Toshiba IC70

[geofy]

I have labelled a few areas.

1> Is this a solder track as the base of AC128 should not be joined to collector of BC108

2 >This solder joint between base and collector look very close

3> The leads from BC109 and BC108 must have been crossed over to give ecb unlike the AC128 ebc configuration

What might be worth doing, print off if you can just the amplifier part of the diagram and then one by one check each lead and mark it off on the print off.

start at the 39K and BC109 base and go from there following each connection and marking any which do not follow the diagram.

The BE and BC forward should show around 600mV if this is a high reading then something is wrong, check this with leads one way and then the other.

Depending on the meter the red to base and black to emitter or collector should give the forward reading for an NPN transistor. But whatever the leads you should get a low reading with leads round one way and a high reading the other way and a highish reading EC CE with base open.

Other components in circiut can give misleading readings if shunting the junctions.

The AC128 should be the opposite as it is PNP and around 300mV

[briskly]

Originally Posted by geofy

I have labelled a few areas.

Thanks again for your help Geof - much appreciated!

1> Is this a solder track as the base of AC128 should not be joined to collector of BC108

Just a solder track. Measured 3.5K between them - will clean it up a bit more.

2 >This solder joint between base and collector look very close

Measured 60K between them - will clean up some more!

3> The leads from BC109 and BC108 must have been crossed over to give ecb unlike the AC128 ebc configuration

Not sure whay you mean here - I thought looking down on the 2SC372 from the pin outputs with the flat edge on the right. E was at the top, C in the middle and B at the bottom - same for the BC183 substitute? I tried fitting a BC109 but the EBC pin arrangement makes this a bit awkward.


What might be worth doing, print off if you can just the amplifier part of the diagram and then one by one check each lead and mark it off on the print off.

start at the 39K and BC109 base and go from there following each connection and marking any which do not follow the diagram.

The BE and BC forward should show around 600mV if this is a high reading then something is wrong, check this with leads one way and then the other.

Depending on the meter the red to base and black to emitter or collector should give the forward reading for an NPN transistor. But whatever the leads you should get a low reading with leads round one way and a high reading the other way and a highish reading EC CE with base open.

Other components in circiut can give misleading readings if shunting the junctions.


I 'll do this and check again


The AC128 should be the opposite as it is PNP and around 300mV


Thanks - will check again

Andrew

[orbanp1]

Hi Andrew,

A couple of points.

There are probably component value errors on the schematics. That is not a factory schematics, but a redrawn one...

Just by looking at it, the 4.7uF bootstrap capacitor in the final amp is more likely 47uF!
Someone also mentioned that the audio jack wiring is suspect.
I plugged the circuit into a circuit simulator, the correct value for the 39K (R5) resistor between the base and the ground of the driver transistor is more like 3.9K (4.7k) according to the simulator!

Now, to the point. You need to make sure that the DC operating point is correct. That is the 3V and the 2.3mA values shown on the diagram. Changing transistors (different beta) will affect the op point. The 3V can be adjusted by selecting the proper value for the 3.9K base resistor. In that voltage also has an effect the bias from the IC, coming from pin 5, 4.4V, through a coil and a 27K resistor (previous page on the schematics). Check with a DMM if the 4.4V is there at the other end of the coil, that is common with the 27K resistor.
When you have an operating point close to half of the supply voltage (3V), you can check the quiescent current. It can be set by adjusting the value of the 39 Ohm resistor (R1, the simulator gave 68 Ohms for 2.2mA).

Good luck, Peter

[Herald1360]

I'd be intrigued to know whether you have a decent model for an AC128. The only ones I've found give a Vbe of around 0.6V which is nonsense for Germanium.

[orbanp1]

Originally Posted by Herald1360

I'd be intrigued to know whether you have a decent model for an AC128. The only ones I've found give a Vbe of around 0.6V which is nonsense for Germanium.

We had a good discussion about this on the DIY-Audio board.
I modeled this transistor from the catalog I have and an Excel application that extracts spice model values from that data. It gave believable results, not like that model you are talking about.

Peter

[briskly]

Thanks Peter,

Originally Posted by orbanp1

Hi Andrew,

A couple of points.

