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Beolab 1700 Speaker No Signal Current High, AUSTRALIA

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Cooker
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Cooker posted on Fri, Jun 2 2017 9:13 AM

Hi all,

With my Beomaster 8000 out of action for the time being (will go to Rudy soon) and thus having no amp to connect my 8002 to, I pulled out a Beolab 1700 from my non working collection of B&O stuff and tried my hand at restoring it as I live in Western Australia and there is no one reputable I know close by that will even look at old B&O. So I thought I'd give it a crack.

 

I ordered the kit from Martin and whilst waiting for the post, studied the service manual in detail and dissasembled the amp ready for the repair kit. I also sanded the wooden top and removed the scratches present and then oiled with boiled linseed oil. Came up lovely.

The repair kit arrived with the usual high quality parts and colour diagram courtesy of Martin.

The original main 3 big power caps were swollen and soft, so I didn't dare turn on the machine until I had at least replaced these. 

I fitted the new 3 large caps and turned on the unit. Light came on and I tweaked the main voltage trim pot to 32V (was only slightly out) and the other speaker Left/Right no signal current trim pots. Specified is 10mV. They were at 1.1V and would not adjust lower then 1V. The rear heat sink was only slightly above room temp, not even warm, so I thought I'd hook up some speakers and see whats up before I continue with the rest of the restoration. 

There was an initial loud bang/crackle when turning on speaker input 1 switch and a quiet bang/crackle when turning on speaker input 2 switch. I played music through PH and Tape. Sounded ok, nothing great, a little bit tinny. 

Turned it off and studied the next step to restore. The speaker bang/crackle was a concern, so I looked at the wiring diagram and saw 2 small capacitors that were in Martins kit that lived near the speaker switch circuit.

I replaced these and turned on the unit again. When I pressed the speaker input 1 and 2, it was silent. Excellant.

I then commenced to replace all the other parts in the kit including new trimmers.

Once completed, I turned on the unit, adjusted the new main voltage trimmer to 32V and then moved onto the speaker trimmers. I was surprised to see they were still at around 1V still. No difference then before. I adjusted them equally to the lowest they would go (1.05V). I played music through the amp again and wow. The amp sounded really really good. Sound quality was massively improved with the tinny sound now gone and the sound quality of Phono via my 8002 was brilliant! With no music playing, all the inputs were dead silent even at max volume. Heat sink was only just slightly warm after 20min of playing music.

My electrical knowledge is 0. This is the first amp I have ever opened and attempted to restore. From a sonic perspective, it would seem a success. But 1V is not 10mV and as such, there must be an underlying problem.

I have not used the amp since for fear of damage either to my speakers or the unit itself.

I liaised with Martin and though he was extremely considerate, it was understandably difficult for him to problem solve half way around the world with a dummy  when it comes to electronic circuitry.

Martin did check I was carrying out the measurements on the correct pins (sent him photos) and I have used two differant multimeters to rule out a faulty meter reading. 

I also checked the new trimmers prior to installation and they were correct.

 

Some info I thought was strange,

* There are 3 large main power caps.

When initially powered on, the large left channel positive pin power cap reads 9.5V and then starts dropping rapidly till around 7V. This takes around 2 minutes to drop from 9.5V down to 7V and it happens in real time on the multimeter screen. Then it slowly slowly starts dropping even lower the longer it is left on. After 10 minutes it is down to 6.4V which is the longest I run it for whilst testing and it is till dropping.

The large right speaker channel positive pin power cap reads a constant 22.3V always. 

The large main power cap positive pin reads a constant 47.5V.

 

The following voltages ALL drop very very slowly over time and always read slightly different every time you take a reading depending on how long the amp has been on for. The longer its on, the lower the voltages.

5TR15,

B - 6.3V

C - 47.5V

E - 7.4V

 

5TR17

B - 6.3V

C - 0/EARTH

E - 5.5V

 

5TR20

B - 22.3V

C - 0/EARTH

E - 21.4V

 

5TR18

B - 22.3V

C - 47.5V

E - 23.5V

 

Can any one shed any light or offer any suggestions to some more checks I should make?

PS: Being in Western Australia, there is no one I know that is reputable to look at old B&O and a few audio experts I have approached decline to touch old B&O. There are a few of us B&O lovers in W.A that share this common dilemma. Is there ANYONE that is confident or specialises in old B&O in Australia? I have a heap of B&O stuff, as do others I know, that need stuff looked at and posting to Denmark or USA is damn expensive. I know, I have sent B&O things overseas for repairs 4 times in 2 years!!!.........

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ALF
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ALF replied on Sat, Jun 3 2017 3:03 AM

Greetings!

where abouts are you located ?

I am in Perth and could have a look at your amp ?!

do you have the service manual for the amp ?

if interested send a PM

cheers

ALF

Cooker
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Cooker replied on Sat, Jun 3 2017 12:28 PM

Thanks Alf. 

 

Found the wrong mV measurment was due to my Beolab 1700 being a later model to the circuit diagram, so the pins to test were different. Measured accross the actual resistor and was able to set it to 0.01mV. Excellant. 

 

There are still a few underlying problems with the Voltage creeping and there seems to be a short somewhere on the left speaker channel.

Will PM you. 

Cooker
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Cooker replied on Sat, Jun 3 2017 12:32 PM

Email sent Alf.

Cooker
Top 500 Contributor
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Amp is all fixed and working a treat! 

My first ever B&O amp restoration and alls well that ends well. Couldn't of done it without Martin! What would we do without you. 

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