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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 Yesterday, 2:51 pm   #81
DMcMahon
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Default Re: Brenell Purchases

Thank you for your inputs, from various previous measurements I think the answer to A & B most likely is No, but would not discount anything at this stage.

I will certainly check out the transformer as you suggest but in-situ, especially as early on I found out that the primary winding wire colours were somewhat different to the schematic, although at the time resistance checks of the primary winding taps made sense even though some of the wire colours were different.

I am also bearing in mind that from day 1 after replacing the 50uF dual can reservoir/filter capacitor (that had little capacitance resulting in very high mains ripple) the HT voltage has apparently been high (compared to schematic values) and also the output winding voltage, even more recently when measured using Avo.

David
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Old Yesterday, 6:10 pm   #82
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Default Re: Brenell Purchases

Mk. 5 Series 2 Update.

Did a barrage of resistance, insulation resistance and voltage tests, will not bore you with the fine details, but everything checked out OK.

Too difficult to remove the mains transformer, using Variac adjusted to suit, set the mains voltage selection switch to the 3 settings of 110V, 220 & 240 volt and got the expected outputs. The primary winding tap various resistances also made sense.

Measured the white/pink wire using Avo wrt to both ends of the output winding and only got a few volts at most, so this result supporting strongly Lawrence's suggestion that it most likely is a transformer screen wire and the DVM high voltage readings were due to capacitive coupling effect.

The blue wire (the actual output winding centre tap) measured correct voltage wrt to both ends of the output winding using DVM & Avo.

Had a look at the Mk.5 power supply for comparison. First time I have inspected this power supply, it is a right mess, everything is very sticky and dirty covered in dead insects etc, especially the cables. Been spoilt by the Series 2 which by comparison was relatively clean.

Very similar power supply with subtle differences. The primary winding taps wire colours agree with the Series 2 schematic, the primary winding tap resistances are very similar to the Series 2, so definitely the mains transformer on the Series 2 is different from the Mk. 5, at least in terms of wire colours, with the Series 2 schematic reflecting the Mk. 5 transformer wire colours.

By resistance measurement the Mk. 5 output winding centre tap is a black wire and it correctly goes to the input side of the 47 Ohm resistor (as per schematic) with the output side connecting to earth. Just these 2 wires connecting to the 47 Ohm on the MK.5

Could not tell if there might be a transformer screen wire on the Mk. 5

Had fun and games putting the Mk. 5 Tape deck back in, it sits on 3 loose rubberised metal spacer mounts (should be 4 but 1 appears missing) and found it very difficult to get the deck in position without the mounts falling inside the case.

Now need to put everything back together on the Series 2, replacing the 470 Ohm resistor and two capacitors and correcting the wiring to the 47 Ohm, nothing so far really explains why the 470 Ohm burnt up ?
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Old Yesterday, 6:48 pm   #83
ms660
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Default Re: Brenell Purchases

If it's burnt up violently I would check for intermittent dead short in the wiring/interconnect plugs and sockets/function switch, a whisker of wire or small blob of solder can sometimes wreak havoc on an intermittent basis.

Lawrence.
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Old Yesterday, 7:34 pm   #84
DMcMahon
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Default Re: Brenell Purchases

When it occurred there was zero current/voltage monitored at the HT connection into the amplifier, which seems odd. This is the only connector really involved, have already closely inspected that connector/pins with nothing untoward seen. I felt (with my finger) the two 10k resistors (including the one that previously got a bit cooked) which connect directly off the HT input to the amplifier and they were cold, that still leaves the direct HT connection to the output transformer circuitry but because there was no voltage/current seen think more likely the issue was at the power supply end.

Seems very strange even if the fault occurred at the power supply end, would have thought would have seen some voltage at the amplifier HT input.

Have visually inspected the wires in the cables from the power supply to the amplifier connector all look good, at both ends at least, will have to meggar them.

David
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Old Today, 12:36 am   #85
DMcMahon
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Default Re: Brenell Purchases

Mk. 5 Series 2 Update.

Everything connected back up with new 470 Ohm 2 Watt resistor (original had gone open circuit) and two new 47uF 350 volts axials (originals measured OK) temporarily mounted on topside of the power supply.

Reconnected the output winding centre tap (green wire) to correct side of the 47 Ohm resistor and the white/pink wire (thought to be the screen of the mains transformer) to the other side (earthed side) of the 47 Ohm.

Rechecked all connections/wires looking for anything that might potentially intermittently short down but all checked out OK.

Powered up and all measurement as before, left running for well over an hour with no change, the various HT resistors were barely warm to the touch at the end.

Tomorrow will reconnect the tape deck and hopefully will be able to do more operational checks.
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