Grundig Concert Boy 1100

[poppydog]

Thanks for the reply Restoration 73.

This evening I have been testing some components on my Concert Boy and came across a ceramic capacitor C664 220pf, I unsoldered it to test it and put it across my digital multi-meter and my analogue multi-meter and there was no movement. Luckily I have a cheap Chinese ESR meter and the results are shown in the attached picture however it doesn't show an ESR reading. All of the other capacitors I have tested I have movements on the meters and an ESR result on the Chinese tester. My question is how do I know if this capacitor is ok? Thank you

[Station X]

ESR readings are only generally used for electrolytic capacitors, which will have values from 1uF upwards.

[poppydog]

Ok, thank you but what test can I perform to check if that ceramic capacitor is ok? Or is my cheap Chinese meter saying it is ok as it saying it is within tolerance?

[WaveyDipole]

220pF is such a small capacitance that it is not likely to effect any movement on a multimeter, analogue or digital. The fact that your Chinese tester is showing 239pF is sufficient enough to determine that there exists, in fact, a capacitance that is close enough to the expected value. Your multimeter test shows that it isn't shorted or leaky. Your capacitor is OK. Don't worry about it. As Station X pointed out, the ESR value is only relevant for electrolytics.

[poppydog]

Thanks for the reply WaveyDipole, that's good news but it isn't, as I have tested oodles of components even around the power supply and the output stage and I have found nothing apart from a few capacitors that were a little higher than specified. The only one that showed any sign of being iffy was C807 and on the sheet it said it should be 10v 470uf, in its place was a 470uf 12v and according to the meter that was reading just under 400uf, so I have replaced that with a 16v one. That is the only thing that has shown any signs of not being right. Most of the resistors I have tested have been spot on their values. In Restoration 73's last post to me he mentioned replacing R650, when I desoldered it it was black underneath.

[poppydog]

This evening I finally managed to get the radio up and running. I replaced AC188, AC187 and AC121, C655, C654, C809, C806, C807, C652 and C651. Out of all those I replaced the only one that was a bit iffy was the C807, it was about 50uf lower than stated. The adjustable resistor R650 I replaced with a Cermet 50ohm trimmer, I had to extend one of the pins to make it fit and after all that I had to remove it as for some reason the needle on the meter was all over the place. So I de-soldered it and after cleaning the original resistor with some Servisol I reinstalled it. For some reason my DMM was not reading any current on no volume at all, no matter how much I adjusted the resistor, however when I turned the volume up it started reading. I plugged my analogue meter in and it read it fine, the needle stayed static at a steady 7.5ma. It's been on for an hour and the transistors are barely warm. It's looking quite possible that my DMM is up the shoot as the meter may have read the current wrong last time. I will try to run it up for a couple of hours every day and make sure the transistors are not getting hot and then perhaps at the weekend I will put it all back together and hopefully that will be the last of it.

Another quick question: Is it normal for when the meter is wired in series with the transistor, when I press the on/off button to turn it on the needle goes full deflection and then settles back down?

Thank you

[Herald1360]

Last question... maybe. It all depends on the exact circuit, how capacitors charge up at switch on, how fast any dc feedback loop gains control etc. etc. Making exactly the same measurement on an identical set might be informative or possibly some transient analysis using a simulation package if any usable Ge transistor models exist.

If it's a suitably short transient event, it's probably not a problem; measuring it accurately to prove that without some sort of storage 'scope is a bit of a non starter.

[poppydog]

Thanks for the reply.

I asked the last question because when it packed up before, it happened as I pressed the on/off switch on, there was a loud hum as if there was a surge, then nothing. Now that I have the meter connected and I turn it on, seeing the needle "hit the bump stop" and go back again it is still in the back of my mind whether the same thing is going to happen again - does that make sense?

[Herald1360]

Definitely. Once bitten........

Depending on the circuit, any problems around quiescent current setup networks can be bad news, especially if an open circuit pot wiper causes excess current rather than too little.

[poppydog]

Originally Posted by Herald1360

Depending on the circuit, any problems around quiescent current setup networks can be bad news, especially if an open circuit pot wiper causes excess current rather than too little.

