Hacker Black Knight RP74MB Quiet Audio

[OldTechFan96]

Hello all,

At the end last year I bought a Hacker Black Knight. It worked fine after the volume potentiometer was fixed. Since the performance did not seem quite right I put it away and focused on other things.

Recently, I brought it out again to have another look. When I switched the radio on It appeared to be dead. With the volume all the way up radio stations along with pops and crackles could be heard, although extremely faint. I had to put the speaker right up to my ear before I heard anything.

I injected an audio signal into the middle tag of the volume potentiometer and a signal could be heard, but it was still faint even at max volume. I bridged the headphone socket and the fault remained the same so I think that it is all right.

I assume that the fault may lie in the audio amplifier section of the Black Knight. What should I do next? Voltage measurements to determine if a transistor is bad? I've noticed that this radio uses a mysterious T2 transistor

I don't have a service manual for the Black Knight but I have one for the Silver Knight. I think that it will be similar enough.

Thanks

[mhennessy]

Yes, the audio sections are identical between Knights...

With this set, assuming the speaker is OK and the connectors are clean, the first question is this: Is it the pre-amp, or is it the power amp?

Full output requires something like 25mV at the input to the power amp, so try injecting that if you can. Or try measuring the audio signal going into the power amp with the volume at max. Either of those tests will narrow it down to the pre or power amp.

If it's the power amp, take a look at C4. If this fails, the main amp loop goes unity-gain (leaving just the gain of TR1). I had that happen to an SP80 once. Obviously, there are many other possibilities, but that one springs to mind. Check by bridging it temporarily with a known-good unit. Generally, the blue Philips capacitors are pretty reliable, but failures occasionally happen.

Don't worry about T2 - that's just setting the output stage bias. I speculate that it's an AC128 or similar that didn't quite meet its spec in some way: http://www.markhennessy.co.uk/articl...sistors.htm#t2

Do you have a 'scope?

[OldTechFan96]

Where is the the input to the power amp? Is it the wire that comes in from the middle of the vol pot that goes to the amp board?

I have found your website a valuable resource when dealing with Hacker radios.

I do have a 'scope but it is not in working order at the moment.

[mhennessy]

No, I'm thinking about the connection between the preamp and the main amp. That'll be on the 4 pin connector on the amp - the core of the coax cable.

What we're trying to do here is the classic "binary search". We have a chain, and at some point in this chain, the signal is getting lost. So we divide the chain in two, and see what we have in the middle. Based on that result, we then know whether we go back or forth for our next move.

In this case, the output of the volume control travels to the tone control PCB (what I've been calling the "pre-amp"). From there, it finds its way to the power amp. The fault could lie in either of those PCBs (or indeed the wiring between them and the speaker), so which one do we need to focus on?

Sorry if that's stating the obvious, but better to be explicit than cryptic

Shame about the 'scope. Personally, I'd want to fix that first of all - I rather depend on my 'scopes. Other engineers get by without, but in my opinion (and indeed experience), I'd say that "get by" is the key part of that statement. I found myself saying all that recently on a radio forum, so won't repeat myself...

In the absence of the 'scope, have you got anything that can be used as a signal tracer? Another radio might do the job, or perhaps the audio sections from a scrap set?

Also, roughly what is the quiescent current? It's always worth checking that first on any none-working radio, as a fault in the output stage could cause all sorts of problems. The lack of signal might be caused by the power supply being dragged down, for example. It's the first thing I look at when I see a radio for the first time.

[OldTechFan96]

I think I know where you mean. I'll get back to you when I measure the voltage present.

If I can't get my scope sorted I'll probably buy a newer one to learn troubleshooting and alignment with.

I have been wanting to buy a Velleman K7000 - Signal Tracer/Injector as it would be a useful troubleshooting tool. There is a stockist locally so I will go and buy one today.

[OldTechFan96]

I've measured the core of the coax lead at point D on the main amp board and I am getting a reading of 0V. The amp and pre amp are definitely receiving power from the battery. I'll have a better look at the circuit diagram tomorrow, it will be good practice.

I bought the aforementioned signal tracer/injector kit and have put it together. Now I need to make some suitable probes.

How do I measure quiescent current?

Thanks

[mhennessy]

Just to be clear, we need a measurement of signal there. Is the 0V reading a signal level, or are you quoting a DC voltage?

Without a 'scope, this might be hard to do. Not all multimeters give good results at low signal levels. As mentioned, the sensitivity of the power amplifier is around 25mV.

