Transistor Voltages....?

[pwdrive]

This relates to a thread on UKVRR, what DC voltages would you expect to measure at the emitter junctions of the record bias supply output transistors Q601, Q602, Q603, Q604?

There are no voltages given on the schematic except for the B+ (23.5 volts)

The tape recorder uses cross-field bias.

Schematic in link below:


  tandberg_64x-62x.pdf (Size: 457.55 KB / Downloads: 15)

Lawrence.

[ppppenguin]

When you say the emitter junctions I assume you mean where the pairs of transistors are connected together. I'd expect approximately half the supply rail, so a little under 12V. Can't really be anything else. If something has failed then the transistors are very ordinary types; almost any small signal silicon should work.

I've attached the relevant bit of circuit diagram to make it easier to follow.

Which reminds me that one day I'll get round to re-capping my 64X. Lovely machine, still working well (for replay, I've not used it to record for many years)  though with a touch more hum than I'd like. Also reminds me that although I have a reasonable paper copy of the service manual I don't have a PDF of it. SO thanks for the schematic. I do have the PDF of the full manual for 62/64 (non-crossfield) models.

[pwdrive]

Thanks for the reply Jeffrey, yes, I thought half rail voltage as well, the OP's getting 25 volts and 26 volts respectively at the emitter junctions of the respective channels, the fault symptoms as far as I can ascertain are one track records and plays back ok (recording and simultaneously monitoring the recorded signal via the play back head etc) The other track sounds weak, thin and trebley, as yet he has no prerecorded tape to test the full playback circuit but he's trying to obtain one, the weak thin symptom was what made me think of tape bias problems on the defective channel but nothing is conclusive yet.

Lawrence.

[ppppenguin]

Assuming the 23.5V rail is correct then the 25/26V readings are misleading. Possibly due to the large amount of AC affecting the meter.

[pwdrive]

Assuming the 23.5V rail is correct then the 25/26V readings are misleading. Possibly due to the large amount of AC affecting the meter.

We think alike...As far as I know he's using a DMM but I don't know what quality it is, of course a 'scope would soon crack it but I don't think he has one.

Lawrence.

[pwdrive]

Bye the way, I also downloaded a copy of the 6x full manual from the site in the link below as it appeared to be very similar circuit wise to the 64x, was hoping it would give those emitter voltages but it didn't, anyways for what it's worth here's the root page for the site I downloaded it from, lot's of Tandberg related stuff in it:

http://sportsbil.com/tandberg/

Lawrence.

[ppppenguin]

Thanks for the link. Looks like the 62x/64x alignment instructions are to be read in conjunction wih the full 62/63 manual. Which I already have so all looks good for when I want to tackle my machine.