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 Post subject: Re: Variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 7:49 pm 
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Not sure what the wattage requirement of the phono is as the service manual doesn't specify. Iit is a Sansui SR-2050C record player.

On the 100 volt tap it can work over a voltage range of 80-100Vrms.

Now to reduce the wattage required I could disconnect the common of the voltage select transformer.

Perhaps I could use the 70 volt tap on the amp then rewire the voltage select transformer to where the motor and circuitry runs off the 250 volt tap of that transformer with the voltage being applied to the 100 volt tap and common.

That would give me a 1:2.5 voltage ratio.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 8:22 pm 
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A PA amplifier is more rugged than a domestic HiFi amp. As Steve says, the 70v line transformer won't hack the pace and replacing it with an ordinary power transformer might do the trick - you may have to keep the line transformer in case it uses a winding for feedback though.

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 Post subject: Re: Variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 8:35 pm 
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Tube Radio wrote:
Not sure what the wattage requirement of the phono is as the service manual doesn't specify. Iit is a Sansui SR-2050C record player.
Measure it.....

Quote:
Now to reduce the wattage required I could disconnect the common of the voltage select transformer.
??

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"Voltage is fun to watch, but it's the CURRENT that does the work."


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 11:13 pm 
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Yes the transformer would draw a little power but maybe not enough to matter.

Anyways this just got real simple.

I found the wire going to the 100 volt tap on the voltage select transformer of the phono and moved it to the 250 volt tap.

I then set the select switch to 100Vac and connected the power cord to the 70 volt output of the amp.

At 61.6Hz (required frequency for proper platter speed) I could get 90Vrms before the sinewave started distorting.

I ran the frequency up to 120Hz and got more output before distortion so that tells me the internal transformer cannot handle 60Hz properly.

So I'll try that 200 watt 70 volt transformer driven directly by the amp output like the original transformer is driven and see how well that works.

Also I can get the platter spinning fast enough in the 45 RPM selection to where I can play 78 RPM records so that opens up some possibilities.

Will then need a headshell with a 78 RPM magnetic cartridge on it and a preamp with the various EQ curves used for 78 RPM records.

So I will look for a variable audio oscillator circuit that I can build and stick inside the amp.

I fed the audio input to the reverb in which is normally bridged to the reverb out which is a pre out main in setup which bypassed the preamp completely.

Works great that way.

I will look at the schematic and disconnect power to the preamp. I'll then recap the amp and power supply which consists of 5 electrolytic caps.

If the 200 watt transformer will work there's plenty of room inside the chassis to mount it.

Also I will remove the plug from the phono and replace it with a unique connector so that I cannot plug it into anything but the amp.

EDIT:

The 200 watt transformer works just fine.

Now I do want to use the existing transformer in the record player as a step up like I have it wired now, but that could lead to potential problems meaning I'd have to modify the power plug so it only plugs into the amp and I won't be able to use a voltmeter like I want without extra wires and it being separate.

So I'll need a transformer to step up the voltage.

I may try the Triad N-68X I have just to see if a 115 to 230 volt transformer will be enough as the amp with that 70 volt transformer has a good bit of headroom left before distortion sets in.

EDIT 2:

Remembered I had parts of an AKAI M7 real to reel which has a multi voltage autotranaformer for operation on different voltages.

Opened up the case and the transformer was still there.

I wired the phono back to stock and set it for 130 volts.

I tried that transformer feeding the input between 0 and 100 taps with the output between the 0 and 230 taps.

Worked like a charm.

I mounted the transformers as seen here

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I found that 61.75Hz produces the proper speed.

Also I found that I can get the motor fast enough in both 33 and 45 to where I can play 78 RPM records.

It's easier on 33 as the load on the motor is lighter due to the smaller capstan size.

Have to take the frequency up gradually though as the motor won't start at that high of a frequency.

Now if it is not good for the motor to run that fast I won't do it.

My plan is to just go with an external audio oscillator.

For voltage monitoring I have one of those older radio shack line voltage monitors that goes from 100 to 130V with the area right around 120V in green.

I just adjust the amp's volume control until I'm within the green on the meter.

The heatsink for the transistors does get quite hot, but the amp has a thermal cutout in series with the primary of the power transformer with the cutout mounted right where the heatsink is so if it gets too hot the amp will shut off.

