Drawing Conclusions From Transistor Voltages - UK Vintage Radio Repair and Restoration Discussion Forum

OldTechFan96 · 25th Jun 2018, 5:46 pm

At the moment I am working on a Hacker Herald RP10. When switched on the set is silent. It used to pop when the AF117 transistors were tapped. Injecting an audio signal into the middle tag of the volume pot yields no sound from the speaker.

An amplifier fault is likely present. The RF/IF section could have hidden faults due to AF117 transistors.

To begin I measured the voltages on each transistor with my Avo 8.

RF/IF Section

T1: E - -0.35V, B - -0.4V, C - -14.5V

T2: E - -1.7V, B - -1.65V, C - -9V

T3: E - -2.1V, B - -2.2V, C - -14.V

Amplifier

TR4: E - -0.6V, B - -0.6V, C - -8V

TR5: E - -0.8V, B - -0.2V, C - -18V

TR6: E - -1.1V, B - -1.2V, C - -18V

TR7: E - -0.9V, B - -0.9V, C - -0.9V

Most, if not all, voltages measured are way off. The problem is that I don't know what conclusions to make from these measurements as to what transistors are faulty.

The high collector voltages on certain transistors certainly point to a fault. I'm not 100% clued up on transistors at the moment but I have found some good resources to comb though.

Can anybody please explain what is going on?

paulsherwin · 25th Jun 2018, 6:16 pm

You're right, they all look suspect. Could the wrong voltage polarity have been applied?

I'm no Hacker expert though.

ronbryan · 25th Jun 2018, 6:38 pm

The voltage measurements should be taken with respect to chassis, rather than from the positive terminal of the battery, the one whose negative terminal goes to chassis, if you want to compare the voltages you measure with the ones in the Hacker service sheet.

The output stage uses a 9-0-9V supply, with the loudspeaker directly coupled from T6 emitter and T7 collector to the chassis connected centre point between the two batteries. The driver stage T5 uses the full 18V and is biassed to have about 9V across the driver transformer primary. The rest of the set just uses one -9V battery (the one with +ve to chassis).

Ron

paulsherwin · 25th Jun 2018, 7:10 pm

Ah, the voltages look a lot more plausible in that case.

OldTechFan96 · 25th Jun 2018, 7:37 pm

Paul and Ron,

Your posts made me rethink how I am powering this set. I had my bench power supply connected to one set of battery terminals and set to 18V. I did not know the the battery arrangement was a bit different than most with the Herald. I hope my possible over voltage has not damaged anything.

I'll try the radio with two PP9s (they are a little bit flat at 8.3V) and retake the voltage measurements.

paulsherwin · 25th Jun 2018, 8:02 pm

You definitely need a 9-0-9 supply. You could use a couple of Poundland PP3s for test purposes. That certainly explains why it's not producing any sound

Alternatively you could use an 18V supply and fit a 1000uF electrolytic between the speaker output and the speaker, and connect the other end to the +ve rail.

Quite a few early transistor sets had this dual battery arrangement with the speaker going to the mid point.

ronbryan · 25th Jun 2018, 9:05 pm

Paul

The RF, IF and first audio stages require just 9V in this case, so an electrolytic providing a centre tap for the speaker output would not provide the correct voltage for the aforementioned stages.

Ron

paulsherwin · 25th Jun 2018, 9:14 pm

OK Ron, I stand corrected.

OldTechFan96 · 25th Jun 2018, 9:54 pm

With two PP9 (8.5V off load) batteries connected the radio all I could hear from the radio is a very faint hiss.

New voltage measurements:

RF/IF Section

T1: E - -0.15V, B - -0.16V, C - -6.1V

T2: E - -0.65V, B - -0.8V, C - -4V

T3: E - -0.8V, B - -0.95V, C - -0.63.V

Amplifier

TR4: E - -0.2V, B - -0.15V, C - -3.5V

TR5: E - +0.8V, B - +0.5V, C - -7.2V

TR6: E 0V, B - -0.01V, C - -7.3V

TR7: E - +8.2V, B - +7.7V, C - 0V

These look a lot better. The suspect transistors are now TR1 and TR4. The collector voltages are close while the base and emitter voltages are off.

paulsherwin · 25th Jun 2018, 10:04 pm

Those audio stages look plausible. Try injecting a signal at the volume control - it's likely the amp is now working.

