MC Preamp no gain when DC Power Applied - Page 1
Forum section: electronics / repair

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 04:29:28 pm
content: With no B+RI & LI = 75mv ppRO & LO = 2v ppWith B+ and no change in signal value from signal generatorRI & LI = 3mv ppRO & LO = 300mv ppHow can a amp circuit create gain with no B+ and then when B+ is applied the gain drops to such low levels?

author: TimFox
author_rank: Super Contributor
timestamp: May 31, 2023, 05:07:38 pm
content: When asking about such an apparently absurd result, it behooves you to post more details about your measurement.1. How do you connect the generator to RI/LI?2. How do you measure the voltage at RI/LI?3. What frequency do you measure at, and is the output at the same frequency (i.e., no important noise or oscillation)?4. How do you measure the voltage at RO/LO? Are your voltage measurements true differential, or does it ground LO?

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 05:53:53 pm
content: When asking aboutsuch an apparently absurd result,it behooves you to post more details about your measurement.1. How do you connect the generator to RI/LI?BNC to RCA to inputs2. How do you measure the voltage at RI/LI?Scope3. What frequency do you measure at, and is the output at the same frequency (i.e., no important noise or oscillation)?1khz4. How do you measure the voltage at RO/LO? Are your voltage measurements true differential, or does it ground LO?scope

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 05:55:52 pm
content: More photos

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 06:03:34 pm
content: With power on

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 06:05:22 pm
content: Input is shown on channel 1 TOPoutput is shown on channel 2 Bottom

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 06:15:47 pm
content: 75mv in and 2v out with no B+

author: TimFox
author_rank: Super Contributor
timestamp: May 31, 2023, 06:17:19 pm
content: Is that a x10 probe measuring the output?The screen-indicated scale on the CRT indicates the voltage with a x1 input, not having scaled with probe connection.Therefore, your output may be 10 times higher than the value shown on the scope.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 06:19:04 pm
content: Set on X1

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 06:21:48 pm
content: The only change taking place is the B+ being turned off and on

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 06:24:04 pm
content: The function of this board is to take the low output voltage of a Moving Coil to the level of a Moving Magnet Cartridge

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 06:30:21 pm
content: Spec sheet

author: TimFox
author_rank: Super Contributor
timestamp: May 31, 2023, 06:42:35 pm
content: Those specs are incomplete. What voltage gain do you expect (apparently flat over audio range, with RIAA in the following MM preamp)?I don't know what is causing your apparent high gain with no DC applied, but I would keep looking for a problem.Without a transformer, you cannot have a voltage gain > 1 with no DC applied.For example, what are you using for a generator, what is its output impedance, what happens when you connect the generator directly to your probe without the circuit?What do you see at the output of the amplifier with no generator, DC on and off?By the way, using a grounded-base input circuit for a low-Z MC cartridge will work, but there is no improvement in noise performance over a grounded-emitter (normal) amplifier.The equivalent input noise voltages of a CB and CE amplifier for a given transistor and operating point are equal.Your circuit is essentially a feedback inverter, with a low-frequency gain determined by R467 and R463 (approximately), and has R467 (180\$\Omega\$) in series with the MC source, degrading its thermal noise.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 07:03:05 pm
content: Those specs are incomplete. What voltage gain do you expect (apparently flat over audio range, with RIAA in the following MM preamp)?MC is 300uV/10ohm, looks to me like the MM input is 3mV/50kI don't know what is causing your apparent high gain with no DC applied, but I would keep looking for a problem.Without a transformer, you cannot have a voltage gain > 1 with no DC applied.It is defying logic at all levelsFor example, what are you using for a generator, what is its output impedance, what happens when you connect the generator directly to your probe without the circuit?During those measurements it was at 2vWhat do you see at the output of the amplifier with no generator, DC on and off?ZeroBy the way, using a grounded-base input circuit for a low-Z MC cartridge will work, but there is no improvement in noise performance over a grounded-emitter (normal) amplifier.The equivalent input noise voltages of a CB and CE amplifier for a given transistor and operating point are equal.Your circuit is essentially a feedback inverter, with a low-frequency gain determined by R467 and R463 (approximately), and has R467 (180Ω) in series with the MC source, degrading its thermal noise.I wish I understood that in greater detail