Nevertheless I will figure a way to add a fan to make the heatsink a bit cooler or I will get a better heatsink.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Nov Thu 01, 2018 2:34 am 
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Here's something I had forgotten about the amp I am using.

BikenSwim wrote:
It looks like the 60 watt amp just uses a higher current power transformer (same voltage) and a lower primary impedance output transformer. If you attached a 4 ohm speaker in place of the output transformer, you would get the same 35 watts so your amplifiers would match. Actually the difference between 35 and 60 watts is less than 3 dB so it would not be very noticeable.


There's also a 100 watt version of this amp.

Here's the schematic

http://www.bogen.com/support/discontinu ... HS35Am.pdf

So it looks like there's a minimum of circuit changes between the three versions.

The fact that the primary impedance of the original transformer is quite low may explain why this amp does so good driving a 70 volt transformer and a subsequent voltage step up transformer.

To completely disconnect the preamp from B+ I need to remove a 560 ohm 3 watt resistor.

There are two plug in modules (could be transformers maybe) in the mic preamp circuitry. Perhaps I should pull them look them up and see if they have any actual value.

I've listened to several records and I am satisfied that the amp and oscillator solution is good enough.

The only investment will be the cost of the capacitors and also of a 56 ohm resistor which looks like it got a bit hot in the past, although it is close to the right value (possibly within tolerance) and the amp still works right.

EDIT:

What I really should do is calculate the voltage ratio from the input of the first transformer to the output of the second transformer and get Edcor to make a single transformer with the same ratio. I suspect that might be better in the long run.

That said is transformer ratio all I need to be concerned about or does impedance matter and if impedance matters how would I find the impedance of the two transformers connected like they are and how would I find how much of a load the amp can actually handle?

Suppose what I could do is get the voltage ratio and see what power transformers have that voltage ratio then select one of those rated at a few amps.

That said if it won't get me any better efficiency then it wouldn't be worth it unless the transformer is lighter than the two transformers I currently use.

Think I'll pull the cover on the amp and measure the voltage ratio when I get home today.

Thinking that to avoid distortion and provide the maximum output I may need to look at transformers with a 240 volt primary as the amp is more than able to put out well above 120Vac unloaded and even with the record player as a load will easily do at least 130Vac (high as that meter will go) and possibly more.

I may also measure the maximum no load voltage just to see what it is really capable of.

Even though the original rectifiers are good I am replacing them anyways given the type of service I am pressing this amp into so that I can be sure it will be reliable.

I also upped the value of the first filter cap to 6800uF and the output coupling cap to 3300uF. I chose good low ESR caps to ensure reliability and long life.

I want to upgrade the power transistor heatsink with something better, but I would need to keep the transistors in the stock location for the thermal cutout devide to function properly and I would need a very specific size of heatsink so I think maybe a fan will be best unless there's a dual TO-3 heatsink that is better and will fit the stock location.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Nov Fri 02, 2018 2:27 pm 
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Thought I had a good kit, but the frequency error is too high for my application.

https://www.sparkfun.com/products/11394

https://jyetech.com/Products/085/Manual_085F.pdf


EDIT:

Think I have found a good kit.

https://www.amazon.com/SainSmart-UDB100 ... p_pl_dp_12


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Nov Fri 02, 2018 7:49 pm 
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Location: Edmonton AB Canada
Where did you get the belt for the Sansui? A too tight belt will always play slow like this, and I've had very bad luck with vendors supplying wrong size belts for various turntables. I have repaired dozens of turntables with belts I ordered online, and playing too slow is a common issue. If you had to stretch it quite a bit to get it to fit, playing slow is almost to be expected. The info I have found online for belt size can sometimes be very unreliable - I would try and go to a forum like audiokarma, and ask people what size belt they use for a specific model. The circumference of the belt should be not less than 10% less than a piece of string put around the belt path.

Case in point, the Pioneer PL-41 is extremely sensitive to belt size, and many of the ones you can buy online are flat our wrong. My apologies if you've already considered this, but it is certainly the easier solution.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Nov Fri 02, 2018 9:27 pm 
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The belt is right far as I know.

Comparing it to the original it looks to be the same thickness and width.

