Tandberg 64x after changing capacitors

[ms660]

I can't see why those voltages are different either, it shows the same voltage differences in the Tandberg 64 & 62 manual, according to that manual the early versions had an ECC83 fitted in that position, the schematic in that manual is with an ECC81 fitted.

Lawrence.

[Levente]

Thanks Graham/ Lawrence

I went to the pharmacy and bought isopropyl 99% alcohol. Cleaned all the heads. It was very..but VERY filthy!

Did another test. Sounding way much better. Adjusted the heads during record/playback. I have not high frequencies (also because the clean heads as you guys suggested!)

The R magic eye became alive more and I can put them equeal visually, however, during the B test, on recording the R channel is still out of volume even tho the R magic eye telling me other wise. And this is the same to play back my recording.

I still got no other tapes on hold as it is not easy.

I think it is progressing really well besides the R channel being out during recording.

Thank you again for all of you the patience and help.

Not sure about that ecc81...why should be a difference. Obviously my knowledge on these is not like yours guys....just found it weird that two same valves next to each other why having different values.


Levente

[Levente]

Hey Guys,

Hope you are having a great Sunday.

Swapped the ECC 81 tubes around and I have the same effect. The channel 2 (track 3-2) is very weak and distorted. The Channel 1 (1-4) sounding really good and when it is not recording,both channels are sounding really good.

I press record listening in "B" test what is being recorded on the tape and this is when the Channel 2 (track 3-2) is getting distorted and weak, however the magic eye shows a nice gain there.

Checked the resistors around that ECC81 which had the lower voltages in pin 1.

R123 82kOhm resistor measures 88kOhm
R122 0.1mOhm resistor measures 108kOhm
R118 20kOhm resistor measures 21kOhm
R130 390 measures the same and the R128 1mOhm meaures 1083kOhm

It seems that the sound for the channel 2 (track3-2) on the way to the recording head has some faults....?


OR

The RCA output has some connecting issues..i have noticed that actually all the RCA ins and outs has a bit of a contact issue when I move around a bit. It seems these older RCAs are loose a bit compare to the new one. However, if i move around and check during the recording, the sound doesn't get better, but it might be an internal fault there which I cant reach....yet....

Levente

[ms660]

If the heads are ok it might be a bias problem, that could be switch contacts or a defective component, unfortunately the schematic doesn't appear to give any DC test voltages for the bias section, but it might be worth comparing the DC voltage at the junction of Q601/Q602 emitters to the DC voltage at the junction of Q603/Q604 emitters when in record mode.

EDIT: If when monitoring the bad channel when recording the sound is low and muffled (lack of treble) that would suggest a head problem, if the sound is thin and distorted but has treble then that would suggest a bias problem, nothing is certain though as diagnosis via the internet can sometime be difficult.

Lawrence.

[Levente]

Thanks Lawrence.

Definitely this one "if the sound is thin and distorted but has treble then that would suggest a bias problem"

Just too mention the C112 a very small paper cap is right next to the eq switch on the record preamp PCB and the side of the switch has scratched out a little piece of the C112. Could this be an issue? The other "funny" thing is that according to the schematic on this PCB, there should be a C116 cap and there is NONE. Where this cap should be there is nothing just a wire...

Where is the bias section in this recorder?

Thank you

Levente

[Station X]

The bias section is marked on the circuit as "Oscillator Board".

[ms660]

Originally Posted by Levente

Just too mention the C112 a very small paper cap is right next to the eq switch on the record preamp PCB and the side of the switch has scratched out a little piece of the C112. Could this be an issue? The other "funny" thing is that according to the schematic on this PCB, there should be a C116 cap and there is NONE. Where this cap should be there is nothing just a wire...

Where is the bias section in this recorder?

So far as I can see C112 is only in circuit when recording at the 7.5 i.p.s. speed, if the symptom is still there when recording and monitoring at that speed then you can leave C112 out as a problem for the moment.