The procedure for measuring Iq is given in the manual. But just a rough idea of what the whole set it taking is all I'm after. It should be in the 20-30mA region, give or take. I'm assuming that you're comfortable using a multimeter to measure DC current, but we can expand on areas like this if there is any doubt...

[OldTechFan96]

My mistake, the 0V is a DC measurement I made with my DMM.

I've never measured current with my DMM before. I imagine that it is measured by placing the DMM probes in parallel with the battery terminals while the radio is switched on. There is a mA range on my DMM.

[ms660]

Originally Posted by OldTechFan96

My mistake, the 0V is a DC measurement I made with my DMM.

I've never measured current with my DMM before. I imagine that it is measured by placing the DMM probes in parallel with the battery terminals while the radio is switched on. There is a mA range on my DMM.

No, measured in series.

Lawrence.

[OldTechFan96]

I hooked up my DMM in series with the battery and measured 00.44mA of current. I'm not 100% sure if made the measurement correctly so I'll have another go tomorrow.

[OldTechFan96]

I've measured 00.30mA current draw on my Bush TR82C so I think that I may have made the measurement correctly.

[mhennessy]

Sounds like it's time to put the radio to one side and go back to basics here. Test your multimeter using a voltage source and a resistor (Ohm's law).

For example, if you have a 1k resistor, how much current should it draw from a 9V battery? And does your multimeter give you a sensible reading when you try it for real?

Verifying the performance of your tools - and your understanding - is fundamental to this game. Give it a go

[OldTechFan96]

Quite right. Faultfinding will be more satisfying when I know more. I'll do some more reading and come back with some questions later.

[OldTechFan96]

Since my last post in this thread I have learned a lot more about the basics of radio and how to troubleshoot them. Also, I've learned more about the tools I have and how to use them effectively. This includes a Hameg 20MHz oscilloscope, which I'm slowly getting the hang of.

Now is a good time to start troubleshooting the quiet Black Knight.

The radio is being powered by its 12V power supply. The current draw is 19mA.

To check if the RF/IF stages are working I probed the cathode of the detector diode D1 with the scope. I saw what looked like a demodulated audio signal and was content that everything was working as it should here.

The input and output of the tone control board (pre-amp?) showed the audio signal at each point. It does not look like any amplification took place between the input and output of the tone board. The audio signal enters the base of TR1 (BC149) and is amplified at the collector. Somewhere within the tone board this amplified signal seems to have been reduced by the time it leaves. I did not know if this was quite right so I moved on to the amp board.

The audio signal is present at point D of the amp and not present at the output. The problem may lie in between these two points.

The audio signal can be found at the base of TR1 (BC149). At the collector the signal is present but is looks to be very weak, as if it has not been amplified. I thought that TR1 could be bad. Since both TR1 and TR2 are BC149s I decided to remove and test them with my DMM and they both test good.

I tried to measure the quiescent current by cutting the test link and inserting my Avo8 in series with the circuit. Unfortunately nothing could be measured. This looks to be a good clue.

This is where I need some help. I wonder if the lockfit transistors are bad? Or is it something else?

Thanks

[john.north]

Hello,
If T2 is leaky there will be no audio at low volume. If the output stage midpoint voltage cannot be set this will confirm T2 as faulty. It will read ok on a tester.
John.

[OldTechFan96]

I measured a mid point voltage of 258mV, it should be 5.85V. I tried to adjust it using RV1 but it remained at about the same voltage. I did get a bit of audio when RV1 was cranked all of the way.

If T2 is faulty what can I replace it with? I might have something to hand.

[mhennessy]

Could you provide a table of DC voltages in the amplifier, from each leg of each transistor to ground?

[OldTechFan96]

Voltage measurement follow as:

TR1, BC149:

C- 4.8V
E- 0.1V
B- 0.7V

TR2, BC149:

C- 4.1V
E- 4.0V
B- 4.8V

TR3, T2:

C- 0V
E- 0V
B- 0V

TR4, BC159:

C-0V
E- 11.47V
B- 4.1V

TR5, AC187:

C- 11.7V
E- 0.3V
B- 0V

TR6, AC188:

C- 0V
E- 0V
B- 0V

[john.north]

Hello,
T2 AC128, OC81 PNP.
John.

[mhennessy]

T4 looks suspicious - try changing that first of all

It's a general-purpose small-signal PNP silicon transistor that can be replaced by many devices. The direct replacement would be a BC548, but almost anything will do the job. Watch out for the pin-out - it's a "base in the middle" type.