I am a novice in the repair of vintage tube and solid state. Regarding the B&K ST 140 it says to bias using a100mV signal RMS @ 20Khz. Looks like I am going to have to buy this test equipment which i don't know what it is. But, with the help of Tektronix I will have answer to buying this test gear. I thought most biasing is done with a DC meter probe across an emitter resistor? Help
Anthony
Anthony
Yes. They want you to warm up the amplifier first. Here is the BK service note.
You should be able to play the amplifier with music for an hour or so and then
quickly check the readings, instead of using a test signal to warm it up.
It's essential to have a scope, generator, and dummy 8R loads if you intend to
continue to work on audio equipment, a minimum of $400-$500 to get started.
The following procedure is for setting the bias and DC in the B&K amplifiers. Read the complete
procedure first. These procedures should be performed by a qualified technician.
1. Plug the amplifier into a variac. Monitor the (-) negative rail of each channel by removing
the fuse and placing a current meter across the fuse clips.
2. Connect an 8 Ω load to each channel. Supply an input signal of 100 mV RMS @ 20 Khz.
3. Slowly turn the variac up to operating voltage while monitoring the rails for excessive
current draw. The meters should not read more than 400 mA DC. If one or both read
more than 400 mA, turn the bias pots (P2) to minimum and retry.
4. Set bias of the amplifier by adjusting the bias pot (P2) mounted on the solder side of the
board to achieve a reading of 200 mA on each channel for models ST 140, ST 140M,
ST 202, and ST 202+. The bias for the Pro 600, EX 442 and M 200 amplifiers is 250 mA.
Setting the bias higher than stated only causes the amplifier to run hotter and burn out
faster. It does nothing for the sound quality of the amplifier.
5. Set the DC out of the amplifier by monitoring across the outputs. Adjust the DC pot (P1)
for 0 VDC ± 5 mVDC.
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Hello Rayma
Thank you for explaining this for me. I actually have the same service note. As an example most Marantz and even Sansui receiver using the same emitter connection and read bias in Mv. Not all but most. My passion and interest starts when I was 11 and a ham radio operator. The electronics theory was taught by RCA of Camden NJ, my home town. I remember my friend showing me an oscilloscope and it seemed easy back then to learn. I am 60 but began very involved in repairs of solid state and tube. I have a basic new scope from Tektronix and hope it will be easy. What made things easier recently was a test fro Peak which has made transistor checking easy.My understanding is to mathe hfe. However, mot output device surprisingly are not shorted but the drivers and pre drivers are at fault. So, what is the name of this test device for injecting this signal. Are there any brands that are basic? Thanks again my friend
Anthony
Thank you for explaining this for me. I actually have the same service note. As an example most Marantz and even Sansui receiver using the same emitter connection and read bias in Mv. Not all but most. My passion and interest starts when I was 11 and a ham radio operator. The electronics theory was taught by RCA of Camden NJ, my home town. I remember my friend showing me an oscilloscope and it seemed easy back then to learn. I am 60 but began very involved in repairs of solid state and tube. I have a basic new scope from Tektronix and hope it will be easy. What made things easier recently was a test fro Peak which has made transistor checking easy.My understanding is to mathe hfe. However, mot output device surprisingly are not shorted but the drivers and pre drivers are at fault. So, what is the name of this test device for injecting this signal. Are there any brands that are basic? Thanks again my friend
Anthony
It's called a function generator, which makes sine, triangle, square waves of adjustable amplitude and frequency.
Not low distortion, but useful enough. Here are a couple of random examples, but there are many more.
I can't really recommend any particular one, though.
MHS-5200A DDS Digital Function Signal Generator -$73.35 Online Shopping| GearBest.com
VICTOR VC2002 Function Signal Generator 5 Digits (0.2 Hz ~ 2 MHz) 7 Frequency | eBay
BTW, please be careful to connect the scope probe ground clips ONLY to ground, not to any other point in the circuit.
You cannot connect the scope probe across the emitter resistor, for example, because neither end of the resistor
is connected to ground. In other words, the probes are single ended, not differential.
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Why do you need the scope ?....is only to verify the input signal of 100 mV RMS @ 20 Khz ?
Is this signal a square or sine wave ?
I have a couple 10ohm 50watt non-inductive ceramic resistors.....so, is 8 ohms essential ?
(I have a Fluke 189 multimeter)
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Just some useful notes regarding updating my B&K st140........(and safety)...
These tips are from my own experience.
I have the st140 version that has a PC board + rectifier that spans between the two large can-type caps.
I re-capped all the electrolytics, including the large cans.
Best practice is to take photos of all the wiring and cap orientation BEFORE removing and replacing anything !!
For the large cans, I chose from Digikey:
PART: 493-8984-ND
MFG : Nichicon / LNT2A223MSE
(22,000uf, 100v)
They fit perfectly, but make sure they come with the proper sized bottom clamp. (my cans came with clamps included).....otherwise, order the correct size clamp.
