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5088This is a wonderful sounding radio. The Grundig 5088 USA has all English wording on the radio. It has four bands, FM, BC, SW1 and SW2. The 5088 has five push button tone controls and four tone knobs. Performance of this radio is surprising. I used a short antenna lead connected between the radio and the forced air supply duct work. The radio picks up many short wave international stations. There is an internal FM dipole and a "PU" input for a phonograph cartridge. A novel rotating ferrite stick antenna is mounted inside and rotated by the left outer knob for best BC station reception. Following are some restoration highlights. Restoration was straight forward. All wax/paper capacitors were replaced with modern 630volt Premium quality PPT Metalized Polypropylene film dielectric, aluminum foil electrode, components. Several resistors were measured bad and replaced. All other resistors were extremely true to their tolerance. The selenium rectifier was under performing, not supplying sufficient B+. The rectifier was replaced with a diode bridge of 1N4007 and a series resistor to compensate the Vfwd (voltage drop) of the selenium array. Alignment revealed the IF stages to be significantly off peak. The dial scale frequency alignments were very accurate and precise except for the lower SW band. Once the IF was properly aligned and the oscillators realigned all the bands and dial scales line up well. All alignments adjusted easily (once the German alignment methods/techniques were understood) and all sensitivities met or exceeded factory specifications. This unit was in service in my 12 year old son's room at a "Torture Test". He fell in love with its detail and the functional EM34 tuning eye. (A "Dimmer" switch has been added to the EM34 to extend the phosphor life.) Listening to WWV (at 10 and 15 Mc) is soothing. I anticipate the rhythmic passing of the seconds and the slowly wavering signal strength will gently lull him my boy off to sleep (Here's hoping). My son gave this radio a good work out. That way I am sure it will survive standard daily service. This is a beautiful unit that I restored not only to look and sound good but also to be a reliable "Daily Driver".
Runway Shots:
Some internal pictures during restoration.
SWII band oscillations:After running the radio for some time (two weeks) it developed an oscillation problem on the SWII band from about 17.5 to 18.3 Mhz. Consulting with the experts at RadionMuseum.org found that component placement near the ECH81 tube. Particularly the capacitor C33 to chassis ground. One can put the radio near oscillation then with a plastic stick move the C33 closer to the tube socket to start oscillation. Moving it away prevented oscillation. The tube was weak on the triode section. That tube was replaced after the capacitor tweaking. The radio has not oscillated under "torture testing" (my 12 year old son) since then. Tuning Eye saver:I have added an STSP miniature switch "with quick disconnects" to the Tuning Eye B+ circuit. It is a simple bat handled switch that bolts through an existing vent hole. It has been soldered in between the B+ switch on the phono pickup switch contacts 4k and 4i and the tube socket. An original terminal strip can be found under the chassis for easy hook up. That is where the tuning eye socket lead wire feeds from above the chassis. All parts are available at Radio Shack. This modification is easily reversed. Thanks to the participants at http://www.radiomuseum.org/
for all their help. An informative discussion about this radio can be viewed
at http://www.radiomuseum.org/dsp_forum_post.cfm?thread_id=41660. Stop in and say Hi.
There are extremely knowledgeable and helpful
people at the
3035 w/3D USA
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As received |
 Look
left |
Look right |
Broken Ferrite Antenna support quickly repaired with Hot Melt glue
gun. Replacing and stringing the dial cord was a different
matter. |
Out of the cabinet shot. |
I ALWAYS store the dial glass separately. I do not want an accident
and have to find a replacement! |
The blue capacitor just under the left side of the switch enclosure was
shorted. You can also see the burnt (open) 1k ohm resistors that the
short took out. |
 The
yellowed paper capacitors can be seen under side. The blue
capacitors are also paper inside. |
Restored and working. Products on the left used to shine the knob brights
and treat the plastic. Glass cleaner took care of the dial
glass. TEST the paint before cleaning. |
No flash and Light Out shot. |
Sorry. I did not take a picture of the restored under chassis. I will post one if I disassemble the unit again. All the paper and blue capacitors have been replaced with yellow tubular caps that are considerable smaller in size but all rated at 630 volts. | In the picture directly above, between the two horizontally oriented blue caps, is a dark circle with three wires. Two electrolytic capacitors mounted to a new terminal strip replace the chassis mounted can capacitor. The wires were moved to the terminal strip and the can cap left in place for appearance sake. Again sorry for not taking an after picture. See the Capacitor Page for an example. |
Selenium Bridge Rectifier retrofit with 1N4007. Selenium rectifiers loose their efficiency over the years. It is common practice to replace them with silicon diodes of similar capabilities. I use 1N4007 diodes with a 1000 volt PIV. That stands up easily in a tube type radio B+. Some applications a series resistor is added to compensate for the much lower voltage drop of the silicon junction diode. This is to bring the B+ back down to specifications. Below is a pictorial of how a flat bridge rectifier was retrofitted.
This is the flat pack selenium rectifier bridge removed from the
radio. A screw driver, pliers and a soft touch will slip the cover
off. |
Once the cover clears the bottom metal container the top flies across the
table and little square metal pieces shower back down on your bench.
Those are the selenium wafers. |
Trim the contacts so one pad per solder contact remains. Drill two
holes in each to receive the 1N4007 lead wires. |
I tapes and labeled the solder pads in accordance to the stamped terminal
identifiers in the metal cover plate (the one that shot across the
bench). |
Install and solder all four diodes. |
Reassemble making sure all the insulating materials are returned inside
the metal enclosure or a high voltage short will occur. That's it! |
I had wanted to hook up some external audio sources to the Grundig. So I took a Radio Shack switch box disassembled it, modified the circuit board and added video quality dual shielded coaxial cable terminated with a three pin DIN. It works well. The output of the Grundig is routed to the two RCA output jacks on the switchbox. This provides for recoding off of the air programs on AM, FM or SW bands. The signal seems a little weaker than standard line output so the recorder's manual gain had to be set high.
Three inputs can be selected with the three push buttons. I have hooked up a DVD player playing Shania Twain "UP" album, A cassette tape deck (I tested the off the air recordings) and a monaural source, my FM amateur radio transceiver. All three sounded GREAT through the Grundig.
Use the tape button on the front panel of the Grundig to play the source going through the switch box.
Schematic Diagram. |
Front view. |
 Rear
view of switch box. |
DIN connector in the Grundig 3035. |
I was curios to the frequency response of this modification. There are a lot of people rightfully concern with audio losses due to cable losses. So I preformed a little test.
Unfortunately, I do not own an Audio Generator so I used the lower end of my RF generator (HP-606a) along with a 100mhz B&K Oscilloscope and a 100k ohm termination resistor (as in the Grundig).
I found no frequency roll off between the frequencies of 56khz and 20 mhz. Therefore, I can conclude frequencies below 56khz down to DC are also unaffected.
Paul
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