I can't see a C116 on the 64x schematic, it is however shown in the 64 schematic, my guess is that it was never fitted in the 64x version, the thumbnails you posted are for the 64x schematic but the 64 layout so far as I can make out.

Bias section location is as per Graham's reply.

Lawrence.

[Levente]

Thanks Guys...

Oh Lord! I am loosing it right now. Each time I switch back on and off now the machine responds totally differently!! Jeeeez! What is going on...

The the Channel 1 is fin and the Channel 2 is weak. Each time i flip around to access the pcb board something is chnanging. Now the V2 tube has contact failure...God...what else is coming up here?

If i move around the tube and the sound level was changing cracking......the potmeter is not responding now for Channel 1.

One hour ago everything seemed improving....i have not changed anything on it. Now is giving me these faults....why?

[ms660]

It's usually down to the law of interference, known in the UK as Sod's Law or Murphy's Law, we've all been there

Look carefully at the wire connections that connect the board to the rest of the circuit, the more times the PCB is moved, the more the wires can weaken and sometimes break or cause the copper printed circuit to crack at those junctions, dirty valve pins and/or valve socket contacts can cause the valve not to function properly, usually intermittently, frustrating I know but don't give up.

Lawrence.

[ms660]

For what it's worth here's a supplement for the 64x, it includes the component layout on the oscillator board (component side only):

http://sportsbil.com/tandberg/62x_64x-align.PDF

Lawrence.

[Levente]

Thanks Lawrence,

I have changed the faulty tube holder V2. It was a lead disconnected at the potmeter for channel 1. Resoldered, now it works.

At this time, I am having the same results as this morning. Channel 2 is weak and "tiny" a bit distorted. Channel 1 is fine. I have tried to adjust the bias head, but no other changes for Channel 2, also again I have tried to adjust the other playback and record head but no better sound or response for channel 2....again this is ONLY hen I am recording and doing the "B" test, and playing back hat has been recorded.

It will be a bit of a mission to remove the wooden cover off chassis and and test it during the recording+ adjusting the bias pcb...but some how I will try...

So this will be my next move. Thanks guys again,

Levente

[ms660]

You need to be careful if you adjust the heads, always count the number of turns (or part thereof) in whatever direction you turn any screws so that they can be returned to their original positions if no improvement is noticed, the number of adjustments in tape decks can vary, I've not studied how many in this deck but typically head height, azimuth, zenith and some times rotational (angle of head gap to tape)

Sometimes preset adjustment pots can develop high resistance between the wiper and the pots track, turning wiper either way by a small amount will normally show that condition up, again make a reference, usually with a marker pen on the pot somewhere to show where the wiper was originally set, if a wiper contact has gone high resistance then a drop of switch cleaner can usually cure that, just a drop or two and work the wiper back and forth then returning it to its original position.

The main preset bias pots in the schematic are R609 (upper channel) and R612 (lower channel) The other two pots are for reducing the bias level when recording at 3.75 ips, R607 (upper channel) R610 (lower channel)

The key thing is not to end up with anything worse by making adjustments.

EDIT: It might be worth doing the voltage checks on the bias amplifier transistors as was mentioned earlier.

Lawrence.

[Levente]

Will do check that....what are those screws on the oscillator board? C602 C604... Is it something like gain adjustment for the recording head? Shall I touch them or measure the voltages first?
/
The tape should not run when I measure the voltages right? Just to put it on recording mode with the break applied...?

Are we moving away from the tubes means the tubes are OK i guess...one little thing when I was swapping round one particular ecc83 I have noticed a tiny little piece inside the tube is "flying around"...the tube still lights up and it seems ok but is this a problem or could cause problems later?


Thank you.

Levente

[ms660]

I wouldn't touch those screws, they are preset capacitors and normally reliable.

Yes, those voltages to be checked in record mode but be careful not to short out any of the transistor connections when probing around because it's possible to destroy a transistor if you do, yes it should be ok to do the measurements with the brake on.