There are probably component value errors on the schematics. That is not a factory schematics, but a redrawn one...

Just by looking at it, the 4.7uF bootstrap capacitor in the final amp is more likely 47uF!
Someone also mentioned that the audio jack wiring is suspect.
I plugged the circuit into a circuit simulator, the correct value for the 39K (R5) resistor between the base and the ground of the driver transistor is more like 3.9K (4.7k) according to the simulator!


I had to replace the resistor between base and ground of the driver transistor as it had broken. I thought I read the colour code as green blue orange (strange tone of orange), making 56k - near to the value on the schematic - and replaced it with this value. Maybe I was mistaken and it was green blue red, making 5.6k? Do you think I should try 5.6k first?


Now, to the point. You need to make sure that the DC operating point is correct. That is the 3V and the 2.3mA values shown on the diagram. Changing transistors (different beta) will affect the op point. The 3V can be adjusted by selecting the proper value for the 3.9K base resistor. In that voltage also has an effect the bias from the IC, coming from pin 5, 4.4V, through a coil and a 27K resistor (previous page on the schematics). Check with a DMM if the 4.4V is there at the other end of the coil, that is common with the 27K resistor.
When you have an operating point close to half of the supply voltage (3V), you can check the quiescent current. It can be set by adjusting the value of the 39 Ohm resistor (R1, the simulator gave 68 Ohms for 2.2mA).

Great - I'll try this once I can get something working. Should I hear some sort of noise in the speaker while fiddling with this?

Good luck, Peter

Thanks, I think I'm going to need it!

[briskly]

Geof,


I've cleaned up the tracks, replaced 2SC372 with a new BC183 but get the following readings in circuit with the diode tester on my DVM:

Black lead on base or (collector for c-e):

BC183 (in place of 2SC372): b-e:620, b-c:720, c-e 200

2SC735: b-e: high, b-c: 216, c-e: 156

AC128 (in place of 2SB364): b-e: 164, b-c:158, c-e:717



Red lead on base (or c for c-e)


BC183: b-e:high, b-c:high, c-e 720

2SC735: b-e: 609, b-c: 216, c-e: 612

AC128: b-e: 1570, b-c:720, c-e:1570


Does this make any sense?


Andrew

[geofy]

If you are sure the transistors are going in the right place, which is difficult to tell from the pictures as I would have expected the ebc configuration to be on the board to match the transistor leadouts, not the twisted arrangement you have apart from the AC128 which seems to follow ebc which matches the transistor leadouts. This needs rechecking. And try measuring a transistor out of circuit to get an idea of readings a good one shows on the meter.

If you are sure the base resistor of the BC109 was 56K then it would be better to stick with these values and not go any lower than 39K. 3.9K sounds a bit to low for this position.

Something around the 2SC735 doesn't look quite right, as if the b-e is back to front.

It isn't necessary to send back the whole of the previous post by the way, this can have parts deleted to make it more compact.

[briskly]

Thanks again Geof,

I'm pretty sure the BC183 configuration is the same as the original - the three pins are in line. When I tried a BC109 I got into a mess with twisted leads! But something does look a bit odd with 2SC735 even though I haven't touched it...

I'll check some transistors out of circuit as you suggest.

I'm beginning to lose hope....

[Station X]

You may find the following thread useful. It describes testing transistors out of circuit:-

https://www.vintage-radio.net/forum/...ad.php?t=42194

[briskly]

Hello again everyone. Well, I took a cold bath, cleaned up the track, checked every component on the board against the schematic one by one, checked the transistor orientation, then replaced all 3 audio amp transistors with new (now BC337 for the 2SC375, BC183 for the 2SC372 and AC128 for the 2SB364) and the diode (BAY 44). Powered up with batteries and......

A puff of smoke and all 3 transistors got very hot. Switched off after about 2 seconds and then measured 15 Ohms between the emitter and collector of 2SC375 (now BC337). Help - Has anyone got any ideas?

