Tandberg 64x after changing capacitors

[Levente]

I meant to reply to you Graham ....sorry..getting lost with the messages...Cheers Graham

[ricard]

Originally Posted by Levente

First is one of those Hunts coupling capacitor picture 1-3.

Disregarding the Hunts capacitor for a moment here, another thing caught my eye in this picture. In the middle of the picture is what looks like a Rifa capacitor, marked 47n X1. As you've replaced all the Rifa Miniprint caps (which I'd failed to notice when I submitted my post at the start of the thread, sorry about that), I'm assuming this is one of the replacements? It does look like a capacitor of similar vintage or at least similar type though as the original Miniprints. My point is that if this is indeed the case, even more modern Rifa caps tend to fail (and often with disastrous results when connected across the mains, although that is of course not the case here) and I wouldn't really recommend using them as replacements.

[Levente]

Hi Ricard,

Yes...i have replaced all of the miniprint caps with these type X1 and X2 caps...its Kemet/Rifa. I was hoping to follow the way how they have built this back in the days and therefore I have chosen these...i really hope they will serve well as I have changed over 20 of these spending about 50£ ....this made me sad a bit now. i just have to leave it as it is i think...i have replaced the hunts cap with LCR components film cap...i have not touched the other Hunts yest however, i have purchased all the replacement already of the same LCR film caps...

Thanks Ricard for your input

[ricard]

Sorry Levente, didn't mean to put you down. I guess the the new caps were brought brand new then? If so as you say they should at least be better than the originals for quite some time. Do you know if they are paper or polyesster? On the picture they just looked vintage so I wondered if they were new old stock components. In this application there is no advantage to using peper capacitors, they were chosen back in the day because they were cheaper than mylar (polyester), and even back then they probably had a certain amount of leakage causing random performance degradation of the amplifiers even when they were new. So using polyester would always be an advantage, for one, no one would have to replace them ever again.

[julie_m]

The failure mode for these capacitors (if they are the sort I think) seems to be that they suffer thermal stress in use, which leads to encapsulation failure and ingress of moisture.

New ones will be fine at least for now, as long as they do not show any signs of cracking or bulging.

[Levente]

Hey Ricard and guys...hope all is well! I had to have a break of the tape recorder as my head was kinda spinning around.

To answer Ricard: No worries Ricard and thanks for your input again. The replacement caps were brand new factory made from Farnell UK. So far they are looking OK and I am sure will serve me few years from now.

Julie, thanks for the input. No cracking on the caps yet, however tiny cracks seen on some of the resistors...

Graham, Lawrence, I hope you are well.

I have been quiet and the tape i am waiting for still hasn't arrived yet and will not be with me before Christmas.

Just now I have replaced the tube holders which had contact failures. ...did a test on the recorder. Sounding really really nice but again, the same thing when I hit the record button. So basically the Right channel is very tiny and weak but to me what is really strange is that magic eye is in the peak so the sound should have been very loud and distorted and not very quiet and distorted. Isn't strange?

Just to test it I have removed the one ECC83 tube which has some pieces flying around inside. I believe this is the V7 tube but could be wrong on that....after I have removed the R channel as silent which is fine but when I hit both the record buttons again, the R magic eye was again in the peak area but of course no sound and i have removed the tube from the playback board.

Does this mean that the issue is not on the playback side if the unit but the recording side or am I completely wrong here?

Cheers guys!

[ricard]

It's hard to really tell until you've verified that playback works properly with a known good tape, but if a recording sounds weak and distorted despite indications that a strong signal is being recorded, my first line of research would be to check if the recording bias is missing for the channel in question. With the 64X there's the additional complication that the bias head may need mechanical adjustment, but if the left channel is fine but not the right, I would start looking for an electrical failure in the bias switching circuitry.

[Levente]

Thanks Ricard, Graham, Lawrence, Julie....Wish you all a peaceful and Merry Christmas!!

It took 2 hours but I have checked all the resistors on both the recording and playback PCB.s

BIG TROUBLE! Specially on the playback pre and cathode follower board:

R201 0,1 mOhm = 801kOhm
R204 1 mOhm = 1245 kOhm
R226 7,5 kOhm = 0

R219 18 kOhm= 10,35 kOhm
R220 18 kOhm= 10,24 kOhm
R221 27 kOhm= 0
R222 27 kOhm= 0
R231 0,68 mOhm= 155 kOhm
R232 0,68 mOhm= 144 kOhm

R234 0,2 mOhm= 135 kOhm
R235 51 kOhm = 0
R236 51 kOhm= 20 kOhm


The record pre and the record amplifier PCB


R123 o,5 mOhm= 290 kOhm
R124 0,5 mOhm= 205 kOhm

R101 5 mOhm= 50 kOhm

R120 3,9 kOhm= 1,8 kOhm


I think we are not near to solve this are we?

