Blaupunkt "Frankfurt" (K Series)

A Chronicle of Repairs to a Blaupunkt Frankfurt Automobile Radio
Latest Revisions are at the Bottom of the Page

Another Achievement from Radio Headquarters


AM/FM/LW Automobile Radio

Blaupunkt model "Frankfurt" (K Series)

Blaupunkt Werke GmbH (Germany)
Serial Number: K150607




Push-buttons labeled: U U M M L

U (Ultra Kurz Welle = UKW = FM) 87-100 MHz
M (Mittel Welle = MW = AM) 550-1500 kHz
L (Lange Welle = LW = Long Wave) 150-290 kHz


This radio was an original, extra-cost option in my 1957 Mercedes-Benz Type 190 sedan, and so the association began on June 17, 1989, when I took ownership of the car.

To verify it was working to its fullest potential, it was sent to radio repairman Wilford Wilkes (Brisbin, Pennsylvania) in 1990. I did not have access to a tube tester, and it was the first vacuum tube radio in my collection. 

It continued to work well from 1989 up until the fall of 2010 when the audio volume dropped off significantly. At that time, it was removed from the car, and the tube sockets were cleaned with DeoxIT. The tubes tested fine, and the minor servicing seemed to fix the problem. Unfortunately, the next summer, during its seasonal maiden voyage (June 25, 2011) there was absolutely no audio of any kind coming from the speaker. I removed the radio, and the resulting experiences were chronicled on this page.

Specification Overview

The radio's serial number (K150607) begins with the letter K, so it may also be referred to as a “K Series Blaupunkt Frankfurt.” The model name “Frankfurt” was used by Blaupunkt for several years and numerous design iterations during the 1950s, so it is important when seeking literature and schematics to specify the “K Series” along with the model name.

Valve (Tube) Line-up

Type Description Reference
EF89 (6DA6) RF voltage amplifier
EF89 (6DA6) RF voltage amplifier
ECC85 (6AQ8) dual-triode for the front end stages of FM radio receivers
Note: located on bottom of chassis
ECH81 (6AJ8) local oscillator
EABC80 (6AK8) AM/FM detector stage
Radio has a separate power supply with a "buzzer" (a.k.a. "vibrator", or "chopper") to convert 12 volt D.C. to pulsing D.C., which looks like A.C.
EL84 (6BQ5) audio amplifier
Note: Located in power supply

Repair History

July 5, 2011

Borrowed an ex-Navy model "TV-7D/U" tube tester and verified all tubes were healthy. Got an extra set of tubes in the bargain. Put the radio back together and installed it in the car, but still had absolutely no audio (or sounds of any kind) from the speaker. Tested the speaker, and it measured 8 ohms and crackled while being tested, so it seemed to be working.

Removed the power supply from under the dash. It was held in place with two large flat-blade screws. Surprisingly, there were no wires (for 12 volt input?) or any other connections, aside from the interconnection cable to the radio). The power supply was in perfect, unmolested physical condition.

The radio itself has three connections:

The power supply has two connections:

The power supply (includes the EL84 amplifier tube) must first get its power from the radio (via the interconnect cable) and then supplies the radio with the B+ high voltage. The power supply includes an output tube type EL84 and a chopper (a.k.a. vibrator). There should be 225 volts coming out at pin 1 of the multi pin connector.

According to the Operating Manual for the B&K (B+K) Precision Model 1460 oscilloscope it features a calibration jack (bottom left of front panel) that generates a 1 kHz signal (5 V p-p square wave). When the probe from the calibration jack is touched to certain parts of the circuit, an audible signal should be heard. Touched the probe to pin 2 of the EL84 tube and there was indeed audio at the speaker. On the other side of the 1 k Ω resistor (directly off pin 2 of the EL84) there was no audio. This test provided some idea of where to start looking for the problem. It could have been the 500 k Ω resistor (annotated “0.5 M” in the schematic - see References), or the 0.5 µF capacitor, or possibly with the interconnect cable. It appeared as though something was shorted to ground.

July 7, 2011

The 6-pin Bakelite plug was broken, and the ground wire (pin 3) was disconnected from the outer braided shield jacket. The insulation on the wires was in poor condition, and there was a dead short between pin 6 (off pin 2 of the EL84) and 3 (ground/shield) so I addressed that.

The insulation was so bad, I was not sure if I could use the existing wires. I re-soldered the pin 3 wire to the braided shield, but still no audio (unbelievable!) out the speaker.

The short between pin 3 and 6 of the connector (on the radio side of the cable) told me I needed to look at that. The power supply was off the radar for the short term.

13-jul-2011: Per Karl Horst (British Columbia, Canada) these connectors were very common on other equipment. Ericsson used them on their intercom systems in the 1960s to 1980s. There is a huge German mail-order house for electronic parts called Conrad. Try searching the internet on both companies and send them these photos, and they may be able to direct me to a source of new connector.

July 11, 2011

Building a new interconnect cable. I bought some audio cable from Radio Shack for the shielded audio line. Its diameter is a little on the large side, so I may be forced to use a regular wire with the braided shield from RG-174 coax (about the diameter of a ball point pen refill). The goal is to get all the wires of the interconnect cable through the bottom opening of the original Hirschmann connector. I will try to do that with the heaviest gauge wire possible - that might mean #16 AWG. I am wire poor at the moment. I have smaller wire and larger wire, but no #16 AWG.

For the outer braided shield jacket (encases all six wires, including the shielded audio line) I was hoping the braided shield from RG-8 coax would work. I do have some RG-8 here, but no RG-174.

