Sign in   |  Join   |  Help

Beomaster 1500 - DC offset problem

rated by 0 users
Not Answered This post has 0 verified answers | 23 Replies | 0 Followers

krais
Top 500 Contributor
The Netherlands
134 Posts
OFFLINE
Bronze Member
krais posted on Mon, May 31 2021 8:34 PM

Hi folks, I'm currently restoring a Beomaster 1500 and ran into a mysterious DC offset problem. 

Originally DC offset measured a little high (around 15mV on both channels), so I decided to replace transistors TR104/105 and TR204/TR205 with matched BC548B BC547B pairs using Ian Fritz's transistor matching circuits (I've done the same on a Beomaster 4400 and was able to reduce DC offset successfully). 

However, after replacing the transistors DC offset now measures 25-30mV on both channels... 

I've checked voltages on PC6 but they seem ok and found no loose connections or cold solder joints. 

The service manual lists BC547B transistors for TR104/204 and BC182C transistors for TR105/205. However the transistors I desoldered were all BC547C transistors. Could there be an issue replacing these with BC547B transistors? Or could something else explain higher than expected DC offset?

Thanks!

 

All Replies

manfy
Top 500 Contributor
135 Posts
OFFLINE
Bronze Member
manfy replied on Tue, Jun 1 2021 8:43 AM

Is that a trick question? Crying Devil

If you pick just any transistor with arbitrary gain values and voltages, one would surely expect that the output is different than with original transistors, don't you think? Do you think that the B&O guys picked the BC547B and the BC182C for this opamp TR104/105 just to annoy you? (see footnote)

I would guess they did it on purpose and for some technical reason. Either they actually calculated the amplifier design or - more likely - they used a standard design and adjusted it to their needs.For today's standards, this is a run of the mill class AB amplifier with feedback to the driver stage. 
The base of TR105 is the non-inverting input of this opamp and carries the feedback from the final output signal. The base of TR104 is the inverting input and it carries the audio input signal. Changing hfe of those transistors will surely have an effect on the characteristics of the amplifier. If you must replace them, then use equivalents with similar characteristics.

Using 30V transistors instead of the original 50V ones is not a good idea. It may work if all other components are ok, but if TR108 shorts for whatever reason this will also send TR104 and 105 to their graves. B&O chose 50V versions; so, use the same or bigger.

------------------------

PS: Sorry for my initial sarcasm. I just find it a bit strange that you take the effort to hand-match modern transistors, but then just replace the original models with some other type without considering their function in the circuit.
The general rule in electronics is: If you need to replace obsolete transistors, you pick models that are as close as possible to the original. This way you have a high chance of success without having to analyze the whole circuit in detail. 
If you're good in electronics design, you can change the original transistor specs quite a bit based on what's going on in the circuit. It does leave you though with the risk that you're overlooking something that the original designers had already identified as a problem and fixed in their commercial product.

krais
Top 500 Contributor
The Netherlands
134 Posts
OFFLINE
Bronze Member
krais replied on Tue, Jun 1 2021 10:31 AM

Oops typo, sorry for the confusion. The replacement transistors I used are BC547B, not BC548B. 

 

The originals were all BC547C even though the service manual lists BC547B for 6TR104/204 and BC182C for 6TR105/205.  

 

manfy
Top 500 Contributor
135 Posts
OFFLINE
Bronze Member
manfy replied on Tue, Jun 1 2021 10:52 AM

In the Fairchild datasheet I see following:

BC547B: hfe = 200-450
BC547C: hfe = 420-800

So yes, I'd consider that a substantial difference.
I didn't find any datasheet on BC182C, but BC182 has 125-500 gain. That is also quite different from 420-800.
You said that the transistors installed initially were all 547C. Did they seem factory-assembled or were they changed by some repair guy?
If they were factory-assembled, I'd stick with the same models. 15mV instead of 10 doesn't seem critical. That makes some 38mA bias current instead of 25mA. Your amp will run a bit hotter and a bit less "energy efficient" than the speced version, but it shouldn't be a problem for a 70W transistor (IC100). 

krais
Top 500 Contributor
The Netherlands
134 Posts
OFFLINE
Bronze Member
krais replied on Tue, Jun 1 2021 11:38 AM

hfe for the BC547B's measured around 400, so I figured that might be ok (given the service manual listed a BC547B as well). But yeah, I can definitely swap them for BC547C's or just put back the originals.

