Toshiba IC70 - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

geofy · 3rd Oct 2011, 6:04 pm

I would conclude from this that the germanium 2SB364 has failed. Try lifting the base or collector lead and re-measure. And recheck the diode. The multimeter should have a diode check range which usually gives around 600 millivolt forward and infinite or high reverse for a good silicon junction, and 300 millivolt forward and high reverse for a good germanium junction.

ukcol · 3rd Oct 2011, 7:22 pm

Hi Andrew

Ref your questions in post 18.

I would follow Paul Sherwins advice. Use a cheap DMM that has a transistor test function. For the 2SC735 pick the transistor with the lower hfe of the two. For the 2SC372 pick the transistor with the higher hfe of the two.

Ref geofy's advice about checking each device before condemming it, I have a different view. Although it is perfectly good advice, in my experience once you have established that at least one device in a DC coupled solid state circuit has failed short circuit, then its a good idea to replace all of them even if they read OK. If you have suitable parts to hand then I would replace all 3 transistors and the diode. If not it may be worth replacing only those that measure obviously faulty and then giving it a try.

Regards

Colin

geofy · 3rd Oct 2011, 8:13 pm

While I agree in some cases one transistor failing can take out others, in this case it looks like the germanium has failed protecting the others in the process by taking all the current, the other silicon transistors are still measuring ok, or will be once the shorted germanium is removed, so why blanket change transistors which are not faulty. The silicon types are far stronger, only a much too large base current could damage them or to higher voltage which hasn't happened here, a BC109 has around 30V CE

If the AC128 is checked out of circuit it will probably be found to be shorted.

Don't know what type a BAY44 diode is but the output stage could be biased to take a PNP silicon transistor. This diode should be checked out of circuit.

paulsherwin · 3rd Oct 2011, 8:29 pm

In general, Si transistors are much tougher than Ge as Geof says, so it's very likely that the Ge transistor has failed protecting the others.

You can replace the 2SB364 with a range of Ge transistors if you don't have an AC128 handy, including earlier output transistors like the OC81. Do this before doing anything else. I wouldn't change any other transistors if they seem to be OK, as you will be subbing different transistor types and this is always to be avoided if possible. The more you change the more likely you are to hit compatibility problems.

Your responses imply that you aren't testing transistors using a transistor tester. This is much quicker and less error prone than trying to measure the resistance between junctions, and even £3 DMMs from market stalls have a suitable basic transistor test function.

briskly · 3rd Oct 2011, 8:55 pm

Thanks again for your help everyone. I'm hoping to start working on it as soon as possible in good light tomorrow - trying the AC128 replacement first. I've just found a multimeter with a transistor checker as well!

Cheers,

Andrew

geofy · 3rd Oct 2011, 9:30 pm

It might be possible to fit two diodes such as 1N914 in place of the BAY44, one taking place of the 39 Ohm resistor, both facing the same way as the existing diode, cathodes towards BC109 collector. And then replace the AC128 with a silicon such as a BC158 or BC558 PNP transistor. The two diodes give the correct bias for the silicon transistor. Although both the BC108 (BC148) and BC158 are audio drivers and not output transistors but seem to be used for this.

briskly · 4th Oct 2011, 9:45 am

OK - I've just removed 2SB364 and it is giving 0 on the hfe test and 2.3 Ohms Base to collector measured with DVM in both directions. The diode BAY 44 is pretty hard to get to - in situ it is measuring 506 on the diode test, -ive terminal of DVM to base of (now missing) 2SB364 and 1466 with +ive terminal of DVM to base connection. 2SB372 is giving 2.4 Ohms C to E measured in both directions.

Can anyone advise what I should do next? Help!

Andrew

ekcobeach · 4th Oct 2011, 10:49 am

Hi Andrew, The Toshiba IC70 was one of the first transistor radios that i purchased new in the late 1960s. It is still going strong. In fact i have three,two of which i bought on the auction site. I also have another somewhere that i bought for spares.I think they are a really great performer and they were way ahead of their time. If you get really stuck and need spares/transistors i might be able to find the donor radio. Colin.

briskly · 4th Oct 2011, 11:19 am

Quote:

Originally Posted by ekcobeach

Hi Andrew, The Toshiba IC70 was one of the first transistor radios that i purchased new in the late 1960s. It is still going strong. In fact i have three,two of which i bought on the auction site. I also have another somewhere that i bought for spares.I think they are a really great performer and they were way ahead of their time. If you get really stuck and need spares/transistors i might be able to find the donor radio. Colin.

