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

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Old 8th Dec 2020, 8:34 pm   #81
cdm1christopher
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Default Re: Identification of unknown PA amplifier

Do you mean the empty spots where extras can be added to the heatsinks? Plus looks like a couple more resistors aswell. If so no.
Was not sure what to put in there.
Tell you what after hour of running it had to turn it down once everything started to warm up and the smell of the the new heatsink grease.
One thing never knew always thought with valve gear but had her running for about 4 hours will be all day tomorrow and on switching her off the cd was still playing through the speakers for about minute and half. Took a while to discharge. Was surprised by that thought the switch was shot on it initially.
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Old 11th Dec 2020, 8:34 am   #82
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Default Re: Identification of unknown PA amplifier

Trev any suggestions on a descent eq module bass/treble that will fit into the op amp. Plus dB meters. Had good look round but nothing really sticks out.
Cheers Chris
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Old 12th Dec 2020, 2:14 pm   #83
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Default Re: Identification of unknown PA amplifier

Main rail 45.6v assuming that's about right?
Readings still bit off on right module noticed one of the small resistors got bit hot and suflex cap 150pf looking bit rough outer casing looks like it's also got hot. I'll change out all resistors started today but missed buying a 12k so everything got put away after that.
Interesting the preset potentiometers set both at 10k there 22k why put a 22k in? Don't get that! Although could be affecting bias on right side. Again I'm guestimating here I'll get some new ones and change those suflex caps aswell might as well go whole hog.
Like say think it has been run hard throughout it's lifetime and looks like it was run until the main caps just gave up after leaking probably went up in a puff of smoke.
Like say sounds fine but need to get those voltages and current same both sides. Not been getting very hot either.
Anyhow solder on.
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Old 12th Dec 2020, 10:19 pm   #84
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Default Re: Identification of unknown PA amplifier

Sorry been away for a few days
If you go to a few auction sites you will find tone control and mixer modules advertised
Same with vu meters or indeed led bars
You say 46 volts is that across each cap or both ? i.e. plus 46 and minus 46
That should be good for about 100 watts into 8 ohm if its the later
Are you in a position yet to post a schematic ?
Regards Trev
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Old 13th Dec 2020, 7:45 am   #85
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Default Re: Identification of unknown PA amplifier

That's it you got it. Like say moved preset potentiometer to 12k on right side left over side at 10k. Make sense as Bins are 100 watt each.
Yeah looked at those not quite what looking for.
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Old 13th Dec 2020, 7:25 pm   #86
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Default Re: Identification of unknown PA amplifier

I'll sort out a schematic by end of next week and post for ya. Still trying to work out those darstan resistors. No info on them at all in relation to the codes well anything.
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Old 14th Dec 2020, 1:51 pm   #87
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Default Re: Identification of unknown PA amplifier

Trev here's some pics of module you should be able to blow up and follow the traces.
I'll list the parts and post. But you should be able to work resistor values out all coded. If not let me know. You should be able to see jump wires.
Tip33c tip34c
Mj15025g
Mj15024
Green resistors 0.3r x 2
The OHMITE gold 75r x 2
Diodes on board standard 1N4005 x 3
Diodes on heatsink IN4934 x 2 on each this are as near match to originals except ns rate is 300 as opposed to 150 for originals. That was closets I could get either it was 75 or 400 above or below so went with 300.
Caps small cans 10uf 63v
Main filters were 2200uf dual cans x 4 63v I used 4 single 6800uf 100v
Bridge Rectifier International KBPC602 X 1
OP AMP 741 (UA741CP) X 2
Red cap dogbone 75pf
Large yellow cap 1.5uf x 1
Green cap glass paper type 0.01uf x 1
1000pf x2
1500pf x2

Think that's it.
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Old 16th Dec 2020, 5:37 pm   #88
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Default Re: Identification of unknown PA amplifier

Mega upgrade. Not a of lot room for new components bit tight.
Test run tomorrow.
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Old 24th Dec 2020, 8:38 am   #89
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Default Re: Identification of unknown PA amplifier

Guys quick question
The bk electronics boards have earth points and theres also a earth to chasis from mains.
Is there a reason why the board earths are switchable on/off. Don't understand reasoning behind it.
Cheers Chris
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Old 24th Dec 2020, 8:50 am   #90
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Default Re: Identification of unknown PA amplifier

Maybe to float the amplifier so it takes its earth from the signal source, to break up an earth loop.

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Old 24th Dec 2020, 9:02 am   #91
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Default Re: Identification of unknown PA amplifier

Ah cheers David I've been reading up about that. I'm just starting to understand how these circuits work. Obviously your looking at dB gain and low distortion around 0.02%. So your looking to increase high current as opposed to high voltages. This is where the mos fets come in. So your basically using small devices along with the diodes to create a valve amp sound. Hence gain, drain, emitter, anode, diode etc and doing away with bulky transformers. Quite interesting. Considering the circuit does not seem that complicated it's not as easy as you think to get the current ratings voltages etc spot on to stop thermal runaway along with other things. Can see why you find it an interesting subject.
I have a huge military AF amplifier 300 watt but I'm gonna strip that and put the bk modules in there. Fan already built in so don't need to worry about heat. Biasing is not easy on these and trying to get the input output voltages right harder than you think. The variable resistor which I stupidly played around with started giving me 5v on the output. Oh deary me. I'm learning.
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Old 24th Dec 2020, 9:27 am   #92
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Default Re: Identification of unknown PA amplifier

Blimey those modules today would cost 350 each in todays money. 700 quid. Thats without anything else so in essence you'd be looking at over grand for proper amp setup. So if you start adding descent tone controls various inputs etc blimey �� starts mounting up. The other one in todays money cost around around 2k. Blimey.
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Old 24th Dec 2020, 10:00 am   #93
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Default Re: Identification of unknown PA amplifier

This is what I'm stripping the cabinet is top class in in mint condition look at the size of those caps plus massive fan housed at the back of the unit. Unfortunately someone has messed with it and only input is for a mic which has been pulled. Thats why guy got it of took no pics of front of it. Plus wires cut left floating around. Solder on caps when there screw terminals? So no schematic to go by too much in there to try and trace it all out. Probably good for parts.
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Old 25th Dec 2020, 5:17 pm   #94
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Default Re: Identification of unknown PA amplifier

That looks to be a military-spec amplifier. Too recent, for modulating an AM transmitter, I think it's for driving a group of public address speakers.

Normally, transistor amplifiers are designed to drive speakers directly, but the presence of a second, smaller transformer in it makes me thing the unit is designed to drive speakers via a '100 volt' line to reduce wire losses.

It'll likely be adaptable to driving speakers, but it's also likely to not be designed to do much bass.

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Old 25th Dec 2020, 5:37 pm   #95
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Default Re: Identification of unknown PA amplifier

Yeah big old beast gonna get stripped and I'll use the chasis for those bk modules plus it already has a large fan at the back with heat sensor so be ideal. As those modules do get hot. Mind you whoever made the chassis up don't think much of the wiring setup. Plus it has had wires fitted in and then all been sprayed black so you have black wringing almost everywhere called lazy me thinks.
I think that floating earth switch is to do with mixers and volume controls as when switched over the circuit don't recognise the resistance of volume so you can't switch it to down to zero signal.
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Old 26th Dec 2020, 11:33 am   #96
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Default Re: Identification of unknown PA amplifier

As this unit is now being broken this thread has served its purpose.

A new thread can be started if required when the New amps are built into the case.

Cheers

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