author: TimFox
author_rank: Super Contributor
timestamp: May 31, 2023, 07:14:42 pm
content: The logical answer to the values you report with B+ off is that you have somehow connected 2 V of 1 kHz to the output, and see 75 mV at the input due to the resistor ratios.Your reported 75 mV is roughly explained by the resistors R467 = 3900\$\Omega\$ in series with R463 = 180\$\Omega\$ feeding R461 = 220\$\Omega\$(3900+180+220)/(220) = 19.6; 2000 mV / 19.6 = 102 mVOf course,with the power offthe other impedances/resistances around that network are not well determined, but that ratio looks suspiciously close to me.By the way, your photo obscures the output impedance label on the generator panel: is it 600 or 50\$\Omega\$ , or something else?600\$\Omega\$ will make a difference, given the low input impedance of your amplifier (roughly 100\$\Omega\$).

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 07:20:31 pm
content: I have thought about that

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 07:24:40 pm
content: Internal wiring photo

author: wasedadoc
author_rank: Super Contributor
timestamp: May 31, 2023, 08:13:01 pm
content: Quote from: TimFox on May 31, 2023, 07:14:42 pmI agree.On the 6 pin connector the two outer pins areoutputsinputs, the two centre are power. The remaining two areinputsoutputs.Edit: Corrected after looking at the photos using a desktop PC with large screen instead of a diminutive phone.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 09:05:14 pm
content: Quote from: wasedadoc on May 31, 2023, 08:13:01 pmThis is where a sketch is worth a thousand words.

author: TimFox
author_rank: Super Contributor
timestamp: May 31, 2023, 09:41:15 pm
content: If you think you have connected the generator to the input properly: with the power off, take the probe (from the other CRO input) and move it to the circuit board and see what's on the junction of R461 and C461.

author: TimFox
author_rank: Super Contributor
timestamp: May 31, 2023, 09:56:51 pm
content: Quote from: wasedadoc on May 31, 2023, 08:13:01 pmOn the conductor side of the circuit board, the outer pins are labeled LI and RI (inputs) and the intermediate pins are labeled LO and RO (outputs).Tracing the circuit to see if those labels are correct is left as an exercise for the reader.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 10:32:10 pm
content: R461 doesn't junction to C461

author: TimFox
author_rank: Super Contributor
timestamp: May 31, 2023, 10:34:24 pm
content: It does on the wiring diagram you posted at the top of this thread., at node "LI".Your excerpt has two parts labeled "R463".Do you have better documentation?If not, you have to trace the circuit yourself.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 10:37:48 pm
content: Quote from: TimFox on May 31, 2023, 10:34:24 pmThose damn Yamaha schematics are very inconsistent from page to page. I have the high res version from Analog Alley.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 10:39:30 pm
content: Quote from: TimFox on May 31, 2023, 10:34:24 pmPlease also see reply #15

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 10:42:05 pm
content: Quote from: pulsed_power on May 31, 2023, 10:37:48 pmThe Analog Alley files are the best available anywhere. Believe me I have tried.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 10:44:21 pm
content: Quote from: pulsed_power on May 31, 2023, 10:42:05 pmI have tried to attach the PDF file here but the forum won't accept it. File size

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 10:46:36 pm
content: Quote from: pulsed_power on May 31, 2023, 10:44:21 pmAn additional agrivation on this forum is it will not accept a Screenshot.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 11:13:01 pm
content: After all of that where do we stand with this effort?

