Grundig Concert Boy 1100 (again)

[1980s_john]

Hi,

An earlier thread included this useful info:

"Not to worry, I've got a scrap 1100 here. The transistor(s) that may have failed are on the heatsink, (extreme right on diagram) T011,T012,T013, the first being a circular type held in a metal C-clip, and the latter two being square and bolted direct to the heat sink. It is most likely one of the last two has failed, but it may be a good idea to replace both T011 is AC121 (AC128 will do) T012 is AC187K, and T013 is AC188K. There may be something on the forum about testing these."

I've looked at the schematic and can't find a reference for the transistors used - is there one please, or is the above based on experience and writing down the observed part numbers?

Regards,
John

[Leon Crampin]

John, the circuit diagram supplied originally with the radio gives all the circuit references and the device type numbers. The information given above is broadly correct. T011 is listed as an AC121S but as stated above, it's not critical as it merely defines the inter-base voltage and hence the quiescent current of the output stage. A germanium device is needed to give a temperature coefficient with respect to voltage which matches the output pair. An AC128 would do.

You can make a reasonable check of these devices using the diode test facility on a DMM or an AVO on the Rx1 range. The b-e and b-c junctions should each test as a germanium diode with very little reverse leakage. Note than the AC187 is npn and will show reversed results.

There is a solder covered break in the track to T013 which allows the quiescent current to be set. It is important to ensure that the adjuster R650 is not intermittent - apply Servisol Super 10 and move the pot several times end to end before setting. The adjustment should give a quiescent current of 7.5 mA at a supply voltage of 9V (important - don't use the inbuilt power supply).

The other big weakness is the smoothing capacitor C806 which sees more than its rated 16V on 240V mains. Replace with a 2200uF 25V type, noting that there are 3 negative connections which need to be linked. A failure can cause damage if the 400mAT fuse is incorrectly substituted.

The tuning capacitor drive spindle bearing needs to be free (heat and light oil should do this) and the user push button switches will also need the Servisol treatment.

Good sets, well worth restoring.

Leon.

[1980s_john]

Thanks! I can see now you pointed it out there's a list of transistors and diodes in red under the circuit diagram, missed it before. I'll remove and check the output transistors, I've already replaced the failed C806 and oiled / exercised the stuck tuning capacitor. I also replaced the selenium bridge rectifier with a silicon one, not sure if this causes problems elsewhere though (due to lower voltage drop).

Regards,
John

[ajgriff]

What's actually wrong with the radio? Can't find any reference to it in earlier threads although I may have missed it. A few voltage checks and basic signal tracing can often isolate faulty components so that random desoldering and testing can be avoided. As long as you're not getting more than about 16V across the rectifier it shouldn't matter as the voltage to the rest of the circuit is largely determined by the zener diode further down the line.

Seventies Grundigs are decent sets as Leon says.

Alan

[Leon Crampin]

The output stages run at full voltage from the power supply when the set is run on the mains. This will give an incorrect setting of the quiescent current. A decent set of batteries is usually very close to 9V, although I usually use a lab supply when setting up the output stage.

Last time I did this job, I made a mental note to solder two pieces of wire to the measuring point bridged by a solder link. By bringing these above the of board, the quiescent current can be checked without extracting the board.

Leon.

[1980s_john]

Originally Posted by ajgriff

What's actually wrong with the radio?

Nothing unusal, same as examples in other threads. Was sat on shelf for years with a stuck tuning dial, having freed it set was dead. C806 was acting as a 20 ohm resistor so was replaced, set now receives FM OK but very quiet and distorts when volume raised. I found a DIN lead and output there is fine, feeding a sine wave in (switched set to tape) gives a distorted sine out (very asymetric). I've just removed T012 and T013, both had good PN junctions (all about 0.2V on diode test in forward direction, high resistance in reverse direction). Hfe test gave 140 for T012 but only 90 for T013, so I think I need a new AC188K.

Above is on a 9V DC supply, on mains get a loud hum + signal. I haven't checked the zener diode yet.

Regards,
John

[ajgriff]

Towers gives AC188/K Hfe as 100 min so not sure that T013 is far enough out to be causing such a significant effect. Are the voltages shown on the schematic for the audio stage correct with the bias set to the specified figures? Also sounds like there's an issue with the mains power supply. You don't say whether or not AM is working ok at the DIN output?

