Beomaster 1400/1600 output stage.

[H_Johnstone]

Inherited both the BM1400 and BM1600 old series receivers.
When I fixed them up, new capacitors and output transistors,
the series regulator (negative) providing single 30V rail, gets
really hot at load. (AC-coupled) Classical problem...

If I provide unregulated supply to the output stage only,
dropping the GND by -15V to a total of 45V, How much
of the output stage do I need to resupply (convert)?
Can I use coupling capacitors in between?
All for minimum modification to the circuit.
(Sch. output stage provided)

Many thanks for your input
Sincerely, Howard J.

 

[GrundigXpert]

Hi Howard,
these receivers, though quite elegant and stylish, with about 9 W per channel on 8 Ohms, aren't race horses. They are for typical home use. In this mode, the regulator transistor should't get so hot, unless there is a short or idle current is way off.
I suggest to stick to the original circuit, as a regulated power supply in low-power amplifiers is a nice way to preserve the dynamic range of the music source when the power supply is not so beefy.
Sincerely
Marco

 

[H_Johnstone]

Agreed, had this been a normal receiver, but the PSU is very beefy, rated at 100% overload on the transformer.
The original wicon capacitor and rectifier had to be replaced anyway, 50/3000uF now 50/27000uF and rectifier 15A.
A load test showed regulator cuts sharp at 4A, causing a "pumping" effect at 4 ohms. Bypassed 8A was no problem.
Some of the old power transistors where shorted due to open bias pot, and repaired with MJE2N3055T (TO220) standard.
Thus the available power is already there to be used. (two separate cooling blocks are provided, well provided for full load.)
I can see that it works, but need the help to stabilze the circuit. It is simple due to its AC-coupling, using 50V/12000 instead.
Your input is very much appreciated.

 

[GrundigXpert]

Excuse me, but what you are telling seems to me like putting an 8V 5300 turbocharged engine on a Yugo frame. The power switch, the transformer and the power supply cabling were not designed to stand such a high surge current at power-on. I am not pretending to change your mind about this, I'm just sad figuring that a 51 year-old work of art like the B&O Beomaster 1400 is turning into a wannabe dragster. Please believe me there's nothing personal.

 

[H_Johnstone]

I am amazed, everybody either states like yourself; do not touch the original unit, others frowns at it as stylish, worthless
of any serious audio concern (AC-coupling), and a few pull everything out, replacing with SMPS and ICEmodules leaving
the rest alone for control and style. We who have at least some workable knowledge can see that there are a few simple
things such as replacement of dried capacitors, high loss rectifiers, heat worn transistors and half open trimpots, that can rejuvenate the unit. The BM1400/1600 units had a casting chassis, double block heat sinks, over size transformer that was
never used given the limits of AC-coupling. This is no longer a problem, capacitors with values close to 20mfd are now
available as well as low loss diodes/rectifiers. A simple NTC will limit start up current should this be a problem. 9W will not
suffice for modern day 8 ohm full range loudspeakers. Therefore, when 15W can be made available for a small effort, this
should be taken into account for a more modern and usable receiver without prejudice. All audio, no attitude. (Sic!)

 

[Oldsansui441]

The original wicon capacitor and rectifier had to be replaced anyway, 50/3000uF now 50/27000uF and rectifier 15A

Hi Howard, as a Beomaster owner l respect your willingness to bring these two beauties back to their former glory but l personally wouldn't use a filter cap value anywhere near that high.

9W will not
suffice for modern day 8 ohm full range loudspeakers

Thing is they were never intended for modern day "inefficient" speakers. The old Beovox's from the 70's were l suspect around 90-91dB.