author: TimFox
author_rank: Super Contributor
timestamp: May 31, 2023, 11:23:20 pm
content: OK--you can't show me any decent, self-consistent, or believable documentation on the layout and schematic.Therefore, you must try troubleshooting it yourself, following what you can see of the circuit.I suggest you connect your generator to what you believe to be "LI" = left input, and also to Ch1 of the CRO.MCAmp.pdf(17.34 kB - downloaded 160 times.) , and start with power off.Take your x1 probe (or x10) and start with where you think LI is on the PCB, then go to the emitter of the input transistor.You should see almost the same thing when you probe those two pointson the PCB.If you pass that test, then you can turn the input level down to a reasonable level for this amplifier, apply DC power, and probe from that point to further points down the two-transistor amplifier chain (CRO AC-coupled).That would be a good start.I contend, given the information given me, that you have accidentally connected the generator to an output of the amplifier.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 11:31:06 pm
content: Quote from: TimFox on May 31, 2023, 11:23:20 pmI have tried to give you everything at my disposal, please see comments above.As you have recommended I will dig deeper with the Scope tomorrow.I don't have the signal generator connected to an output, please see my posted photos.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 11:33:13 pm
content: Quote from: pulsed_power on May 31, 2023, 11:31:06 pmI have turned the signal generator output down to the noise floor and the input vs output stays linear.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: May 31, 2023, 11:35:21 pm
content: Quote from: pulsed_power on May 31, 2023, 11:33:13 pmOne more interesting fact is I have two of these MC boards, therefore 4 channels and they all act exactly the same.

author: JohnG
author_rank: Frequent Contributor
timestamp: June 01, 2023, 12:04:36 am
content: Here are some random things you can try, just for troubleshooting:1) Connect both probes to the same point on the circuit at the same time. Make sure they measure the same.2) Without B+, but with the applied signal, measure the voltage at B+ to make sure you are not rectifying any input (long shot, very unlikely, but hey, it is possible to rectify the input signal and get DC power).3) When B+ is off, disconnect from the power supply and connect the B+ to ground with a 100 ohm or smaller resistor, then redo the measurement and see if anything has changed.4) Send a photograph of the probes to show exactly how they are connected to the PCB when making the measurement. Are they grounded properly, etc?John

author: pulsed_power
author_rank: Frequent Contributor
timestamp: June 01, 2023, 12:19:32 am
content: Quote from: JohnG on June 01, 2023, 12:04:36 amThis unit does have some very strange ground loops and is very critical to these grounds. I have chased it through all boards with great difficulty. This MC has been removed from the unit and is running external at this time.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 01, 2023, 03:44:19 am
content: Quote from: pulsed_power on May 31, 2023, 04:29:28 pmAs the circuit shown is really two separate preamps, one for theleftchannel & the other for therightchannel, with only the "common" rail in common, it would seem unlikely that both channels have the same problem.This increases the likelihood that it is a testing error.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: June 01, 2023, 03:18:21 pm
content: I have checked the DCV at all transistors and the values are correct per the schematic.

author: TimFox
author_rank: Super Contributor
timestamp: June 01, 2023, 03:44:59 pm
content: As I stated above, you need to trace the signal from the input to the output with the same probe.DC voltages only indicate that your DC power connection is correct.Since the documentation available to you (that you shared here) is woefully inconsistent, there is a limit to what others can diagnose remotely.Specifically, the schematic is inconsistent with the PCB layout drawing, which in turn has duplicate part references that do not correspond to the circuit diagram.Unfortunately, that means that you have to trace the circuit yourself in order to get reliable documentation, and then trace what is actually happening.Rule of thumb: when the apparent results are obviously physically impossible, start by verifying the actual connections.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: June 01, 2023, 03:47:17 pm
content: 1) Connect both probes to the same point on the circuit at the same time. Make sure they measure the same.2) Without B+, but with the applied signal, measure the voltage at B+ to make sure you are not rectifying any input (long shot, very unlikely, but hey, it is possible to rectify the input signal and get DC power).Measures 0.00vDC3) When B+ is off, disconnect from the power supply and connect the B+ to ground with a 100 ohm or smaller resistor, then redo the measurement and see if anything has changed.0.00vDC4) Send a photograph of the probes to show exactly how they are connected to the PCB when making the measurement. Are they grounded properly, etc?

