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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 29th Nov 2020, 4:34 pm   #61
Trevor
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Default Re: Identification of unknown PA amplifier

Chris dont worry re the capacitance the board is designed for two extra output devices in any case I feel you are over thinking it in this respect
I will as soon as I have time construct the circuit But I have a lot on at the moment
Hoping to retire in 3 months 68 next week and will have time to play
Trev
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Old 29th Nov 2020, 5:06 pm   #62
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Default Re: Identification of unknown PA amplifier

Blimey 68 should have been 65. Still good news Trev nice one you'll probably find your more busy once retired than when you where working. But more enjoyable not beholdent to anyone.
Okay I'll leave off the spec sheets. Bit of a perfectionist find it hard to ignore things. Ha ha bit of OCD there.
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Old 30th Nov 2020, 10:17 am   #63
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Default Re: Identification of unknown PA amplifier

Little bit of confusion here now the darstan resistor is measuring between 0.46r and 0.3r thats the two green ones. The reading on case is B27K this is for sure a code. Unfortunately I'm not able to track this down but I'm confident it's a very low ohm resistor as per readings above.
The grey ones again 72 ohm 11 again is a code on removing it on the underneath is 27r which it reads spot on at 5%.
So you where right re "are you sure its 27k".
So solved
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Old 30th Nov 2020, 12:02 pm   #64
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Default Re: Identification of unknown PA amplifier

For that green resistor the value is too low for you to get an accurate reading from a regular DMM.
You can get an accurate reading with a four wire meter.
The readings you are getting include the resistance of the leads.
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Old 30th Nov 2020, 12:24 pm   #65
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No my meter I use goes from 1r to 0.0001 not a general run of the mill job. The 0.3r error on that is +/- 1% using my low ohm meter. Other taken from normal dvm.
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Old 30th Nov 2020, 12:28 pm   #66
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Default Re: Identification of unknown PA amplifier

‘cat walks in ...’

That’s hilarious ! I’m glad it was only a spark...

Cats huh, they do have superpowers.
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Old 30th Nov 2020, 12:48 pm   #67
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Yes she does it everytime. Don't want to know me then as soon as I'm testing she's in ****** nightmare the spark to be honest was about 2" inches like small bolt of lightning ha ha. Cat weren't bothered just sat there purring away as I was fing and blinding ha ha.

Trev pic attached that unit rubber booted is a small inductor. Not cut that off yet see if it has any ratings on it. Otherwise I'll remove it and measure. So got the power transistors plus new diodes to go with them original parts are
IN4934 guess what Don't make em anymore thats unusual ha ha. Substituted. So be interesting to see how it sounds I'll leave one side as is and compare it to one with new components
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Old 30th Nov 2020, 12:57 pm   #68
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Here's some pics of tranny don't think posted any.
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Old 30th Nov 2020, 9:31 pm   #69
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Default Re: Identification of unknown PA amplifier

interesting link re amps of this type
https://dextrel.net/diy%2520master/60w-korrekti.html
Do not worry re the inductor never known one to fail
Trev
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Old 30th Nov 2020, 10:18 pm   #70
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Yeah very interesting. Quite a few similarities but also quite some differences. I think the main HT was 143v, then 16v, 14v 12 v etc I'll need to re check. The voltage on the one sent is incredibly low in comparison. On output side this runs from 4 ohm minimum load and up. So I'd assume it runs from 4r to 16r loads require higher output. Still once I've replaced the board I shorted be interesting to see what voltages are. Biasing pretty simple on this compared to valve equipment for sure no effort at all. Threw me do not connect speakers or any load not used to that one ha ha. Instinct is oh dear that's the op knackered.
Not worried about inductor just for your reference.
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Old 30th Nov 2020, 10:59 pm   #71
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Got that wrong voltages getting late straight off the tranny. Odd not connecting speakers not used to that one normally that would be a pop or nothing at all when you re connect speakers then 150 quid plus down the drain. Bias setting easy as pie on these blimey.
Inductor was just for your reference noticed not used one on the link you gave me.
Anyhow all good fun mind you some of those components aren't junkyard parts for sure not price wise anyway.
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Old 1st Dec 2020, 9:44 am   #72
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Just quick questions having not really worked on these before one the caps I've put in can handle up to 5.6v ripple current is that adequate? Secondly is it okay to put Y2 caps across the line on mains input? I know you have the 4 fuses on the mains in via the bridge rectifier but was just thinking along the lines of slowing down the inrush current.
Obviously on valve gear it's just habit of putting them in full stop.
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Old 2nd Dec 2020, 12:30 am   #73
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Default Re: Identification of unknown PA amplifier

"R27K" on a resistor could well mean 0.27Ω 10%. The R is used to indicate ohms and in the position of the decimal point. 27Ω 10% in same format would be "27RK". Can get nicely confusing when "K" is used in the same way for kilohms as in "27KK" or "4K7K"
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Old 3rd Dec 2020, 9:58 am   #74
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Default Re: Identification of unknown PA amplifier

Yeah when I removed it it was marked underneath 27R.
But yes that was rather confusing for sure. But I have noticed looking at these type of circuits they do use rather low resistors seems like a few act as shunts to stop an inrush of current.
I will add some Y2 on the line and ground which should give a soft start up. Works on valve gear so should be no reason why it won't on this circuit. Nearly got all my parts just waiting on two transistors from Canada. He's posted priority international for me so hopefully they will be here today or tomorrow.
At least then I can get that side working again.
I'm also going to look into those addition areas where additional parts can be added. I'd assume another transistor poss another NPN and few resistors and a poly cap poss 400v type.
I'll get new parts in first.
Cheers Herald
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Old 4th Dec 2020, 9:01 am   #75
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Here ya go Trev pic of inductor on ferrite rod enamelled copper wire around 1mm diameter and 11 turns. No markings so have not measured uH.
Hope of some help pic attached.
All best Chris
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Old 4th Dec 2020, 12:22 pm   #76
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Default Re: Identification of unknown PA amplifier

Those chokes are fitted to amplifiers that drive speakers with passive cross overs in them.
They allow modern transistors to be used as an output pair without the speaker cable becoming a transmitter aerial.
The value is not critical as long it is low enough not to drop the audio signal and high enough to reduce RF.
Some designers wind them around a resistor.
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Old 4th Dec 2020, 1:21 pm   #77
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Yep it is around a low value resistor. Rated 2%.
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Old 8th Dec 2020, 3:08 pm   #78
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Well thats all sorted new tp33/34c, (lucky to get those) in new power transistors mj series as recommended by Trev with new diodes new bridge rectifier and 2 new little 6 legged transistors forgot what called. All NOS except diodes and mj. So I definitely short circuited that board for sure. Put X2 across the lines straight off mains so no thud now nice soft start. So job done got mains voltage down from 249v to 234v. Unfortunately I'm right next to the mains power supply for Birchington so it always sits at 245v on occasions shot to 250v.
Was gonna re do all resistors but changed my mind only on one board. Considering no tone controls pretty good sound. Time will tell all new parts and caps need burning in run it in for a week. Also need to check voltages give it a couple of days to bed in.
Still not been able to find this chasis. Or the modules found similar with tp42c none with 33 or 34.
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Old 8th Dec 2020, 3:50 pm   #79
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Could not resist pic of her running and the PA speakers running her through. Nice boooooom. Ha ha.
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Old 8th Dec 2020, 9:11 pm   #80
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Default Re: Identification of unknown PA amplifier

Well done So pleased you have sorted it ! did you add the extra transistors?
Best regards Trev
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