Pye Black Box amplifier PCL83 version

[Edward Huggins]

It was not so much cost-shaving back in those days as these were relatively low volume products compared to TVs - but the continuous tinkering was often the culprit.....This circuit relied very much upon the components maintaining their tolerance. This was fine of course back in 1961 when all were new. As far as I can remember R&A (in house company) drivers were used. Not so on the later 1004 BB model.

[Michael Maurice]

I have measured the two resistors R27 and R28 and they are both within spec of each other.

The anode voltage on the anodes are 223V the same on each anode

Originally Posted by Steve_P

I wonder how much the circuit has to be modified to take a PCL82?

The Bases need to be rewired but apart from that...? On Second thoughts the heater voltages....

Just a thought

Cheers,

Steve P.

You probably could use a couple of ECL82's and wire the heaters so they are in parallel, however ECL82's are too tall and would interfere with the deck mechanics.

I could put a 2.2R resistor across the speaker terminals to model a 1.5 ohm speaker.

So really have no choice but to keep to the original design.

Originally Posted by Edward Huggins

It was not so much cost-shaving back in those days as these were relatively low volume products compared to TVs - but the continuous tinkering was often the culprit.....This circuit relied very much upon the components maintaining their tolerance. This was fine of course back in 1961 when all were new. As far as I can remember R&A (in house company) drivers were used. Not so on the later 1004 BB model.

Edward, all but a few resistors which have now been replaced are within tolerance, the hunts capacitors have been replaced with 47nF replacing 40nF's and I have repaired probably tens of these with no problems.

[behindblueeyes]

No idea if this is helpful, but when I repaired one of these which arrived with a burned out power transformer a few years back (with extensive help from the people on this forum) I ended up replacing the suspect PCL83s with 30PL1s, which were reckoned to be more robust.

Apart from the speakers, output transformer and switches/pots I basically replaced everything else in accordance with the wiring diagram and a pencil and paper to try and make sense of the rats nest of components. Fortunately I didn't encounter the issue you're experiencing.

It gets used at least once a week and has been fine ever since (fingers crossed).

[Michael Maurice]

Thanks for that Blueeyes, What I may do is to rebuild another amplifier with a PCL83 and then swap the O/P transformers to see what happens but that will have to wait for a couple of weeks.

[Steve_P]

Why not an ECL83?

Cheers.

Steve P.

[Michael Maurice]

ECL83 have 6.3V heater, PCL83 has 12.6V heater Anyway from what i can gather the ECL is as bad as the PCL.

[peterpiper]

Hi, have you tried reversing the polarity of the speaker winding?

Peter

[Michael Maurice]

Yes Peter I have, it made a very loud feedback noise

[Michael Maurice]

I've now found another black box lurking in that black hole called a storage unit!

I'm servicing it now but thought I'd take cold readings of the original transformer (which is good) and the rewound one.

All readings taken from the Anode tag and are in ohms


Original Rewound

1st Anode: 105.4 104.4
1st G2 20.3 33.9
2nd G2 17.6 30.1
2nd Anode 82.6 107.4

To me, these readings show the rewound one to be incorrect.

Any else care to comment.

[Radio Wrangler]

The original shows an imbalance in the DC resistance of the two primary halves, as you'd expect due to one winding over the top of the other, and the different lengths of copper due to their different average diameters.

The re-wound one is quite balanced. I wonder if the anode windings are interleaved layers, bifilar wound or just scramble-filled with two runs of wire?

The re-wound one could be quite different in terms of inter-winding capacitance.

No guesses about what's going on with the Williamson taps!

David

[Herald1360]

Might be interesting to excite half of the primary with about 50v ac and do some voltage checks to establish winding ratios and see how they check out.

[Michael Maurice]

The second black box amplifier has now been serviced and is working well.

[Radio Wrangler]

Inter-winding C tests may yield some clues, too.

David