author: pulsed_power
author_rank: Frequent Contributor
timestamp: June 01, 2023, 03:53:52 pm
content: Quote from: TimFox on June 01, 2023, 03:44:59 pmAll is extremely well understood and appreciated. My common humor says that the Yamaha Documentation Team was constantly under the influence of Sake!

author: TimFox
author_rank: Super Contributor
timestamp: June 01, 2023, 03:55:07 pm
content: Not shōchū?

author: pulsed_power
author_rank: Frequent Contributor
timestamp: June 01, 2023, 03:57:21 pm
content: Quote from: TimFox on June 01, 2023, 03:55:07 pmAll is possible, and on Fridays they mixed in mushrooms.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: June 03, 2023, 05:39:16 pm
content: Step by step signal flowGarbage in = Garbage out

author: pulsed_power
author_rank: Frequent Contributor
timestamp: June 03, 2023, 05:57:36 pm
content: Quote from: JohnG on June 01, 2023, 12:04:36 amAny additional thoughts based upon my reply to you?

author: pulsed_power
author_rank: Frequent Contributor
timestamp: June 04, 2023, 07:38:13 pm
content: Check this @#$!# out!

author: TimFox
author_rank: Super Contributor
timestamp: June 04, 2023, 07:44:46 pm
content: But, what results did you get when tracing the signal along the PCB?

author: pulsed_power
author_rank: Frequent Contributor
timestamp: June 04, 2023, 08:10:13 pm
content: Quote from: TimFox on June 04, 2023, 07:44:46 pmYou were correct all along, I had signal + connected to signal - and vice versus

author: pulsed_power
author_rank: Frequent Contributor
timestamp: June 04, 2023, 08:43:29 pm
content: I have now lost B+ in the Main Amp, I will need to track it down.I did connect the MC Amp B+ to a 50vdc power supply and it works fine.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: June 07, 2023, 12:31:16 am
content: What is the purpose of the Sleeve Shunt on this DC Power Plug?

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2023, 02:51:36 am
content: It's a switch that disconnects when the mating plug is inserted.These were common on 1/4" 'phone plugs, often used to short an input when disconnected on a guitar amplifier.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: June 07, 2023, 02:56:39 am
content: I mounted one one the MC Amp Box and another one on the Integrated Amp to get the power to run the MC Amp. If this is shorting the +vdc at the Integrated Amp than that is not good, will blow the Power Supply. I am not getting voltage to the MC Amp.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: June 07, 2023, 03:06:09 am
content: I am using a two conductor cable with connection from Center Pin to Center Pin and Sleeve to Sleeve. Nothing connected to the Sleeve Shunt. No +vdc at the MC Amp.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: June 07, 2023, 06:18:39 am
content: Quote from: pulsed_power on June 07, 2023, 12:31:16 amThese are commonly used when the unit can either be powered from an external source or an internal battery.Inserting the DC plug disconnects the battery leaving the power coming from the plug instead.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: June 07, 2023, 07:38:40 am
content: Quote from: Audiorepair on June 07, 2023, 06:18:39 amWow! Now that makes good sense, Thank You for the clear explanation. I was also thinking the Center Pin was for +v, with that configuration it appears the +v should be on the Sleeve?

author: wasedadoc
author_rank: Super Contributor
timestamp: June 07, 2023, 09:22:40 am
content: Quote from: pulsed_power on June 07, 2023, 07:38:40 amNot necessarily. Could be switching negative side.

author: pulsed_power
author_rank: Frequent Contributor
timestamp: June 09, 2023, 03:44:15 pm
content: Quote from: TimFox on June 04, 2023, 07:44:46 pmThank you for all your assistance. As stated before I did in fact find the input/output cause for failure. I have also corrected the power plug issue and all works very well. Input 20mv = Output 400mv