Many thanks Colin! This set means a lot to me as I was only 14 when I bought it new and it cost about 6 month's pocket money.... I've always thought it performed superbly.

Andrew

geofy · 4th Oct 2011, 1:19 pm

Quote:

Originally Posted by briskly

OK - I've just removed 2SB364 and it is giving 0 on the hfe test and 2.3 Ohms Base to collector measured with DVM in both directions. Can anyone advise what I should do next? Help!
Andrew

Looks like the 2SC372 is duff as well, unless the 10n capacitor across c-e 2SC372 has a short, this should also be checked before fitting a new transistor. The diode and 2SC735 are probably still ok. With the 2SC372 removed the diode should measure around 600mV in one direction and high in the other on the diode range, if not then this will have to be replaced. If it is ok then fit new transistors in these positions, recheck the 2SC735 it should give a similar 600mV b-e and b-c forward and high reverse and a high reading c-e and e-c.

paulsherwin · 4th Oct 2011, 2:05 pm

Be careful that you are checking the correct pinout. Silicon transistors in particular have a variety of possible pinouts. This also applies to any replacements you select.

If you fit the transistor in the test socket you should see a significant hfe reading when the wires are inserted correctly, though the reading is more accurate with Si transistors. It's worth experimenting with all the possible connections until you are certain everything is correct. Check a known good transistor with known pinouts before doing anything else so that you know what to expect.

briskly · 4th Oct 2011, 3:18 pm

Quote:

Originally Posted by geofy

Quote:

Originally Posted by briskly

OK - I've just removed 2SB364 and it is giving 0 on the hfe test and 2.3 Ohms Base to collector measured with DVM in both directions. Can anyone advise what I should do next? Help!
Andrew

Looks like the 2SC372 is duff as well, unless the 10n capacitor across c-e 2SC372 has a short, this should also be checked before fitting a new transistor. The diode and 2SC735 are probably still ok. With the 2SC372 removed the diode should measure around 600mV in one direction and high in the other on the diode range, if not then this will have to be replaced. If it is ok then fit new transistors in these positions, recheck the 2SC735 it should give a similar 600mV b-e and b-c forward and high reverse and a high reading c-e and e-c.

Thanks Geoff,

I've just taken 2SC372 out -the devil of a job and I'm afraid a track might have been damaged. I've checked the diode and it is now 526 mV in one direction, high in the other. I don't think the 10n capacitor is shorting. About to fit the new trannies....

geofy · 4th Oct 2011, 3:40 pm

Make sure you have taken Pauls advice about the different pin outs. This particularly applies to plastic cased transistors. The track will lift if it gets too hot, only keep an iron on for a few seconds and let the track cool between attempts, or just cut the leads and remove the bits, you should have a solder sucker for this. The diode should be ok.

briskly · 4th Oct 2011, 5:00 pm

I'm getting into a bit of a mess since the track started lifting - I can't seem to see what is happening very well as the light is fading - I'm hoping to begin again tomorrow. Thanks again for the help.

Colin - I don't suppose you'd consider selling one of your sets?

Cheers,

Andrew

briskly · 5th Oct 2011, 1:27 pm

After getting no sound - just a click at switch on, voltage at "3V" point of 5.75V relative to Earth and current drain measured at battery of 10mA I've taken the two new trannies - 2SC372 (now BC109)and 2SB364 (now AC128) out.

The track is damaged in a few places and I need to try to fix it. Also, the 39K resistor between the base of 2SC372 and earth has broken -it is coded green, blue orange - 56K rather than 39K. Is this important?

Colin - if you haven't a spare set for sale (don't mind a battered one!), is there any chance you could send me a pic of the track around this section sometime as it is a little tricky to fathom out!

