How to determine new replacement meters?

I wish to obtain 2 new meters for a small 40 watt Toshiba SC335 power amplifier. One is frozen and checking with a DMM reads OL. The needle of the other swings when nudged and it's resistance measures about 600 ohms.

As I have just come to understand from watching an "understanding meters" video, these are small ammeters and knowing the full scale is important. I do not have the variable power supply as was mentioned in the video (something needed to determine this) plus movement of both needles is compromised. I did try "loosening the screw" and used compressed air to blow out debris but this did nothing. Both meters were opened by some previous owner for reasons unknown except that new fuse lamps were "made to fit".

I located online a model SQ10 meter made by Anders which has nearly identical physical dimensions to what I need and it's coil resistance is 600 ohms. The data sheet states it as being 0 - 500 micro amps. I think I can remove the original meter's dial scales intact and would hope to transplant them to the new ones.

My question is, would a 500 micro amp meter be suitable for this amplifier?
 
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Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
A 500 uA meter is all the specification you need for a power meter movement in a domestic power amp, where precision is unlikely. There are usually trimpots to adjust FSD (full scale deflection) in any case. Like any electronic work, you need instruments to check this by driving the amplifier to maximum rated output voltage level with a fixed sinewave signal of around 1 kHz (no load required) and adjusting the meter FSD according to the scale.

However, it's unlikely that the meter scale mounting screws on typical replacement meters will make it easy for mounting the Toshiba scales. I've tried to do this several times and never been satisfied until I had the artwork professionally copied onto card overlays. Even then, the meter cases were too deep and would not fit in the available space.
 
A 500 uA meter is all the specification you need for a power meter movement in a domestic power amp, where precision is unlikely. There are usually trimpots to adjust FSD (full scale deflection) in any case. Like any electronic work, you need instruments to check this by driving the amplifier to maximum rated output voltage level with a fixed sinewave signal of around 1 kHz (no load required) and adjusting the meter FSD according to the scale.

However, it's unlikely that the meter scale mounting screws on typical replacement meters will make it easy for mounting the Toshiba scales. I've tried to do this several times and never been satisfied until I had the artwork professionally copied onto card overlays. Even then, the meter cases were too deep and would not fit in the available space.

Thanks Ian and I'm going to order those Anders meters. Like you say, precision is not important as long as they move and have the same range of movement. Should they be grossly out of whack I'll address that by the trim pots. Any adjustment will be eye the piece of test gear you mention means a Sunday drive to the country and then another to return it.

I experimented on the Toshiba meter's dial scale and am pretty confident of getting them off. Removing those from the Anders might be different.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Heh, heh - These days, a great sinewave generator is just a mouseclick away....here: Download | Audacity®

It seems from a hand drawn schematic, that the meter sensitivity is set by R31, a fixed 8.2 k resistor in the right channel and balanced for appearance (I'd say) by R32, a fixed corresponding 5.6k resistor + VR03, a 5k trimpot in the left channel. 'Should be a snap to adjust the FSD for whatever you want with R31 and make them balance nicely too. TOSHIBA SC-335 STEREO AMPLIFIER SCH Service Manual free download, schematics, eeprom, repair info for electronics

Good luck with the refit :)
 
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Thanks again Ian. I did not consider that a sinewave generator was available online. Too stuck in the past I guess because my thoughts went immediately to hardware.

Downloading the SC335 SM through your link was unsuccessful and in the process my homepage was hijacked by DOC to PDF by My Way but I managed to delete it. Not really that good at computer tasks unfortunately.

When I reach the point of needing the schematic, my intention is to download the SC330 SM from HiFi Engine. That model is a year older and 5 watts less but the outward appearance is the same so I'm thinking the meter circuit is the likewise.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Yes, there are problems with the way the Electrotanya site organizes its download menu.
Just wait about 20 seconds after selecting the manual you want and seeing the preview appear. After this, the text below the image will change. "get manual" appears and that's what you click on. Anything else that looks inviting, is going to wind up up in a crapware download and goodness knows how much they will corrupt your browser security or at least spoil your day. Other than that, the site is a good, fast resource with no registration for up to 2 downloads per day - some of which are hard to come by anywhere else.
 
Are these VU meters? PM me a photo with dimensions, I have some left from my service days. E

Yes, VU meters.

Toshiba SC335 meter front.JPG

Toshiba SC335 meter side.JPG

Toshiba SC335 meter back.JPG

Max width - 64mm
Bezel opening width - 60mm
Max height - 54mm
Bezel opening height - 32mm
Max depth - 28mm to end of barrel. Add 9mm for terminals.
 
Yes, there are problems with the way the Electrotanya site organizes its download menu.
Just wait about 20 seconds after selecting the manual you want and seeing the preview appear. After this, the text below the image will change. "get manual" appears and that's what you click on. Anything else that looks inviting, is going to wind up up in a crapware download and goodness knows how much they will corrupt your browser security or at least spoil your day. Other than that, the site is a good, fast resource with no registration for up to 2 downloads per day - some of which are hard to come by anywhere else.

Right, followed your instructions and got the download. Actually the same as the link sent by irrebeo.
 
Even safer, and easier than a software generator, download one or more of these ready to use MP3 test tones; 440Hz or 1kHz is what you need for that setup.

