Grundig SV41 Signalverfolger (signal tracer)

When entering the are of old tube receiver repairment, I found that a signal tracer device is crucial. With this device, you can trace a signal in the stages of the radio device and identify the stage that is not working anymore, so it is a great tool to troubleshoot radio receivers.

Grundig SV41 signal tracer.

Signal tracers are not produced anymore, I think they passed when receivers and amplifiers were designed with ICs, where you have multiple or all stages of an electronic circuit in a single chip. Then a signal tracer is if no help anymore. So, usually a used a signal tracer usually must be found.

ebay is your friend here and after some days checking, I got the Grundig SV41. Grundig is a famous name in germany, but today chinese-owned as far as I remember. The signal tracer is from the 70ies I guess.

It can be operated from battery or external power supply at 9V=. My device showed dead and leaked batteries in the battery pack section. The external power supply connector was broken, maybe during delivery. I replaced the external power supply and cleaned the battery pack.

Battery section at the top (black box) and a single PCB with old style components like AC187 (germanium transistor)

The device offers a 1 Khz signal generator with attenuation -10 .. -60 dB, an amplifier, a speaker and a panel meter. It has connectors for generator output and signal input.

„Power“ amplifier with two germanium transistors, Speaker at right
PCB seen from the bottom

Hickok 539A tube tester

Having 3 tube radios from 1950 .. 1960ies, I though it’s time to also have a tube tester.

You can have the simple ones, but also a lab level device. All these tube testers come from 1950 .. 1960 years. You can have one for european tubes only or more general ones with american and european tubes. So I decided to get an american version. All modern amplifier tubes are american brand, so this is a good approach.

Tube testers are expensive items. Finally I found a Hickok device -The #1 vendor of tube testers back in the days.

My Hickok 539A is a complex and beautiful device.

Hickok 539A overall view

While all/most german tube testers use pile of paper cards to set up the tester for a special tube type, american tube testers usually use rotary switches to individually set up the device a tube to test. In the picture above the selector row contains 7 switches for setting up the device for a particular tube.

Filament voltage can be set up in a wide range between 0.6V and 117V. An expected Micromhos range (in german: Steigung Ia/V) can be set between 3.000 and 30.000.

Filament switch, Micromhos switch, shunt regulator

Gate BIAS can be set with a voltage regulator in range 0..10V or 0..50V. Shorts inside the tube also can be identified (Shorts switch).

Bias regulator

Before using the device for any measurement, the device can be somehow „calibrated“ to AC input (110V US).

Calibration to AC supply voltage

After calibration, the tube can be inserted in one of 11 tube connectors.

11 standard tube connectors

Inside of the device, the following pictires were taken.

Inside the device. 2 transformers at the top left, tube data „display“ at the bottom, and many other switching and regulation devices….
A whole bunch of multi-layer switches
Power supply tubes
The date code, February 1952

For us Europeans: 1000 Micromhos is 1mA/V …

Testing unknown tubes

Knows tubes can be selected by entering a code like ff-abcd-e, example EV-2781-9 with the rotary dials in the middle of the device labeled „Selectors“. These dials allow to connect the tester inputs to the tube socket pins. From the left, the dials are for Filament 1 (f), Filament 2 (f), Grid gate (g1), Plate (a), Screen gate (g2), Cathode (k) and Suppression gate (g3).

For known tubes, there are lists containing the code ready to enter. For example, for EF80 the code is EV-2781-9. For the first triode inside an E80CC, code is EV-7608-0, for the second triode EV-2103-0.

For unknown tubes, testing is still possible. The code to enter then is not contained in any list but must be developed by checking tubes data sheet.

For example, the EF98 has a 7 pin miniature socket. Filament voltage is 6.3 volts. Pin numbering is:

1=g1, 2=cathode, 3=f, 4=f, 5=anode/plate, 6=g2, 7=g3.

EF98, seen from bottom side

From pin numbering we have (in order of code letters):

34-1562-7

Socket numbering for the sockets used in 539A:

Hickok 539A socket numbering

Historically, Hickok has introduced two ideas that complicate things.

Pin numbering in Hickok 539A differs from convention for some sockets. For e.g. 7 pin miniature, the numbers are reordered for pins 1,3,4 to 3,1,8. Don’t ask why. So, the adjusted code is:

18-3562-7

For the filament pin part („18“), numbers are not used but a combination of characters. Check the following table:

Socket pin number Filament 1Filament 2
0AP
1BR
2CS
3DT
4EU
5FV
6GW
7HX
8JY
9KZ
Mapping of filament pin numbers to code characters
Scan of mapping table

So for pins 1,8 we get B,Y. This is the same as J,R. So, the adjusted code is:

JR-3562-7 (I guess BY-3562-7 is also correct)

Required BIAS voltage (gate voltage) can be taken from data sheet. Plate voltage is fixed, on my device about 130 volts. Of course, there is no Micromhos target value because there is no list entry in Hickok lists. But this value also can be taken as a landmark from the data sheet.

This example with EF98 is already some special case, because the EF98 is a battery tube, designed for low plate voltage. It’s plate voltage maximum from datasheet is 50 volts. Because the Hickok 539A delivers about 130 volts if adjusted to 100 volts line, this is already too much. But it is possible to adjust line voltage to lower values than 100 volts and then the tube can be tested – at least if the test runs only for a short time.

This allows to do some basic tests also for unknown tube types. If you have a new and good tube of the same kind, even comparisons of the tube to test can be done.