It does not seem the BM1500 was worked on before, so the original transistors seem to be factory installed.

I guess the broader question I have is: what are the main contributing factors to DC offset (forgive my limited knowledge of AB amplifier circuit fundamentals)? My initial assumption was that dc offset is mainly dependant on the matching of transistor pairs 6TR104/105, 6TR204/205 and thus a relatively large DC offset while using matched transistors could point to an issue. But reading through some more threads it seems that DC offset can also be introduced by the power transistors? If the latter, would it make sense to experiment with some transistor pairs with different hfe matching values in order to reduce DC offset to below ~10mV?

 

Dillen
Top 10 Contributor
Copenhagen / Denmark
12,988 Posts
OFFLINE
Founder
Moderator
Dillen replied on Tue, Jun 1 2021 12:37 PM

25-30mV DC on the outputs of this amplifier is not a problem. I wouldn't worry about it.
Any (serious) imbalance in the differential amplifier would have a much larger effect (from 10ths of volts to several volts) on the outputs.

Martin

manfy
Top 500 Contributor
135 Posts
OFFLINE
Bronze Member
manfy replied on Tue, Jun 1 2021 12:39 PM

Oh goodness! I just realized that you talked about the DC offset on the speaker terminal, but I've been thinking of the 10mV setting of the idle current all along! Crying So, sorry for that! I deleted my reply to avoid further confusion.

In any case, I still agree with Martin.

Granted, 0V at no input signal is the ideal value for a distortion-free output signal, but a minor DC offset of 25mV is quite certainly not audible.
In the service manual it shows a value of 0V +/-200mV at the speaker output. So that's what B&O considers "within spec".
They did not add any DC offset adjustment potentiometer like they did in the BM8000, which means they don't see a minor offset as problematic.

krais
Top 500 Contributor
The Netherlands
134 Posts
OFFLINE
Bronze Member

No worries Manfy and thanks to you and Martin for the quick help.

 

I did run into another problem with the BM1500 though...

When testing the output amps I noticed that the amplitude of the left channel is around 25% higher than the right channel. First I suspected a problem in one of the power amp circuits but after a number of measurements it seems that the problem is introduced in the preamp stage, in the treble/bass circuit to be precise.

In the pic below measurements I did with a multimeter in AC mode at various points in the preamp stages (after injecting a 1KHz signal from an audio signal generator through Tape 1, volume halfway).   


krais
Top 500 Contributor
The Netherlands
134 Posts
OFFLINE
Bronze Member
krais replied on Wed, Jun 16 2021 8:21 PM

I've checked the treble and bass potentiometers for corrosion but they seem fine.

The electrolytic caps in the preamp circuit already have been replaced and I could not find any obvious faults when measuring the other components in that area (in circuit). No obvious cold solder joints as far as I could see.

Any ideas on next steps? Thanks!

manfy
Top 500 Contributor
135 Posts
OFFLINE
Bronze Member
manfy replied on Thu, Jun 17 2021 8:15 AM

Is that channel imbalance audible in the speakers?
How far do you have to push the balance slider to get a balanced signal on both channels (according to the multimeter)?

krais:

I've checked the treble and bass potentiometers for corrosion but they seem fine.

How did you check that? Oxidation is not really easy to see with the naked eye. Did you try any sort of cleaning?

