Transistor voltage measurements in absence of circuit diagram - UK Vintage Radio Repair and Restoration Discussion Forum

Jolly 7 · 2nd Mar 2018, 9:50 pm

In the absence of a circuit diagram, is it possible to gather any useful information from voltage measurements as to whether a particular transistor is not working ? e.g.here are the RF transistor voltage measurements using a 5.8V supply for a radio I've been trying to fix for ages: Transistor 1: vBE 0.04V, vCE 2.44V; Transistor 2: vBE 0.1V, vCE 4.72V; Transistor 3: vBE 0.24V, vCE 4.45V; Transistor 4: vBE 0.25V, vCE 4.04V.

Nuvistor · 2nd Mar 2018, 10:17 pm

What make and model radio, someone may have a similar circuit, are they germanium or silicon transistors and what function does each transistor perform, I presume they are all not RF transisors?

Jolly 7 · 2nd Mar 2018, 10:33 pm

It's a Loewe Opta Dolly 8 export for which the exact diagram is unavailable even from the Loewe Opta museum website. There are somewhat similar radio circuits but they use different transistors and have different wavebands. Yes they are all RF transistors in order from one to four before the AF circuit starts, which is functioning normally.

Nuvistor · 2nd Mar 2018, 10:57 pm

This is guess work, the set appears to be made circa 1960 so the transistors will be germanium.
Tr3 and 4 are probably IF transistors, that is if you have numbered them Tr1 aerial circuits and Tr4 last one before the audio. Tr3 and 4 voltages Look ok.
Tr1 and 2 are a bit more difficult, they could be osc and mixer or RF amp any mixer/osc, it’s conceivable that Tr1 is mixer osc and Tr2 is another IF amp, I find it more difficult to advise on those voltages without knowing their role.

I would ignore the voltages for now and inject an IF signal into Tr3 and 4 and confirm the IF stages are working. It could be as simple as the dectector diode being faulty.

TrevorG3VLF · 3rd Mar 2018, 5:58 pm

Either Q1 or Q2 will be the oscillator and maybe have strange voltages. The other should have Vbe 0.24V for a germanium transistor.
I assume that the collector is negative.
What are the transistor types? Have you got some AF117s or similar?

You can test the oscillator by listening for it on another radio.

G6Tanuki · 3rd Mar 2018, 6:25 pm

Be aware that probing an oscillator-transistor (or self-oscillating mixer) may introduce sufficient extra capacitive loading because of the test-leads that the transistor may stop oscillating - at which point you're no longer measuring anything sensible.

[The solution to this is to use a very-high-resistance meter, and use a high-value resistor as the 'probe' - a typical modern DVM will have an input-resistance of a Megohm or more, so you can use a resistor of, say, 100K as your probe with little likelihood of stalling the oscillator]

GeoffK · 3rd Mar 2018, 7:13 pm

Have you tested the detector diode?

Jolly 7 · 3rd Mar 2018, 9:21 pm

Quote:

Originally Posted by GeoffK

Have you tested the detector diode?

Yes the detector diode is a OA91 which is ok. A Schottky diode works fine as a replacement too. All the IF stages are working properly so I am concentrating on the oscillator section with a newly fitted transistor, an AF178. This replaces an ASZ20, originally an O170. There are myriads of very fine wires going through the oscillator coils connected to the bandchange switches and one seems to be connected to AF178, which is now TR1. TR2 is also an AF178, TR3 and TR4 are both GT322Bs, replacing the original OC169s. Maybe the oscillator transistor receives its power through an oscillator coil ?

Nuvistor · 3rd Mar 2018, 9:37 pm

Can you confirm the osc is stopped, has stated in earlier post it should audible on another radio.

Jolly 7 · 3rd Mar 2018, 9:55 pm

Quote:

Originally Posted by Nuvistor

Can you confirm the osc is stopped, has stated in earlier post it should audible on another radio.

Excuse my ignorance. The radio produces a whistle on a nearby radio on MW when I turn the shaft of the tuning capacitor. e.g. I was able to blank out a station on 1296 kHz and hear a whistle. Is this evidence enough that the oscillator is working? The reason why I thought of investigating the oscillator transistor is because of the low vBE.

I am attaching a photo of the oscillator coils connected to the bandchange switches.

Hartley118 · 3rd Mar 2018, 9:56 pm

I've found when groping in the dark without a circuit diagram, it's worth using the 'diode test' function of a DMM (with the set switched off) in order to check that every transistor has two working junctions. It requires some experimenting with the probes if the pin-outs are uncertain, but a silicon junction will typically read 0.5 to 0.7V forward drop and a germanium junction 0.2 to 0.3V.

If a transistor exhibits two working junctions, it's likely to be OK. Certainly a zero reading indicates a likely internal short unless of course there's a coil connected across the junction.

Martin

Nuvistor · 3rd Mar 2018, 10:06 pm

The osc is usually the Intermediate Frequency higher than the station the radio is tuned to. Example, faulty radio tuned to 1Mhz (300m) with an IF of 470Khz you should hear a signal at 1.47Mhz on the test radio.
If that’s the sort of result you get I would say the osc is working. If there is no result I would say the osc as stopped, if say the signal is heard not on or near the frequency you expect, it could mean the osc is off adjustment but working.

