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
I second sonavor's advice.
It's not really normal for contacts to slip out of their housing. It happened to me several times but only because I had taken those contacts out intentionally and later was careless when putting them back.In any case, since it happened to you twice, carefully check every single connector. Loose contacts can make you go in circles when trying to narrow down the source of a problem.
Once the connectors are confirmed good and in their right socket, check for potential short circuits (particularly in the path of preamp, tone control, output amp). You can get a good idea of whether the resistance values on supply points are good by comparing left to right channel. It's unlikely that both channels show the very same fault.
If all that looks good, use the dim bulb tester and start with power-on tests.You might be lucky and all is good now. The fact that the inrush current limiter relay did not engage is a good explanation for the smoke, but of course it's not a guarantee that this was the only problem.
Good luck with your troubleshooting!.
I double checked all connectors - no more loose pins. I did find that P84 on the uProcessor board was not plugged in (probably slipped out when fiddling with the cables). Probably not important as this cable connects to the FM board.
The P37 ground pin on the left output amplifier might have been disconnected as well (not 100% sure as this may have come off when checking resistances on that board).
Resistance checks on the signal and power inputs of the preamp, tone control, output amp boards did not show any short circuits. The only issue I found that resistance for the left and right signal lines to the tone control board (P27-1/4) were not consistent. The cause for this was a dead tantalum cap C128 in the volume control circuit.
PS: Also checked PC4 for burn marks just in case, but could not find anything obvious.
Hi there
I´ve a question to the BM 800 in generally:
Did someone measured the Standby and On Powerconsumption?
Mine is taking about 10 to 15 Watt in Stby and about 70 Watt in On State.
Is this OK or not?
Thanks for reply.
Berst Regards
Christian
Well, I don't have actual measurements, but your values look ok.
The manual specifies a power consumption of 20-700W, so that matches your standby consumption of 10-15W. This value seems reasonable considering that the standby transformer is always powered on when the unit is plugged in.70W as idle consumption (i.e. unit is powered on but without input signal and volume set to minimum) is fairly low for a class G amplifier and it shows that the designers did pay attention during the design process.
Let's see what values krais measures on his BM8000.
rgds, manfy.
Following up on Sonavor's advice on using a variac for the startup test... I spotted a used high quality combined variac / isolation transformer (Germany, 1980s) on Ebay and managed the win the auction with a very reasonable bid.
Newbie question on the variac startup procedure: I understand that the 7RL1 and 7RL2 relays must be bridged before the voltage on the variac can be slowly increased while watching for any excessive current draw. But at what point should I turn on the amplifier (microcomputer)? I assume the uprocessor will not turn on if the supply voltage is considerably lower than 5V but needs to be switched on at some point to control the 15V line going to the output amplifiers.
Good question!I don't have much experience with using a variac, but based on the circuit diagram and based on theory alone I'd say it doesn't matter.
Once you jumper 7RL1/RL2 the output amp is connected to the variac as power source and if there is a shortcircuit after the main transformer, a high current will flow once you increase the voltage. The uP now only selects the input source - and what I remember from last time, the uP doesn't do any other power switching. (But I'm not totally sure about that!)
Better wait for a binding answer from Sonavor!
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[edit:] I just checked the service manual and you're right. The slave processor 9IC4 pin 14 does also switch on the +/-15V, which is the supply for tuner, pre-amp, volume control and tone control.But even if there is a short circuit or a problem on one of those boards, it should not affect the power drawn from the main transformer. The 15V regulators are normally short circuit safe, i.e. the 3-pin regulator will decrease the voltage and limit the current.
Concerning the processor, I'm not quite sure how it will handle a gradual low-volt startup. It doesn't have an internal hardware watchdog, which means it can be in an undefined state in such a brownout condition. There is some circuitry controlling the reset-pins, but in low volt condition that circuitry might be in an undefined state too.I don't believe that this type of startup will cause any damage to your system, but the actual startup behaviour of the processor might be unpredictable. The chip is specified to need a supply voltage of 5V +/-5%, so it might not start operating while the supply voltage is too far below that.
I hope Sonavor will be back to share his experience on variac startup on uP-controlled units.
Before doing the full power up test, I thought I would do some basic voltage tests on the PSU board with P51 disconnected (connection to the relays that I have not yet bridged). So I hooked up the variac and dim bulb tester, raised the voltage to see if there was any excessive current and measured the 5V, +15V and -15V rails.
