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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 21st Oct 2020, 10:32 pm   #61
DMcMahon
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Default Re: Brenell Purchases

Have now powered up and done some quick measurements, monitoring the HT voltage and current while slowly bringing up the mains voltage from Variac.

Only got to 150 volts mains with the HT current already up to 45 mA (highest I have seen it) and HT voltage 165, then the EL84 Output pentode started flashing/arcing internally, together with loud horrible noises from the speaker. Mains ripple also seemed higher than normal at around 400mV p-p.

Switched off and removed the valve and repeated. At 200 volts mains, HT voltage was 356 volts and HT current 3 mA and ripple 30mV, I think these figures are very similar to previous values seen with no EL84 fitted. I did not take the mains voltage up to 240 because the HT voltage was already at 356 volts and power supply capacitors only rated at 350 volts.

Not sure yet if the EL 84 has developed an internal breakdown problem or something to do with the external connections to the valve.

David
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Old 22nd Oct 2020, 10:32 pm   #62
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Default Re: Brenell Purchases

Thoroughly checked out all the circuitry around the EL 84 but nothing untoward found.

Repeated tests as above and now the problem has gone away. Able to go up to 240 volts mains voltage without excess HT current or the EL 84 arcing, with typical good results as seen before.

Results at 240 volts (Playback mode) :-

Variac AC current 0.16A
DC HT voltage +332V (schematic value = 285V)
DC HT current 40mA
EL 84 Anode voltage +325V (schematic value = 270V)
EL 84 Screen grid voltage +290V (schematic value = 250V)
EL 84 Cathode voltage +9.6V (schematic value = 8.5V)
HT mains ripple 240mV p-p

Still have the very original issue of the HT voltage being too high. The value of 332 V above is 47V higher than stated value on schematic and no obvious reason found. The AC voltage across the EZ 80 full wave rectifying valve anodes is 615V which is 45V higher than stated value on schematic.

I know the values shown on the schematic are approximate but my values do seem just too high to me.
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Old 22nd Oct 2020, 11:11 pm   #63
Ted Kendall
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Default Re: Brenell Purchases

Quote:
Originally Posted by DMcMahon View Post
Still have the very original issue of the HT voltage being too high. The value of 332 V above is 47V higher than stated value on schematic and no obvious reason found. The AC voltage across the EZ 80 full wave rectifying valve anodes is 615V which is 45V higher than stated value on schematic.

I know the values shown on the schematic are approximate but my values do seem just too high to me.
Contemporary Grundig manuals give voltages as read by an Avo 7, and it is likely that Brenell specify something similar. A modern meter is going to give a higher reading as it will load the circuit less.
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Old 22nd Oct 2020, 11:25 pm   #64
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Default Re: Brenell Purchases

Thank you Ted, a very good point.

The Brenell documentation states Avo 8 for the Mk. 5 Series 2 and Avo 7 for the Mk. 5, I know from a few comparisons on other similar machines, surprisingly the difference between Avo and high impedance DVM is not that great but will recheck the Brenell using Avo.

David

Last edited by DMcMahon; 22nd Oct 2020 at 11:29 pm. Reason: Update
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Old 23rd Oct 2020, 10:36 am   #65
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Default Re: Brenell Purchases

Saw the e-mail about post #62 and arrived to find Ted had said just what was on my mind. I discovered the excessive voltage and circuit loading effect thanks to Lawrence while doing my Ferrograph...
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Old 23rd Oct 2020, 10:58 pm   #66
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Default Re: Brenell Purchases

Repeated all the main DC and AC voltage measurements using two DVM's and two Model 8 Mk. 3 Avo's.

The two DVMs gave as good as identical results.

One of the Avo's gave almost the same results, in fact on the higher voltages the AVO read a few volts higher.

The other AVO gave somewhat lower results, a few results are:-

+HT - DVM 333V, Avo 320V
EL 84 Screen Grid - DVM 291V, Avo 280V
EL 84 Cathode - DVM 9.63V, Avo 9.5V
EZ 80 anode to anode - DVM 616V, Avo 580V

Have no idea of the calibration accuracy of the 2 Avo's, obviously the one that read a little higher than the DVMs is suspect. Where possible changing the Avo voltage range to a higher or lower range generally gave the same results for the same voltage.
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Old 24th Oct 2020, 8:13 pm   #67
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Default Re: Brenell Purchases

Replaced the cooked 470 Ohm HT resistor and the cooked EL 84 G2 10kOhm resistor, also replaced high value 33k on the Target of the EM 87.

