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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 6th Feb 2018, 4:09 pm   #1
mathologie
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Default Brenell Mini 8.

Hello there.

I am brand new to this Forum!
Born in England living in France ever since I still feel very British but i am better in French than English.
I Had a big 8 track Brenell that I had to sell in teh 90' because I could not synchronise it to video. I made music for films.
The sound was fantastic. So I got a Brenell mini 8 a few years ago and the sound is great too. The tape are expensif and the machine is now getting old and a bit tricky when rewinding sometimes. It stayed a few month in dampness and every now and then à few capacitors die.

My plan is to replace all the 63V 100 uF (Philips) capacitors in the preamps about a hundred ! To start. I changed the ones for the motors because one of them started to smoke very seriously
The Preamps heat up a lot. The ventilator was so noisy I had to change it for a more silent but maybe weaker one.
A French forum indicated this web site (see below) for me and the suffix FC for the model/serie.
But I couldn't get more details and I am not sure if these capacitors will respect the original sound.
So if someone can help me with my choice her is the info I have found.
I don't care in this case if the condensers don't look vintage as long as the sound is as good as can be without spending a lot on it: I don't care if they are not radial if the sound is the same.

https://www.mouser.co.uk/Panasonic/P...yzvjj0Z1z0j6cv

Thanks.
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Old 10th Feb 2018, 10:01 pm   #2
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Default Re: Condensers for Brenell mini 8 preamps

I used to regularly fix one of these for someone and the fault was nearly always one of the tantalum capacitors on the amplifier boards. I would guess that they were originally chosen for their small size but modern electrolytic capacitors are much smaller than those from 30 or 40 years ago. I would think that the ones you link to should be fine but check out the CPC website as they may be slightly cheaper from there. I traced out the circuit ages ago but don't have it to hand at the moment.
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Old 11th Feb 2018, 1:25 am   #3
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Default Re: Condensers for Brenell mini 8 preamps

Here is the manual it is on the AHB website:
http://www.allen-heath.com/media/MIN...NER-MANUAL.pdf

The schematic is here:
http://www.saturn-sound.com/services...20diagrams.htm

Pretty sure that the schematics used to be on the AHB website, the other later machines are there.
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Old 7th May 2018, 1:20 pm   #4
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Default BFX84 replacement transistor

Hello

Still trying to repare may Brenell Mini8.

Does anny one know a real equivalent to the BFX84 used in the seventies?
My electronic dealer sold me BC141 but the HFE is much higher if I look on the data sheet and also when I test it on my digital meter.
My BFX84 give 18 or 20 an other one 56 and yet an other 104 while the BC141 give a reading of 182 !
If I test two by two with the diode position it gives .52V to .53V for the old BFX84 and .54V for the BC141.
From these readings can one be sure the BFX84 are functionnal? I have other old one they give only similar values. The data sheet sais HFe 20 to 30 I don't understand all this. I think it will drive me mad but I can't find any deecent reparator here in Paris.

Thanks for your help.
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Old 7th May 2018, 9:32 pm   #5
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Default Re: Brenell Mini 8.

Those appear to be the driver transistors for the erase head - are you having problems with erasing on some channels. Have you checked the simple things like the relay contacts first?
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Old 8th May 2018, 6:22 am   #6
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Default Re: Brenell Mini 8.

All a transistor does is turn on A) hard - saturation,B) turn off "hard" or C) if used as an amplifier stay turned on "in the middle" Whichever, if it's turned on there will be about 0.6v if you put one probe on the base and the other on the emitter, DMM set to DC volts.

So you can tell if a tranny is working by measuring this voltage drop, the tricky bit is reading the circuit
so you know what the tranny should be doing, whether it is supposed to A,B or C . To further complicate things old trannys can go leaky causing weird faults.

Question is are you replacing the tranny's because there is a fault, or because your think putting new trannys in will make the Mini 8 work better/ you've read somewhere that it's the thing to do?

Changing components wholesale is dodgy, it can lead to the thing your trying to fix not working at all. One other note of caution, tech's/repairer's often won't touch a device that's been "got at" . If they get a device and bag of bits they're liable to run away.

One last thing, if you have two channel's, one working, one not, you can compare the two, use the working channel as a reference.