Cheers,

Andrew

briskly · 6th Oct 2011, 4:53 pm

I wonder if anyone can help? I've repaired the tracking and soldered in two new trannies - BC 183 in pace of 2SC732 and an AC128 in place of 2SB364, together with a new 56K resistor in the base of 2SC372 to earth (not 39k as marked on the schematic). I still just get a click on switch-on, with no signal. The voltage at the 3V common emitter point is 4.7V. The voltage at the base of 2SC735 is 5.4V instead of 3.5V. Any ideas? Could it be that 2SC735 has blown and/or something else? The diode. BAY44, is reading 490 mV forward and infinity backwards.

Thanks,

Andrew

geofy · 6th Oct 2011, 6:18 pm

Have you fitted the BC183 correctly, looking from the top with the flat part facing left the pinouts are top C, middle B, bottom E. It is possible the BC108 is faulty, as it is not conducting, removing this and checking for diode function should show this, though usually this can be tested in circuit, sometimes a transistor (and I prefer to call them transistors not trannies!) can still not work correctly even if it measures ok on the diode test. The 56K resistor might be a bit to high so the 39K value could be tried as the BC183 has a low gain compared to a BC109.

Other possibilities is you have damaged a transistor soldering it in, the germaniums are more susceptible to heat damage and must be soldered quickly , a heat shunt clipped to the leads can help but makes soldering a bit more difficult.

briskly · 6th Oct 2011, 6:35 pm

Thanks Geof,

I'm sure the BC183 connections are correct, but a short seems to be showing between the collector and emitter. I'll try replacing it tomorrow when the light is good. I'm a bit concerned as I bought a couple of BC183s and BC337s and none of them give a sensible hfe reading on my multimeter - it just goes offscale.

Andrew

geofy · 6th Oct 2011, 8:13 pm

Forget about hfe for the moment, check the transistor just like the diode, with leads connected to base and emitter and then the collector, a 600mV reading for both b-e and b-c forward and high reverse show a good transistor. A high reading between emitter and collector with nothing connected to base should also be indicated.

A few pictures of what you are doing might help.

briskly · 7th Oct 2011, 1:18 pm

Thanks Geof,

Here are some photos. The 10n capacitor is partly disconnected. I've measure b-e forward at 600 mV, reverse high. B-c forward and reverse is high. E-c forward is high, reverse is 144, with base in circuit. These are all for the BC183 replacement for 2SC372.

Voltage at the 3V common emitter point is 4.85. I've also measured a resistance of only 200 Ohms between BC at 2SC735 with B+ive.

Andrew

3rd Oct 2011, 6:04 pm	#21
geofy Retired Dormant Member Join Date: Jun 2006 Posts: 1,798	Re: Toshiba IC70 I would conclude from this that the germanium 2SB364 has failed. Try lifting the base or collector lead and re-measure. And recheck the diode. The multimeter should have a diode check range which usually gives around 600 millivolt forward and infinite or high reverse for a good silicon junction, and 300 millivolt forward and high reverse for a good germanium junction.

3rd Oct 2011, 7:22 pm	#22
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Toshiba IC70 Hi Andrew Ref your questions in post 18. I would follow Paul Sherwins advice. Use a cheap DMM that has a transistor test function. For the 2SC735 pick the transistor with the lower hfe of the two. For the 2SC372 pick the transistor with the higher hfe of the two. Ref geofy's advice about checking each device before condemming it, I have a different view. Although it is perfectly good advice, in my experience once you have established that at least one device in a DC coupled solid state circuit has failed short circuit, then its a good idea to replace all of them even if they read OK. If you have suitable parts to hand then I would replace all 3 transistors and the diode. If not it may be worth replacing only those that measure obviously faulty and then giving it a try. Regards Colin

3rd Oct 2011, 8:13 pm	#23
geofy Retired Dormant Member Join Date: Jun 2006 Posts: 1,798	Re: Toshiba IC70 While I agree in some cases one transistor failing can take out others, in this case it looks like the germanium has failed protecting the others in the process by taking all the current, the other silicon transistors are still measuring ok, or will be once the shorted germanium is removed, so why blanket change transistors which are not faulty. The silicon types are far stronger, only a much too large base current could damage them or to higher voltage which hasn't happened here, a BC109 has around 30V CE If the AC128 is checked out of circuit it will probably be found to be shorted. Don't know what type a BAY44 diode is but the output stage could be biased to take a PNP silicon transistor. This diode should be checked out of circuit.