July 12, 2011

Received the Sam's Photofact documents. Set 393 / Folder 7

July 14, 2011

July 16, 2011

Soldered the new wires onto the Hirschmann power plug, but the glue let loose, and the two sections came apart again. There was simply too much stress on the plug due to the size of the #18 AWG (stranded, cloth covered) wires. Needed to get smaller gauge wires and re-glue the plug. Ordered new wire (#20 AWG, stranded, non-cloth covered) from Radio Daze.

July 25, 2011 (Monday)

Fashioned a new interconnect cable; soldered it into the radio, and got audio out of the speaker. Unfortunately it was only the buzzer being amplified. The 6-pin plug came apart again (the glue was not holding) and I decided to let Radio Daze finish it up.

July 26, 2011 (Tuesday)

Brought the radio, power supply, and Bakelite speaker surround to Radio Daze during the noon hour. Also brought the donor radio (with good Bakelite plug and interconnect cable) and power supply. Brian said he had the Sam's Photofact (Set 393 / Folder 7).

August 17, 2011 (Wednesday)

Got an email from Brian at Radio Daze:

OK Jeff, we're all set. I had to rebuild the power supply. After about 10 minutes, it started to smoke a resistor, and there were boiling noises from the supply. Swapped out the caps in the power supply, and replaced the damaged resistor in the receiver portion. Oh by the way, its got a nice new shielded cable between the two units (using the Hirschmann plug from the set donated by Senen Racki). I made a new cable; didn't use the one from the other set (from Senen Racki) because its condition wasn't much better than your original. It's been running for a couple of hours now, I'll run it tomorrow some more, but I'd say come and get it when you get a chance. Brian 

August 19, 2011 (Friday)

Picked up the radio and power supply at Radio Daze at about 8:00 a.m. on my way to the AWA Conference. Total repair bill was $174.66. Installed it in the car on Saturday August 20, and it sounded great.

Table 2
Replacement Parts for Blaupunkt Frankfurt (K Series)
Compiled: 07-jul-2011 through xx-jul-2011. Placed order: multiple days
Vendor 1: RD (Radio Daze) -
Vendor 2 (for reference): AES (Antique Electronic Supply) -
Original Schematic Part Number (German/European annotations) Original component and composition. Reference Sam's Photofact Set 393 / Folder 7 Value (original) Value (replacement)
Composition of new part Vendor (Part number) Price ($) Quantity (actual) Purchased / Ordered Date Total ($)
unknown (off pin 2 of EL84) R42 1 k Ω 1 k Ω metal oxide power, 2W AES (R-F1K) 2.00 1 5 (per package) 07-jul-2011 2.00
unknown (below 1 k resistor off pin 2 of EL84) R43 0.5 M Ω 470 k Ω metal oxide power, 2W AES R-F470K 2.00 1 5 (per package) 07-jul-2011 2.00
unknown (below 0.5 M resistor - goes to pin 3 of interconnect cable) C60 0.4 µF 0.47 µF, 630 v polypropylene, radial AES (C-LD47-630) 1.39 1 5 07-jul-2011 6.95

Built a new power interconnect cable. The #18 AWG wire (reference next five rows) turned out to be too large a diameter to fit through the opening of the Hirschmann power plug. Had to order #20 AWG instead. The two foot length was barely long enough so also ordered three feet of #20 AWG, and tried again. 

N/A wire  red #20 AWG stranded red #18 AWG stranded red #18 AWG stranded, cloth covered  RD (W6ST-18RED) 0.50 per foot ~2 feet 2 feet 15 -jul-2011 1.00
N/A wire  white #20 AWG stranded white #18 AWG stranded white #18 AWG stranded, cloth covered  RD (W6ST-18RED) 0.50 per foot ~2 feet 2 feet 15-jul-2011 1.00
N/A wire  blue #20 AWG stranded blue #18 AWG stranded blue #18 AWG stranded, cloth covered  RD (W6ST-18RED) 0.50 per foot ~2 feet 2 feet 15-jul-2011 1.00
N/A wire  black #20 AWG stranded black #18 AWG stranded black #18 AWG stranded, cloth covered  RD (W6ST-18RED) 0.50 per foot ~2 feet 2 feet 15-jul-2011 1.00
N/A wire  yellow #20 AWG stranded yellow #18 AWG stranded yellow #18 AWG stranded, cloth covered  RD (W6ST-18RED) 0.50 per foot ~2 feet 2 feet 15-jul-2011 1.00

Ordered #20 AWG cloth covered, stranded wire (next five rows) to re-create a new power interconnect cable

N/A wire red,  rubber insulated red, #20 AWG, stranded, cloth covered red, #20 AWG, stranded, cloth covered RD (WCST-20RED) 0.50 per foot ~2 feet 3 feet 18-jul-2011 1.50
N/A wire white, rubber insulated white, #20 AWG, stranded, cloth covered white, #20 AWG, stranded, cloth covered RD (WCST-20WHT) 0.50 per foot ~2 feet 3 feet 18-jul-2011 1.50
N/A wire blue, rubber insulated blue, #20 AWG, stranded, cloth covered blue, #20 AWG, stranded, cloth covered RD (WCST-20BLU) 0.50 per foot ~2 feet 3 feet 18-jul-2011 1.50
N/A wire black, rubber insulated black, #20 AWG, stranded, cloth covered black, #20 AWG, stranded, cloth covered RD (WCST-20BLK) 0.50 per foot ~2 feet 3 feet 18-jul-2011 1.50
N/A wire yellow, rubber insulated yellow, #20 AWG, stranded, cloth covered yellow, #20 AWG, stranded, cloth covered RD (WCST-20YEL) 0.50 per foot ~2 feet 3 feet 18-jul-2011 1.50
future use



c o n t a c t / r a d i o w o r l d
Blaupunkt "Frankfurt" (K Series)
Established: July 8, 2011
© Black Sparrow Photography / Jeffrey P. Miller (N2AWA)