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Old 14th May 2018, 10:47 pm   #1
crackle
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Default ITT Touring Studio 104

I thought I would mention this repair here in case anyone else has similar problems with one of these sets. This is the model in the Radio Museum.

I bought this radio as not working. A German friend translated the description for me, he said the description went like this; "He's saying it'd be a pity to put this radio into the metal press (no this isn't a phrase in German either . He goes on to say the outside is pretty good, but the innards are more or less trash. The keys are not working and the electrolytic capacitors are done for (Elektrolyt Kondensator = Elko). He assumes more is damaged."

So that was the description, and although it was rather confused it was basically correct, the radio was completely dead, and the interlocking for the push buttons had problems.

I spent a good 15 minutes trying to get the cabinet apart, mistakenly I assumed the back would come off like many of the UK ITT models. No this was simple in the end, the chassis just lifts out complete with speaker once the 3 screws in the back have been removed.
I had a quick inspection, and saw that the mains fuse was missing, ah a simple repair, no of course not.

The missing fuse was not the problem because it did not work on a 12v DC supply either. But what did work was the puffs of very smelly smoke which started emitting from a green transistor shortly after I connected up my bench supply, the smoke got right up my noise and it smelled as bad as a Riffa capacitor that is about to blow apart.
I switched off and resigned myself to a long repair, which was rather inconvenient as I already had another radio in bits on the bench. But you know how it is, you just want to try one more thing.

The smoking transistor was a little BC 252 in a green plastic package. Note the nice thick legs, not like the flimsy thin ones you get on similar transistors now days.
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It forms part of a voltage regulator supplying 7 volts to the RF & IF sections, The voltage needs to be stable as the FM tuning is by varicap diodes.
I am not sure what the cause of the transistor to burn out was, but it could possibly be the 22uF tantalum capacitor connected downstream from the transistor. The problem was I was unable to properly test it as one leg broke away as I was removing it from the printed circuit. I replaced it with a 33uF quality electrolytic.

The new transistor and capacitor in place and the circuit. The greyed part of the circuit applies to the 104A model, and is not present in my radio.
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There were no apparent shorts on the RF supply rail, the resistance varied from just over 2k to 4k depending which wave band was switched on.
I connected a 7 volt supply to the RF rail and it looked healthy enough drawing about 15mA from the bench supply. I then tried testing for an audio signal around pin 8 on the audio output amp. (a TBA810), nothing was found, but there was some hiss and small amount of static pickup.

This may be dirty switches I thought, there was signs of tarnish on the silver plated trimmers. I decided to clean all the waveband switches, there are 12 push buttons in all and the circuit board had to be removed from the inner chassis in order to get to them so I could spray with switch cleaner.
I gave the radio a couple of hours for the Servisol to evaporate and went and tried again, this time I used a more sensible test point, marked as 5 on the schematic, this was immediately after the radio section and before any switching associated with the 5 pin DIN socket for pickup or tape, and also before any volume or tone controls.
Here I found a signal and was able to tune in to stations on both the MW and FM bands.

Great I thought, pop in a new transistor to replace the BC 252 put it all back together and the radio is working..

That was when the real problems started. I could not get the output amp to work. For some reason there was no supply voltage going to it.
It was difficult to follow the printed tracks as they were ducking and diving from one side of the board to the other, and under large components like the main FM variable resistor and large electrolytics.
This fault took ages to locate, but to cut a long story shorter I found the problem. It was a through the board connection where the circuit track went through to the other side and back again in order to cross under another track. There was 12 volts one side of the bridge but no volts the other.

Photo of the bad link under the board and the layout drawing showing the component side tracks in pink and the under side tracks in grey.
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I couldn't work this out as it all looked good with original clean solder through the holes and on the copper. After some considerable heating with the iron I managed to get a good flow of solder around and through the hole to make a good connection.

I connected up the 12 volt supply and this time the audio amp was working.

Although it is all working now, I cant reassemble the radio until some new panel bulbs arrive, they cant be fitted unless the circuit board is removed from the inner chassis, (stupid design)
My plan is to replace the two 6v 1.2w lamps which were wired in series, with 14v 1w bulbs. These will be wired in parallel and under run on the dedicated 13v supply from the transformer. I also ordered some 63mA fuses as well to replace the missing 50mA fuse. It will help as the set will be working on 240v rather than the 220v it was designed for, and besides the later model used a 63mA fuse.

Mike

Last edited by crackle; 14th May 2018 at 11:00 pm.
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Old 14th May 2018, 11:03 pm   #2
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Default Re: ITT Touring Studio 104

I have had one of those via holes go open circuit on a TEK scope.
Putting a bit of tinned copper wire through it fixed it for good.
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Old 15th May 2018, 6:44 am   #3
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Default Re: ITT Touring Studio 104

A good quality scan of the service manual covering the basics of this radio can be obtained here. https://freeservicemanuals.info/en/s...nal%20104a.pdf
Although this is for the 104A it is an update to the 104 service information, it appears the circuit is almost the same except for an additional preamp stage to the audio amp.
For more detailed service information including circuit descriptions and alignment on the 104 and 104A see Electrotanya. https://elektrotanya.com/schaub-lore.../download.html
But the scanned images of the PCB layout are not as clear as the 104A one above.