It fits around the platter snug enough to where it won't fall off and I do not have to use undue force to put it around the motor capstan.

The motor doesn't seem to bog down much if at all when I stop the platter.

Also it ran the exact same slower speed with the belt that was on it when I got it which was lose around the platter but still seemed to be tight enough for the motor to drive the platter without slipping.

I may go with a single transformer for the power supply though.

I need one with a ratio of 1:8.72 so a transformer with a 13.76Vac secondary will work. I can either use a 12.6Vac transformer or maybe a 15Vac transformer. If the 15Vac transformer will work the load on the amp will be less.

The Bogen is built like a tank and with how the amp is done it is a real low impedance that drives a transformer to make all the other necessary impedances including 70 volt which made it perfect for this use. Don't know what the primary impedance of the original transformer was but in the CHS-35 I figured the primary impedance was around 4 ohms based on the no load voltage output of the 4 ohm tap being the same as the input to the primary and the CHS-60 is basically the same only it uses a transformer with a lower primary impedance.


Last edited by Tube Radio on Nov Fri 02, 2018 9:36 pm, edited 1 time in total.

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 Post subject: Re: Variable frequency AC power supply
PostPosted: Nov Fri 02, 2018 9:31 pm 
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Location: Edmonton AB Canada
Tube Radio wrote:
The belt is right far as I know.

Comparing it to the original it looks to be the same thickness and width.

It fits around the platter snug enough to where it won't fall off and I do not have to use undue force to put it around the motor capstan.

The motor doesn't seem to bog down much if at all when I stop the platter.

Also it ran the exact same slower speed with the belt that was on it when I got it which was lose around the platter but still seemed to be tight enough for the motor to drive the platter without slipping.


Does the motor have a phase shift capacitor, and is it good? Has it been cleaned and lubed properly? (motor bearings and main bearing). Was it a 50Hz unit and pulley was changed but not cap? Is motor getting correct voltage? Is belt running too high or low on pulley? (bad motor mounts can make motor sag, and belt run on wrong part of pulley)

32.5 is too slow, something is wrong. That deck was pretty decent when new, should be working better than that.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Nov Fri 02, 2018 9:48 pm 
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Motor has been cleaned and lubed and has no capacitor that I know of. At least the schematic shows none and the motor ran fine when I fed 120Vac to it out of the phono.

The motor is receiving the proper voltage.

Motor mounts are right and the capstan was adjusted so the belt rides in the center as it should.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Nov Fri 02, 2018 10:02 pm 
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Location: Edmonton AB Canada
Tube Radio wrote:
Motor has been cleaned and lubed and has no capacitor that I know of. At least the schematic shows none and the motor ran fine when I fed 120Vac to it out of the phono.

The motor is receiving the proper voltage.

Motor mounts are right and the capstan was adjusted so the belt rides in the center as it should.


Sounds like you've tried everything. How long does motor keep spinning with no belt, when you shut power off? Is main bearing absolutely free, and turntable takes a while to spin down, if you remove belt, and spin it by hand?

Just as an experiment, try this quick and dirty fix. If you wrap a little bit of scotch tape around the motor pulley, you can probably get it dead on.

Increasing size of small pulley will speed up the turntable.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Nov Fri 02, 2018 10:07 pm 
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Yes when spun the platter takes a good while to slow down.

Motor slows down like one would expect a small AC motor with permanent magnets would.

I could try the scotch tape, but the power supply option does give me the capability of 78 RPM operation.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Nov Tue 06, 2018 2:25 am 
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The function generator came in today.

So far I am well pleased.

For the time being I mounted it in the box it came in. At some point I may mount it in the amplifier. The only issue there is I either have to use a voltage divider and regulator to power it from the B+ of the amp or I have to add a power supply just for the function generator. Plus I also nave to get four switches then mount them and I also have to find a panel mount rotary encoder and mount it.

The backlight is quite bright so I will figure out how to dim it.

Here's a couple photos.

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The caps, diodes and resistor came in today as well.

I went ahead and installed them.

Here's the output waveform of the amplifier. The sinewave is very clean.

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Unfortunately I found the frequency has to be changed slowly in order to get it to run at 78 RPMs in 33 and this generator must not change frequency smoothly enough if I do it in 1Hz increments as I could not get it to run at 78 RPMs in 33. haven't tried 78 in 45 yet.