Alan

[Leon Crampin]

hfe is very dependent on collector current, so consult the data when checking. These sets do not show exactly half the supply voltage at the speaker coupling capacitor, but the voltages shown on the diagram are pretty close if you correct for 240V mains. The amplifier supply is usually about 17V in normal UK mains operation.

If you read the voltages from the circuit, be careful to interpret correctly the dotted rectangles around the values. These give the measuring conditions.

The electrolytics generally are suspect if made by our friends ERO. You could usefully check the bootstrap capacitor.

You did common the 3 negative connections to the original smoothing capacitor - I hope.....

Leon.

[1980s_john]

Thanks for the tips, I'll try them when I'm back next week. I checked all the electrolytic caps with an LCR meter and all bar one were OK. Are the two output transistors meant to be a matched pair? I only have a simple transistor checker on a DMM so no idea on test collector current.
Regards,
John

[ajgriff]

Originally Posted by 1980s_john

.....feeding a sine wave in (switched set to tape) gives a distorted sine out (very asymetric).

Does this mean that you have an oscilloscope? If so just track the sine wave through the audio signal path to isolate the fault. Still could be an electrolytic. LCR testers are not always 100% reliable in this respect as they don't replicate working conditions. Could even be something simple like a dry solder joint if it's not a transistor problem.

Don't know whether or not any attempt was made to match the output transistors but I do know that if one of them is leaky distortion does occur. Sometimes it's best to replace both transistors if one of them is failing due to stress. Trouble is AC187/Ks and AC188/Ks are becoming increasingly difficult to source at a sensible price although there are workable, but not perfect, alternatives.

Still got to sort out the mains power issue and possibly AM reception at some point though. Best of luck with your efforts. Should be well worth it in the end.

Alan

[ajgriff]

Originally Posted by Leon Crampin

There is a solder covered break in the track to T013 which allows the quiescent current to be set. It is important to ensure that the adjuster R650 is not intermittent - apply Servisol Super 10 and move the pot several times end to end before setting. The adjustment should give a quiescent current of 7.5 mA at a supply voltage of 9V (important - don't use the inbuilt power supply).

Don't forget to do as Leon suggests before carrying out further voltage and signal tracing checks.

Alan

[1980s_john]

Hi,
I've got the radio working fine on a 9V DC supply. I ordered both AC187K and AC188K transistors, I tested these for DC gain and leakage (I'll write about this desperately) and they had much higher gain (and so more Iceo leakage!) than the old ones, they work just fine. Having set the quiescent current to 10mA (sorry - didn't check the specs - blame my faulty memory!) radio sounds good on FM and seems to work OK on all bands.

On mains, hum is still very bad, I have yet to check the zener diode and also try the tape output/input to see which side of the break the hum is introduced. I think I might fit some diodes to reduce the bridge rectifier output voltage, and check / replace the zener diode, but this will be for another day, very happy with the radio now (still needs a good clean), thanks for all the help.

Regards,
John

[1980s_john]

Originally Posted by 1980s_john

I tested these for DC gain and leakage (I'll write about this desperately)

Apologies I meant 'separately' (of course).

[poppydog]

Originally Posted by 1980s_john

Hi,

An earlier thread included this useful info:

"Not to worry, I've got a scrap 1100 here. The transistor(s) that may have failed are on the heatsink, (extreme right on diagram) T011,T012,T013, the first being a circular type held in a metal C-clip, and the latter two being square and bolted direct to the heat sink. It is most likely one of the last two has failed, but it may be a good idea to replace both T011 is AC121 (AC128 will do) T012 is AC187K, and T013 is AC188K. There may be something on the forum about testing these."

I've looked at the schematic and can't find a reference for the transistors used - is there one please, or is the above based on experience and writing down the observed part numbers?

Regards,
John

The quote your referring to is by restoration73 on my thread way back in 2015, how time flies. I never did count how many members chipped in to help but it was quite a few
If your concertboy sounds anything like mine still does then I think you will be happy, the sensitivity on fm is impressive.

poppydog