I took my first real vacation from work in 5 years and this was my project: a Telefunken 512WL radio from 1935.
The radio came to me in an operational state so I wouldn't call this a restoration or repair. It was more of a 'renovation'.
This is a very simple three tube radio in a fairly compact form factor. It sports an AF7 driver, RES964 output pentode and RGN1064 kenotron. I removed the RF section and simplified the AF section as much as possible. This mainly involved putting the AF7 into triode operation with a plate resistor instead of the native choke. The choke will be recycled in another project.
All the remaining resistors were tested for value and in the case of capacitors, leakage. Thankfully, no modern substitutes had to be used. A photo of the original circuit is below. Only very high quality Siemens manufactured parts.
In the original schematic volume control was done capacitively in the RF section before the grid of the AF7. Since I have installed a RCA line input which I will only use with digital sources that already have volume controls, a fixed 1Meg resistor was placed between the grid of RES964 and ground. The signal (again, from digital sources with 0 DC offset) is fed directly into the grid of AF7. The chassis has a service stamp from 1941, but I could not identify what was replaced other than the output tube, which is a later ST shape. I have purchased a globe RES964 for the sake of completeness.
Despite its bashful appearance (the front glass dial broke in shipping and I decided to just do without it), the radio sounds wonderful. It sports a 7" electromagnet speaker which is probably the best speaker I have in house. It simply sounds more interesting than the various 10-12" Telefunkens I have from 1938. The speaker surround has suffered a number of tears which cause it to wheeze at higher volumes but I decided to just leave it as is rather than risk glueing and worsening the sound. I will eventually remove the front intermediate panel and grill cloth and fix the speaker directly to the case, as the grill and panel reduce clarity to the point where it's better to listen to the radio from its rear!
In its current form the radio will serve as a control path which I will use to evaluate the sound of various passive components by placing them in-between the plate of the AF7 and grid of the RES964. Due to the high input impedance of the RES964 grid, various components can be installed in this location to have their sound colouring evaluated prior to installation in another circuit.
The radio came to me in an operational state so I wouldn't call this a restoration or repair. It was more of a 'renovation'.
This is a very simple three tube radio in a fairly compact form factor. It sports an AF7 driver, RES964 output pentode and RGN1064 kenotron. I removed the RF section and simplified the AF section as much as possible. This mainly involved putting the AF7 into triode operation with a plate resistor instead of the native choke. The choke will be recycled in another project.
All the remaining resistors were tested for value and in the case of capacitors, leakage. Thankfully, no modern substitutes had to be used. A photo of the original circuit is below. Only very high quality Siemens manufactured parts.
In the original schematic volume control was done capacitively in the RF section before the grid of the AF7. Since I have installed a RCA line input which I will only use with digital sources that already have volume controls, a fixed 1Meg resistor was placed between the grid of RES964 and ground. The signal (again, from digital sources with 0 DC offset) is fed directly into the grid of AF7. The chassis has a service stamp from 1941, but I could not identify what was replaced other than the output tube, which is a later ST shape. I have purchased a globe RES964 for the sake of completeness.
Despite its bashful appearance (the front glass dial broke in shipping and I decided to just do without it), the radio sounds wonderful. It sports a 7" electromagnet speaker which is probably the best speaker I have in house. It simply sounds more interesting than the various 10-12" Telefunkens I have from 1938. The speaker surround has suffered a number of tears which cause it to wheeze at higher volumes but I decided to just leave it as is rather than risk glueing and worsening the sound. I will eventually remove the front intermediate panel and grill cloth and fix the speaker directly to the case, as the grill and panel reduce clarity to the point where it's better to listen to the radio from its rear!
In its current form the radio will serve as a control path which I will use to evaluate the sound of various passive components by placing them in-between the plate of the AF7 and grid of the RES964. Due to the high input impedance of the RES964 grid, various components can be installed in this location to have their sound colouring evaluated prior to installation in another circuit.