Bits floating around in valves is not unusual, so long as they cause no shorts the valve should be ok.

EDIT: Ref post#112 "The other two pots are for reducing the bias level when recording at 3.75 ips, R607 (upper channel) R610 (lower channel)"... That should be 1 & 7/8th ips not 3.75 ips. Sorry about that!

Lawrence.

[Levente]

Hi Lawrence,

I have measured these

R610 2V
R609 18V and alays varies...sometimes down to 5V

R607 2V
R612 i had no access to it, i will need to focus on that one tomorrow....

And if I am not mistake these are all VAC voltages right?

Thank you!

[ms660]

The DC voltages I suggested measuring are described in post#104, it's a simple test that would indicate as to whether or not the bias output transistors are operating as they should be, in other words something to check while you're waiting for a known good recorded tape to check that the playback heads and amplifier are ok, we need to know if that's the case because you have replaced capacitors and it's possible a fault has been introduced.

Regarding the AC measurements you've made in the bias circuit on those resistors, yes, the voltages around there are AC but of relatively high frequency (85kHz) it's likely that the test meter you're using isn't suitable for measuring at that frequency.

I would do the measurements suggested in post#104 and report back, meanwhile you need to source a known good recorded tape to test the playback function, I wouldn't go fiddling around too much with anything in the meantime.

Lawrence.

[ms660]

As an addition to post#116 you could do a rough check on the condition of the playback amplifier by touching a meter probe on points 17 & 18 on the playback amplifier board when in play mode, you should get a hum from the speakers or whatever from both channels, the loudness of the hum from each channel should be more or less the same if all's well, the other meter probe doesn't have to be connected, this test is best carried out when playing back a section of the tape that has no recording on it or at least a quite recording. Do it with the playback volume set part way in case the hum is very loud.

Lawrence.

[Levente]

hi Lawrence,

I have measured the following DC Voltages on the oscillator board:

Q602 24VDC
Q601 22 VDC
Q603 24 VDC

Q604 bouncing between 24-30 VDC

I will get a test tape soon, but not sure what else to check on this...that microswitch for the bias circuit had few very fine pins broken off i remember as I found these inside the machines back in June..but I am not sure that has something to do with the lower volume on the left channel during recording....Thank you!

[ms660]

Are those the emitter voltages? If not then see below:

Here's how to identify the emitters, referring to the oscillator board layout in the last link that was posted Q601's emitter lead is the one on the left side of that transistor, Q602's emitter lead is on the right side of that transistor, the voltage measurement, whatever it is, should be the same on both emitters of that pair.

Q603's emitter lead is on the right side of that transistor, Q604's emitter lead is on the left side of that transistor, again the voltage measurement, whatever it is, should be the same on both emitters of that pair.

That bias switch only comes into circuit when recording on the lowest speed but if you want to test it then power off and set the speed selector to the lowest speed and measure the resistance from points 33 & 34 to chassis, it should be 0 ohms (short circuit) or as good as in both cases.

Then do the same for the two higher speeds, you should get a higher resistance reading, hopefully in the k ohm range (thousands of ohms) that resistance reading should be the same for both the higher speeds.

Lawrence.

[Levente]

Hi Lawrence,

Yes, I have followed your post regarding the emitters, printed the osc circuit and found these 3 pin vert small looking emitters and measured them. Unless, i have measured them incorrectly. There are only two out of the 3 pins where i could measure the DC voltage and measured each pins on each emitters against each other...

Are these readings shows some issue?


Thanks for the note of the bias switch, i will check the 33 44 later this evening.

I found a guy on ebay who sells great original master tapes. I have decided to bid on one oh his tapes as If i buy one, i want to buy something for good and to enjoy. It will take a bit of a time to arrive but until then maybe I should just carry on w my digitam meter to exclude more stuff...?
Thanks!
Levente