3rd Oct 2011, 8:29 pm	#24
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: Toshiba IC70 In general, Si transistors are much tougher than Ge as Geof says, so it's very likely that the Ge transistor has failed protecting the others. You can replace the 2SB364 with a range of Ge transistors if you don't have an AC128 handy, including earlier output transistors like the OC81. Do this before doing anything else. I wouldn't change any other transistors if they seem to be OK, as you will be subbing different transistor types and this is always to be avoided if possible. The more you change the more likely you are to hit compatibility problems. Your responses imply that you aren't testing transistors using a transistor tester. This is much quicker and less error prone than trying to measure the resistance between junctions, and even £3 DMMs from market stalls have a suitable basic transistor test function.

3rd Oct 2011, 8:55 pm	#25
briskly Retired Dormant Member Join Date: Oct 2010 Location: Chelmsford, Essex, UK. Posts: 98	Re: Toshiba IC70 Thanks again for your help everyone. I'm hoping to start working on it as soon as possible in good light tomorrow - trying the AC128 replacement first. I've just found a multimeter with a transistor checker as well! Cheers, Andrew

3rd Oct 2011, 9:30 pm	#26
geofy Retired Dormant Member Join Date: Jun 2006 Posts: 1,798	Re: Toshiba IC70 It might be possible to fit two diodes such as 1N914 in place of the BAY44, one taking place of the 39 Ohm resistor, both facing the same way as the existing diode, cathodes towards BC109 collector. And then replace the AC128 with a silicon such as a BC158 or BC558 PNP transistor. The two diodes give the correct bias for the silicon transistor. Although both the BC108 (BC148) and BC158 are audio drivers and not output transistors but seem to be used for this. Last edited by geofy; 3rd Oct 2011 at 9:42 pm.

4th Oct 2011, 9:45 am	#27
briskly Retired Dormant Member Join Date: Oct 2010 Location: Chelmsford, Essex, UK. Posts: 98	Re: Toshiba IC70 OK - I've just removed 2SB364 and it is giving 0 on the hfe test and 2.3 Ohms Base to collector measured with DVM in both directions. The diode BAY 44 is pretty hard to get to - in situ it is measuring 506 on the diode test, -ive terminal of DVM to base of (now missing) 2SB364 and 1466 with +ive terminal of DVM to base connection. 2SB372 is giving 2.4 Ohms C to E measured in both directions. Can anyone advise what I should do next? Help! Andrew

4th Oct 2011, 10:49 am	#28
ekcobeach Rest in Peace Join Date: Mar 2005 Location: Wilmington, East Sussex, UK. Posts: 230	Re: Toshiba IC70 Hi Andrew, The Toshiba IC70 was one of the first transistor radios that i purchased new in the late 1960s. It is still going strong. In fact i have three,two of which i bought on the auction site. I also have another somewhere that i bought for spares.I think they are a really great performer and they were way ahead of their time. If you get really stuck and need spares/transistors i might be able to find the donor radio. Colin.

4th Oct 2011, 2:05 pm	#31
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: Toshiba IC70 Be careful that you are checking the correct pinout. Silicon transistors in particular have a variety of possible pinouts. This also applies to any replacements you select. If you fit the transistor in the test socket you should see a significant hfe reading when the wires are inserted correctly, though the reading is more accurate with Si transistors. It's worth experimenting with all the possible connections until you are certain everything is correct. Check a known good transistor with known pinouts before doing anything else so that you know what to expect.

4th Oct 2011, 3:40 pm	#33
geofy Retired Dormant Member Join Date: Jun 2006 Posts: 1,798	Re: Toshiba IC70 Make sure you have taken Pauls advice about the different pin outs. This particularly applies to plastic cased transistors. The track will lift if it gets too hot, only keep an iron on for a few seconds and let the track cool between attempts, or just cut the leads and remove the bits, you should have a solder sucker for this. The diode should be ok.

