Monday, January 15, 2018

Grundig - RTV 320 U

  Picked up this tuner-amp (c. 1968). I was drawn to its having a shortwave band. Note that the high frequencies are on the left.
  Blew it out, cleaned it up a little - it's basically functional.
  The previous owner tried rigging in an RCA jack scheme to circumvent the euro speaker connectors and it had no dial lights.
  The back panel serves as in "interlock" of sorts. I removed the AC cord, drilling out the rivets. I may take a shot at fabricating a replacement back panel (from fiberboard).
  I found an ebay seller with the euro connectors and another with a DIN-to-RCA cable. The Phono and Tape inputs are via DIN jacks.
  I replaced the dial lights. I must have been the first person to have done that. That's probably because doing so requires pulling the knobs and removing, extricating, the dial glass (a total PITA for dial lights). The lamp bases were practically frozen in their sockets. Curiously, they run from their own transformer secondary.
  The left and right audio power amps are each powered from separate secondaries of their own. Those Grundig designers were generous with transformer windings.
  The AM and SW switches have an issue, I have to tap them to get a circuit engaged. I should go back in, those contacts may benefit from some DeoxIT in/on the contacts. [see Follow Up]

  Tuning between FM stations results some noise like what you get from a noisy volume pot. Haven't encountered that. [I've determined that this results from the receiver's lack of 'FM Muting'.]
  The Volume pot is not noisy - which is surprising. Likewise, neither Treble nor Bass pot exhibit any noise and Balance is clean, too.

"Auf Bayern, mit Liebe"

  Follow-Up: Took the chassis out and sprayed some DeoxIT into the AM and SW select switches and it seems to good effect. They're sort of open at the back: spray and work 'em, wait, work 'em again, and repeat.
  The back panel calls out "7V" lamps and, given my 6.3V #40's, I was bugged about the line measuring 6.8v(ac) - so I put a couple of diodes back-to-back in series, knocking that down to '6v'. [The two dial lamps and the one behind the tuning meter are in parallel.] Their brightness was verging on garish and reducing that, v_lamp, "to spec" made things much nicer.
  [Hadn't looked at the circuit board first time around, but noted that the screws securing it to the chassis were soldered over.]
  I was considering putting this in the front room as my 'daily driver', but it doesn't have a tape monitor loop.
  Tuning SW stations requires extra patience because it doesn't have a fine tuning control. Nonetheless, I'd have been especially happy to have had this during the international broadcasting era. Sadly, there's not much to tune in anymore.

  11FEB2018 - Good results with a couple of Bluetooth receiver configurations, connected with DIN to RCA adapter found on (where else?) ebay.

Here's a hotlink to a page where there's a link to a PDF containing the schematic. (Page Down till you find it.)

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