 

krais
Top 500 Contributor
The Netherlands
134 Posts
OFFLINE
Bronze Member
krais replied on Thu, Jun 17 2021 6:44 PM

manfy:
Is that channel imbalance audible in the speakers?
How far do you have to push the balance slider to get a balanced signal on both channels (according to the multimeter)?

The imbalance is not huge but definitely audible (stereo image shifted to the left). The balance slider needs to be set to +0.5 to compensate.

 

manfy:
How did you check that? Oxidation is not really easy to see with the naked eye. Did you try any sort of cleaning?

I measured resistance with a multimeter in circuit for both channels. Resistance ranges were lineair (no peaks or dips) when moving the sliders and I noticed no significant difference between channels. 

manfy
Top 500 Contributor
135 Posts
OFFLINE
Bronze Member
manfy replied on Thu, Jun 17 2021 10:16 PM

Yeah, 0.5 on the balance slider sounds a bit too high.

Check that the caps in the tone control filters are balanced. They have a surprisingly high impact on the voltage level. Preferrably check and match them out of circuit. C112 and C114 should be matched to each other and to C212, C214. Same procedure with C115, C116 and C215, C216.

Post the actual values of those caps and I'll run them through a Spice simulator.

krais
Top 500 Contributor
The Netherlands
134 Posts
OFFLINE
Bronze Member
krais replied on Fri, Jun 18 2021 10:03 PM

Thanks! Here are the values:

 

C112 9.94 nF

C114 10.17 nF 

C212 10.01 nF

C214 10.27 nF

 

C115 68.73 nF

C116 67.98 nF

C215 70.65 nF

C216 68.58 nF 

manfy
Top 500 Contributor
135 Posts
OFFLINE
Bronze Member
manfy replied on Sat, Jun 19 2021 9:40 AM

Thanks!
So, I connected a 1kHz sine signal to the input, at 0dBV = 1.0Vrms, 300ohms and measured rms values at following points:
1) Signal-In = output signal generator
2) Filter-Out = cathode of C117
3) Signal-Out = cathode of C121

all voltages in mVrms in order 1/2/3
- ideal cap values as per schematics: 994m/4.89m/952m
- actal values L channel: 994m/4.87m/947m
- actual values R channel: 994m/4.88m/948m

For the fun of it, I changed C112 to 5nF and C114 to 20nF:
- 5n/20n highpass: 994m/3.67m/664mV

As you can see this change lowered to output by more than 25%, which constitutes the proof of concept that these filter caps have a significant impact on the signal level. The change of caps will of course change the cut-off frequencies, hence the overall characteristics of the tone control, which may or may not be desirable.

So, I'd say we forget about fine-tuning the caps for now and look at the output transistors TR101/201.
Since the output signal is fed back to the filter network, this transistor will have an impact on the level of the output signal, thus different hfe in left and right channel will result in imbalance.
Solder back your tone filter caps and check the hfe of both transistors. Since I don't trust most of those "component guessers" and their guessed values, just write those values down for reference and swap TR101 with 201 and viceversa. Now make the very same measurement as you did in post #?? (above from June 16th) and compare with the results from that day:

-) if the R channel shows higher output levels now, one of your transistors is the culprit
-) if the problem remains unchanged, the imbalance stems from some passive components in the tone control circuit. In this case I'd start measuring the DC working points around the filter and TR101/201 amplifier stage.

[edit] PS: Just out of curiosity, what multimeter or cap-meter did you use to measure the caps? They seem oddly consistent. If you used a professional cap-meter, the measuring frequency and ESR values would be interesting and could be enlightening!

Dillen
Top 10 Contributor
Copenhagen / Denmark
12,988 Posts
OFFLINE
Founder
Moderator
Dillen replied on Sat, Jun 19 2021 10:00 AM

Are C117/C217 new?
If not, I suggest you replace them. Ideally also C111/C211/C121/C221.
If they are tantals originally, I suggest fitting tantals again.

Martin

Page 1 of 2 (24 items) 1 2 Next > | RSS
Beoworld Security Certificate

SSL