Andy.
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Old 8th May 2018, 7:42 am   #7
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Default Re: Brenell Mini 8.

Quote:
Originally Posted by Diabolical Artificer View Post
One other note of caution, tech's/repairer's often won't touch a device that's been "got at" . If they get a device and bag of bits they're liable to run away.
Experience has taught them that running away is often the best option.

What some people will do trying to fix something is amazing. A van was pushed down to dad's garage... "Wouldn't start" Hmmm, that bolt holding the alternator on looked awfully like one of the high-tensile ones that should be holding the big end caps on, therefore.... Oily hand prints still on the exhaust manifold.... Just think what someone without a clue could do with a TV or an AC/DC radio. The van made a loud bang a couple of days later. The non-isolated chassis is far less obvious... and deadly.

If a set is full of certain types of components and a significant number have degraded, then the right thing may be to change the lot. Then it comes down to the quality of work and care, or else doing them in a small quantity at a time. My workmanship is pretty decent (aerospace experience) and my fault-finding skills are good (getting prototypes going where you don't even have the clue that it once worked or even could work) so I wade in confidently when it looks like a clean sweep is economical and forestalls highly likely further failures.

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Old 8th May 2018, 7:45 am   #8
mathologie
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Default Re: Brenell Mini 8.

Quote:
Originally Posted by jamesperrett View Post
Those appear to be the driver transistors for the erase head - are you having problems with erasing on some channels. Have you checked the simple things like the relay contacts first?
Yes They are the drivers for the erase and record bias signal.
I have tested most connections but like NONE of the tracks record or erase any more Geoff (from Servicesound) Who is a specialist in Brenell told me it must be the Master Oscillator.
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Old 8th May 2018, 9:04 am   #9
mathologie
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Default Re: Brenell Mini 8.

Quote:
Originally Posted by Diabolical Artificer View Post
Question is are you replacing the tranny's because there is a fault, or because your think putting new trannys in will make the Mini 8 work better
Thanks for all the answers.
No I am not changing parts for my pleasure.
I had'nt turned on the machine for six months while changing the 100µFcapacitors+ I had a little flood in the studio (dampness). And When I turned on the machine recording didnt work any more.

Then I contacted ServiceSound.
Checked the cables. The connectors. Had a look at the relays.
I looked at the PSU and noticed a leaking big 3300µF capacitor in the AC/DC filter. Measured tensions. Much too low if I remember. Changed the capacitor after a lot of fuss (trying to find a good one).
No difference except tensions are good.
Beside on the 40V PSU board a cracked resistor marked .33 OHM (R14 on the 40 V PSU) that was more than 45 Ohm. I changed that also. But still recording does not function. Maybe that caused a problem?
Tried to understand the requiered tensions on that board but it is still a little mysterious.
I am thinking to send the PSU to ServiceSound in England. But still hoping I can avoid it.
Just trying to track that problem down a little. So I dont send the PSU while the the problem is in the tape recorder for instance or if it is one transistor burned out.
If I could solve the problem without changing any thing I would be so pleased. I will go through the connectics again.
I will put the photos to illustrate about the Brenell mini 8 that is a beautifull machine with a lovely modern and dynamic sound.
(It is also very interesting to try and understand how these things function) I get caught into it!
Maybe I should go back to school trie to study electronics! I think I am getting to old for that.
In anny case thanks for your help.
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Old 10th May 2018, 6:07 am   #10
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Default Re: Brenell Mini 8.

"Experience has taught them that running away is often the best option" Exactly.

For a tape machine to record it has to do quite a few things, first it has to get all mechanism's lined up, on older machines usually done with intricate cogs, rods and levers, so you should be fine there. But still worth checking out.

Then it has to erase the tape by putting AC (from the oscillator) on the erase head. To check that, use your DMM, if it's half decent it should be able to "read" it.

It also has to route the signal from the input's, to the record head, usually done with complicated switching (switches/relays). If a relay on your board isn't being turned on, no signal to the heads.This is the hard bit. Schematics are very hard to read where switching is concerned, an oscilloscope is invaluable here as is a sig gen, which I think you don't have. You could build a simple buffer and 10x gain amplifier with a single op amp to drive a small speaker as a "sniffer" to see if the sig in, is getting to the record heads. Without a sniffer, your going to struggle.