Well, there is now an issue with both the +15v and -15V rails (which were fine in my previous tests). The voltages at the C33, C34 capacitors are fine (26.6V, -26.4V respectively). However, the regulated voltages (measured at 6IC5, 6IC6) increase steadily when raising the input voltage but at some point (Variac set at ~120V) the regulated voltages drop all of a sudden to almost nothing (60mV at +15 voltage regulator, -0.4V at -15V regulator).
At first I suspected the voltage regulators but I measured almost no voltage at the input pins of the regulators. So there must be a problem somewhere between the filter capacitors and the voltage regulators... to be continued
krais: ....but at some point (Variac set at ~120V) the regulated voltages drop all of a sudden to almost nothing (60mV at +15 voltage regulator, -0.4V at -15V regulator).
....but at some point (Variac set at ~120V) the regulated voltages drop all of a sudden to almost nothing (60mV at +15 voltage regulator, -0.4V at -15V regulator).
Hmm, I'd say that suggests that at 120VAC the 5V rail has risen enough so that the uProcessor can start up and initialize its outputs, and that's when the 15V rails are being switched off by the uP.When you try to switch on the BM8000 now (via selector buttons), the uP will switch the 15V rails on again. Can you see the standby indicator at 120VAC? If yes, then you can take the standby indicator as a sign for a working uP.
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[edit:] Come to think of it, the same thing should happen with 7RL1 when P51 is connected. The relay would be switched off once the uP is ready (and provided that the rail that feeds the relay has reached a voltage level that can drive the relay). And when you power the BM8000 on via the selector buttons, the uP will switch 7RL1 back on.
Why don't you just connect the multimeter between Variac and BM8000 to measure the input current? Set it to a 10A range and monitor the current while cranking up the variac voltage. If it exceeds normal idle current too much, you can interact and off the unit. Spassmaker talked about 70W in on-state, i.e. about 300mA as normal idle current.
Just make sure that you switch on the BM8000 as soon as the uP is up and ready, or else you're only measuring the standby current.
Thanks! Indeed when I switched on the BM8000 with the selector buttons the +/-15V lines measure ok. I was thrown off because the voltage drop off points are a little different (100VAC for +15V line and 120VAC for -15V line) and the standby indicator comes on only at 180VAC so I did not realize that the uProcessor was active.
manfy:Why don't you just connect the multimeter between Variac and BM8000 to measure the input current?
There is a built in AC meter on the variac, so no need to use a multimeter.
manfy:Just make sure that you switch on the BM8000 as soon as the uP is up and ready, or else you're only measuring the standby current.
I'm not sure that's going to work. Even with the variac at 180VAC the BM8000 switches off immediately when pressing one of the selector buttons. Only when I first increase the variac to 230VAC and then press one of the selector buttons the device stays on. Wondering how others are able to set the no load current trimmers with the variac power on procedure, perhaps power the uProcessor board with an external 5V supply?
Jacques
Thinking this through some more... I could unplug P76 from the uProcessor board so that there's no power going to that board. This way the uProcessor will not activate and therefore cannot turn off the +/-15V lines. With the 7RL1, 7RL2 relays bridged I could then slowly raise the voltage on the variac while monitoring the current draw without even having to switch on the BM8000.
Anything I'm overlooking here?
But P76 is a 10-way plug. What do the other pins do?
Seems like a hassle - and it requires that you can read and understand circuit diagrams very well.Even if you pull only pin P76-7 to disable 15V rail switching, you should be very sure that this pin ONLY performs this function!! Additionally, free-floating pins can cause the strangest problems. When the uP sets a low signal it is switching its output to GND. Some of the electronics connected to that signal might depend on that!
I think I'd just do a two way test:
1) keep P51 pulled. This way you disable the main transformer. Turn the voltage up to 120V or whenever you can power on the BM8000 via selector buttons. Increase voltage to 230V and keep monitoring the current. If that works fine you know that the standby transformer and all rails it supplies are ok.
2) jumper the relays and slowly turn the voltage up to 230V while monitoring the current. Do NOT power on with selector buttons.If that's ok you know that the main transformer and all its rails and connected electronics works.
By adding the mains current of both operations you know the idle current of the unit.If both subsytems work fine individually, there's a high chance they will also work in normal operation. Of course it's not a guarantee, because the electronics on the standby transformer might activate some parts of the output amp that can cause a problem. But that seems farfetched.
If you're excessively worried, you could also replace F2 with 1A rapid-blow for the final startup test with both transformers. If something is really wrong this could limit the current on the secondary side and could avoid serious damage.