Rechecked all voltages all OK. Refitted the power supply and amplifier, rechecked everything OK, no more sign (yet) of the HT overloading/EL 84 arcing internally.

Tomorrow will refit the tape deck and hopefully do some tape transport checks etc.
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Old 26th Oct 2020, 11:30 am   #68
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Default Re: Brenell Purchases

Mk. 5 Series 2 Update.

Refitted tape deck for tape transport checks (no tape fitted). Removed amp module to be able to keep an easier track of (monitor) the HT voltage and current.

Cleaned up the burred shaft of the Fast wind control. Found several potential donor grub screws to replace the broken grub screw in the Fast wind control knob, but all fractionally wrong size/incorrect thread, so borrowed the control knob from the Mk. 5

Play works in all 3 speeds and seems strong, 15 ips is noticeably noisy.

FFD and RWD works well and seems strong & pretty quiet.

No obvious sign of the previously measured lowish insulation resistances for the capstan motor run capacitor and the 4 suppression CR snubbers causing any issues.

Playback switch rotation is pretty stiff, if the unit turns out to be a runner will investigate that later.
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Old 26th Oct 2020, 12:33 pm   #69
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Default Re: Brenell Purchases

Loaded a 7" Maxell 35-90B tape which is very good condition, had no idea if any recordings on it. It is a back coated tape so probably not ideal with the Brenell. Cannot currently lay my hands on my 8 1/4" tapes.

Playback works well (3 3/4 ips) with good quality sound and plenty of volume available.

Recording on tape is old radio recording with music, must be 4 track as also sometimes getting a reverse track playback together with the normal playback.

FWD & RWD work very well, pretty smooth and fast, a little noisy (vibration ?) in Rewind.

Getting fast playback sound in FWD and RWD as the tape contact release pin is not fully lifting the tape away from the PB/Record head, although even when tape is manually lifted away there is still some playback sound.
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Old 26th Oct 2020, 12:34 pm   #70
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Default Re: Brenell Purchases

Just realised from the photos that I have not fitted the adjustable tape guide !
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Old 26th Oct 2020, 7:23 pm   #71
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Default Re: Brenell Purchases

Remember when fast winding/rewinding to quickly move the switch to the opposite function to use the motors to slow the tape. Just stopping will probably cause a tape spillage or stretch the tape if the brakes snatch.
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Old 26th Oct 2020, 8:41 pm   #72
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Default Re: Brenell Purchases

Thank you vidjoman for reminding me, I had forgotten, memory is not as good as it used to be

No obvious tape spills or snatching brakes seen so far.

David
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Old 26th Oct 2020, 9:54 pm   #73
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Default Re: Brenell Purchases

The rhomboid knobs are still available from RS Components not sure of catalogue number but a quick search should find them.
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Old 27th Oct 2020, 10:15 am   #74
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Default Re: Brenell Purchases

Thank you, that is good to know.
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Old 27th Oct 2020, 10:32 am   #75
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Default Re: Brenell Purchases

Mk. 5 Series 2 Update.

Mini disaster today, powered up to do more tests, found the amplifier was dead with the tape deck still working. Then noticed a burning smell so switched off to investigate.

Was still monitoring the HT voltage and current and prior to noticing the smell, there was no voltage/current at the HT input to the amplifier, so consider the issue purely at the power supply this time.

Found the new 470 ohm (2 watt) resistor at the power supply has burnt up (open circuited) must have generated a fair bit of heat as the resistor and reservoir capacitor soldered connection at the EZ 80 cathode have broken away.

Will replace the resistor and both 47uF reservoir and filter capacitors. The capacitors are only rated at 350V and have been stressed maybe with higher voltages occasionally.

Will probably move them to the topside of the power supply.