[Station X]

Please remember that some resistors will be shunted by other components, so you won't get accurate readings if you measure them in circuit. You'll need to disconnect one end of them to get a true reading.

I still think your best approach is to see what happens when you play back a pre-recorded tape.

[ms660]

As Graham said.

Eg: Looking at the schematic you'll see that there are parallel resistive paths across R219/R220.....R231/R232...R101 for instance.

Lawrence.

[ricard]

Originally Posted by Levente

Thanks Ricard, Graham, Lawrence, Julie....Wish you all a peaceful and Merry Christmas!!

The same to you and everyone else on the forum. This year wishes like these seem to be sadly needed more than usual.

Back to the topic at hand ... I agree with Graham and Lawrence that when in-curcuit resistors tend to be shunted by other components, yielding inaccurate measurements. The most common failure mode for especially carbon composite resistors is an increase in value.

Hence, while I haven't actually referenced the schematics so I don't know where the resistors are located in the circuit:

R201 0,1 mOhm = 801kOhm
R204 1 mOhm = 1245 kOhm

R204 I would say would be a typical increase in value in a machine of this age (i.e. from 1 Mohm to 1.245 Mohm, where with 10% tolerance it should be at the most 1.1 Mohm), and R201 would seem to really have gone high, having increased by 800% from 0.1 Mohm to 0.8 Mohm.

Typically, measuring an anode resistor would yield fairly accurate readings, as the only shunt would be the coupling capacitor (the valve anode itself is essentially just a lump of metal in a vacuum when the heater is not operating). For the others, I would think that the rest of the circuitry would explain the low readings.

I also agree with the previous posters that playing back a known good tape would still be the first thing to do to ascertain what more needs to be investigated.

[Levente]

Hello Guys and Happy New Year. I have started it with a viral chest infection and just recovered. Didn't touched the tape recorder until now, as I have pre recorded and well sounding tape, at least sounded very good on another tape machine.

On the Tandbergg, the symptoms I am having while I hit the play are exactly the same as before. L channel very weak the R channel not so weak. I am saying "not so weak" now even previously said the R channel is fine, as I have played this tape on an AKAI tape recorder and sounded pretty huge and fat....now on the Tandberg is not so...so I beleive the R channel is also weaker as it should be but the L channel is nearly out...

All in all, the symptoms are the same as before.

On other thing while I am playing and touching or getting closer to the C205, I am getting a hum...kinda a 60 cycle hum....there is something not normal I guess....

Can I draw the conclusion that the problem should be somewhere on the playback PCB and not the recording side of it? and not on the oscillator board either?

Thanks guys!

Levente

[ms660]

Touching C205 would induce hum, that's normal, you can use that effect as a rough test for the playback amplifier, set the playback level to a suitable volume and touch C205 and note how loud the hum is, then touch C206 and note how loud the hum is, if the playback amp is ok then the level of hum from both channels should be roughly the same assuming that there are no shorts or leakage on the head side of those capacitors.

Lawrence.

[Levente]

Thanks Lawrence,

The C205 definitely has a nice amount of hum however the C206 has none. These capacitors I have replaced few months back with brand new ones.

Shall I check the soldering again or how about those styroflex capacitors on the row of

201-203------202-204 and the resistors between them?

Levente

[ms660]

It's possible that one of those capacitors relating to the faulty channel is faulty but there's another hum test you can do first to give a better picture as to what's what, you will most likely need to turn the playback volume up higher for this test, touch the grid (pin 7) of V5b and note the level of hum, then touch the grid (pin7) of V6b and note the level of hum, if those two levels are roughly the same then that would suggest the fault lies between the grids of the valves just tested and the input to the previous valve on the faulty channel.

Lawrence.

[Levente]

Thanks Lawrence,


Do I need to remove the tubes for this test?

...only one pin should be touched at the time right? i will use the pin of my voltmeter for this I hope would ork...

The volume on the recorder is/was on the maximum level.

Thanks Lawrence!

[ms660]

Valves fitted, power on, switched to playback, volume up...Yes, use the probe of your voltmeter, if fingers contact the probe then make sure you touch the grid and not HT Yes, one grid at a time so you can hear what each channel is doing separately for comparison, just like you did for the C205 and C206 hum test.

Lawrence.

[Levente]

Okay...just tested it

The first tube I believe is the V5 pin 7 has huge hum..much louder than the C205 cap had...

Th other tube V6 pin 7 has no hum, just a tiny little noise when i touch it for a second...like an electrical tiny little pop, but definitely no hum there at all..

[ms660]

Ok, so far so good, on to the next stage, the hum might get quieter in this test, this test is the same procedure as the last test except you now need to put your meter probe on Pin 2 of V7a and then on pin 2 of V8a, compare the hum levels.

Lawrence.

[Levente]

Thanks Lawrence,

The V7 and V8 are the opposite tubes right? The ECC82s on the same PCB? Just wanted to be sure I am doing things right...