You might as well whip out the AF117s and either zap the tin whiskers or replace them with something else. Any Ge types with an Ft higher than 3MHz are likely to work in an AM only set like this - in fact Si types like 2N3906s or BC558s are likely to work.

ronbryan · 25th Jun 2018, 10:32 pm

Transistor T3, the second IF amplifier, seems to have the collector shorted to everything else. Try the usual remedies with it (pick one:- cut screen lead, zap it, heat it up near the bottom of the can whilst making sure it does not pop off, replace it) and see if that helps.

Ron

OldTechFan96 · 25th Jun 2018, 10:58 pm

Just had a look at my notes. The collector voltage for TR3 should be -6.3V.

Apologies for any confusion.

paulsherwin · 25th Jun 2018, 11:40 pm

You seem to be overthinking this.

1. Confirm the amp is now working
2. Cut or lift the screen lead of the AF117s

Assuming the radio now works after a fashion, you can plan the proper permanent replacement of the AF117s. As I said earlier, there are lots of options depending on your budget and attitude to authenticity.

OldTechFan96 · 10th Jul 2018, 9:18 pm

I replaced TR1 with a BC557 and the set burst into life. The voltages around TR1 are now what they should be. The AF117 B-E junction reads 0.2V both ways. It should only read 0.2V one way.

I might replace the other IF transistors at a later point.

Now I'll start with the radio cabinet. Are the brass fixtures of the Herald lacquered? How can I go about removing it?

I'll see to what extent red shoe polish/cream can restore a faded back and front.

MODS: Could you please change the thread title to 'Hacker Herald RP10 Repair' as the current title does not seem suitable.

25th Jun 2018, 5:46 pm	#1
OldTechFan96 Octode Join Date: Jul 2017 Location: Newcastle Upon Tyne, Tyne & Wear, UK. Posts: 1,186	Drawing Conclusions From Transistor Voltages At the moment I am working on a Hacker Herald RP10. When switched on the set is silent. It used to pop when the AF117 transistors were tapped. Injecting an audio signal into the middle tag of the volume pot yields no sound from the speaker. An amplifier fault is likely present. The RF/IF section could have hidden faults due to AF117 transistors. To begin I measured the voltages on each transistor with my Avo 8. RF/IF Section T1: E - -0.35V, B - -0.4V, C - -14.5V T2: E - -1.7V, B - -1.65V, C - -9V T3: E - -2.1V, B - -2.2V, C - -14.V Amplifier TR4: E - -0.6V, B - -0.6V, C - -8V TR5: E - -0.8V, B - -0.2V, C - -18V TR6: E - -1.1V, B - -1.2V, C - -18V TR7: E - -0.9V, B - -0.9V, C - -0.9V Most, if not all, voltages measured are way off. The problem is that I don't know what conclusions to make from these measurements as to what transistors are faulty. The high collector voltages on certain transistors certainly point to a fault. I'm not 100% clued up on transistors at the moment but I have found some good resources to comb though. Can anybody please explain what is going on? Attached Thumbnails

25th Jun 2018, 6:16 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,960	Re: Drawing Conclusions From Transistor Voltages You're right, they all look suspect. Could the wrong voltage polarity have been applied? I'm no Hacker expert though.

25th Jun 2018, 6:38 pm	#3
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Drawing Conclusions From Transistor Voltages The voltage measurements should be taken with respect to chassis, rather than from the positive terminal of the battery, the one whose negative terminal goes to chassis, if you want to compare the voltages you measure with the ones in the Hacker service sheet. The output stage uses a 9-0-9V supply, with the loudspeaker directly coupled from T6 emitter and T7 collector to the chassis connected centre point between the two batteries. The driver stage T5 uses the full 18V and is biassed to have about 9V across the driver transformer primary. The rest of the set just uses one -9V battery (the one with +ve to chassis). Ron

25th Jun 2018, 7:10 pm	#4
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,960	Re: Drawing Conclusions From Transistor Voltages Ah, the voltages look a lot more plausible in that case.

25th Jun 2018, 7:37 pm	#5
OldTechFan96 Octode Join Date: Jul 2017 Location: Newcastle Upon Tyne, Tyne & Wear, UK. Posts: 1,186	Re: Drawing Conclusions From Transistor Voltages Paul and Ron, Your posts made me rethink how I am powering this set. I had my bench power supply connected to one set of battery terminals and set to 18V. I did not know the the battery arrangement was a bit different than most with the Herald. I hope my possible over voltage has not damaged anything. I'll try the radio with two PP9s (they are a little bit flat at 8.3V) and retake the voltage measurements.

