Beomaster 1000 – stereo decoder board compatibility

[HatOfTheCat]

FIXED 

I’m working on a Beomaster 1000 – type 2301 (white key, Danish market version with Danish radio station names on dial instead of channel numbers).  The only thing not working nicely now is the FM stereo decoding.  The indicator lamp does not come on at all for several minutes after power on and then comes on permanently at a low glow but will briefly flash brightly when tuning is being adjusted but will not stay bright once the tuning stops being moved.  This receiver is fitted with a decoder board that is NOT the same as in most documentation but does appear in one data sheet I found as very definitely being for Beomaster 1000 (same board appears to have also been used with Beomaster 900 too).  Without double checking (my fault) I followed the procedure in the service manual for a type 2313/2314 that said to adjust the “sensitivity” of the indicator lamp to adjust the trim pot on the decoder board… that didn’t help as what I adjusted seems instead to have been a rim pot for “separation”… no improvement anyway, I’ve reset the trim pot to where it started from and nothing has changed.  What I do have is another decoder board from a type 2317 which looks to be same design as is detailed in most documentation for the Beomaster 1000 of all types and also seems to match the schematic that was in lid of my type 2301 (schematic covers types 2301 to 2305 so they must have been making more than one type simultaneously).  Question is can I just try using this decoder from type 2317 as a substitute ?  Picture of the decoder board that is in my type 2301 attached.  The one from the 2317 probably needs electrolytic capacitors replacing as at least one looks a bit weepy.

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[ummagumma]

Can’t help you, but good luck!

I was glad to see the FM board was still in my 2317, plus the schematic inside the chassis folded up in an envelope. How cool is that? Even though I’d found a PDF online.

[Dillen]

An IF stage slightly off correct alignment can cause this.
As pronounced as this, it could be the last stage before the detector.
Don’t attempt to do any alignment unless you have the proper instruments for it.
There are ways to do it without, but it usually requires more than average experience.

I don’t think it does, but if your Beomaster uses AF116/AF117 transistors in the IF stages, one or more of them could also be bad (or just weak). AF125/AF126 are not as fault-prone.

Martin

[HatOfTheCat]

I’m certainly not going to dive into attempting an alignment Martin as I have neither the skills nor equipment to do one.  However just to confirm, would need for alignment not have other symptoms such as poor reception etc ??  The radio sound is crystal clear with no distortion at all and stations are easily found and tuned in.  The tracking may be slightly out on the dial (less than 0.5Mhz) but that’s it.  Below is clearest schematic I’ve found for the decoder board that I have (mine does look to have the resistor mod for Beomaster 1000 that it notes in paragraph at the bottom).

I’ve only had 30 minutes today to look at this problem.  First I double checked with two channel scope that the receiver is only putting out mono sound from FM, confirmed that.  I’ve checked those transistors (hard to see the labelling as they are in between larger components) and I could only see AF126 type and none of the AF116/AF117 ones so that’s good. I also checked the MONO switch isn’t simply stuck on internally and it looks like it is definitely off (it is faulty and doesn’t latch but it doesn’t seem to go on at all as gummed up inside).  I’ve also checked the 4 germanium diodes on the decoder board as apparently they are of a type that can fail but all checked ok, about 0.4v as they should be.  I also had a look at pin 7 on the connector (violet wire) as that is the one connected to the stereo indicator lamp and collector of TR4 which switches it on, was -28v as long as FM selected but didn’t change when moving tuning and near to zero when any other source selected.

 

 

[HatOfTheCat]

FIXED AND WORKING

Was nothing wrong with the IF stage, it was just the decoder board that was not working properly.

I recapped the decoder board I have from a later Type 2317 and swapped that in (it had several obviously leaking caps).  No adjustments made.

https://youtu.be/hDRFIXJcw_Y

I’ll leave fixing the original board for a rainy day (just needs transistors replacing and likely a recap too).

 

[HatOfTheCat]

Well after using the later version 0001 decoder board happily for 2 months it then decided to stop switching on the STEREO lamp, decodes okay but the circuit part on the board that triggers the lamp has failed somehow.  The 38kHz signal is strong and correct just before it.  Have spent quite some time trying to get it working again… various new parts (there aren’t many in this part of the circuit and I think I’ve changed them all) but no joy.  So I turned back to the original decoder board and installed that, it has all new replacement capacitors and transistors, a quick tweak of the two cores as per the schematic I posted to get the max level for the 38kHz signal at test point and it WORKS !! So I now have a fully original Beomaster 1000 Type 2301 with early model decoder board (as used in a Beomaster 900 too)… and a spare “broken” later board …I’ve put fixing that on the back burner.

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