WAV version available for purists ;)

Only trick is to set your player to repeat so you have continuous tone as long as you like.

There is a minuscule silence when file stops and starts again, no big deal

Download Audio Tone Files
 
Sorry, the movements I have are different. The closest I get are for a 30x50 mm opening, no internal lamp. If you can fit them, they are yours! E

Thanks for the offer but I think I'll proceed with the Anders SQ10. I popped the the bezel off the metal face plate which the meters actually fit into and it's a piece of plastic. That's good because I can do the small bit of enlarging with a file and the internal lamps are the right voltage. There's nothing which holds the meters in place so friction fit is a must have. I e-mailed Anders in the UK and was told that the dial scale can be removed if done carefully so a transplant should be pretty easy.

The vendor (Newark Element 14) told me by phone that they are shipped from the UK not North America as I thought so delivery is 2 to 3 weeks.

Pity the meters will cost a bit more than twice what I paid for the amp. Unless I start doing new capacitors, I will be into it for $110 CDN. Only gave it a brief listen through some Minimus 7's but it seems to sound OK so maybe new meters will be it.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
I dunno about these meters. Anders are marketing these Chinese origin display devices but at a retail $25 CAD or $24 AUD, I think you would be able to source similar quality and dimensions elsewhere for less. I include a link to a similar product available retail here (the bezel is detachable) and as our $ is about even presently, at least you get the picture on price. http://www.altronics.com.au/p/q0490-vu-meter-with-backlight/

A real VU meter is a different animal, designed along with its drive circuit for specified needle ballistics among other things that make them much more expensive but not needed here: https://en.wikipedia.org/wiki/VU_meter
 
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I dunno about these meters. Anders are marketing these Chinese origin display devices but at a retail $25 CAD or $24 AUD, I think you would be able to source similar quality and dimensions elsewhere for less. I include a link to a similar product available retail here (the bezel is detachable) and as our $ is about even presently, at least you get the picture on price. http://www.altronics.com.au/p/q0490-vu-meter-with-backlight/

A real VU meter is a different animal, designed along with its drive circuit for specified needle ballistics among other things that make them much more expensive but not needed here: https://en.wikipedia.org/wiki/VU_meter

Thanks Ian, I appreciate the advice and feedback but I ordered the Anders meters earlier today knowing full well they are over priced. There are stores in Vancouver, near to where live that sell meters which range from $10 to $12. Chinese made of course but none meet size requirements. At least the Anders will need little modification to make them fit and the lamps inside are the right voltage.

I discovered Anders meters a couple of years ago when I wanted to replace a pair in the (hang my head in shame) first 8 track recorder I restored. The only ones even close to the right size were Anders but I did have to add illumination. They worked perfectly and I had to adjust nothing so I have no problem with the quality of the product. Besides, they are just eye candy in this Toshiba amp anyway.
 
So the new meters are installed and operating.

Ian, in an earlier post you described calibrating them. The adjustment pots are clearly marked and I have a 1000hz test tone recorded to CD. You mentioned needing to measure while adjusting and I was told likewise by someone else. Thinking the new meters had their VU scales, that person said to adjust the meters to read "0" dB (40 watts, the amp's rated output) when reaching 17.88 volts at the speaker terminals.

This procedure involved a dummy load and checking the voltage across it's leads. But you said "no load necessary". Was it this dummy load you would be referring to or is it something else?

If I must have one, what do I need for this dummy load? I have a 25 watt load resistor but it's 100 ohms so therefore, useless I assume.

It was also explained to do this quickly so that the amp is not damaged while running at full power with no speakers connected. I believe I recall reading several times that it was relatively safe to do this with an SS amp but not so with a tube amp.

Since the the VU scales are gone, I assume I will be adjusting to achieve the 40 watt mark instead of 0dB?
 
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Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
The dummy load is required for a true power measurement but all these these meter amplifier display types are wired only as simple voltmeters and these cannot "know" what current is flowing in the output stage so that power could be measured directly. If the suggestions you read are aimed at a tube amplifier, there is certainly a need for them to be loaded but that is a safety issue about tube amplifiers only.

As discussed earlier, the meters are only for a relative output voltage measurement, usually marked as power into an assumed, fixed 8 ohms resistance. You don't need a load on a solid state amplifier for this, just produce and calibrate the meter to the AC output voltage that equates to 40W dissipation in an 8R resistor.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
A VU meter is normally used with 0dB as the output maximum, about 2/3 FSD, similar to power meters on amplifiers of this vintage. As the scale is drawn to focus attention on 0dB, it makes sense to follow suit as the max. rated output ref. point. When the scale is switched to 5W maximum as it can be, the same scaling factor will remain as before, so whatever point 40W indicated on the high scale, 5W will be the same point on the low scale (within the scaling resistor precision and calibration accuracy) You'd have to question calibrating the higher scale with a lower one or at only 12% FSD because errors are then multiplied. Rather the reverse would be better, if that's what you meant, and since the maximum rated power is probably the more significant reference point.

If the original power meter scales are transplanted and used, calibrated to 40W as before, you still have the same 8:1 relationship between FSD of the ranges so the 5W scale will be there too, just the same. As the OP says though, it's for cosmetics really, just amp. bling as they always were.