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 just finished restoring a BM4400, in great cosmetic shape, but poor electronic shape.
I replaced all the components in both completely dead and shorted Output amp stages. Sometimes I am astounded at how these amps get to be destroyed so thoroughly.
All parts are highest quality Nichicon and Panasonic, and the semi's are all original BC5xx, matched where necessary. New original ST TIP142 and 147, and BC139 in the constant current generator.
Basically a complete rebuild from zero, in the output stages.
All main rail voltages check just fine.
It's working almost just fine, with the Bias adjusted to 13mV (across BOTH Emitter resistors = total 0.36ohm) and stable, on both the Left and Right channels.
When I switch on, there's a momentary buzz, and then a low-level-hum, audible at zero volume (not at listening volume), but definitely there.
I checked the DC voltage at the speaker outputs (junction of R286/R47 and R186/R46), and the Left channel shows 52mV and the Right channel shows 56mV.
The RL1 is not activating to shut off the speakers - probably 50mV is below the activation threshold.
I applied freeze-spray on the 0TR100/200 BC139 Constant current generator to see if that would change anything - no change at all.
I also monitored DC voltage at the speaker outputs during power on, and the highest it showed, right at power on, is 700mV, and then decreasing to ~50mV, where it remains stable.
I am normally accustomed to not hearing this buzz and hum at power on (on any amp), and I don't remember ever seeing a speaker output DC of anything above a couple of mV. So this 50mV has me stumped.
Any ideas?
Menahem
Menahem Yachad: ... all original BC5xx, matched where necessary...
... all original BC5xx, matched where necessary...
Did you also match the pair in the differential amplifier? And what type did you use here?
Martin
Just read it again and realized you have the same offset in both channels.50mV is not the end of the world, but please tell me that you did not fit low-ESR caps in the output stages and as power supply filter caps?And have you checked for oscillation with a good scope?What happens if you ground the inputs to the output stages (TP107/207)? Still hum?
I've done further checking.
The only TP which is not according to the SM, is TP111, 211 - the Base of the Feedback TR113/213 in the Diff Amplifier = 64mV. That's out of spec - 0+/- 50mV.
So, I'm focused on the Diff Amp right now.
In the Diff Amp, I used matched BC550C, which I think may be causing oscillation (The SM's specs for these stated BC237C and BC547C, but the other specs were BC182B). I'll check that tomorrow.
I can't think of a better scope than my Tek 2465B!
I'm planning on replacing the Diff Amp with BC546B, with a lower HFe of 300.
I'll also change C138/238, according to the SM - even though it's new, perhaps the output DC messed it.
I don't use super-low ESR caps in amplifiers anymore. The regular long-life 105°C with a slightly-higher ESR is my standard choice.
If that doesn't work, I'll ground TP107/207, and see what happens.
BC546B would be a good choice for the differential amplifier (or BC549B if you can get them - they are basically the same, only with better noise specs).It's a good handrule to never use C-rated transistors in B&Os output stages. Things get too eager.Normally you don't have to match the two transistors scientifically. In most cases it'll be fine if you can just fit two transistors fromthe same production batch. They are often placed physically close together to experience the same operating conditions, temperature etc. for the same reason.
OK, so here's what I did - for the time being I'm working on the LEFT channel only (in order to determine results, before later duplicating any changes on the RIGHT channel as well)
I removed the Diff Amp 2x BC550C HFe 467/465
I installed 2x BC546B HFe 300/300 - same batch, same DoM.
I removed the C138 47uF, and replaced it with a new one.
TP111 went UP from ~50mV, to 94mV.
I was really disappointed!
Then I removed C132 1uF, which connects the PreAmp to the Output Amp.
No change at TP111.
Then I connected the junction of R153/C232 (C232 is still uninstalled) to PCB ground.
TP111 dropped to 33mV.
I'm sure that you're about to teach me something significant here!
If the offset got worse, try swapping the two transistors and see, if it gets better.
Jacques
chartz:Did you replace the MPSH54 transistors Menahem? I seem to remember I had trouble with those until I could get hold of NOS ones. Offset was too high (but no hum). Why did you put Wimas everywhere?
Why did you put Wimas everywhere?
Yes, I replaced them with brand-new OnSemi MPSA55 from Mouser - I had no option - all the TR's were toast.
I also have 2SA970's in stock - they are listed as a substitute - I'll try with those
The Wima's are not "everywhere" - I use them ONLY in the coupling positions in the signal path, and only 4.7uF and smaller. Much improved signal quality, over an electrolytic.
Dillen: If the offset got worse, try swapping the two transistors and see, if it gets better.
Thanks Martin - I'll try that tomorrow.
But in addition, can you explain the significance of the massive drop in the Offset voltage at the Feedback side (94mV to 33mV), and also Why is there still 33mV, and where should I look to start eliminating that?
The DC offset can come from many places, but it's a relatively small offset so could point to a problem in the differential amplifier circuit.This circuit often carries a DC offset trimmer in larger amplifiers, exactly to counter for hfe mismatches and other component tolerances.
Not sure what to say, really. But I would take a second look at the components you fitted in MPSH54 positions.
MPSA55 and 2SA970 may be quite similar transistors in many ways, but they will not always be good replacements for MPSH54.MPSH54 is not any transistor. It does something that no other transistor will.Many repairs coming in here has something else fitted instead of MPSH54, presumably because the real thing can be difficult to find.I always revert to MPSH54, which often solves many strange issues.I cannot tell you exactly why, but I have a feeling it has something to do with internal capacitances (rarely taken into account when listing equivalents).
It was chosen by B&O for a reason.