Mike
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Old 15th May 2018, 10:59 pm   #4
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Default Re: ITT Touring Studio 104

I forgot to mention, typical of a German radio the AM preset tuning gang was almost seized. The main AM tuning gang was Ok probably because it was used more.
A little heat from the soldering iron on the spindle and copious quantities of sewing machine oil, with lots of turning back and forth and it finally became nice and free.
Here are a few photos.
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Old 19th May 2018, 4:22 pm   #5
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Default Re: ITT Touring Studio 104

I have a little problem with this radio. On switch on there is a fairly loud click/thump in the speakers. I dont believe this should be happening.
Removing components from the printed circuit board in this radio is quite difficult as they are mounted close to the board and soldered on both sides, including under the close fitting radial capacitors.
So rather than potentially destroy all the capacitors to remove them just to sub in a new one to see if it makes any difference to the thump.

I have tested 2 other radios which use this IC and they switch on fairly quietly.

I was hoping that maybe someone who knows the TBA810 can point me to the components which are going to be the cause, or give me some tests to perform to try and determine which component/s may be faulty.

Below is part of the circuit involving the TBA810 ic.
circuit.doc
Thanks
Mike

Last edited by crackle; 19th May 2018 at 4:31 pm.
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Old 19th May 2018, 4:26 pm   #6
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Default Re: ITT Touring Studio 104

Have a look at the data sheet for the chip.
There might be a mute pin that can be held with an R/C network during power up.
Also have a look at the circuit diagram from something that switches on quietly for a couple of extra components around the chip.
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Old 19th May 2018, 8:21 pm   #7
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Default Re: ITT Touring Studio 104

Pin 4 is bootstrap, what is bootstrap.
Is there anyone with experience of the TBA810?
thanks

Mike
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Old 20th May 2018, 12:25 am   #8
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Default Re: ITT Touring Studio 104

Bootstrap is a positive feedback kind of thing where the feedback is used to boost the drive to one of the output transistors. Usually the top one of a pair of NPNs.
There is normally a reasonably big electrolytic capacitor connected to it.
The speaker capacitor is always bigger.
http://www.alldatasheet.com/datashee...CS/TBA810.html
Pin 7 ripple rejection.
Increasing it will hold off the base drive to Q8 and the upper output transistor at power up at the expense of it being distorted for a very short time if it is too higher value.

Last edited by Refugee; 20th May 2018 at 12:39 am. Reason: Data sheet detail addid
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Old 20th May 2018, 12:02 pm   #9
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Default Re: ITT Touring Studio 104

Simon, are you saying that the capacitor on pin 7 prevents the switch on pulse, so check to see if it is OK.

Thanks
Mike
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Old 20th May 2018, 1:34 pm   #10
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Default Re: ITT Touring Studio 104

It might be that the original capacitor is duff.
If it is a Callins it will certainly be duff.
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Old 20th May 2018, 4:19 pm   #11
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Default Re: ITT Touring Studio 104

These are all quality ITT branded electrolytic capacitors, and their ESR still shows as better than most modern replacements.
I tacked a 100uF cap onto the PCB pads with C375 on pin 7 and it made no difference, I also tried a 1000uF cap and that seemed to delay the pulse but the speaker diaphragm still makes a very large excursion on switch on and off.

Mike
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Old 20th May 2018, 7:50 pm   #12
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Default Re: ITT Touring Studio 104

Have you checked / replaced the speaker coupling capacitor?
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Old 20th May 2018, 11:06 pm   #13
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Default Re: ITT Touring Studio 104

Once we have checked the speaker coupling capacitor we need to start looking for a small burst of RF during power up.
There is a simple way of making a scope flash up a blank raster during the thump if it is RF.
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Old 21st May 2018, 6:51 am   #14
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Default Re: ITT Touring Studio 104

It does the the thump when the radio is switched to "amp". The Output capacitor "passed" on my Peak ESR meter.

Mike
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Old 21st May 2018, 10:53 am   #15
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Default Re: ITT Touring Studio 104

Does the thump vary with the volume control?
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Old 21st May 2018, 11:53 am   #16
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Default Re: ITT Touring Studio 104

No it is just the same with the volume down.

Mike
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Old 21st May 2018, 11:58 am   #17
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Default Re: ITT Touring Studio 104

The only thing left is the comp capacitors.
An open circuit one could make a little burst of RF.
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Old 21st May 2018, 3:26 pm   #18
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Default Re: ITT Touring Studio 104

Sorry, I may be being dim but what are "comp capacitors".

Thanks
Mike
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Old 21st May 2018, 5:10 pm   #19
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Default Re: ITT Touring Studio 104

Have a look back at the data sheet I linked to a few posts back.
It shows a pin called comp with the capacitors that stop it oscillating at HF.
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