P.O loudspeaker amplifier

[yestertech]

This has been on my round to it pile for a considerable time. As I recall, I picked it up at one of the NVCF events. I think it cost about a tenner and was marked as “not working“
The plan was to use it as an audio ‘sniffer’ for fault finding.
The DC fuse was blown it but quick checks on the power rails revealed nothing I miss.
Having replaced fuse, there was no signs of life until the fuse popped again.
It contains a single PCB with a 10 discrete transistor amplifier, which was quite difficult to get to due to the stacked construction.
I delved straight in and checked the output and driver transistors which were fine of course.
I then packaged up all the bits and tucked it away in a cupboard for another day.

That day came yesterday when I rediscovered it in side cupboard. I found all the hardware and three transistors which had been removed for testing. These were replaced, the driver transistor substituted for a new one with longer leads, to supplement the missing very flimsy tracks, which had lifted on desoldering.
A fresh was fitted, and there were slight signs of life as the input power was increased.
There was a lot of hum present, and connecting a signal generator produced a very ‘warbly’ thin output. As the power was increased further, the hum also increased and I sensed a slightly warm smell coming from somewhere – at this point, the fuse went again.
All the remaining transistors checked out okay, as did all the electrolytics.
At this point, I spied several tantalum capacitors on the PCB, and, having been kindly sent the service data by a forum member, confirmed that two of these sat across the power rails, and one was in fact getting rather warm. Both were replaced with ordinary electrolytics and the unit then powered up OK but with a very loud hum drowning the signal. After much head scratching, I realise that with less than 100% power correct DC conditions were not being applied. On increasing to full mains, the hum instantly vanished leaving a small amount of white noise at full gain. Finally…..success
The unit was reassembled, retested and then I turned my attention to the rather forlorn looking case. This is a simple, wooden box, painted grey and was just grimy, with various stickers attached. Once these were removed, I gave it a rubdown with T-cut and a good buffing which looked much better. The knobs were removed and cleaned and then the rather grubby panel was T- cutted also. Thankfully the printing survived this treatment !
The only thing which really lets it down now, is the rusty chrome handle end caps. My chrome polish has currently gone AWOL, so this will have to wait. A quick spray of “back to black“ improved the look of the handle strap.
So, after a very long wait, I now have my signal tracer !
It has a handy differential input and plenty of gain to search out the very smallest of signals.
Hopefully it will prove useful.

Andy

[bobbyball]

A good job well done then

[Radio Wrangler]

There's definitely more satisfaction after one that's put up a bit of a fight, doubly so on one that's done the rounds and been given up on before!

David

[McMurdo]

I like it, was it for one specific job or was it a general-purpose thing?

[yestertech]

I imagine it was for general monitoring of lines and checking for faults etc. It would have had the ability to be used in exchanges or other comms centres, as well as for field service, as it has a battery option ( 2 x PP9)

A.

[FIXITNOW]

I have one off these somewhere but its missing most of its internals was going to Bluetooth it, but this looks like another good use for it with one off those
Velleman signal tracer kits

[Hartley118]

Originally Posted by yestertech

I imagine it was for general monitoring of lines and checking for faults etc. It would have had the ability to be used in exchanges or other comms centres, as well as for field service, as it has a battery option ( 2 x PP9)

A.

Yes, it's for line monitoring, bridging a '600 ohm' telephone line, but I do recall working in a PO repeater station where the commonly monitored line was the Extel private wire racing commentary service!

Martin

[newlite4]

There was one of these in the varnished wood case option on the BVWS stall at Punnets Town earlier this month. I had not seen one before then. Handy bit of kit for the bench.
Neil

[CambridgeWorks]

I remember seeing those around 1990-91 at a scrapyard/ electronics breaker and turned my nose up at them back then! Could have bought maybe 50p each.
Plywood box with a loudspeaker and amplifier? Who would want such junk! So never bothered.
Around same time loads of the wb1400 early warning system gear, brand new in boxes was broken up. Was I interested buying very cheaply? No!
These days, those cold war things are worth serious money!
Hindsight is a wonderful thing.
Rob