

It must have taken a while to arrive at the resistors that have replaced RV3. I admit that I lack patience with Spice

[orbanp1]

Originally Posted by mhennessy

...

The gain is on the high side because the bass control network was left unconnected - it is a loss network. If you set R8 to 0k (bass at min) and join R9 to the collector of Q5, you should find the gain is nearer to 45dB.

It must have taken a while to arrive at the resistors that have replaced RV3. I admit that I lack patience with Spice

Indeed! The gain is in that case is 43.6dB!
Putting in pots and stepping through their values for the tone control is a lot more time consuming.
I concentrated on the DC voltages.

Finding RV3 value was not bad, only about three iteration.

Peter

[Argus25]

Originally Posted by kirstyd

All measurements were taken with the meter set as it is in the photo

It is obvious from these figures why your amplifier has severe cross over distortion and sounds awful at low volume.

The base to base voltage of the output transistors, T5 and T6 is only 60mV. It should be around 400mV to get both of the output transistors into initial conduction. (200mV per transistor) So the question is why ? This voltage is developed across the two series 150R resistors as a result of T4's collector current. From your measurements T4's base-emitter voltage is about 150mV (it should be around 200mV but its not ridiculously low), so it should be generating some collector current in the 150R resistors to get at least more than the 60mV that is there.

The 150 R resistors are shunted by the OC71 (T3) which is the "vbe multiplier" transistor. Its job is to control the output transistors inter base voltage downward to about 400mV and also to temperature compensate the voltage downwards.

Assuming the two 150R, the 680R and the 330 R resistors in T4 collector circuit are ok (and T4 is ok..probably) I would suspect that T3, the OC71 vbe multiplier transistor had emitter to collector leakage, or is near shorted out.

With the amp de-powered, what is the resistance from the emitter to the collector of the OC71 (T3) ? And check the resistors.

[Argus25]

A bit more information might help you too:

You will notice if you look at T3's (OC71) circuit, there are two 150R resistors in series with the junction of them on the base and the ends of them on the collector and emitter.
When enough current passes via those 150R resistors, the voltage developed across the one between the emitter and the base rises to around 200mv (0.2V). This is the typical base-emitter threshold voltage for a germanium transistor (unlike 0.6V for a silicon type). This causes the OC71 transistor to pass collector current, which then shunts the voltage across the two 150R resistors downwards again. So it is an enclosed DC negative feedback circuit set up to generate very close to 0.4V across the two series 150R resistors (between the bases of the two output transistors). It was done this way because as the OC71 heats up, this generated 0.4V drops downward, preventing the output transistors getting thermal runaway.

It still is possible that the collector current in T4 is too low, and T4 is faulty. It is a bit unusual that its base-emitter voltage is a bit low. But, aside from the cross over distortion you are hearing, which one would expect from the very low output transistor bias, the amplifier is still working, which would be unlikely if T4 had partially failed, but not impossible.

[mhennessy]

Hi,

Kirsty sent the amplifier to me, and I can confirm that it was indeed crossover distortion, as the earlier voltage measurements suggested. The culprit was T3, which had a dead short between collector and emitter. This is a later version which uses a "T2" rather than an OC71. I replaced it with an AC128.

The noise floor was reasonable, but I decided to replace the Lockfit transistors anyway. This made it about 6-9dB quieter, and the BC108s look the part. The hardest thing was finding exactly the right shade of green sleeving for the base leads! The noise floor is about 1mV, depending on how you measure - the spec says -60dB w.r.t. to 1.5 watts, which is 4.7mV RMS, so it's now well within spec. I thought it was worth paying attention to this as the set is used quietly late at night.

Otherwise, everything else was fine. It uses the blue Mullard caps that have a near-100% reliability record in transistor sets, so it came as no surprise that they measured fine (and were left well alone). I tightened up the contacts of the 5-pin connector and went over a few solder joints, finalised the settings of the presets, and double-checked all was well a couple of days later.

The photos show the crossover distortion on the 'scope, and the amplifier connected to my test box, which might be of interest to those who service these sets. Using parts from a scrap chassis - including the wiring - and a spare die-cast enclosure, it's very easy to test an amplifier without needing a radio to connect it to. The BNC on the top left is the input signal, the 3 pots are volume, bass and treble, power goes in on the red and black 4mm connectors, and the speaker output leaves via the BNC on the top right - this goes to a 'scope or distortion analyser or whatever else you might need. The toggle switch chooses between 15 or 30 ohms impedance (and it has a centre-off position for open-circuit). It's very simple, but I wish I'd built it years before I did!