Hi, I apologize as I probably wasn't clear in using the variac and dim bulb tester. Especially on the Beomaster 8000. They are useful tools for an initial check to make sure something isn't shorted when first powering on and you thought there was a problem (smoke) coming from the start up circuit. Once you are past that the variac isn't very useful on the BM8000 as the Beomaster will just come up in standby when fully powered from the line voltage (as it should). The variac could be useful in monitoring the current when the Beomaster switches to full on but it sounds like you are also past needing that. You also don't need the dim bulb tester in the circuit now. I agree with Jacques that at this point those devices can confuse what is really going on. I suggest returning to connecting power to the Beomaster 8000 by just plugging it into line power and then measure the power supplies. If activating a source to turn on the Beomaster causes the protection circuit to remove power then start disconnecting power from some of the boards (i.e FM, preamp). If you suspect a problem with the output amplifiers you can pull the rail voltages from them before plugging the Beomaster in (being careful they aren't left where they can touch something conductive). However I remember you checked the output amplifier boards pretty thoroughly earlier.Leave the power supply board connections in place as those are need to check out all of the other voltages. Let's see what the present state of the Beomaster 8000 standby and power on state is - what DC voltages are present or not present.
-sonavor
sonavor:Hi, I apologize as I probably wasn't clear in using the variac and dim bulb tester. Especially on the Beomaster 8000. They are useful tools for an initial check to make sure something isn't shorted when first powering on and you thought there was a problem (smoke) coming from the start up circuit.
No worries Sonavor, an isolation transformer and variac were both already on my wishlist. So the device is a welcome addition to the workshop even if not that useful for the BM8000.
Update: I did a power up test with the relays bridged and found no further issues. The 55V rails measured ok and there was no high current draw.
After that I proceeded to power up the BM8000 without the variac and dim bulb tester. At first glance all was well: the double relays are now working and the uProcessor switches on without issues.
However, I did check the idle current on both output boards and while the iniitial values were as expected (~16mV) the voltage quickly started to drop towards 0V on both boards. This is not something I noticed on my earlier tests so not sure what's going on. I'll double check the wiring and do some more voltage checks this evening.
krais:However, I did check the idle current on both output boards and while the iniitial values were as expected (~16mV) the voltage quickly started to drop towards 0V on both boards. This is not something I noticed on my earlier tests so not sure what's going on. I'll double check the wiring and do some more voltage checks this evening.
It's unlikely that both output amps suffer from the very same fault, so I'd guess that this rapid decrease in idle current on both amps at the same time is caused by some controlling circuitry that acts on both amps. As per block diagram there's only the fault switch and the clipping circuitry that's doing that.If the supply voltages to the amps are ok, I'd focus my troubleshooting on the fault switch and the clipping circuits next.
Looks like you were right, Manfy.
Yesterday I started checking the base voltages and the 5V/15V/-15V lines all measured ok. The +/-55V lines are being cut off immediately when switching on. I measure the relay control voltage on P51 and indeed that voltage drops when powering on, which causes the relays to switch off.
Next I did some voltage checks on the fault switch circuit on PC6. Values in yellow are with P51 detached (relays not activated) but uProcessor switched on, values in green are with relays activated and disengaged. Definitely something that is triggering the fault switch...
So I figured there were 2 possibilities. Either #1 there is an issue on one of the output boards that is sending a trigger signal to the fault switch circuit or #2 something is wrong with the fault switch circuit itself. To eliminate #1, I began testing with one of the output amplifier boards disconnected. Starting with the left board disconnected, the Beomaster powered on and the relays did not disengage anymore.
So it looks like there is an issue with PC5L... Will do some more troubleshooting this weekend.
Excellent! Good progress!
So, there's definitely a problem in the left output amp. The fault switch on PCB6 works as it should.Now I see 2 potential sources of the problem:
#1) The output darlingtons really overheat, which causes the PTC R251 to rapidly increase its resistance and activate the fault switch correctly. You can check that by touching the output transistors near the thermistor R251. If they are hot then R251 is doing its job correctly and you have to continue troubleshooting in the output transistor path.
#2) If the transistors are cold, then there is something pulling the voltage at the loudspeaker output towards a negative DC value and that's what's sending the fault switch signal (between R253 and R254) to the 12.5V you're measuring. If so, you should look at the offset circuitry (TR201/202/203).
Update on PC5L troubleshooting: I removed the board from the chassis and hooked it up to external power supplies to do some voltage checks. Immediately, I noticed an issue when powering up the board. The idle current voltage was at -10mV without the 15V rails being applied and the -55V rails was drawing unusually high current (~70mA).
After that I spend hours checking pretty much every component on the board as well as traces and solder connections around the offset circuit but I could not find anything wrong. Finally I noticed this...
