ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hi all
I have always wanted a BM4400 for myself, and I was rewarded after a patient wait for many years with a dead unit.
HOWEVER …. after replacing all the electrolytics and Poti’s, initial examination shows the following:
1. With P11 Left and P12 Right cables connected, the Dim Bulb Tester illuminates with power applied, and remains lit.
2. Continuing with P11 Left and P12 Right cables still connected, and with the shorted LEFT 0IC102 TIP146 now desoldered, then the DBT illuminates momentarily, RL1 kicks in, and the DBT extinguishes
3. With PL12 Right connected and PL11 Left removed, then the DBT illuminates momentarily, RL1 kicks in, and the DBT extinguishes
4. With PL11 Left connected and PL12 Right removed, then the DBT illuminates momentarily, RL1 does NOT kick in, the DBT extinguishes, and the Overload lamp illuminates
5. With PL11 AND PL12 removed, then the DBT illuminates momentarily, RL1 does NOT kick in, and the DBT extinguishes, and the Overload lamp does NOT illuminate
Initial checks show all remaining Output amp transistors on the rear panel to be OK
So, it looks like the Left and Right Output Amps each have a different problem.
I’m continuing to do checks of all the components in the Output Amp sections, but in the meantime, if anyone has any pointers, feel free!
Question (in the meantime):
I removed both 5TR115/215 (the MPSH54 PNP drivers), and the HFe measures 64 and 35 respectively. To me that looks very low.
However, the Cbc (Current between the base and collector) is critical to prevent RF oscillation, and that’s why B&O changed this driver to the MPSH54, from the BC5xxB (HFe 200+) in all previous versions.
So, HOW critical is THIS gain to prevent oscillation and breakdown?
What would the optimal HFe for these drivers be, and what other common drivers could be used instead?
Menahem Yachad
Jerusalem ISRAEL
I cannot help you with your component issue except to say the 4400 is worth waiting for.
Im sure some here knows a way to sort it.
I had to replace the driver transistors (one channel) il mine. They were leaky or DOA when I got it, although the amp sort of worked. I also had a bad MPSH54.
Worst of all, the protection circuit was also malfunctioning (one dead transistor here too).
It has been in daily use for four years now with no further problems.
Menahem, you have electronic skills, I trust you will figure it out.
Jacques
Jacques - thanks!
OK, you replaced the MPSH54 driver - with what?
Did you pay any attention to the HFe of the replacement transistor?
This is going to be a slow restoration, as I have to fit it in between my other work, which customers pay for ;-)
I've got it up and running (somewhat) just from replacing the electrolytics, so that's something!
All the bumble bee 68nF polyester capacitors in the PreAmp are cracked, so that's the next thing on the agenda. I've never seen such wholesale degradation of polyesters before - pictures to follow.
Bad MPSH54 transistors are not uncommon and they are known to cause symptoms like this,Replace with new MPSH54. (There's really nothing else that good though BC556B could come close).And I wouldn't worry too much about the hFe of the driver, there's enough going on at its emitter to stop worry,ing about hFe , though Iwould expect the two identical transistors to be a bit closer than yours seem to be.It's not common though, to see them cause blown output stage transistors and it's rare to find just one blown transistorin a DC-coupled complimentary amplifier like this. I would look closer at that output stage.
Was the problem also present before you started upgrading?
Martin
I got it completely dead, and filthy.
I air-compressed it to the point I was willing to put in on my workbench, DeOxit-ed the switch-bar and each switch, until each was moving correctly.
Connected it via the DBT - illuminated bright steady, and still completely dead.
Was disgusted with a few random ESR measurements on the electrolytics, and replaced all the electrolytics wholesale as a very first step.
Connected it up, as in the first post above, and for the very first time saw some signs of life (PSU voltage where there was no voltage before), even though the DBT still illuminated.
So, next week I will continue the troubleshooting.
I don't have any MPSH54's, but I will put in 2 BC556's as a first step.
You do have a TIP142/147 matched kit for all the 8 transistors, right? I'm not certain that I want to do that just yet, until I've done proper troubleshooting.
The TIP142s/147s should ideally be matched but, having said that, it's usually fine to just fit some from the same batch.They will be close enough.Minute variations in hFe is largely countered for by the emitter resistors anyways.
If you can get MPSH54, do it.Also if it means waiting a bit for delivery.
I've basically nailed it, but I still want to check some more things, to optimize the circuits.
The key is to work with the DimBulbTester, and each successive correct replacement will show a progressive dimming of the bulb element, until there is no illumination at all.
To sum up, in order of replacements:
1. All electrolytic capacitors with Wima MKS2, Panasonic FC, and Nichicon HE, AND the cracked 68nF bumblebees with Panasonic stacked films, and all Trimpots with Piher PTC Cermets.
