the value of old test and repair equipment

tony duell ard at
Sun Aug 7 00:56:37 CDT 2016

> Though I have some background in Telcom, I'd never heard of the HP 3370(B)
> so had to check it out.  Here are a couple of take-away quotes from a
> thread at:

Did you also notice that one of the contributors to that thread goes 
by the username of 'tonyduell'? That's where mine came from :-)

> "The A suffix boxes were designed to test phone lines for modems for CCIT
> standards (UK & Europe), the B suffix (B-for-Bell) were the for the
> American market. Designed and built in South Queensferry.

Yes and I don't believe it. Mine is a -B. I got the manual -- Keysight
(was Agilent, was HP) have what they claim is the operating manual to
download. Well the first 50 pages are the operating manual. The rest
is the service manual with full schematics [1]. Anyway, it's the manual
for the -B. But there is an option (A44 assembly) to add the extra
protection zeners and fuses that was only fitted to UK models (and mine
has it). Since a Bell modem would cause a lot of problems on an old
UK phone line, I do not believe the -B version was US only.

[1] Well, schematics of everything (all the PCBs, PSU, etc) apart from the
rotary encoder used on the manual tuning knob. And guess what, my 
rotary encoder is not giving any outputs. One line is stuck high (I think), the
other seems to be floating. Still, there are not that many bits inside it. At least
I have schematics of the 30-odd plug-in PCBs.

> You use a pair of 3770s one at each end of a leased phone line connection.

You can use a single 3770B to transmit to itself if you have access to both
ends of the line in the same room. Or if you want to test a simulated line,
repeater amplifiers, etc.

The use of 2 instruments, one effectively controling the other, requires a
factory option in them. Mine has it, but it was not standard.

> "They were around 12 kilobucks each back in the day. I think the 3770 did
> level, flatness and distortion, I think the 3771 did group delay come to
> think of it. My pilot tone filter probably did go in the 3770."

I've not looked at all the features yet (I am still reparing it) but there is
certainly some delay-measuring feature in the 3770B

> As I was assuming, they are used in pairs at opposite ends of the line.
> Neat device.

Oddly, there's a lot of digital circuitry in there (the transmit 'oscillator' is
a digital direct synthesiser with a couple of HP tricks, for example) but there
is no HPIB (or other digital) interface on the unit. 


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