Trio Kenwood VT-151 AC Voltmeter

[david77]

Today I got another old AC millivoltmeter, this time a Trio Kenwood VT-151.
This thing is autoranging so the classical range switch is missing. Back in the 70's and 80's lots of japanese companies produced look-alikes of the HP 400 series of millivoltmeters, Trio did that too, of course but not with this instrument, this seems to be one of their own creations.



Unusually the meter uses an N connector for the input - luckily there was a BNC adapter included.

The unique feature of this meter is the range display, that's the black rectangle on the right.
A series of bulbs light up and illuminate a two coloured display that shows the range in V and in dB.





Let's take it apart!



Ok. This doesn't look like the insides of high quality measurment equipment, looks rather like the insides of 1970's japanese A/V equipment. No FR4 boards, lots of wires and messy wiring looms, soldered wire wraps, components thrown on the boards in all orientations but not straight, lots of bodge jobs.
Now, for an instrument like this it is what you'd expect to find. Not only was it made by a radio manufacturer but these instruments were dirt cheap compared to R+S, HP or Fluke.

The really shocking part comes now: The main wiring loom between the back and the front.
They lumped mains and signal wires into one, using the same thin wires for mains and signals.





I really don't like the look of that. One expects to find unisolated main contacts on the fuse holder and voltage switch in equipment of this age.

[ciccio]

I have one old TRIO audio oscillator (don't remember the model but it is older than this one, maybe end of 70s).

It is built with the same style, and like this  the mains wiring is done, front to rear, with a dual conductor screened cable (in yours is the larger red cable)  about 2 X 0.22 mm2.
This was a common practice in these years, and I'm still amazed by the fact that, after more than 30 years, the insulation is still good.
I know this because last summer I tested all the lab's instruments for insulation resistance at 5 kV, to check the new insulation meter..  The N connector was standard in these years (you could plug a banana in it, and there was a banana ground on it's side) and the cheaper equipment had only two banana sockets.
My TRIO, which was "inspired" by HP model 200, was copied by countless companies,  starting from GW - Instek (I have one of their audio oscillators) and ending in non name  clones. 

[david77]

I also have a Trio audio oscillator the AG-202A. It is actually not a bad piece of kit, if memory serves I measured it with only 0,1% distortion. The AG-202A seems to be a bit newer than this VM, it's got a properly grounded case and the mains wiring was more conventional if I remember correctly.

The red screened cable is actually not mains. The white wire visible at the bottom of the case in the last picture is one of the wires to the mains switch.

[tekfan]

Very nice meter. Classic Japanese construction.  It reminds me of Kenwood and Sansui amps inside. Is the connector really an N connector or a PL connector? I don't think you can put a banana plug into an N connector. Does this meter also have a relative function?

I always liked the dual channel Kenwood meters with two needles in one meter movement - one for each channel. You can quickly see if there's any difference between the channels.

[david77]

Yeah, you are right of course it is a PL connector, I mixed the two up a bit  .

It really reminds one of japanese Hifi gear, doesn't it? It's even got a problem that I have seen in some 70's vintage japanese Hifi equipment. Failing Hitachi 2SC458 transistors.
It didn't give a reliable output at first, so I thought I'd do a recap. While I did that I saw the type of NPN transistors used in there and that started to niggle on my mind.
The recap didn't improve the accuracy of the meter, it was always a few mV short compared to my HP 400 and could not be adjusted to show the proper value..
Finally I remembered problems with the 2SC458 in some old Akai taperecorders, the transistors there often just die or become noise generators and loose gain.
So I started replacing them with BC547C starting from what I thought was the last amp stage before the meter. And after 7 transistors I finally hit the bad one. There must be at least 50 of the 2SC458's in this instrument.
I've compared the obviously broken 2SC458 with another one using the hFE function on one of my DMM's, it showed 43 for the broken one and 372 for the other one.