Have previously planned to replace the 2 capacitors with a new twin can 50uF 500V with a very local collection (to save postage) but currently they have gone out of stock.
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Old 27th Oct 2020, 12:50 pm   #76
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Default Re: Brenell Purchases

The two 47uF 350V capacitors checked out fine capacitance and ESR wise (not checked on Meggar), but still will renew them, the 470 Ohm resistor is open circuit.

Decided to double check the mains transformer output to the EZ 80 and have found something very confusing. I disconnected the transformer output wiring (have never disturbed this area before) designated B and measured the centre tap of the output winding to each end of the winding and got totally unbalanced voltages 380V and 165V. Previously (before this fault occurred) with everything connected had measured balanced voltages OK.

Did resistance checks of the output winding and found that the centre tap wire was not the centre tap, there being no resistance between that wire and either end of the output winding (winding itself measures OK).

The Series 2 schematic shows the centre tap connecting to a 47 Ohm resistor (I will call it the input here) and then on the output side of the 47 Ohm resistor the schematic shows connections to the wiper of the humdinger, earth and one side of the loudspeaker.

I do not really know the function of the 47 Ohm.

The transformer wire connected to the input of the 47 Ohm is white/pink, according to the schematic should be the output winding centre tap, but as explained it is not, however somehow it has a connection voltage wise.

Found another transformer wire (green) connected to the output side of the 47 Ohm which by resistance measurement seems to the centre tap of the output winding, so the centre tap connection is on the output side of the 47 Ohm as opposed to the input side as shown on the schematic. There are no other transformer wires shown on the output side (apart from the 2 sets of heater windings) so the white/pink wire is currently a mystery. The schematic does not show a transformer connection to the output side of the 47 Ohm.

I had mentioned early on in the Thread that some of the primary winding wire colours do not match the schematic, unfortunately the schematic does not show wire colours for the 3 output windings.

So first need to find out what the mystery white/pink transformer wire is, will recheck all the primary winding taps. Will have a look at the older Mk. 5 Power supply transformer wiring also.

Last edited by DMcMahon; 27th Oct 2020 at 12:59 pm.
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Old 27th Oct 2020, 1:03 pm   #77
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Default Re: Brenell Purchases

The 47 ohm will be to help limit the maximum hot switching current through the rectifier, the other wire might be a connection to the screen of the transformer, that would normally be connected to ground/chassis, the center tap of the HT winding should be connected to the one end of the 47 ohm and the other end of the 47 ohm should be connected to ground/chassis.

Lawrence.

Last edited by ms660; 27th Oct 2020 at 1:18 pm. Reason: missing word
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Old 27th Oct 2020, 1:32 pm   #78
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Default Re: Brenell Purchases

Thank you Lawrence, that makes a lot of sense, so if that is correct then the 2 wires are swapped over (which is what I was thinking but could not understand what the white/pink wire is). The green centre tap wire currently goes to output side of the 47 Ohm (Earthed) and the white/pink wire (not sure what actual colour it is) goes to the input side of the 47 Ohm.

Would you expect to see any voltage (large in this case) from the transformer screen to either side of the output winding ? I did check that the white/pink wire does not electrically connect to earth.

It possibly does look like the soldering at the 47 Ohm resistor ends has been disturbed before but not by me.

David
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Old 27th Oct 2020, 1:38 pm   #79
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Default Re: Brenell Purchases

Putting the mystery of the pink/white wire aside is it possible that a) the HT is wired up incorrectly? Yes B) that you have a short on one side of the HT to the centre tap? Yes.

My suggestion is to remove the transformer feed it from you variac not at full voltage and then to measure the volts on each of the wires in all permutations.

This will establish what each wire is make notes of course and then wire it back up to the power supply as per the schematic.

It is possible this machine never worked if it was wired up incorrectly in the first place.

Other solutions may of course be simpler but I think you need to establish if the power transformer is shorted on one or more of its windings.
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Old 27th Oct 2020, 1:45 pm   #80
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Default Re: Brenell Purchases

Connecting a high impedance digital volt meter to a disconnected screen connection can sometimes result in a voltage being displayed due to capacitive coupling (or leakage) between the screen and the windings.

I would configure the 47 ohm as per the schematic.

EDIT: Post crossed.

Lawrence.
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