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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 13th Mar 2017, 9:58 pm   #1
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Default Baird Varsity HT voltage question.

I have a very quick question on the Baird Varsity tape recorder (I think this may be known as the 101 model). Does anyone know the exact HT voltage at the reservoir capacitor on this model?

Quick story - I took this machine on as a repair for a local 'radio operator', as he's given me some other tape recorders to either use/repair/break for parts. I've also repaired a 50s Grundig valve radio for him in the past. This tape recorder had been well and truly 'got-at' by someone in its previous life. Once I started on it I realised that I'd probably made a mistake by saying that I'd fix it for him. There's no problem with the deck as I think that he'd already done some basic servicing on it. He has since told me that other than removing the bottom cover and seeing all the 'soldering' around the smoothing can and the 'hanging off' rectifier, that he decided to leave the electronics alone and let someone else have a look at this side of it.

I found that the chassis couldn't be removed due to the fact that a couple of the mounting studs revolved with the fixing nuts, so I at first tried to do the necessary repairs with the chassis in place - not a good idea! The problem is that the control panel fascia is stuck on and the rear of the studs are under it. This panel has also degraded and shrunk making it not line up with the socket holes correctly, so trying to remove it in this condition may well have ruined it altogether.

This chassis uses those metal end caped resistors that are (sometimes) notoriously unreliable and I'd already found one such and replaced it. There seemed to be bad solder joints that someone had been trying to rectify sometime in the past. There's a problem, this unit uses separate 'stand-off' circuit boards connected to the main board via a row of pins and if a soldering iron is applied to the joint where the pins meet the main board, the heat runs up the pins which unfortunately have a soldered join half way up which melts as well. This had happened to this circuit board in several places and when I then noticed that whoever had removed the chassis before, had completely trapped the mains supply wiring to the deck motor between the top of the chassis and the woodwork of the case, I realised that the chassis would have to be removed, not only to get at those melted pin connections, but for safety with regards to that dangerously trapped wiring!

With some hot air applied, the chrome trim and the control panel cosmetic fascia came off without damage, exposing the slotted top of those chassis mounting studs and the chassis was able to be removed. This is a strange design with a ECL86 triode / pentode preceded by a single OC44 transistor. Someone had at sometime in the past obviously found that the original selenium bridge rectifier had gone a bit 'high resistance' and had tried to remove it, perhaps with the chassis still in place. They'd got one of the bolts out and the other was half out, but they'd left the rectifier swinging about with it's bent metal flange flopping about dangerously close to the circuit board solder connections - as you can see in one of the pictures below. Also below, you can see one of the little stand off boards which I'd re-soldered two of the pins that had come un-soldered, but one was actually missing as you can see, so I had to make a new wire one for the one on the end.

The rectifier has now been replaced with a bridge made up from four diodes with series 'watty' droppers in the AC feed and it's now all put back together and working well - or perhaps I should say "as well as a machine of this type can work"! The HT had originally been around 150 volts and dropping lower with time with the old rectifier. It's now stable at around 295 volts (the smoothing can is rated at 350 volts working) with the new rectifier and series resistor. So that's the question - it's all up and running, but I've 'guestimated' the HT voltage for the time being.

That wasn't such a quick story as I thought it was going to be when I started this post. Any other comments on this recorder are obviously welcome as well as the main question with regards to the HT voltage - pictures below:-
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Old 15th Mar 2017, 1:41 am   #2
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Default Re: Baird Varsity HT voltage question.

I thought it was a bit of a long shot to find out if anyone knew the HT voltage for this recorder. I didn't actually choose that voltage I quoted, but rather estimated what the forward resistance of the original bridge might have been and picked a couple of series resistors of the appropriate value (and which I also just happened to have to hand) to stick in the AC feed to the bridge. So it happened to end up at just over 295 volts, which after a good soak test seems to be fine. Out of interest, the voltage without any resistors was around 318 volts.

There's a very annoying feature with this particular machine in that the lid cannot be closed with seven inch spools fitted, even though the deck can obviously accommodate these - the designers ought to have been shot (not literally, of course). This certainly isn't the most interesting of machines, or the best performing by a long way, but its owner particularly loves the BSR TD10 tape deck and he has at least one other machine with the same deck fitted. I have to say that I quite like the good old work horse that is the three speed TD10, also the TD2. The most annoying feature of this decks operation is the force that's needed on the three way control when working the 'wind' motion which nearly pushes the unit off the table, even when well lubricated, but at least it's 'positive' in its operation. I notice how the edge of the deck is worn due to years of fingers gripping the side to get 'purchase' on this 'clunky' control while operating it.
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Old 15th Mar 2017, 7:04 am   #3
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Default Re: Baird Varsity HT voltage question.

Sounds like you had your work cut, fair play to you for seeing it through.

Andy.
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Old 17th Mar 2017, 3:35 am   #4
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Default Re: Baird Varsity HT voltage question.

Thanks Andy.

I certainly spent far too much time on it, but it got to be a bit of a challenge in the end. I have to admit that it's a bit of a mess inside and I've added to the melt marks on some of the chassis wiring with the soldering iron due to trying to do the repairs with the chassis at first still in place. A proper restoration would involve replacing it all, but it would never be worth it and he only wanted it working. I've stuck that trim back on so I hope it doesn't have to come off ever again, or that I don't get asked to repair it again at any time in the future. I also replaced the obligatory 13 amp fuse in the plug with a 3 amp one. He said he'd pay me for repairing it and asked me how much it was going to be, but I told him that seeing as he's given me bits and pieces in the past that I'll basically just charge him just for parts. Also, he's told me that he's just found a box of valves that I can have and when I asked him over the 'radio' to quote me some of the type numbers, it sounds like there'll be some useful types among them that I can make use of, so all's good.
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