Transistor Specification - UK Vintage Radio Repair and Restoration Discussion Forum

Jamesdb · 27th Aug 2014, 7:50 pm

Hi, another newbie question here. While repairing my Philips RL570 1968 radio I noted down that the push pull AF driver transistors AC128 measured on my DMM an hFE of 13 and 24. On a data sheet I see that they should have an hFE of 70 to 80. Is that acceptable or am I getting a much weaker output than I should be getting? I have to say the radio is not as loud as my Hacker Hunter. Thanks. James

Sideband · 27th Aug 2014, 8:20 pm

I very much doubt your DMM is suitable for GERMANIUM transistors which is what the AC128 is. As far as I am aware, DMM's will only give an indication of the gain of silicon transistors. Germanium transistors have not really been used since the 1970's, long before the concept of the modern DMM so it was never in the design critera of DMM's to measure 'old fashioned' germanuim transistors.

In case you are wondering why the DMM will not measure germaniums properly, it's because the bias requirements for germanium and silicon are very different.

Jamesdb · 27th Aug 2014, 9:01 pm

Thanks Sideband, I was wondering how they could be so far out but still working.

jimmc101 · 27th Aug 2014, 11:06 pm

Typical DMMs with an hFE test range (such as the DT830B) measure the collector current a base current of 10uA (and Vce ~ 3v).
This does not show the AC127 and AC128 at their best, their current gain falls off at low currents (Ic < 1mA). (The AC128s current gain peaks at around 70mA.)

As Sideband has said these meters are not calibrated for germanium transistors, a simple series resistor is used to define the base current so if anything they should read high! (Lower Vbe for germanium gives more base current.)
Jim

Refugee · 28th Aug 2014, 9:26 am

I have a home made transistor tester from the 1970s.
I built it to test all types of transistor that I was likely to encounter at the time.

Modern darlingtons do need the internal resistors allowed for but can be tested at up to 100ma from the internal power supply with the limits of an external bench power supply being the only thing to stop me going above a couple of amps.
It is still in regular use.

paulsherwin · 28th Aug 2014, 1:12 pm

As has been said, simple hfe testers in basic DMMs don't test Ge transistors well, but they're adequate as a go/no go test. It's unlikely that the gain of the transistors will have fallen over time. If they fail it's normally because they go short or open circuit.

28th Aug 2014, 1:46 pm

Quote:

Originally Posted by Jamesdb

I noted down that the push pull AF driver transistors AC128 measured on my DMM an hFE of 13 and 24

Pretty close for a 'pair' 50 odd years old, they will work OK.

27th Aug 2014, 7:50 pm	#1
Jamesdb Retired Dormant Member Join Date: Jan 2014 Location: Staplecross, East Sussex, UK. Posts: 27	Transistor Specification Hi, another newbie question here. While repairing my Philips RL570 1968 radio I noted down that the push pull AF driver transistors AC128 measured on my DMM an hFE of 13 and 24. On a data sheet I see that they should have an hFE of 70 to 80. Is that acceptable or am I getting a much weaker output than I should be getting? I have to say the radio is not as loud as my Hacker Hunter. Thanks. James

27th Aug 2014, 8:20 pm	#2
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,580	Re: Transistor Specification I very much doubt your DMM is suitable for GERMANIUM transistors which is what the AC128 is. As far as I am aware, DMM's will only give an indication of the gain of silicon transistors. Germanium transistors have not really been used since the 1970's, long before the concept of the modern DMM so it was never in the design critera of DMM's to measure 'old fashioned' germanuim transistors. In case you are wondering why the DMM will not measure germaniums properly, it's because the bias requirements for germanium and silicon are very different. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

27th Aug 2014, 9:01 pm	#3
Jamesdb Retired Dormant Member Join Date: Jan 2014 Location: Staplecross, East Sussex, UK. Posts: 27	Re: Transistor Specification Thanks Sideband, I was wondering how they could be so far out but still working.

27th Aug 2014, 11:06 pm	#4
jimmc101 Heptode Join Date: Mar 2009 Location: Portsmouth, Hampshire, UK. Posts: 674	Re: Transistor Specification Typical DMMs with an hFE test range (such as the DT830B) measure the collector current a base current of 10uA (and Vce ~ 3v). This does not show the AC127 and AC128 at their best, their current gain falls off at low currents (Ic < 1mA). (The AC128s current gain peaks at around 70mA.) As Sideband has said these meters are not calibrated for germanium transistors, a simple series resistor is used to define the base current so if anything they should read high! (Lower Vbe for germanium gives more base current.) Jim Last edited by jimmc101; 27th Aug 2014 at 11:12 pm. Reason: Last paragraph added

28th Aug 2014, 9:26 am	#5
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: Transistor Specification I have a home made transistor tester from the 1970s. I built it to test all types of transistor that I was likely to encounter at the time. Modern darlingtons do need the internal resistors allowed for but can be tested at up to 100ma from the internal power supply with the limits of an external bench power supply being the only thing to stop me going above a couple of amps. It is still in regular use.

28th Aug 2014, 1:12 pm	#6
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,974	Re: Transistor Specification As has been said, simple hfe testers in basic DMMs don't test Ge transistors well, but they're adequate as a go/no go test. It's unlikely that the gain of the transistors will have fallen over time. If they fail it's normally because they go short or open circuit.