25th Jun 2018, 8:02 pm	#6
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,960	Re: Drawing Conclusions From Transistor Voltages You definitely need a 9-0-9 supply. You could use a couple of Poundland PP3s for test purposes. That certainly explains why it's not producing any sound Alternatively you could use an 18V supply and fit a 1000uF electrolytic between the speaker output and the speaker, and connect the other end to the +ve rail. Quite a few early transistor sets had this dual battery arrangement with the speaker going to the mid point.

25th Jun 2018, 9:05 pm	#7
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Drawing Conclusions From Transistor Voltages Paul The RF, IF and first audio stages require just 9V in this case, so an electrolytic providing a centre tap for the speaker output would not provide the correct voltage for the aforementioned stages. Ron

25th Jun 2018, 9:14 pm	#8
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,960	Re: Drawing Conclusions From Transistor Voltages OK Ron, I stand corrected.

25th Jun 2018, 9:54 pm	#9
OldTechFan96 Octode Join Date: Jul 2017 Location: Newcastle Upon Tyne, Tyne & Wear, UK. Posts: 1,186	Re: Drawing Conclusions From Transistor Voltages With two PP9 (8.5V off load) batteries connected the radio all I could hear from the radio is a very faint hiss. New voltage measurements: RF/IF Section T1: E - -0.15V, B - -0.16V, C - -6.1V T2: E - -0.65V, B - -0.8V, C - -4V T3: E - -0.8V, B - -0.95V, C - -0.63.V Amplifier TR4: E - -0.2V, B - -0.15V, C - -3.5V TR5: E - +0.8V, B - +0.5V, C - -7.2V TR6: E 0V, B - -0.01V, C - -7.3V TR7: E - +8.2V, B - +7.7V, C - 0V These look a lot better. The suspect transistors are now TR1 and TR4. The collector voltages are close while the base and emitter voltages are off.

25th Jun 2018, 10:04 pm	#10
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,960	Re: Drawing Conclusions From Transistor Voltages Those audio stages look plausible. Try injecting a signal at the volume control - it's likely the amp is now working. You might as well whip out the AF117s and either zap the tin whiskers or replace them with something else. Any Ge types with an Ft higher than 3MHz are likely to work in an AM only set like this - in fact Si types like 2N3906s or BC558s are likely to work.

25th Jun 2018, 10:32 pm	#11
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Drawing Conclusions From Transistor Voltages Transistor T3, the second IF amplifier, seems to have the collector shorted to everything else. Try the usual remedies with it (pick one:- cut screen lead, zap it, heat it up near the bottom of the can whilst making sure it does not pop off, replace it) and see if that helps. Ron

25th Jun 2018, 10:58 pm	#12
OldTechFan96 Octode Join Date: Jul 2017 Location: Newcastle Upon Tyne, Tyne & Wear, UK. Posts: 1,186	Re: Drawing Conclusions From Transistor Voltages Just had a look at my notes. The collector voltage for TR3 should be -6.3V. Apologies for any confusion.

25th Jun 2018, 11:40 pm	#13
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,960	Re: Drawing Conclusions From Transistor Voltages You seem to be overthinking this. 1. Confirm the amp is now working 2. Cut or lift the screen lead of the AF117s Assuming the radio now works after a fashion, you can plan the proper permanent replacement of the AF117s. As I said earlier, there are lots of options depending on your budget and attitude to authenticity.

10th Jul 2018, 9:18 pm	#14
OldTechFan96 Octode Join Date: Jul 2017 Location: Newcastle Upon Tyne, Tyne & Wear, UK. Posts: 1,186	Re: Drawing Conclusions From Transistor Voltages I replaced TR1 with a BC557 and the set burst into life. The voltages around TR1 are now what they should be. The AF117 B-E junction reads 0.2V both ways. It should only read 0.2V one way. I might replace the other IF transistors at a later point. Now I'll start with the radio cabinet. Are the brass fixtures of the Herald lacquered? How can I go about removing it? I'll see to what extent red shoe polish/cream can restore a faded back and front. MODS: Could you please change the thread title to 'Hacker Herald RP10 Repair' as the current title does not seem suitable.