NOS MPSH54 can from time to time be found on Ebay - else here:https://www.beoparts-shop.com/product/transistor-mps-h54-motorola/
OK, I understand that you're focusing on the MPSH54 as the solution.
I checked my parts box, and I have 6 Brand-new NOS MPSH55, which is listed on the same MPSH54 original datasheet from Motorola, showing it as a low noise RF transistor. The only difference between the 54 and 55 is shown as max HFe.
These have the gold leads.
I checked the HFe, and it reads 39. The minimum HFe per the datasheet is 30.
I installed it in the left channel, and the Relay went into protection.
I removed it, and reinstalled the MPSA55 (Hfe 160), and the relay did not activate - but still reading 94mV at TP111.
So, perhaps my MPSH55 HFe is too low at 39.
Can you check the HFe's of your MPSH54's (if that is your shop)?
Menahem Yachad: MPSH55...I installed it in the left channel, and the Relay went into protection.
MPSH55...I installed it in the left channel, and the Relay went into protection.
Sure you got the pinout right? They have reverse E/C from most other TO-92.
Pinout correct - double-checked before installation! But thanks.
More progress:
I'm concentrating in the area around the Diff Amp.
I replaced TR112/113 BC546B HFe 300 with 2x BC546C with HFe 477. Down to 69mV.
So HFe is very important here - drop of 25mV with Higher HFe.
This is the best progress so far - I'm not intending to change these again.
Removed TR115 MPSA55, replaced with BC560C HFe 450 – 68mV – not significant
Replaced C140 5pF with 6.8pF – 63mV - drop of 5mV. The measured values are not exact anyway, and expecting drift. I was looking for a favorable change in mV, and that’s what I got.
Checked (very carefully) all the semis for correct readings under power.
Replaced D102/202 Z2.7V – both reading 1.8V, with NOS ITT Z2.7V (reading 2.66V) – I’ll guess that they were both damaged when DC caused the Relay to trip in.
L/R DC Offset now reading (using DBT) -29mV minus – I probably ignored the Minus sign in all my previous readings.
Just to see if there would be an improvement, I removed Diff Amp 2x BC546C, and installed 2x BC546B – No. DC Offset increased to -49mV.
Reinstalled the 2x BC546C again.
Today’s score is (net AC, without the DBT):
TP111 Left = -26.7mV,
TP211 Right = -28.4mV
(now within the SM Spec of 0 +/- 50mV)
Speaker line (junction of 2x 0.18Ω Emitter resistors)
Left = +15.6mV,
Right = +13.8mV.
Bias is dead stable on both L&R at 13mV .
So, in summary, up to now, the components which made the most impact on reducing the high mV DC Offset are:
TR112/113 TR212/213 Differential Amp – Optimum = 2x BC546C (HFe 477), instead of BC546B
D102/202 Z2.7V
I have the MPSH54 on order – when it arrives, I’ll remove the 2x MPSA55’s (TR115/215), and see if it makes any difference – I’m very curious to see if it does.
Ha - you made me laugh!
I can just picture you reading my post, and then saying to yourself "Hm, I should get up and measure my BM4400, just to make sure!"
I saw a note somewhere about replacing R176/276 180Ω with a 220Ω. Is that something that any of you know about?
I'm certainly going to give it my best shot at getting it under 10mV.
I've invested a fortune of effort in this so far, I'd like to do the job as right as possible.
Menahem Yachad: Ha - you made me laugh!
Glad to have made your day then
Believe this or not - I cleaned the BM4400 with compressed air, and the DC offset dropped to 12mV on both channels. It was not that dirty to begin with, but coming to the end of the job, I wanted to improve the look as much as possible.
Today, I'll polish the Rosewood panels.
FYI - Another little trick I employed to protect the output stage semi's during this troubleshooting, is to remove the 4 emitter resistors, and replace them with 1Kohm 5W resistors. That drops the current to a couple of nA. Do NOT readjust the bias trimpots - leave them as they were before changing the resistors. This enables me to change components freely, without fear of blowing the transistors, while tracking the causes of excessive DC offset.
Of course, always use a DBT when poking around like this. It makes a slip of the probe a non-event.
The other change I made to the circuit, was to reverse the mounting position of the right-side resistor of each of the 2 emitter pairs, so that I can measure across both of them easily, from the top of the PCB, and measure each of them individually.
On both channels, I have identical Bias - left emitter 6.4mV, right emitter 6.9mV = 13.3mV total.
Last job will be to install the MPSH54's when they arrive.
I got the MPSH54's today.
First thing is power to the BM4400, to confirm the Baseline measurements.
Speaker Output L/R = +12mVDC
Now, I installed the MPSH54's
Speaker Output L = +2.8mVDC
Speaker Output R = +0.23mVDC
TP111 Left = -35.9mV,
TP211 Right = -37.9mV
I had to readjust the Bias again, after installing the MPSH54's.
So, the MPSH54's are the icing on the cake.
In summary (after the full restoration), these were the troubleshooting steps which provided the most significant gains:
1. Replace Diff Amp TR112/113 and TR212/213 with 2x matched BC546C (not BC546B) - the SM specifies the C-grade, if the BC54x is used.
2. Replace D102/202 Z2.7V – both reading 1.8V, with NOS ITT Z2.7V (reading 2.66V)
3. Replaced C140 5pF with 6.8pF
4. Replaced TR115/215 with MPSA55 first - that brought the 0mV rail down to 12mVDC, and afterwards the MPSH54 brought it down to 0.2mVDC.
My conclusions - the MPSH54 is ideal (I also don't know why), but if you can't get them, then the MPSA55 will also do an excellent job, to within 10-12mV.
Restoration complete!
Good. Now you know – that you don’t know