Hope this is of interest,

Mark

[Philips210]

Hi

It's good to know that you sorted out the problem Mark. Hugo's diagnosis in post #50 was correct re T3 being the culprit.

I like your test facility for the Hacker amplifier boards. I have some of the correct plugs to fit the boards from some scrapped receivers so I may well try building such a set up. Do you have a circuit diagram of your test box?

Regards
Symon.

[mhennessy]

Hi Symon,

Thanks for the feedback. No, I've not drawn up a circuit diagram - it's hardly worth it. Later on this evening or over the weekend, I'll try to find a photo of the insides (or take it apart and take a snap) - that'll explain all. Remind me if I forget.

Cheers,

Mark

[Argus25]

Good news it's fixed !

There is something about the Hacker Sovereign II radio. I think it is a combination of that amplifier, the speaker and the cabinet acoustics.

It is by far the better sounding transistor radio in my collection. Sometimes, after a glass of Chardonnay and listening to some great 60's music on it, on a lazy Sunday afternoon, it sounds better than a fully fledged HiFi and way better than any iPod stage. Whether some of that is "psychoacoustics" I wouldn't care.

So these radios are always worth any time & effort put in to repair them well.

[ajgriff]

Also pleased to hear that the Hacker will soon be restored to full health with its newly refurbished amplifier.

Have a feeling that a glass of Chardonnay, good music and a lazy Sunday afternoon in Queensland would improve most aspects of life, acoustic or otherwise.

Alan

[mhennessy]

Hi again,

As promised, here is a picture of the inside of my test box. It was a bit of a rush job, but it works well enough...

The pots came from scrap sets. The treble pot is 5k antilog and the bass control is 50k log. That's lucky, as another bass pot can be used as the volume control. The knobs for these came from Farnell IIRC - the shaft diameter is 4mm. You'll see that the metal box is crudely connected to signal earth via a solder joint on the side of the treble pot.

The power is obvious enough, with a protection diode in inverse-parallel. As I always connect this to current-limited bench power supplies (usually set to around 30mA initially), the 1N400x is more than adequate.

On the output side, the left-hand pole of the switch chooses between the two 15 ohm resistors (or neither when in the centre-off position). The right hand pole is only used as convenient earth tags - DPDT is what I had in stock, but SPDT would be fine instead. Between the speaker connection and the BNC connection is a 51 ohm resistor to protect against short-circuits. I used PTFE sleeving to insulate the wires - it looks good apart from when the flux wicks up the end. Keep the input and output earths separate, naturally...

It's worth saying that this covers many of their sets - both the Sovereign I and II, the many Heralds that use the same amps as those Sovereigns (RP30, 32, 32, 35, 36) and the Hunter RP38A too. Also some other, simpler sets that use a similar plug arrangement. If you service these sets, I highly recommend one of these - it'll only take an hour to build, and you'll quickly recover that with the time savings you'll get in return.

Hope this helps,

Mark

[Philips210]

That's excellent Mark. Thanks for the details and pic. I assume the speaker connection was chosen to be BNC simply to allow easy connection to the oscilloscope. I suppose a BNC 'T' piece adaptor, together with a BNC to 4mm sockets adaptor, is used for connection to the 'scope and speaker?

Regards
Symon.

[mhennessy]

Hi Symon,

No, the BNC is for test gear only - there is a 51 ohm in series with it, so connecting a speaker would load it down too much. That's quite deliberate - I don't particularly want to listen to test tones . I do have a powered speaker connected to the Y-out of my 'scope, should I really want to listen to an amp; otherwise I can connect a test speaker directly to the amp instead of the test box.

You could fit a pair of 4mm terminals to the box for connection to a speaker, but there's a risk of confusing them with the power input, and you'd have to be sure you've set the impedance switch to high-Z when using the speaker. Seems hardly worth it - you can listen to it when it's back in the radio once it's passed all the technical tests.

Cheers,

Mark

[Philips210]

Thanks for clarifying that Mark. I could use a 2-pin DIN socket for the test speaker should I decide to add this feature.

Your powered loudspeaker also looks an excellent project to build and very well documented too. I'd love to be getting on with building these projects but have had a huge amount of domestic projects to complete, but will definitely get around to making a start before too long.

Regards
Symon.