Hi all

I have a Beomaster 3400 in for repair here.

The symptoms were fluctuating L&R channel indicator lights, then after a few seconds, the selected FM station would cut out, randomly and continuously jumping to adjacent stations.

With the scope, I narrowed it down to the indicator light circuit, with DC ripples on the circuit.

To isolate the ripple, I desoldered pins 5 & 6 of the TCA420A - these pins are the Left and Right AF output of the IC.

With these 2 pins disconnected from the rest of the circuit, the pins continued to show DC ripple, and the rest of the circuit is free of ripple.

Bingo!

Does anyone have any good spares, and any ideas (from the gurus) why this happened? All caps were already replaced a while back, and a double check on the circuit shows zero ripple on the replaced caps. Input voltage is dead-on at 14.9VDC, so no overvoltage there.

Menahem

Did you replace the little round rectifier mounted on the power supply board near the voltage regulator Lightning fast payment, anytime again  ?

Martin

Hi Martin,

I put in a new B40C1500 about 18 months ago, when I first got this in for repair.

Is it possible that this rectifier is already faulty (the 26V main circuit is stable), or is it more likely that the previous rectifier damaged the TCA420A?

I measured the B40C's + terminal on the scope, and it's showing ripple, but I'm not sure that it's more than normal.

Assuming it's no good, what do you think about replacing with 4x SB260 Schottky rectifiers?

In the audiophile world, there's a lot of talk with using Schottky's instead of regular bridges.

Besides that, to reduce the ripple, would you increase the coupling capacitor from 470uF to 1000uf or even more?

Menahem

If the bridge rectifier was already replaced, I don't think it's the problem.
I have a little stock of odd parts from scrapped machines and I'm sure
I can find a TCA 420A for you to try. PM me.

Martin

Martin

Thanks so much for your kind offer.

In the meantime, I did some further troubleshooting (as I do not have a TCA420A to pop in), on the circuit as follows:

The Driver amplifiers TR2 and TR3 were replaced with a new pair of matched low-noise BC550C. I always find it a good idea to replace the driver pair, if there is any problem on a circuit, especially output stages.

At the Stereo Demodulator, I was getting a discrepancy on the collectors of TR10 (5.4V)  and TR11 (6.4V). I replaced both of them with matched BC546B, and now have 6.1V and 6.2V - much better.

I replaced the original IC2 and IC3 SPS5418 Darlingtons (which had hFe's of 3700 and 4400), with a pair of matched MPSA13's (hFe's 19800 and 19900), and the L&R Tuner indicator lights are MUCH more responsive now.

On TR6, I had a trace of ripple, so I replaced it with a BC556B.

Lastly, I took your advice on the Bridge, and jumped on the Schottky bandwagon - I joined 4 11DQ10 1.1A diodes, and put them instead of the B40C.

Then I looked again on the scope, and I was still not happy with the ripple on the + point of the new bridge. I put in a new Nichicon HE 1000uF 50V cap, in parallel with the existing 470uF, and now, on the scope, the ripple is less than half.

I have now let the BM3400 run for over 12 hours, with no station fluctuations at all, just incredibly clean FM, even on distant stations. I'll run it for 72 hours straight, and if no failure happens, I'll call it a clean repair.

I can only assume that prior to my original restoration, this unit had been stressed with the original lousy capacitors, and even though I replaced them all, there was residual damage to some of the semiconductors. This BM3400 is in daily use now, so the stress effects showed up finally.

So for the time being, I won't need the IC - but I'll take you up on it if I do.

Regards

Menahem