A trace connecting ground pin P37 with the ground plane had been completely burnt up! That explains the high DC offset leading to the fault switch circuit kicking in. After restoring the board by adding a jumper wire I completed the idle current and DC offset procedures and the output amp looks in good condition again.
However, you have to wonder what could have caused such a wide trace to burn up. In fact, it seems likely to me that this was the cause for the smoke appearing when testing the BM8000 a few weeks ago (instead of the resistors on PC7 overheating). I'm really struggling to understand what could have caused such high current going through that trace.
I am kind of surprised none of the transistors gave out. It's hard to say what happened considering you ended up also finding some bad connector wiring after the smoke incident. Nice to be able to check out the output amplifier assembly outside the Beomaster. When you re-install the left channel output amplifier carefully check the heatsink mounts and clearances on the power transistors with the BM8000 metal frame. Check the right channel again too while you are there.
Yeah, I forgot to mention that heatsink mount on TR206 was almost touching the leads. The heatsink mount must have moved when mounting the heatsink to the chassis. Very well possible that at some point the leads were shorted.
Another thought: when the smoke appeared I was testing an audio input signal with headphones plugged into the BM8000. Could this have something to do with it perhaps? I noticed on diagram 2 that the headphone signal ground is connected to pin 37 on PC5 (just next to the burnt trace).
I would recheck all of your ground connections to make sure you having missed any. Are you headphones still working? They aren't shorted to ground? The headphones get the audio signal the speakers would get except the headphone path goes through a voltage divider. That cuts the signal about in half. The ground for the headphones and speakers should be the same ground. It gets confusing because in the board and chassis connections involving the output amplifiers ground, signal ground and chassis ground all tie together. Go through that part of the schematic and verify the grounding connections at the points shown. Make some measurements with the headphones plugged in (and the Beomaster not plugged in).
Hey guys, finally another update on the BM8000 project. I have been quite busy the last 2 months and needed to finish a few side projects so did not have a lot of time to work on it.
So let's continue where I left off, which is putting everything together including the repaired left output board and doing some basic tests. Immediately I found another issue: the idle current on the left channel was rising much more quickly than on the right channel (ramping quickly from 12mV to 20mV within a couple of mintues). Yet another issue with the left power amplifier board it seemed...
I removed the output boards from the Beomaster for the 3rd time ( ) to investigate. When testing the idle current on the individual board with external power supplies, I noticed that there was an initial voltage of about 0.8V across the 0.18ohm emitter resistors (without the 15V rails being applied) on the left board vs 0.0V on the right board. I suspected one of the emitter resistors (when measuring I noticed that resistance seemed to drift) and decided to replace these. This solved the issue of the initial voltage on the left channel.
So... after assembling the Beomaster again I fine-tuned both the idle current and DC offset trimmers. Good news: the problem on the left channel has indeed disappeared and the idle current on both channels is now quite stable. I adjusted the idle current trimmers so that the initial voltages across the emitter resistors is 16mV and stabilize after warming up for an hour around 21mV.
Another improvement: replacing these old speaker switches (one of which was faulty) with modern encapsulated switches.
I took the base PCBs from the old switches and used a rotary tool to create new slots to fit the new switches. The switches have been secured to the base plates with some blobs of CA glue and baking powder.
Next: it was finally time to test whether there's an audio signal on the speaker outputs. I hooked up a 0.5V sine wave from an audio signal generator to TP1, connected 8ohm dummy loads to the speaker terminals and slowly increased the volume while measuring AC voltage on the speaker outputs with a multimeter.
Good news: the Beomaster did not blow up this time
More good news: I measured an AC voltage that was increasing while adjusting the volume on both speaker outputs.
The bad news: the voltage measured was in the range of 0-20mV.
I plugged in my headphones and indeed the signal was so small that I could not hear anything. So somewhere in the signal path there is a problem... more troubleshooting to do.
Next thing I did was probing various points in the signal path with an oscilloscope. I soon was able to determine that the problem must be somewhere in the Tone Control board (PC4). At the signal input of this board (P27) I measured a 1.5V signal while at the inputs of the output amplifier boards there is no signal coming in.
Also, I did not measure any signal at TP203 which means that the problem must be somewhere in the volume attenuator stage.
Did some more measurements... the signal is there at R136/236 but nothing at R137/237 (pin 2/6 at IC6). So this seems to indicate an issue with both AD7110 ic's.
However, this raises a number of questions:
Are the binary signals OK?
Martin
Dillen:Are the binary signals OK?
Apologies for the newbie question but how do I measure this? Am I right in assuming these binary signals are continuous dc voltages on D1-D6 so I should measure voltage between pins 3-8 and DGND (pin9) while adjusting volume?