Does this mean I am still advised to change R650? I don't know what was wrong with that Cermet 50ohm trimmer I bought, it just wouldn't adjust properly. It was the only thing I could find that was close to what I had, even though the pin spacings were a mile out. If anybody could point me in the right direction for a replacement for one of these it would be greatly appreciated.

[Herald1360]

R650 should be 50ohm and good quality. If its wiper loses contact, quiescent current will take off. The whole thing isn't helped by lack of any emitter resistors for the output pair. Bias stabilisation relies on good thermal linking between output devices and TR11.

From the outline of R650 on the Trader sheet, it looks like a common sort of preset- can you post a dimensioned sketch of its footprint? That would help to find something suitable. A and B dims on snippet attached.

[poppydog]

I will measure and post tonight, thanks

[poppydog]

Please see attachment as requested, hope somebody can help. Sizes are approx.
Thank you

[Herald1360]

I've had a poke around and come to the conclusion that finding anything less than 100R is unlikely. So.....

Maybe get something like this:

http://www.ebay.co.uk/itm/Iskra-Encl...EAAMXQatBSblM~

and connect a 100R fixed resistor across the ends of the track under the PCB. It should work well enough.

Or this:

http://uk.farnell.com/bourns/3386p-1...-th/dp/2328570

and judiciously extend the wiper leg with some tinned copper wire.

If you can find a Bourns 3386G-1-500F anywhere it would be ideal, but I couldn't find any in stock. Mouser offers them on 7 weeks lead time!

[poppydog]

Thank you Herald1360, however great minds think alike with regards to your Farnell link! Please see photos. I extended the middle pin then tested the Cermet one before installation and the meter says it was absolutely fine, however when I soldered it in place I couldn't get the reading down below 10 to 12ma. It didn't like it at all.

When you say 100R does 100R mean 100ohm?

Thanks

[poppydog]

That Bourns trimmer in your link even has the same part number as my Cermet. "Badge engineering" comes to mind. I am not going to waste much more time on this but what about:

http://www.ebay.co.uk/itm/2522030881...%3AMEBIDX%3AIT

I will email the seller later for the dimensions if they will do the job. Rather expensive but I may be able to off-load some of them elsewhere.

Thanks

[Herald1360]

Aha!.

Mmmm, if a good 50R (yes ohms) pot didn't work , I would be getting leery about the bias regulator or output transistors. At one end of its travel the regulator should be hard on with only about 200mV or 1 Ge Vbe across it, which should be low enough to turn the output stage off. You do have a Ge transistor in there? A silicon one wouldn't do.

The R usage is fairly common- it's more obvious if you see resistor values like 4R7, akin to 4K7 or 4M7 for different multipliers. It looks a bit odd on a low value metalclad wirewound where you can get things like R1 or R01 (0.1 and 0.01 ohms) printed on the body.

[poppydog]

Thanks Herald1360, so with regards to the adjustable resistor, the replacement cermet one I bought turns out to be faulty, but luckily I bought two (just in case of cock-up) and I have installed this this evening and it works perfectly. It adjusts nice and smooth, the needle on the meter is very steady. However it is still not right. There is a fault and it's intermittent, on a couple of occasions this evening for no apparent reason (I still have the meter in series with the transistor) the needle on the meter has slammed over and stayed there. However when you turn it off and then turn it back on again it settles back down. I found that if I gently tap the vertical circuit board (the one with the ferrite rod on) with the end of a screwdriver that will make it do the same. I was wondering if the on/off switch could be the culprit. Being intermittent this could turn out to be a nightmare!

[Herald1360]

Eeeek!

Possibly literally- the problem may be caused by a bad joint or fractured track causing ultrasonic oscillations that drive the amplifier to full output. More likely, the bias network has an intermittent connection somewhere. This may have been the original fault!

Can you monitor the voltage between the output transistor bases as well as the current? Does that go up when it happens?

If the set's output dies with the current increase, you could use this symptom instead of the current meter as your problem indicator and just use one meter.

[Mike Noble]

Hi, I have two of these Grundig's and both have had intermittent joints where the resistor board solders to the main board. One had occasional loud popping noises from the speaker and one would suddenly go quiet.
Re-soldering these joints cured both. Worth a go. Mike