TIPs for mechanically fitting these new large(er) cans......
* Disconnect power cord.
* Discharge the old cans by waiting for 5 minutes or so after your last power-down, and use a multimeter on DC volts to verify there is less than 1 volt across the cap's contacts. (shock hazard)
* On the PC board, mark the polarity of the old caps with a Sharpie.
* Unscrew old cans and then their old clamps.
* Slip the new clamp onto the new cans, and loosen crimp bolt/screw a bit on these new cans (so they can be twisted to fit).
* Fit the new can's into the overhead PC board with the included contact bolts.
* Spin the clamps to fit into existing holes on the bottom of the chassis (my amp version has several pre-drilled holes, but you may have to drill one or more holes, and vacuum any metal shards away)....making sure you spin the clamps to have access to tighten the clamp crimp !!
* Insert the clamp to the chassis using the old mounting bolts, but leave those screws slightly loose, including the tightening screw on the clamp as mentioned above.
* Once everything is aligned and loosely fit, tighten the can's bolts to the PC board, , then tighten the clamps to the chassis, then tighten the crimp clamp to secure the caps inside the mounting clamp.
* Basically, the reason for all this, is you don't want to create any bending-stress on the overhead PC board as it contains the Rectifier, etc.
I also added a 1uf/100v axial film cap for additional DC smoothing..... wired directly across each large can, with the 1uf cap's wires snuggled around the can's screw terminals (just above the flat washer), making sure the connection is tight and won't wiggle loose over time. Perhaps a better way would have been to solder a circular lug to each end of the 1uf's wires....making a flatter more secure connection. In fact, I'm going to re-do this now.
All re-capping was done with leaving the primary amp circuitboards in place ; I did not remove them. (I'm good at soldering) If you feel it would be easier to remove them, you will need to unscrew the power FETs... which means it would be a VERY GOOD idea to also get new "T-03 sil pads" for the power FETs. Don't reuse the old ones.
The four 100uf/100v electro caps are polarized, so make sure the new ones are correctly installed. There are two per board/channel.
I believe the other original 47uf electro's were NP (non-polarized) types.
However, positive reports have been to change the Input NP electros (one per channel), to a film type....around 10uf or higher, I assume for DC blocking.(these are the 47uf caps nearest to the transformer)
I swapped these out for...
PART: 1928-1631-ND
MFG : WIMA / MKS2B051001N00JSSD
These 10uf films have the same spacing as the old NP electrolytics...BUT you'll need to solder them to the underside of each board. (space issue). For ease, I soldered the lowest pin first, then slid the upper pin into place and soldered that second pin.. They will not fit snug to the board, FIY.....because you need to fit your soldering tip in there somehow.
The other two electro 47uf's in the middle of the board(s) can be either NP or polarized. I chose Silmac polarized types, but be sure to notice that there is a "+" marking on the PC board. Follow that orientation ! (or use NP types)
At that point I fired-up the amp with nothing attached and with the chassis cover applied., and checked the DC Offset at the speaker terminals.
After a 30 minute warm-up...I got readings of...
-14mV one channel
and
+24mV other channel
This is considered acceptable. (under 50mV from zero, either way)
But if I want to adjust the offset closer to 0mV....I have to adjust the trim pot on the boards. (the ones without the resistor across it)
There can be a few issues here......
* The ST140 pots are old and "open-types (not sealed) and probably dirty.
* The pots are "single turn" and can be hard to adjust precisely.
* They are hard to access, and the risk to personal SHOCK is rather high.
Solutions......
*Swap-out these pots for newer 10-turn 1/2 watt "sealed" types, with the adjustment screw facing upwards (not sideways like in the original). (Buy trimmers with "straight in-line" pins, and bend to fit the original triangle formation)
* Flush the old ones with a chemical/solvent/potentiometer cleaner, turning the old pots back and forth several times to self-clean them..
* Make a 45 degree tool out of strong plastic where you can adjust them from above.
* Make a very long plastic adjustment tool that can keep your hands away from the caps, scooted under the rectifier board.
I don't feel safe sticking my fingers in and around the powered circuit.... and neither should you. Always use a plastic adjustment tool for your safety, and safety of the circuit.
I have not tried either of these for DC offset adjustment methods..yet..... but I'm considering them. Suggestions welcomed.
(post note)
I was trying to calibrate a 1960's Gen Rad unit, which had unsealed trim pots. Nothing worked with any accuracy. But by simply turning the internal pot several times back and forth, it was likely that the pot's wiper "self cleaned" the dirt (or oxidization) off the carbon strip inside the pot. Then it worked.
These tips are from my own experience.
I have the st140 version that has a PC board + rectifier that spans between the two large can-type caps.
I re-capped all the electrolytics, including the large cans.
Best practice is to take photos of all the wiring and cap orientation BEFORE removing and replacing anything !!
