Beomaster 4000 Distortion and Nasty Hissing Noise

[AlexHempel]

So I'm repairing a Beomaster 4000 for a friend. After some work, I got it to bias correctly and went to play some music. The first time I listened, no major issues. I did notice that, after being turned off, it took a second or two for the music to fade out. I assume this is a function of being a cap-coupled design and/or not having a speaker protection relay.

Upon being powered up a second time, there was an initial burst of static through the speakers -- this faded within a second or two. During playback, I could hear audible distortion in the left channel. When I cut the power, the speakers again made a loud static noise. In the right channel, this persisted for at least ten seconds after the power was switched off. It might have been occurring in the left channel as well, but I was sitting next to the right channel speaker so that's what I noticed.

I'm wondering if anyone recognizes this behavior. I did not work on the unit at all in between listens, although I did flip some preamp switches. I'm assuming this is a power amp issue -- maybe failing caps? However, I'm a little perplexed as to why it manifested the second time but not the first. Maybe some component was close to failure and having some power through it caused degradation? Any insights very much appreciated.

 

[AlexHempel]

Ok, I'm an idiot and did my initial offset test on the wrong speaker terminals. Tried the right ones this time, it's passing a ton of DC. Time for some new caps...

 

[Oldsansui441]

Hi Alex, I have had a 4000 in the past and l am sure it is output capacitor coupled. You will see DC on the outputs without a load present. In saying that, as you probably know already those red/brown coloured ROE caps are notorious for leaking and will show evidence of this leakage if bad. The orange ones seem to always fair much better.

A turn on thump with this receiver will be quite normal. Usually best to leave speaker switch engaged on these as it reduces this thump.

 

[AlexHempel]

@Oldsansui441 Ok, I was measuring without a load -- that explains it! Just re-did the measurement with a load and the DC still seems too high, albeit not alarmingly so. Once it stabilizes, I'm getting about 75mv on one channel and 45mv on the other. I'm guessing this is indicative of leaky coupling caps?

Unfortunately, the noise I'm getting at turn-on and turn-off isn't really a thump. It's more like a loud static/hissing sound, and I'm assuming it might be related to the distortion I'm hearing on the left channel.

 

[Oldsansui441]

Yes, they could be leaking very slightly, although it could also be another component/s in the amplifier circuit slightly out of spec, either way it doesn't look like anything terrible is going on. Do you have a way of testing those output caps? The large caps in these old Beomaster's are "generally" ok compared to other brands.

I would start with a visual of all of those ROE caps to begin with in regards to the distortion in the left channel and the other problems. If the transistors are still all original they are usually pretty bulletproof in these.

As you have probably noticed already, the switches on these aren't exactly the easiest to gain access to for cleaning .

 

[AlexHempel]

I have tested the coupling and filter caps with an ESR meter and they gave an acceptable reading, but I don't have a leakage tester (which would be more useful in this instance). Before even turning the amp on, I replaced the amp board trimmers because they were fairly corroded and provided excessive resistance. I also replaced five resistors on the small board next to the caps because they had drifted out of tolerance and the PCB showed signs of excessive heat. In the process, I did a fairly thorough visual inspection of all the boards but didn't notice any issues. I also checked a few of the orange caps with the ESR meter but didn't notice any problems.

This unit has another issue: the stereo switch won't depress. As you mentioned, these switches are an absolute nightmare to work on. I think the solution will be hardwiring a jumper such that the stereo switch is always engaged. Afaik this prevents the amp from being used with headphones, but I checked and the owner doesn't mind.

 

[Binkman]

Didn't they make a move called "Beo master"? these projects don't come up often. International AU to EU posted here.

 

[AlexHempel]

Yeah, I jumped on the project because it sounded like a fun challenge. The design seems -- for lack of a better word -- dated. Cap coupling, all boards have soldered connections (as opposed to wire wrap or plugs/sockets), adjustment requires a ton of trimmers. Seriously, there have to be like 20 trimmers in this thing, I have never seen that before. Removing the boards, transformers, or switches is a major pain because they're hardwired in. My Sansui Six (began production around the same time) is direct coupled with socketed boards and other nice conveniences. But there's no denying the Beomaster looks slick, and I expect the cap coupling/low damping factor/relatively high THD to give it an interesting warm sound once it's all fixed up.

 

[Binkman]

sounded like a fun challenge.

Not too many people would take that proj. on as 'fun'.

Seriously, there have to be like 20 trimmers in this thing

 

[AlexHempel]

Ok, I tried powering it up with speakers again today. The DC spike at turn-on is too brief for my auto-ranging DMM to display, but it's pushing the drivers to what looks like maximum excursion. I assume this isn't normal? At this point I'm really leaning towards a full recap, since everything else seems to be working fine.

 

[Oldsansui441]

but it's pushing the drivers to what looks like maximum excursion. I assume this isn't normal?

Max excursion doesn't sound normal at all. I would expect maybe half woofer excursion.

At this point I'm really leaning towards a full recap, since everything else seems to be working fine.

Sounds like a good call at this point, especially if the unit looks untouched by anyone else in the past and all transistors look factory. Great idea on those bias trimmer replacements also .

