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Old 18th Jul 2019, 3:10 pm   #1
Celestial
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Default Hacker Sovereign II RP25A

Getting no audio at all from this radio. The battery supply on load is about 16.5 volts. I have tested the speaker. This is working fine. I can verify with a tracer that a very strong signal is present at the collectors of the transistors on the AM IF board. I have also traced a signal at the input plug on the audio amp board. The amplifier board fitted to this radio is the A205. I can trace a very strong but slightly distorted signal on the base of T2 (BC148 is fitted in this position). I cannot trace a signal on the collector of this transistor. I have taken this transistor out and tested it on my transistor tester and according to that it is working fine. (The gain figure is about 156). When I touch the collector lead on TR4 all I hear is just a background hiss. The same conditions apply on subsequent transistors. Is it possible that there is a problem with the 300pf bypass capacitor i.e not passing any audio onto the next transistor? I have checked all other components on this board and they all seem to be working correctly.
Any help would be very much appreceiated. Thank you.
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Old 18th Jul 2019, 4:22 pm   #2
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Default Re: Hacker Sovereign II RP25A

If I were you I'd replace that BC148. They are notorious for mysterious problems which don't show up on a standard transistor test. Any Si general purpose type will do - BC548, 2N3904, 2N2222 etc.
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Old 18th Jul 2019, 7:13 pm   #3
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Default Re: Hacker Sovereign II RP25A

The PCB isn't designed for a "Lockfit", at least mine wasn't. The Lockfits must be in a properly designed PCB or can develop internal fractures.
I replaced the BC148 on my amp board (in circuit voltages suggested something odd). I used a 2N3904. It's really the same thing. Now excellent.
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Old 19th Jul 2019, 7:20 am   #4
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Default Re: Hacker Sovereign II RP25A

Thank you everbody for your replies.
I have replaced the BC148 transistor with a BC548. This has not solved the problem however. If I am correct the signal comes into the base of the transistor and is amplified by the device and appears at the collector if the device is working properly?
The 300pf capacitor (C7) in the diagram is a polystyrene type. Do they suffer from any faults? I have also examined the underside of the board with a strong magnifying glass to see if I can see any breaks in print which might stop the circuit from working. I cannot find any.
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Old 19th Jul 2019, 8:17 am   #5
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Default Re: Hacker Sovereign II RP25A

May be a sillily question but you have checked the headphone skt, they are notorious for being O/C?
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Old 19th Jul 2019, 8:45 am   #6
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Default Re: Hacker Sovereign II RP25A

Polystyrene only get damaged by too much heat from a soldering iron.
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Old 19th Jul 2019, 9:22 am   #7
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Default Re: Hacker Sovereign II RP25A

Not so sure about that. An obscure (in the sense that I didn't expect it to be what it was) fault that fooled me into a complete rework and siliconization of the amp in my VHF Herald turned out to have been a perfect looking but dead short polystyrene capacitor.

Before that I too expected undamaged polystyrene caps just to be OK.
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Old 19th Jul 2019, 9:37 am   #8
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Default Re: Hacker Sovereign II RP25A

Any component CAN be open, short or faulty. But faulty resistors, loose rivet mica caps, dodgy connectors, dry PCB joints or cracks, snapped wires that look connected etc are all more common than faulty polystyrene caps. You can get faulty polyester / met poly caps or ceramic too, even in transistor stuff.

Obviously a methodical fault finding approach is needed.
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Old 19th Jul 2019, 9:54 am   #9
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Default Re: Hacker Sovereign II RP25A

Quote:
Originally Posted by Nuvistor View Post
May be a sillily question but you have checked the headphone skt, they are notorious for being O/C?
Hi Frank,
Yes I have checked this socket. I know that they can be notorious for completely silencing a set.
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Old 19th Jul 2019, 9:59 am   #10
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Default Re: Hacker Sovereign II RP25A

Gentlemen,
Thank you all for your replies on my topic.
It looks like I will have to go back to the drawing board on this one.
So far it has caused me a lot of frustration trying to fatham out what is causing this fault.
I am not a professionally trained electronic technician merely a hobbyist.
Thanks and all the very best to everybody on the forum.
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Old 19th Jul 2019, 3:59 pm   #11
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Default Re: Hacker Sovereign II RP25A

Double check the leads are in the right order on the replacement transistor.
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Old 20th Jul 2019, 7:57 am   #12
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Default Re: Hacker Sovereign II RP25A

Hello Mike,
I have carefully checked the lead placement on a transistor tester and also my databooks before inserting the new transistor. The leads exactly match the legend on the PCB. I may also check TR1 as well as a BC148 Lockfit transistor has also been fitted in that position. I will certainly check the 300pf polystyrene capacitor as well. If that all fails, then a check of the solder side of the PCB would be useful to check for any dry joints etc.
Thanks once again for all your help.
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Old 20th Jul 2019, 9:14 am   #13
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Default Re: Hacker Sovereign II RP25A

You say that no signals are appearing on the collector of the transistor so have you checked any resistors that are in the collector and emitter circuit? Anything open circuit here or even a dodgy cap pulling down the supply could cause this.