2. 0IC102 - shorted TIP146, with replacement TIP147
3. 5TR115 and 5TR215 MPSH54 (HFe 60 and 30 something), replaced with MPSA55's HFe 170
4. 0TR100 and 0TR200 BD165,(HFe 11 and 30) replaced with BD139's HFe 140
5. 5TR1 BC256B HFe 41, replaced with BC556B HFe 260
6. 5TR2 BC142 HFe 28, replaced with BC142 HFe 223 (the highest I've ever seen on this transistor)
The Bias current mV is pretty stable at 13mV both sides, but wandering a bit, so I still want to check those TR pairs in the amplifiers which should be matched, AND in the tuner sections, replace all those old drifty ceramic caps with new Murata C0G/NP0 types for super-stable reception.
I'll keep you all updated, with pictures later.....
Menahem
The wandering idle current is most likely not because of the output stage hFe as such, and it's definitely not because they aren'tmatched pairs (it would merely shift the DC-offset - but they'd have to be severe off to cause any problem).It's more likely because of bad thermal contact tothe cooling fin, both for the transistor in the constant current generator and for the output stage transistors.If it keeps creeping upwards, it's typically because one of the resistors in the idle current/constant current generator is drifting with temp.
Idle currents will almost never be completely steady.
But - replacing only one output stage transistor in a push-pull output stage like this is not recommendable.The other(s) will have suffered overloads (to varying extent).
Thanks Martin
I've still got a good bit of work to do, to get it to where I want it as perfect.
I will be replacing all the semiconductors and a few resistors in the Output Amp stage.
There are a few transistors (TR214, 219 and 220) much hotter than normal, and and therefore showing a too-low voltage 29V on the main 35V rail.
However, it is running now, which is much more than could have been said for it a few weeks ago.
It will be a while until I can get back to it, as it is a non-priority project.
OK, I had a few hours today to get back to it.
Still a problem.
As per the picture, I replaced all the TO-92 transistors and ceramics in the output amps, and selected resistors around the constant-current source.
R160 560R
R163 22K
R164 2K2
R165 56R
R169 18K
R170 18K
R171 330R
R172 330R
D104 1N4148
Same as above for Right channel
Not yet replaced - the 2 10,000uF caps, and the 8 TIP141/146's.
When connected to the DBT, all is OK, no relay clicks, but the 35VDC rails measure as 32VDC - expected due to the slight resistance of the DBT.
Bias resistance is dead-stable at 13mV.
In short, everything is just fine - same on both channels.
However, when I disconnect the DBT, and connect the BM4400 direct to AC power, the relay clicks (first-time only).
So, I disconnect the AC plug, and connect again, then the BM connects fine, no relay clicks.
No fuse blows, and I can measure.
L&R 35VDC + and - rails are dead stable at + and - 35.3VDC,
BUT
the Bias resistance on both channels climbs rapidly from 13mV (original setting) to 17mV, at which point I smartly disconnected the AC power.
Is this the point at which I replace the 8 Darlingtons?
Why is there the difference between the circuit's stability with the DBT, but the instability without the DBT?
"the Bias resistance on both channels climbs rapidly from 13mV (original setting) to 17mV,"Resistance is not measured in mV.
You should set the idle with the Beomaster running at correct mains. Remember that the amplifier runs off unregulated power supplies.It's supply voltages are greatly depending on the mains.Idle at 17mV is a little high but no cause for panic. Can't you just adjust it down?
If the idle keeps creeping upwards, there is still a problem somewhere. If it creeps in both channels, this would almost certainlyhave something to do with what you've put in.
Bias resistance - brain fart!
"this would almost certainly have something to do with what you've put in"
Each new component installed improved the situation, so I can't see blaming them for that.
For example, the new resistors got the bias voltage dead stable (on the DBT). On my last post, I mentioned that the bias voltage was creeping (on the DBT), and all the voltage readings at each replaced resistor absolutely equal between the L&R channels.
After installation of each new component, the measurements were done to confirm that the system was OK - but only via the DBT.
Only once I had everything OK on the DBT, did I connect directly to Mains AC.
That's how I always do repairs, so that I don't blow fuses due to stupidity - which happens sometimes.
I can adjust the Bias voltage - no problem - I have it adjusted now to 13mV, via the mains AC, but it still creeps very slowly upwards.
When I switched it on Mains AC (this time, the relay didn't click), I quickly adjusted both trimmers from 17 to 11mV, and it crept upwards from there, equally on both channels, but is now waving around 13mV - still not stable enough for me.
Do you still recommend replacing all the TIP Darlingtons at this stage?
If an idle current drift fault is present in both channels, the problem will have to be something common.In this case it cannot be the power supply (unless the rectifier is acting up) and it's rare to see the same faultin both channels of the same Beomaster unless it has something to do with something that was done to both channels - likereplacing certain components in pairs.I take it, you didn't use metal oxide resistors?C-range transistors?Schottky barrier diodes...?Always adjust idle when cold (livingroom temp), check and readjust after 5min with power on (no load/input).(It will almost always have crept upwards a couple of mVs, as much as 5-6mV is not uncommon - and no cause for panic).If the idle still drifts upwards after the 5 minutes, check the condition of the cooling paste.Transistors hFe depends heavily on their temperature (higher temp= higher hFe unfortunately).With bad thermal contact to the heatsink, they can heat up seperately, even if only ever so slightly (and without the constantcurrent generator sensing transistor feeling it).