 

[AlexHempel]

Yep, everything looks factory except the screws. I think the owner realized it was broken and opened it up thinking they might be able to fix it, because some screws were missing or stripped. They probably gave up when they got a look inside! Transistors are definitely stock, they have little dots of paint which I assume was either for matching or quality control. Put together a Mouser cart for the recap and price isn't too bad anyways.

 

[Oldsansui441]

They probably gave up when they got a look inside!

Haha, yes very possible.

Transistors are definitely stock, they have little dots of paint which I assume was either for matching or quality control.

Yes, that's correct some will be for matching gains etc and others probably for quality control.

Put together a Mouser cart for the recap and price isn't too bad anyways.

Should come out very nicely when you are done. I still have a Beomaster 4400 which replaced the 4000, directly coupled and has plenty of power, sounds excellent. If you think this one is fun have a look inside a Beomaster 6000 quad when you get a chance.

It's funny, the 4000 does seem a little aged in design compared to others like you mention, but then in the mid-late 70's the Beomaster's had some features that the Japanese would have been envious of, ie soft touch controls, solenoid/motor operated tone controls etc. Their amplifier sections etc seem to be very robust, their use of plastic hinges, plastic din connector surrounds, plastic tone slider contact housings etc not so much.

 

[AlexHempel]

Ok, I was a bit unfair to the coupling caps. After running the amplifier on load resistors for about an hour, the DC offset has settled down quite a bit. Now down to less than 10mv on both channels, and it's dropping steadily. And when I hook it up to speakers, the woofer excursion isn't nearly as bad as before -- now it's down to about half. The hissing noises at power-off are also getting quieter, although still present. I can only assume the caps needed a bit of re-forming after sitting for such a long time. Unfortunately, the output still sounds a bit distorted, so I'm not out of the woods yet, but maybe some more cap reforming will also take care of it. If not, I might finally have an excuse to buy a scope so I can properly adjust the four trimmers on the amp board.

I'll probably still recap fully because the owner seemed to want that and I'd like to have as much longevity is possible. Also, tantalums make me nervous. But it is a shame to remove good caps.

 

[Oldsansui441]

That is good news, as you say it could have just been sitting for many years unused. Maybe you could do all of the regular wet e caps, then if still not 100% go in for the tantalums in the tone circuit etc?

 

[AlexHempel]

I'm really amazed at how well the caps reformed. I'm down to less than 1mv offset on both channels and no more noises when the unit is turned on and off. This has allowed me to hear other issues more clearly. On the left channel there's a high noise floor, with a persistent static that can be heard with no input signal. However, when playing music the left channel sounds pretty much fine. On the right channel, there's 60hz mains hum, but it's quieter than the left channel and not really noticeable unless one's ear is right to the driver. However, the signal on the right channel is distorting, especially at one specific frequency that produces a sort of crunchy noise. I think the right channel issue may be fixable when I properly adjust the amp board trimmers, but the left channel issue might be a noisy transistor? I'm wondering if I should add transistors to my Mouser cart, since I don't want to order twice.

 

[Oldsansui441]

Hi, yes a constant hiss or static definitely sounds more like transistor noise. May be worthwhile adding some to your list. You could desolder one of the small coupling caps at the front end of the power amp section to narrow down whether the noise is in the left power amp section or an earlier stage.

 

[AlexHempel]

Recapped the amplifier board today and, surprise, it actually fixed the distortion issue on the right channel. I always recap when I do restorations, but this is the first time it's actually addressed the problem! Hopefully transistor and filter/coupling cap replacement tomorrow will clean up the remainder of the hum and popping. I'll post pics once it's all done.

 

[Oldsansui441]

Recapped the amplifier board today and, surprise, it actually fixed the distortion issue on the right channel. I always recap when I do restorations, but this is the first time it's actually addressed the problem! Hopefully transistor and filter/coupling cap replacement tomorrow will clean up the remainder of the hum and popping. I'll post pics once it's all done.

Nice work, great to hear you have sorted that distortion problem. I agree, more often than not caps aren't usually the cause of a specific problem. Yes, some photos when you are finished would be terrific .

 

[AlexHempel]

@Oldsansui441 Here you go! Better late than never? I actually finished it a week or two ago but got distracted. Listened critically for a few hours with the friend who offered me the job, seems good as new. Sound is a bit warm and a little less resolving than my other amps, but overall I was impressed. I'd say it's a fun, rocking sound.

Still have a slight amount of power supply hum in the right channel, but it's only audible with one's ear right to the driver, shouldn't be an issue. I re-mounted all the level adjustment trimmers backwards so they're accessible through the bottom panel cutouts (although I set them with a scope, so I hope nobody touches them). One of the trimmers was reading in the megs from the wiper to the legs, so they definitely needed to go. If anyone is trying to decide whether to restore a Beomaster 4000, I'd say go for it, as long as your switches and pots are working! The boards themselves aren't too bad to work on -- the amp board can be recapped without removing anything so long as you have a thin, long soldering iron and some good dexterity, and the preamp board can be easily unscrewed. The tuner board is more of a PITA, but it only has one electrolytic, and you can replace it without removing the board (this requires a thin/small iron and desolder braid). However, working on the switches and pots would be an absolute nightmare given the proprietary nature of the whole assembly, so I'd be hesitant when buying a Beomaster 4000 in unknown condition.