Now what manufacturer in their right mind does not include screws with a capacitor that has screw type terminals?

Had the same happen with a variable resistor that could be PC board mounted or panel mounted. No nut included.

So far this function generator is quite good.

In fact I would go so far as to recommend it for a general purpose function generator.

Has a counter function and also a frequency counter function.

I can use the frequency counter with my RP-190 to serve as an indicator of the proper speed.

Also if I switch it to counter or frequency counter it still outputs the signal.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Nov Tue 06, 2018 3:23 am 
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Tube Radio wrote:


Now what manufacturer in their right mind does not include screws with a capacitor that has screw type terminals?



Mallory. :( We kept 8-32 and 10-32 and some 12-24 screws on hand to supply with their CGS line of computer grade caps. My whole NOS boxed Sangamo white computer grade caps have no screws either. I can't speak for Sprague and CDE.

I think it's similar to the bushing nuts for test equipment manufacturers - With Tek, you got the pot, no hardware. Those were separate part numbered parts. :evil:

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"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Nov Tue 06, 2018 5:33 pm 
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Perhaps that is just standard practice as how is the manufacturer to know what length screws are needed for a particular application?

I dimmed the backlight by inserting a 470 ohm resistor in series with the existing 100 ohm resistor.

Got the brightness down to a better level.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Nov Mon 26, 2018 10:19 pm 
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I did decide to mount the function generator in the amplifier.

I found a good deal on some quality pushbuttons on Electronics Goldmine so I bought six as I needed two for a scope clock build anyways.

I found the correct panel mount rotary encoder with D shaft at Mouser and it has a switch.

I thought about it and may get a single coil SPDT latching relay and use the pushbutton to actuate it so that I have a convenient way of turning the output on and off. Got to thinking and given the relay will be SPDT I could use it to switch the output of the function generator between the amp input or a jack on the rear of the amp so that I can use it for other things besides just driving the amplifier.

All I need to do is cut the rectangular hole for the display.

That also means I will need to move the level control of the function generator to the front panel.

Not sure how that will affect the output level as I go higher in frequency though.

The pot shows an in circuit resistance of 1.137K So I will need to measure what it is out of circuit.

It also is wired so that the wiper and one terminal are tied together so it simply lowers the resistance across the output.

Would love to find a schematic of it so that I could see how it is wired.

If the value is 1K I have one of those from a Heath scope with the proper D shaft so that I can use an original amplifier knob with it. I'll put the rotary encoder where the MIC 1 control is since the buttons will be mounted to the left of that control. The level control will be mounted where the MIC 2 control was. The master level control of the amp will still be used.

Now I would love to mount a small digital line powered AC voltmeter in the amp, but the front panel won't have room unless I do it where the logo is and that would only mess up part of it. Besides I really don't want to screw up the logo.

The solid state amplifier logo where the LCD will be put I don't care about as it is on a removable plate anyways which covers two holes for optional controls.

Now it is possible to take the Radio Shack 100-130 Vac voltmeter remove the pins that plug into the outlet, mount it to the amp top then wire it in using a connector so that I can still remove the cover if necessary.

What I would like to do is find a variable AC load and see how much of a load the supply can actually handle.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Dec Wed 05, 2018 8:25 pm 
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I finally got the stuff together to mount the function generator inside of the amp.

Concerning the mistakes I made with cutting the hole is there a way I can fix that?

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The wiring is a mess I know. Will fix later.

Here's the waveform. Not quite as clean as it was previously.

Attachment:
20181205_002539-800x600.jpg
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Here's the problem part of the waveform as seen with the scope's 5X mag turned on.

Not sure yet what is causing it.

Attachment:
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At first I was getting some sort of waveform which was a few cycles of a sinewave with a larger positive peak then a space of no waveform followed by the same sinewave section with the level controls turned all the way down and nothing connected to the output except the scope probe with the scope set to the lowest V/DIV setting. Thought about it and I had knowingly created a ground loop. I disconnected the ground wire from the shielded cable feeding the audio to the amp at the function generator's output since I used the existing terminal block and it was easy to remove the ground there.

That eliminated the waveform.

I don't quite understand the level control the function generator has.