2nd Mar 2018, 9:50 pm	#1
Jolly 7 Octode Join Date: Feb 2018 Location: Southampton, Hampshire, UK. Posts: 1,063	Transistor voltage measurements in absence of circuit diagram In the absence of a circuit diagram, is it possible to gather any useful information from voltage measurements as to whether a particular transistor is not working ? e.g.here are the RF transistor voltage measurements using a 5.8V supply for a radio I've been trying to fix for ages: Transistor 1: vBE 0.04V, vCE 2.44V; Transistor 2: vBE 0.1V, vCE 4.72V; Transistor 3: vBE 0.24V, vCE 4.45V; Transistor 4: vBE 0.25V, vCE 4.04V.

2nd Mar 2018, 10:17 pm	#2
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Transistor voltage measurements in absence of circuit diagram What make and model radio, someone may have a similar circuit, are they germanium or silicon transistors and what function does each transistor perform, I presume they are all not RF transisors? __________________ Frank

2nd Mar 2018, 10:33 pm	#3
Jolly 7 Octode Join Date: Feb 2018 Location: Southampton, Hampshire, UK. Posts: 1,063	Re: Transistor voltage measurements in absence of circuit diagram It's a Loewe Opta Dolly 8 export for which the exact diagram is unavailable even from the Loewe Opta museum website. There are somewhat similar radio circuits but they use different transistors and have different wavebands. Yes they are all RF transistors in order from one to four before the AF circuit starts, which is functioning normally.

2nd Mar 2018, 10:57 pm	#4
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Transistor voltage measurements in absence of circuit diagram This is guess work, the set appears to be made circa 1960 so the transistors will be germanium. Tr3 and 4 are probably IF transistors, that is if you have numbered them Tr1 aerial circuits and Tr4 last one before the audio. Tr3 and 4 voltages Look ok. Tr1 and 2 are a bit more difficult, they could be osc and mixer or RF amp any mixer/osc, it’s conceivable that Tr1 is mixer osc and Tr2 is another IF amp, I find it more difficult to advise on those voltages without knowing their role. I would ignore the voltages for now and inject an IF signal into Tr3 and 4 and confirm the IF stages are working. It could be as simple as the dectector diode being faulty. __________________ Frank

3rd Mar 2018, 5:58 pm	#5
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Transistor voltage measurements in absence of circuit diagram Either Q1 or Q2 will be the oscillator and maybe have strange voltages. The other should have Vbe 0.24V for a germanium transistor. I assume that the collector is negative. What are the transistor types? Have you got some AF117s or similar? You can test the oscillator by listening for it on another radio.

3rd Mar 2018, 6:25 pm	#6
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Transistor voltage measurements in absence of circuit diagram Be aware that probing an oscillator-transistor (or self-oscillating mixer) may introduce sufficient extra capacitive loading because of the test-leads that the transistor may stop oscillating - at which point you're no longer measuring anything sensible. [The solution to this is to use a very-high-resistance meter, and use a high-value resistor as the 'probe' - a typical modern DVM will have an input-resistance of a Megohm or more, so you can use a resistor of, say, 100K as your probe with little likelihood of stalling the oscillator]

3rd Mar 2018, 7:13 pm	#7
GeoffK Heptode Join Date: Mar 2016 Location: Essex, UK. Posts: 602	Re: Transistor voltage measurements in absence of circuit diagram Have you tested the detector diode? __________________ *Geoff*

3rd Mar 2018, 9:37 pm	#9
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Transistor voltage measurements in absence of circuit diagram Can you confirm the osc is stopped, has stated in earlier post it should audible on another radio. __________________ Frank

3rd Mar 2018, 9:56 pm	#11
Hartley118 Nonode Join Date: Aug 2015 Location: Cambridge, Cambs. UK. Posts: 2,198	Re: Transistor voltage measurements in absence of circuit diagram I've found when groping in the dark without a circuit diagram, it's worth using the 'diode test' function of a DMM (with the set switched off) in order to check that every transistor has two working junctions. It requires some experimenting with the probes if the pin-outs are uncertain, but a silicon junction will typically read 0.5 to 0.7V forward drop and a germanium junction 0.2 to 0.3V. If a transistor exhibits two working junctions, it's likely to be OK. Certainly a zero reading indicates a likely internal short unless of course there's a coil connected across the junction. Martin __________________ BVWS Member

3rd Mar 2018, 10:06 pm	#12
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Transistor voltage measurements in absence of circuit diagram The osc is usually the Intermediate Frequency higher than the station the radio is tuned to. Example, faulty radio tuned to 1Mhz (300m) with an IF of 470Khz you should hear a signal at 1.47Mhz on the test radio. If that’s the sort of result you get I would say the osc is working. If there is no result I would say the osc as stopped, if say the signal is heard not on or near the frequency you expect, it could mean the osc is off adjustment but working. __________________ Frank