The volume is controlled by a binary signal from the CPU.Binary signals are either high or low and their pattern tells the volume setting.
Thanks, Martin
Yes, I'm measuring high (4.8V) and low (0V) binary signals when adjusting volume.
Are the ICs powered?All connectors plugged in etc.?
Dillen: Are the ICs powered?All connectors plugged in etc.?
Yes, I double checked everything. Also measured all voltages in that area as specified in the service manual and those seem fine.
Power voltages on AD7110 (IC202):
VDD (pin 14): +4.87V
VBB (pin 10): -2.97V
The +/-15V voltages on opamp IC6 are present as well.
Also, while I could not measure any signal with the multimeter on TP203, with the oscilloscope I can see a very small signal (with increasing amplitude when adjusting volume). So the AD7110s seem to be at least partially functioning.
Finally another update on the Beomaster 8000. Over the last couple of weeks I did a lot of troubleshooting on the PC4 board but it looks like the board is ok after all... Here’s the troubleshooting steps I took:
- After eliminating the supply voltages and IC6 opamp as possible causes for the loss of signal I focussed on the AD7110s. I tested the AD7110s outside the BM800 on a breadboard using external power supplies (IC6: +/-12V, VDD: +5V, VBB: -3V) but found no issues (attenuation was working as specified in the datasheet).
- Next up was testing the tone control board outside of the chassis. I hooked up +15V/-15V/5V supply voltages, inserted a 1VRMS 1KHz sine wave and hooked up an oscilloscope. I was able to measure the signal coming into the AD7110s but found no signal at the output of opamp IC6. So I spend hours checking solder joints and components on the volume circuit to no avail. That’s when I finally realized that R115/R235 were pulling S3 high with connector P29/P28 detached, resulting in a 48dB attenuation. After shorting S3 to ground, the signal was back. I can now confirm that the volume circuit is working as expected and was able to measure the signal at the PC4 output connectors.
So... it seems like I just wasted a lot of time troubleshooting a board that was perfectly ok. I guess I underestimated the logarithmic nature of volume attenuation resulting in very small voltage measurements at low volumes. Also, I could not hear any signal over my headphones (which was probably a separate problem) leading me to believe there was a problem upstream.
krais: So... it seems like I just wasted a lot of time troubleshooting a board that was perfectly ok.
So... it seems like I just wasted a lot of time troubleshooting a board that was perfectly ok.
Not a wasted effort as you now know a lot more detail of the inner workings of the filter and tone control board...and thanks for sharing the information.A nice bit of troubleshooting I would say. Good job.
Thanks, Sonavor. You're right, troubleshooting the BM8000 has been a great learning experience so far...
Yesterday, I reassembled the BM8000 and did another round of tests. Using an oscilloscope I tested the speaker output after inserting a 1KHz 23mV(RMS) sine wave at the Tape 1 inputs. At first I could only measure a speaker output signal at the left channel but after poking around a bit both speaker outputs were working. I measured ~2.7 VRMS at the speaker outputs at max volume which is according to specs in the service manual.
Next, I fed TP1 with a 1.5V(RMS) input signal and did some sound tests. Finally: sound coming out of the speakers! Very happy that finally after all this time there is at least something resembling a working Beomaster.
However I did find a number of new problems…
PS. I just realized that I haven't reinstalled the metal shield box on the microprocessor board yet. Thinking this could be the reason for the speaker hum so I'll reinstall the shield to see if that makes a difference.
One more issue...
5. Look at the balance indicator and you'll see that the decimal volume digit is displayed, albeit with very low brightness. So when adjusting volume I can see both the volume indicator and the digit in the balance indicator changing (the left volume digit does not work, probably due to a broken connection). Probably a LED multiplexing issue, hopefully not a problem with one of the PC9 ICs.
Today I reinstalled the metal shield box but that did not make any difference. Still a slight hum from the speakers (audible from about 1m distance).The only progress is that the left volume digit and another led segment started working again. I guess some intermittent contact issue.
Regarding #3: I tried swapping one of the AD7110s with a replacement part that I purchased. No difference, still a clearly audible signal at 0.0 volume.
Interestingly, the low volume signal over the speakers is audible even when the Beomaster is put into standby mode (until the reservoir caps discharge). I guess the output amps remain active in that state and amplify a signal that for some reason is not fully muted,
Is the hum you are hearing the 50Hz line voltage frequency? If so then it sounds like there is a grounding issue somewhere. Check that the shielding grounds on the source input box are all connected. From there check the grounds to the chassis ground points.