4th Oct 2011, 5:00 pm	#34
briskly Retired Dormant Member Join Date: Oct 2010 Location: Chelmsford, Essex, UK. Posts: 98	Re: Toshiba IC70 I'm getting into a bit of a mess since the track started lifting - I can't seem to see what is happening very well as the light is fading - I'm hoping to begin again tomorrow. Thanks again for the help. Colin - I don't suppose you'd consider selling one of your sets? Cheers, Andrew

5th Oct 2011, 1:27 pm	#35
briskly Retired Dormant Member Join Date: Oct 2010 Location: Chelmsford, Essex, UK. Posts: 98	Re: Toshiba IC70 After getting no sound - just a click at switch on, voltage at "3V" point of 5.75V relative to Earth and current drain measured at battery of 10mA I've taken the two new trannies - 2SC372 (now BC109)and 2SB364 (now AC128) out. The track is damaged in a few places and I need to try to fix it. Also, the 39K resistor between the base of 2SC372 and earth has broken -it is coded green, blue orange - 56K rather than 39K. Is this important? Colin - if you haven't a spare set for sale (don't mind a battered one!), is there any chance you could send me a pic of the track around this section sometime as it is a little tricky to fathom out! Cheers, Andrew

6th Oct 2011, 4:53 pm	#36
briskly Retired Dormant Member Join Date: Oct 2010 Location: Chelmsford, Essex, UK. Posts: 98	Re: Toshiba IC70 I wonder if anyone can help? I've repaired the tracking and soldered in two new trannies - BC 183 in pace of 2SC732 and an AC128 in place of 2SB364, together with a new 56K resistor in the base of 2SC372 to earth (not 39k as marked on the schematic). I still just get a click on switch-on, with no signal. The voltage at the 3V common emitter point is 4.7V. The voltage at the base of 2SC735 is 5.4V instead of 3.5V. Any ideas? Could it be that 2SC735 has blown and/or something else? The diode. BAY44, is reading 490 mV forward and infinity backwards. Thanks, Andrew

6th Oct 2011, 6:18 pm	#37
geofy Retired Dormant Member Join Date: Jun 2006 Posts: 1,798	Re: Toshiba IC70 Have you fitted the BC183 correctly, looking from the top with the flat part facing left the pinouts are top C, middle B, bottom E. It is possible the BC108 is faulty, as it is not conducting, removing this and checking for diode function should show this, though usually this can be tested in circuit, sometimes a transistor (and I prefer to call them transistors not trannies!) can still not work correctly even if it measures ok on the diode test. The 56K resistor might be a bit to high so the 39K value could be tried as the BC183 has a low gain compared to a BC109. Other possibilities is you have damaged a transistor soldering it in, the germaniums are more susceptible to heat damage and must be soldered quickly , a heat shunt clipped to the leads can help but makes soldering a bit more difficult.

6th Oct 2011, 6:35 pm	#38
briskly Retired Dormant Member Join Date: Oct 2010 Location: Chelmsford, Essex, UK. Posts: 98	Re: Toshiba IC70 Thanks Geof, I'm sure the BC183 connections are correct, but a short seems to be showing between the collector and emitter. I'll try replacing it tomorrow when the light is good. I'm a bit concerned as I bought a couple of BC183s and BC337s and none of them give a sensible hfe reading on my multimeter - it just goes offscale. Andrew

6th Oct 2011, 8:13 pm	#39
geofy Retired Dormant Member Join Date: Jun 2006 Posts: 1,798	Re: Toshiba IC70 Forget about hfe for the moment, check the transistor just like the diode, with leads connected to base and emitter and then the collector, a 600mV reading for both b-e and b-c forward and high reverse show a good transistor. A high reading between emitter and collector with nothing connected to base should also be indicated. A few pictures of what you are doing might help.

7th Oct 2011, 1:18 pm	#40
briskly Retired Dormant Member Join Date: Oct 2010 Location: Chelmsford, Essex, UK. Posts: 98	Re: Toshiba IC70 Thanks Geof, Here are some photos. The 10n capacitor is partly disconnected. I've measure b-e forward at 600 mV, reverse high. B-c forward and reverse is high. E-c forward is high, reverse is 144, with base in circuit. These are all for the BC183 replacement for 2SC372. Voltage at the 3V common emitter point is 4.85. I've also measured a resistance of only 200 Ohms between BC at 2SC735 with B+ive. Andrew Attached Thumbnails