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Beomaster 1500 - DC offset problem

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krais
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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!

 

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manfy
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manfy replied on Sat, Jun 19 2021 10:50 AM

Thumbs Up (this thumbsup is in reference to Martin's post!)

Particularly C111/C211 seem critical!
I just simulated different values and ESR from 0 to 3500ohms for all those caps. C117 and C121 showed little change in the signal level (I suppose because of the amplifier feedback to the filter network, which "balances itself" and adjusts the gain level accordingly).
But minor changes in capacitance or ESR of C111 showed a direct impact on the output signal level!

PS: Just a disclaimer: My spice simulations are just simulations and as such they are based on an "ideal world". Real world conditions may be quite different because of parasitic C/R/L values present in every circuit and component and transient effects in a real world environment!

krais
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krais replied on Sat, Jun 19 2021 7:09 PM

This is awesome, thanks so much!

 

manfy:
So, I'd say we forget about fine-tuning the caps for now and look at the output transistors TR101/201.

I already tried replacing TR101/201 with matched transistors but that did not make a difference.  

 

manfy:
-) 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.

Yes, I'll do some DC measurements tomorrow and report back.

 

manfy:
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!
  

I used a XJW01 LCR meter at 1KHz (not professional by any means but accuracy should be decent). ESR measures around 50 ohm for C112/114/212/214 and 8ohm for C115/116/215/216.  

krais
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krais replied on Sat, Jun 19 2021 7:13 PM

Dillen:
Are C117/C217 new?
If not, I suggest you replace them. Ideally also C111/C211/C121/C221.

 

Thanks Martin. Yes, C110/210, C111/211, C117/217, C120/220, C121/221, C122/222 are all new.  

krais
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krais replied on Sat, Jun 19 2021 7:14 PM

Dillen:
Are C117/C217 new?
If not, I suggest you replace them. Ideally also C111/C211/C121/C221.

 

Thanks Martin. Yes, C110/210, C111/211, C117/217, C120/220, C121/221, C122/222 are all new.  

manfy
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manfy replied on Sun, Jun 20 2021 2:54 PM

krais:

Thanks Martin. Yes, C110/210, C111/211, C117/217, C120/220, C121/221, C122/222 are all new.  

Yes, but since you have a real LCR meter, you really should check C111 and C211 carefully. E-caps usually have a +/-10% or +/-20% tolerance, and that includes brandnew ones. At least part of your channel imbalance could originate from these 2 caps.

krais:

I used a XJW01 LCR meter at 1KHz (not professional by any means but accuracy should be decent).

Not too shabby! That seems to be a real LCR meter that measures with a sine wave signal and not one of those $10.- testers that uses tricks & obscure firmware to guess the component and value.
Looking at the specs that go around on the net, this tool might actually use the dedicated LCR chipset ES51919/51920 by Cyrustek. If so, the basic capabilities of the meter is good! I have an Appa703 and a DerEE DE5000. Both use that chip and both devices have a good reputation.

The only real problem with those low-cost no-name devices from China is that you don't know the quality standards the maker applied. You rarely get a calibration certificate. It's a bit of hit and miss. If you're lucky, you might get a meter with same quality as a $400 branded meter.

 

krais
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krais replied on Sun, Jun 20 2021 9:40 PM

The C111/C211 electrolytic caps measure as follows: 

C111: 2.03 uF, 6.98 ohm ESR @ 1KHz

C211: 1.94 uF, 8.27 ohm ESR @ 1KHz

 

Thanks for the info on the XJW01. I considered a DE5000 but that was a little beyond my budget (with the likely addition of import charges).

 

krais
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Found the problem...

 

Very much ashamed to admit it, but my previous statement that the bass/treble potentiometers measured ok turned out to be incorrect. The problem is the bass potentiometer: the resistive strip does not make contact with the right terminal (see arrow). When I did my original measurement I must have only measured resistance between the left terminal and the wiper (between R122 and R125 on the preamp board), not between the other terminals. As resistance was linear throughout the range, I concluded that there was no oxidation and the potentiometer could not be the cause. Such a stupid mistake...

 

Am I right in assuming that it wouldn't be feasible to repair the potentiometer as it cannot be soldered?


Dillen
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Dillen replied on Tue, Jun 22 2021 9:29 PM

Clean it as good as you can.
Secure it using a small amount of strong 2-comp. glue and when fully set overpaint the joint using conductive paint/glue.
Alternatively, replace the potentiometer.

Martin

krais
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krais replied on Fri, Jun 25 2021 12:00 PM

Thanks Martin! I'll take a stab at repairing the potentiometer using this method.

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