It is a 1K variable resistor that as it is turned CW the resistance lowers and it is connected between the output and input of an OP-AMP (forget if + or - input) which causes the level to increase as the control is turned CW.

I did notice something though.

The outlet strip I used with the amp so I could plug in the voltmeter and phono must have something in it that has a lower resistance as the frequency is increased as I noticed the B+ decreasing as I turned up the frequency with the outlet strip being the only thing connected. Will never need to take the supply above 100Hz though in its current use.

The eventual plan is to clean up the wiring and use the proper standoffs so the display can be flush mounted with the panel.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Dec Wed 05, 2018 8:37 pm 
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Location: Jackson, TN
Tube,

That is a remarkable project! You are a bulldog when it comes to developing a solution for a problem.

I would never have considered this solution for speed adjustment, let alone going on to design and build it.

Keep on truckin!

Tim


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Dec Wed 05, 2018 8:59 pm 
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Tkilboy wrote:
Tube,

That is a remarkable project! You are a bulldog when it comes to developing a solution for a problem.

I would never have considered this solution for speed adjustment, let alone going on to design and build it.

Keep on truckin!

Tim


Thanks.

I must give credit where credit is due though.

Many times the members of this forum have helped me when I was unsure of how to do something.

The credit to the idea of the variable frequency supply goes to someone over on Vinyl Engine who had done that for a phono using a 30 watt tube amp.

Same principal the commercially available variable frequency AC supplies use only this one produces way less wattage and was several hundred dollars cheaper.

I've thought about eliminating the resistors on the output used to make the level control less sensitive and use an audio transformer to do the same thing which would isolate the ground as well, but as I found with a two transistor two transformer amp capable of 50Vrms output one has to be very careful when using transformers in high gain situations as feedback can happen due to magnetic coupling of the transformers.

Two thinks I don't like.

1. The rotary encoder I used takes four clicks to change something on the display.
2. The control possibly needs to be fed through a debounce circuit as sometimes the digits change multiple times instead of once after four clicks.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Dec Thu 06, 2018 7:07 pm 
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I do want to go to a single transformer to step up the output.

If I take the 60 watts max power of the amp and the amp output voltage required to get 120Vac out of the transformer which is 13.76Vrms I get a current of 4.36A.

I want to minimise the transformer losses as current increases so would a transformer with 5 amp secondary be ok?

I could also use a 27.5Vac to 240Vac transformer as well.

Problem is I can only readily find 12 volt and 15 volt transformrs. Don't want to use a 12.6vac transformer as that will give a ratio of 1:9.52 which will put more of a load on the amp. A 15Vac transformer will give me a ratio of 1:8 and the amp might run out of headroom before I reach 120Vac.

Trying to avoid having Edcor make one as that will be expensive.

I could go with the existing two transformers, but would really like a single transformer solution as it would get rid of the heavy 200 watt 70 volt transformer.

I might experiment with a 120Vac variac run in reverse and see if 15Vac will work. If so that will be better as it will lighten the load on the amp and is a transformer I can easily find.

EDIT:

Using a variac 15Vrms will just get me to 130Vrms before the amp starts distorting without a load.

While that will work I don't like it as its running the amp close to its limit.

I'll try the variac set to 12.6Vrms and see what I get.

If that works better then I'll find a 12.6Vac 5A transformer which should be real easy to find.

EDIT 2:

12.6 volts I don't like all that much.

15 volts I can make work.

EDIT 3:

Found another potential ground loop source.

The regulator is mounted to the amp chassis which is at ground potential and I did not use any sort of insulator.

I'll first remove the regulator from contact with the chassis and see if that does it. If so I'll add a transistor insulator.

There is a ground point near the regulator which was original to the amp and is where the added filter cap, regulator and generator are grounded. The main ground where the rest of rhe amp circuit is grounded is close to the power transformer.

Would using the one ground for the added generator be causing the ground loop issue?

Given I have some SPST toggle switches I'll add one to switch the generator's output on and off. Won't use the generator for anything else so no need to worry about if the switch or anything else can handle the full 2MHz.

The idea for the switch is so I can instantly turn the voltage on and off without switching the amp off and back on again.

Now the counter function does seem to load down the tach out of the 45 player so I may build a circuit that will provide a higher impedance for the tach out or maybe I could add said circuit to the 45 player's circuitry.


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