Pye P75: How not to do it. - UK Vintage Radio Repair and Restoration Discussion Forum

G6Tanuki · 19th Jan 2019, 8:32 pm

A decade back some friends bought themselves a house; it came with a horrible late-90s-style kitchen, all light-walnut wood-veneer and MDF. So traumatically early-Britpop it just had to go!

The Pye P75 that had sat on a shelf in this old monstrosity got gifted to me, the 'new' kitchen having been equipped with a Bose SoundDock so they could play music from their iPhones.

I only recently got round to looking at the P75; it had been unused for 10 years. Replaced the old figure-of-8 single-insulated mains lead with something modern, and pre-emptively replaced a few known-weakness capacitors. First-sound waa fine - OK, a bit deaf but a replacement ECH42 solved that. The big problem was a persistent background audio-crackle.

OK, so the EL41 is known to be a valve that has issues. Hard-wiring its grid to ground didn't stop the crackle. Replacing the EL41 with a 5KOhm resistor from anode-to-cathode didn't stop it either! There was another noise-beast to slay.

Testing pointed at the output-transformer having 'internal issues' - so I grabbed a similar-spec transformer from a scrap radio and fitted it: Joy! Lots of audio and no crackles!

So I dragged-out the original output-transformer and fitted the one from the scrap-set in its place. I only needed to drill a single mounting hole in the P75's chassis to make this substitution, because the substitute-transformer was about half an inch longer.

But - what's this? utterly-feeble audio and lots of distortion!

Get out the 'scope; there's not much voltage-swing showing on the EL41's anode. Swap-in a new EL41 - no difference.

Drink Gin-and-Tonics, go to bed.

Wake up the next morning, drink coffee, get enlightened! I'd wired the primary of the output-transformer wrong! It's got a hum-bucking winding - I'd connected the hum-bucking winding to the poor EL41's anode, which explains the pathetic audio...

Rewire, and - oh, happiness! TalkSport on MW is now really punchy and full of volume!

snowman_al · 19th Jan 2019, 9:28 pm

Are you sure the sequence is correctly described?

Maybe the G&Ts came before the fitting of the opt...

19th Jan 2019, 8:32 pm	#1
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Pye P75: How not to do it. A decade back some friends bought themselves a house; it came with a horrible late-90s-style kitchen, all light-walnut wood-veneer and MDF. So traumatically early-Britpop it just had to go! The Pye P75 that had sat on a shelf in this old monstrosity got gifted to me, the 'new' kitchen having been equipped with a Bose SoundDock so they could play music from their iPhones. I only recently got round to looking at the P75; it had been unused for 10 years. Replaced the old figure-of-8 single-insulated mains lead with something modern, and pre-emptively replaced a few known-weakness capacitors. First-sound waa fine - OK, a bit deaf but a replacement ECH42 solved that. The big problem was a persistent background audio-crackle. OK, so the EL41 is known to be a valve that has issues. Hard-wiring its grid to ground didn't stop the crackle. Replacing the EL41 with a 5KOhm resistor from anode-to-cathode didn't stop it either! There was another noise-beast to slay. Testing pointed at the output-transformer having 'internal issues' - so I grabbed a similar-spec transformer from a scrap radio and fitted it: Joy! Lots of audio and no crackles! So I dragged-out the original output-transformer and fitted the one from the scrap-set in its place. I only needed to drill a single mounting hole in the P75's chassis to make this substitution, because the substitute-transformer was about half an inch longer. But - what's this? utterly-feeble audio and lots of distortion! Get out the 'scope; there's not much voltage-swing showing on the EL41's anode. Swap-in a new EL41 - no difference. Drink Gin-and-Tonics, go to bed. Wake up the next morning, drink coffee, get enlightened! I'd wired the primary of the output-transformer wrong! It's got a hum-bucking winding - I'd connected the hum-bucking winding to the poor EL41's anode, which explains the pathetic audio... Rewire, and - oh, happiness! TalkSport on MW is now really punchy and full of volume!

19th Jan 2019, 9:28 pm	#2
snowman_al Octode Join Date: Dec 2018 Location: Llandeilo, West Wales, UK. Posts: 1,092	Re: Pye P75: How not to do it. Are you sure the sequence is correctly described? Maybe the G&Ts came before the fitting of the opt...