Personally I'd change all the electrolytics on the amp board, I did a Sovreign back in the winter for a friend and every electrolytic was either dried up and open or short circuit!

You sound like you are pretty clued up with what you are doing so just keep going at it, we are are here "on tap" if you need help.

Cheers
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Old 20th Jul 2019, 2:30 pm   #14
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Default Re: Hacker Sovereign II RP25A

Worth checking the DC voltages on T2. I was about to quote the service sheet values but not convinced they are correct!
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Old 20th Jul 2019, 8:21 pm   #15
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Default Re: Hacker Sovereign II RP25A

Try replacing TR4 , especially if an AC128. These can fail short and pull down the output from TR2.

Regards
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Old 20th Jul 2019, 10:25 pm   #16
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Default Re: Hacker Sovereign II RP25A

Voltage measurements with a DMM at the audio transistor junctions might help in eliminating some of the guesswork from this frustrating fault finding exercise.

Alan
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Old 22nd Jul 2019, 3:33 pm   #17
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Default Re: Hacker Sovereign II RP25A

Quote:
Originally Posted by Hunts smoothing bomb View Post
You say that no signals are appearing on the collector of the transistor so have you checked any resistors that are in the collector and emitter circuit? Anything open circuit here or even a dodgy cap pulling down the supply could cause this.

Personally I'd change all the electrolytics on the amp board, I did a Sovreign back in the winter for a friend and every electrolytic was either dried up and open or short circuit!

You sound like you are pretty clued up with what you are doing so just keep going at it, we are are here "on tap" if you need help.

Cheers
Hi,
Sorry for the delay in coming back to you. I have been rather busy just lately.
I have in fact chanegd all the old electrolytics on the audio board. Philips were the ones fitted. I know that they can drift "all over the shop." The audio connection one to the speaker is 500mfd. I tested this one and on my little Atlas machine it was reading 820mfd!! I know that this would not stop the circuit working. I cannot understand how they drift so much from their orginal value.
I have also tested the resistors in the part of the circuit associated with TR2. I have tested them on my 20M/ohm per volt meter and they all appear to be more or less on value. I did change TR1 for a BC184 to try and see if that would clear the problem. In fact, it did not. The distorotion has become worse if anything when I trace the signal through. Perhaps this transistor does not have the same characteristics as the original BC148 Lockfit that was fitted originally.
What I have detected by holding an ear very close to the radio spearker is that there is a very faint switch on thump when the switch is operated. If you test all round the circuit voltage is present. I have used my DMM to test. I have not yet changed the AC128 transistor as yet which is the driver transistor fitted. I know that they are similar to the old OC81D transistor.
I will have another look at it when I get a few moments to myself.
Thank you very much indeed to everybody for your help in trying to resolve this problem.
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Old 22nd Jul 2019, 4:36 pm   #18
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Default Re: Hacker Sovereign II RP25A

The BC184 should be fine.

Try changing the speaker electrolytic. An electrolytic reading high on test is often a sign of something amiss. Use anything reasonable you have to hand as the value isn't critical.

I agree with Alan though, you should take more voltage readings and compare them with the service data.
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Old 23rd Jul 2019, 7:43 am   #19
Celestial
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Default Re: Hacker Sovereign II RP25A

Quote:
Originally Posted by paulsherwin View Post
The BC184 should be fine.

Try changing the speaker electrolytic. An electrolytic reading high on test is often a sign of something amiss. Use anything reasonable you have to hand as the value isn't critical.

I agree with Alan though, you should take more voltage readings and compare them with the service data.
Hi Paul,
I have already changed all the old Philips electrolytics including the speaker link.
I have disconnected both the resistors in the TR2 circuit and tested them. They are all more or less the correct value. I obtained a copy of the service sheet for this radio. I find it very confusing to use as there were so many modifications made during the production of this and other Hacker radios of the period.
When I have a few minutes again I will have another look at the radio.
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