If it merely hunts a mV up and down I wouldn't worry too much.It's rare to see a completely steady idle.Slight variations in the mains alone is enough to cause small deviations.
The channel that had the shorted TIP - yes, I would replace the other TIPs in that channel. But not with the idle fault in mind.The other one, no reason.
I never use metal oxide resistors - they have high residual internal noise, so they are not suitable for audio.
Of course, being the cheapest resistors, they are found in all kinds of audio equipment, their high noise levels notwithstanding.
Here, because the original precision carbon resistors are all but unavailable, I have used Vishay Dale CMF Metal-Film 0.5W (where the original spec was 1/8W) and Vishay Philips PR02 Metal Film 2W Power resistors, where the original spec was 1/2W.
Metal Film's have a stricter derating factor than carbon, so I am extra-generous with the rating factor.
The Metal films which I installed, are all running quite cool - nothing more than room temp.
These are my standard resistor choices, in other similar projects, with no problems.
If you feel that a specific resistor (out of those which I listed previously), should absolutely be carbon, I'll source it and replace the MF which I installed.
All transistors are B range, exactly as per the Service Manual - I learnt that about 10 years back on my BM3400 ;-)
The only non-SM resistor is the MPSA55 instead of the original MPSH54's.
No Schottky's - I don't do any mods, until a machine is working perfectly, in its original configuration.
The only point of interest is that R119 219 are slightly hot. They were replaced, but before they were replaced, they were EXTREMELY hot to touch.
Now they are slightly hotter than warm, but they are still the hottest of all the TO-92 transistors, which are not hot at all.
So the next stage is to definitely replace all the cooling paste, and the TIPs in the same channel.
OK, progress.
A complete overhaul of the output amps, and disassembly and cleaning of the relay - no more relay problems.
I started the Calibration procedures - everything fine until the calibration of 2L1 and 2L2, which are connected to 2IC1 TCA420A pins 5 and 6.
No matter how much I adjust the new R24 Poti (to achieve 0mV differential between pins 5 & 6), the voltage doesn't budge.
Pin 5 11V, and Pin 6 8.5V
Also, as soon as I connect my battery Voltmeter to Pin 6, an audible buzz is heard.
I would have suspected the TCA420A immediately, but subsequent checks make me suspect the 2IC2 TCA750 also.
2TR2 FET gate measures 11V, instead of 0.1V. I replaced the FET with a J310 - perfectly acceptable substitute - but same readings.
Also another issue which makes me suspect the TCA750, is this:
Without AFC, the radio receives fine (albeit distorted, because I cannot solve the Pin 5 and 6 issue above), but when I switch in AFC, the reception becomes dead silent across the band.
The TCA750 AFC input is Pins 8 and 9, exactly the 2 rails above, which I cannot zero between them.
I also cut the traces (the red lines) between PCB2 and PCB3, to see if that would induce a change, but not.
Any thoughts much welcome.
If you get the MPSH54 !!!!!!!!!
I have not yet found a supplier - apparently the NTE159 is an equivalent transistor ?!!
Should you know a source pease let me know, as I am too in the middle of bringing back my BM4400 from the dead with what looks very much like power-amp fault(s)
Cheers - ALF
The NTE159 is an equivalent for hundreds of transistors. Not all are suitable for this position.
In Canada, there is NettyElectronics - he has lots of NOS parts.
I bought the MPSH55 from him - same transistor with a higher voltage rating.
NTE produce no transistors themselves.They buy in overstock from others and rebadge it.That's not necessarily bad in itself, it's just a hint that you cannot be sure toget the exact same thing twice even if it has the same NTE designation etc.As Menahem mentions, NTE's cross-reference guide is also very "large" on replacement parts, so what would've been thousands andthousands of different spec'ed types from other producers are referenced down to not a lot morethan about a hundred NTE types.Whatever they buy in, it just gets rebadged with the nearest designation in their own numbering system.(And sold at a solid overprice if you ask me).If your circuit is not critical and will accept "any and all" within a certain range, then NTEs willbe fine but really only if they happen to be the only choice in the drawer for the given task.Everything else would be a tighter spec'ed solution - and usually cheaper.And often less noisy.What makes NTE transistors go noisy, I cannot say.The handling related to the rebadging, maybe.Or perhaps it's related to just one of the original brands.Difficult to say without knowing exactly what you sit with.
Get the real thing if possible.
thanks Menahem,
that is the source where I bought a lot of MPSA55 :-))
also found that Little Diode had some on offer, but quite pricey.
other than yours my project has not yet had a happy ending - still searching for a serious fault !
don't want to see another set of TIP141/146 exploding !
cheers
ALF