For the large cans, I chose from Digikey:
PART: 493-8984-ND
MFG : Nichicon / LNT2A223MSE
(22,000uf, 100v)
They fit perfectly, but make sure they come with the proper sized bottom clamp. (my cans came with clamps included).....otherwise, order the correct size clamp.
TIPs for mechanically fitting these new large(er) cans......
* Disconnect power cord.
* Discharge the old cans by waiting for 5 minutes or so after your last power-down, and use a multimeter on DC volts to verify there is less than 1 volt across the cap's contacts. (shock hazard)
* On the PC board, mark the polarity of the old caps with a Sharpie.
* Unscrew old cans and then their old clamps.
* Slip the new clamp onto the new cans, and loosen crimp bolt/screw a bit on these new cans (so they can be twisted to fit).
* Fit the new can's into the overhead PC board with the included contact bolts.
* Spin the clamps to fit into existing holes on the bottom of the chassis (my amp version has several pre-drilled holes, but you may have to drill one or more holes, and vacuum any metal shards away)....making sure you spin the clamps to have access to tighten the clamp crimp !!
* Insert the clamp to the chassis using the old mounting bolts, but leave those screws slightly loose, including the tightening screw on the clamp as mentioned above.
* Once everything is aligned and loosely fit, tighten the can's bolts to the PC board, , then tighten the clamps to the chassis, then tighten the crimp clamp to secure the caps inside the mounting clamp.
* Basically, the reason for all this, is you don't want to create any bending-stress on the overhead PC board as it contains the Rectifier, etc.
I also added a 1uf/100v axial film cap for additional DC smoothing..... wired directly across each large can, with the 1uf cap's wires snuggled around the can's screw terminals (just above the flat washer), making sure the connection is tight and won't wiggle loose over time. Perhaps a better way would have been to solder a circular lug to each end of the 1uf's wires....making a flatter more secure connection. In fact, I'm going to re-do this now.
All re-capping was done with leaving the primary amp circuitboards in place ; I did not remove them. (I'm good at soldering) If you feel it would be easier to remove them, you will need to unscrew the power FETs... which means it would be a VERY GOOD idea to also get new "T-03 sil pads" for the power FETs. Don't reuse the old ones.
The four 100uf/100v electro caps are polarized, so make sure the new ones are correctly installed. There are two per board/channel.
I believe the other original 47uf electro's were NP (non-polarized) types.
However, positive reports have been to change the Input NP electros (one per channel), to a film type....around 10uf or higher, I assume for DC blocking.(these are the 47uf caps nearest to the transformer)
I swapped these out for...
PART: 1928-1631-ND
MFG : WIMA / MKS2B051001N00JSSD
These 10uf films have the same spacing as the old NP electrolytics...BUT you'll need to solder them to the underside of each board. (space issue). For ease, I soldered the lowest pin first, then slid the upper pin into place and soldered that second pin.. They will not fit snug to the board, FIY.....because you need to fit your soldering tip in there somehow.
The other two electro 47uf's in the middle of the board(s) can be either NP or polarized. I chose Silmac polarized types, but be sure to notice that there is a "+" marking on the PC board. Follow that orientation ! (or use NP types)
At that point I fired-up the amp with nothing attached and with the chassis cover applied., and checked the DC Offset at the speaker terminals.
After a 30 minute warm-up...I got readings of...
-14mV one channel
and
+24mV other channel
This is considered acceptable. (under 50mV from zero, either way)
But if I want to adjust the offset closer to 0mV....I have to adjust the trim pot on the boards. (the ones without the resistor across it)
There can be a few issues here......
* The ST140 pots are old and "open-types (not sealed) and probably dirty.
* The pots are "single turn" and can be hard to adjust precisely.
* They are hard to access, and the risk to personal SHOCK is rather high.
Solutions......
*Swap-out these pots for newer 10-turn 1/2 watt "sealed" types, with the adjustment screw facing upwards (not sideways like in the original). (Buy trimmers with "straight in-line" pins, and bend to fit the original triangle formation)
* Flush the old ones with a chemical/solvent/potentiometer cleaner, turning the old pots back and forth several times to self-clean them..
* Make a 45 degree tool out of strong plastic where you can adjust them from above.
* Make a very long plastic adjustment tool that can keep your hands away from the caps, scooted under the rectifier board.
I don't feel safe sticking my fingers in and around the powered circuit.... and neither should you. Always use a plastic adjustment tool for your safety, and safety of the circuit.
I have not tried either of these for DC offset adjustment methods..yet..... but I'm considering them. Suggestions welcomed.
(post note)
I was trying to calibrate a 1960's Gen Rad unit, which had unsealed trim pots. Nothing worked with any accuracy. But by simply turning the internal pot several times back and forth, it was likely that the pot's wiper "self cleaned" the dirt (or oxidization) off the carbon strip inside the pot. Then it worked.
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