It seems I have 3 options, which I'd very much appreciate any help with:

1. Continue trying to fix it with your help.

2. Build a new power amp on vero board and cut out the old section to replace it. If so - has anyone a suggested simple 250 - 500 mW output 6V circuit using easy to find silicon transistors?

3. Use something like the LM386N IC instead of a vero board circuit.This seems to give a 20dB gain without many extra components. But I think I will need much more gain than this - any ideas?

Thanks in advance for your help,


Andrew

[frsimen]

Andrew,

Intrigued by the trouble you’ve had, I built a lash up version of the circuit, omitting the capacitors, as I was only interested in the DC performance. I used a 1N914 for D1, a BC237B (much the same as a BC109B) for Q1 (2SC372), a BC107B for Q2 and a 2SB172 for Q3. The result was an amplifier which when fed from 6V had 2.85 volts on the emitters of Q2 and Q3 and a total supply current of 5mA. Changing Q2 to a much lower gain transistor, gave much the same Q2 and Q3 emitter voltages and about 4.5mA total supply current. So, the circuit as shown should work with the transistors you are using.

My suspicion is that you have something connected the wrong way round. D1 incorrectly wired or the base lead of any of the transistors swapped in error for the collector will result in a very significantly higher current flow. My suggestion would be to put coloured sleeving on the transistor leads to allow you to identify each of them as they are fitted. Be aware that the 2SC372 and 2SC735 have a different lead arrangement from the BC183 and BC337 you have been using, so double check that you have things right. Finally, before trying for the first time, add a current limiting resistor of say 100-150 ohms in series with the 6v supply to the radio. If you have made a mistake, the resistor will get hot, but nothing will be damaged.

Of your other ideas, yes, you could do those. You could just build another version of the amplifier on veroboard. The LM386 could also be pressed into service, but it would be better to try to get the original arrangement working, if that is possible.

Paula

[briskly]

Originally Posted by frsimen

Be aware that the 2SC372 and 2SC735 have a different lead arrangement from the BC183 and BC337 you have been using, so double check that you have things right.

Paula

Thanks for this - it looks like I've been stupid and not followed Geof and the others' advice from earlier. Could you please tell me the lead arrangement for the BC183 and BC337

[frsimen]

Andrew,

Both the BC183 and the BC337 have the same arrangement. With the leads facing you and the flat side down, the leads from left to right are:

E B C.

The BC183L is different, but they're not that common these days. Flat side down they go B C E, which is the same as in the 2SC372 and 2SC735.

Good luck!

Paula

[Herald1360]

I wonder....

Beware the lead out difference between the BC183 and the BC183L .

See pics for info:

Beaten to the draw!

[briskly]

Thanks Paula and Chris - I'll have another go!

Andrew

[briskly]

I've replaced all the transistors - 2SC735 with BC108B (all I could get), 2SC372 with BC109 and 2SB354 with a genuine Toshiba - hoping that the arrow mark on the case indicates emitter. I also replaced the diode again. Using a 100 Ohm resistor after the on/off switch for safety - voltage 5.2V at + connection on board.

Still no life - the voltages are still wrong - only around .2V at the common emitter point and the diode - terminal. A glimmer of hope though - a very,very faint sound of stations I tune to in headphones. I'm wondering what to do next. Checking the BC109 with a diode checker reveals high reading in both directions between base and emitter and collector and between collector and emitter - maybe this is faulty? Any help appreciated.......

[Herald1360]

Hi,

a 2SB364 (nearly fooled me but a 2SB354 is a 100V 15A stud mounting device!) is TO1 and the spot is collector

Hopefully it's survived.....

[Hybrid tellies]

My heart goes out to you, I have had similiar output problems with my much beloved ITT Colt.After trying out different configurations of output transistors and biasing diodes without success I had to order transistors and diodes directly from ITT. Your Toshiba uses a very intertesting settup in the output stage and like others have never come across one mixing Ge and Si devices before. My advice don't lose heart especially having gone so far, suitable equivalants should still be available and if all fails building a small IC amp is still an option.

[Herald1360]

If it has died, an AC128 or AC188 should do as substitute, maybe an OC81 too.

Possibly cheaper option to just use silicon pnp and increase the 39R resistor to get bias about right.