Still the "muck about for a few hours/days/weeks" approach can work, it ain't easy though. Your best bet is to check voltages or tensions as you call them. Get the schematic a bit of paper and write what you find down, post here.

It's hard for us this end to advise or know what your doing as English isn't your first language and being Englishmen, we don't speak French as a rule being too lazy, prefering to shout and wave our arms about at foreigners instead. That and we can't see the thing. Try posting some good pictures, it really helps, and gives us something to look at in a post, rather than having to read all the text : )

Bon chance, Andy.
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Old 10th May 2018, 1:47 pm   #11
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Default Re: Brenell Mini 8.

That circuit with a 40v supply needs a transistor with at least an 80v rating to be reliable.

BFX84 is rated at 60v

2N5682 looks like it may be a better replacement.

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Old 10th May 2018, 3:17 pm   #12
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Default Re: Brenell Mini 8.

Hmmm....

With reference to post #8 showing the multivibrator/oscillator, if it was not running, I would not be surprised at all. For this type of oscillator to start it requires a moderate amount of current gain in each transistor and a small imbalance that is nearly always there.

But, the design is defective; with each cycle of oscillation the base-emitter junctions probably get reverse biased well past their zener voltage. Relentlessly zenering the junctions slowly degrades the transistor's current gain, ultimately doing this, it will get to the point, one day (now maybe), where the oscillator does not start.

So you are probably right that these transistors are defective now. When you replace them you could put B-E diodes on the base to emitter junctions to protect them. This will change the oscillation frequency, but, you can fix that by altering the capacitor values to get back to the original frequency.

Of course this osc might be running and you have another connection/wiring fault etc, but if there is no activity from the osc on the scope, it will need new transistors.

As a quick test put a 10R resistor across one of the emitter resistors and see if the oscillator starts.

Last edited by Argus25; 10th May 2018 at 3:29 pm.
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Old 10th May 2018, 3:33 pm   #13
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Default Re: Brenell Mini 8.

THere's lots of volts for base bias, so the added drop of a series diode in the base lead would probably not upset things much and would protect the b-e junction.

Reverse biasing to avalanche breakdown the b-e junction of a transistor is the easiest way to kill one there is.

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Old 8th Jul 2018, 9:14 pm   #14
mathologie
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Default Re: Brenell Mini 8.

Hello,

I took my Brenell to a guy who said he could repair it but finally he couldn't and he gave it back to me asking for 20€. I think he didn't know anything about tape recorders but we did test the voltage coming out of these BFX84's on his oscilloscope and here was a beautiful sine wave of around 16VAC between the two collectors and about the same voltage coming out of the final transformer. The specialist at Servicesound in England mentioned 100mV between these collectors so I am really troubled. The R12 resistor had been replaced by a 10 Ohm instead of 100 Ohm so I changed but no significant change.

Through testing the tracks one by one alone in the deck I noticed two of them still record and erase just about perfectly. I wonder if these two track are not just a little stronger than the others and managed to not burn out with the too strong voltage of the oscillator? What do you think? What AC voltage does a master oscillator output usualy on such a kind of machine? Is there any way someone could calculate it?

Thanks for your answers.
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Old 12th Jul 2018, 11:11 pm   #15
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Default Re: Brenell Mini 8.

BFX84's are available here
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Old 13th Jul 2018, 6:27 am   #16
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Default Re: Brenell Mini 8.

That circuit is a power oscillator. When running it should have a high AC voltage on the collectors of the transistors, and a high voltage coming out of the transformer.

C5 resonates with the transformer and should make the output sinusoidal.

The injection of the bias signal to the recording head needs this high voltage. It seems that this circuit is working OK. That mention of 100mV is probably the result of some confusion and is likely to be meant somewhere else.

Having two channels working confirms this.

I do not believe the oscillator voltage has destroyed six channel cards. I believe it is OK, and there is also a filter trap to keep the bias voltage from going backwards into the output of each recording amplifier.

Bad relay contacts are a high possibility, dirty connectors too. If these are OK, then it comes down to following a signal through a bad channel to find where it stops. AND you have a good channel to compare it against. This should not be difficult to repair.

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