Tenma 72-1040 Dual Channel AC Millivolt/dB Meter
SCompRacer
Posts: 9,077
Posted this here since it needs a tweak of calibration. I need to find a service manual which are scarce as hens' teeth.
Finally got a reasonably priced dual channel AC millivolt/dB meter for calibrating R2R decks. A Tenma 72-1040, under $150 shipped. Leader is a great brand and highly recommended, but the prices are typically much higher for those. This Tenma is a rebranded GW Instek (Good Will Instruments) GVT-427. These are very good meters and sometimes fly under the radar selling for less. There are Chinese knockoffs for $200-250 but some say stay away.
Two channel meters are nice as you don't have to unplug and plug L and R channels while calibrating. I made a two into one switchbox, so I just had to flip a switch. The calibration tones don't last long and flipping the switch and moving the adjustment tool to the correct pot takes some time, so this is more efficient. (The holes in the boards for pots are not marked. I just have a picture in the manual to follow). For me, I'm less likely to hurry and adjust the incorrect pot. No having to rewind tape and replay the tone either.
Small scrape on side, some nicks and scrapes here and there.
Looks familiar...
These are still manufactured as GVT-427B models. New ones ~ $600-700 bucks. Single channel available, GVT-417B.
My single channel Tenma 72-450. Notice the GW at top left of meter face. It has served me well and will continue to do so if the other one is not accurate and cannot be calibrated. It needs to be twice as accurate...lol
We set signal gen to 1kHz sine and .775VRMS (Root-Mean-Square), or 0dBu.
And one channel is slightly off... The lower scale is dBm which is essentially the same value as dBu. The 0 dB or dBV scale above is 0 dB = 1.0VRMS. All the decks I serviced the manuals specify 0dBu = .775VRMS so everyone is on the same page calibrating.
I need a service manual for the Tenma 72-1040. All I have found was an owner manual for the single channel 72-450.
Finally got a reasonably priced dual channel AC millivolt/dB meter for calibrating R2R decks. A Tenma 72-1040, under $150 shipped. Leader is a great brand and highly recommended, but the prices are typically much higher for those. This Tenma is a rebranded GW Instek (Good Will Instruments) GVT-427. These are very good meters and sometimes fly under the radar selling for less. There are Chinese knockoffs for $200-250 but some say stay away.
Two channel meters are nice as you don't have to unplug and plug L and R channels while calibrating. I made a two into one switchbox, so I just had to flip a switch. The calibration tones don't last long and flipping the switch and moving the adjustment tool to the correct pot takes some time, so this is more efficient. (The holes in the boards for pots are not marked. I just have a picture in the manual to follow). For me, I'm less likely to hurry and adjust the incorrect pot. No having to rewind tape and replay the tone either.
Small scrape on side, some nicks and scrapes here and there.
Looks familiar...
These are still manufactured as GVT-427B models. New ones ~ $600-700 bucks. Single channel available, GVT-417B.
My single channel Tenma 72-450. Notice the GW at top left of meter face. It has served me well and will continue to do so if the other one is not accurate and cannot be calibrated. It needs to be twice as accurate...lol
We set signal gen to 1kHz sine and .775VRMS (Root-Mean-Square), or 0dBu.
And one channel is slightly off... The lower scale is dBm which is essentially the same value as dBu. The 0 dB or dBV scale above is 0 dB = 1.0VRMS. All the decks I serviced the manuals specify 0dBu = .775VRMS so everyone is on the same page calibrating.
I need a service manual for the Tenma 72-1040. All I have found was an owner manual for the single channel 72-450.
Salk SoundScape 8's * Audio Research Reference 3 * Bottlehead Eros Phono * Park's Audio Budgie SUT * Krell KSA-250 * Harmonic Technology Pro 9+ * Signature Series Sonore Music Server w/Deux PS * Roon * Gustard R26 DAC / Singxer SU-6 DDC * Heavy Plinth Lenco L75 Idler Drive * AA MG-1 Linear Air Bearing Arm * AT33PTG/II & Denon 103R * Richard Gray 600S * NHT B-12d subs * GIK Acoustic Treatments * Sennheiser HD650 *
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So I found a GW 417B/427B service manual. A longer shield is used. Three adjustment pots where indicated, one MIA.... But all is well now! Oh, I have verified my signal generator outputs the voltages selected with a Fluke 87 True RMS meter and a 179.
1kHz tone, 1.0VRMS. Adjust VR101 so meter points at 1.0VRMS.
So where is VR102?
Oh, there you are, as VR201.
1kHz tone, 1.0VRMS. Adjust VR101 and VR201 so meter needles points at 1.0VRMS.
This must be a test, or a mistake. "200kHz tone at 3VRMS. Make sure meter points to 1.0VRMS." No, I don't think so....
200kHz @ 3.0VRMS.
0dBu/dBm, .775mvrms.
-8dB, 308mvrms.
All calibrated and ready to go.
Salk SoundScape 8's * Audio Research Reference 3 * Bottlehead Eros Phono * Park's Audio Budgie SUT * Krell KSA-250 * Harmonic Technology Pro 9+ * Signature Series Sonore Music Server w/Deux PS * Roon * Gustard R26 DAC / Singxer SU-6 DDC * Heavy Plinth Lenco L75 Idler Drive * AA MG-1 Linear Air Bearing Arm * AT33PTG/II & Denon 103R * Richard Gray 600S * NHT B-12d subs * GIK Acoustic Treatments * Sennheiser HD650 * -
