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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 7th Dec 2020, 4:59 pm   #1
TerryB44
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Thumbs up Grundig TK20 and Truvox R82. Help Required Please.

Hi Everyone,
I have a Grundig TK20 and a Truvox R82 reel to reel tape recorder. Both are fine mechanically but both have audio issues. The Grundig plays with good volume for around 20 minutes before slowly fading until it reaches a point that increasing the volume control has no effect. The Truvox had a similar problem but has now changed to a hum which varies with the volume control and the audio level is extremely low, hardly audible in fact.
If anyone can pinpoint what is likely needed to be replaced and it is a simple job I'll give it a go, otherwise is there anyone out there in my area, Merseyside, who would be prepared to take a look at them for me. I have recordings dating back to the early 1960's when I was in a band and I would like to be able to listen to them again!
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Old 7th Dec 2020, 5:22 pm   #2
Ted Kendall
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Two points here - the most likely culprit on either machine is likely to be ageing capacitors, but the best remedy is not wholesale replacement without further investigation. If your main purpose is to preserve old tapes, it may be more useful to have them transferred to digital files. I do this for a living, as do others...
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Old 7th Dec 2020, 8:23 pm   #3
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Hi Ted, I have the facility to digitise the tapes if I can get the machines to work. I would like the Truvox in working order preferably as it is two speed and most of my tapes are 71/2 ips recordings. Thanks for your suggestions anyway.
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Old 8th Dec 2020, 12:07 am   #4
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Hello Terry,
As Ted suggests it could well be bad capacitors but not easy to prove without various tests, which would involve working on open chassis with various potential dangerous voltages present, so not advisable unless you know what you are doing, for safety reasons.

Some people would recommend wholesale capacitor replacement but a big job unless you really know what you are doing and there is potential for creating new problems and some maybe even many could be fine anyway.

There are 2 main areas/capacitor types to check out:-

1. The rectified DC + HT voltage, should be around +285V dc with a several volts max AC mains ripple. Should be checked/monitored for the period that the volume issue develops, for the R82 this is at electrolytic capacitor C37 (there are also lower associated voltages after this). For the TK 20 the HT voltage should be around +260 volts at C31. Low HT voltage/high ripple could indicate bad reservoir/smoothing electrolytic capacitors.

2. Grid coupling capacitors, normally paper (di-electric type) in these vintage of machines. There is a sticky Thread about these in the "Components and circuits" category. Basically they should not pass DC voltage, so little if any +ve voltage should be measured at the Control Grids of the valves, particularly important at the output valve.

David
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Old 9th Dec 2020, 10:55 am   #5
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Hello David,
Thanks for the info, and NO I certainly won't be messing with open chassis measuring voltages, I don't have the expertise for that. You would probably find the fault in no time but unfortuately you're too far away for me to let you have a look, but, I can replace components easily enough. On the Truvox is C37 likely to be a can type or just a standard axial electrolytic?
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Old 9th Dec 2020, 2:30 pm   #6
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

If still original part then most likely will be a metal can capacitor. I see Terry that you posted the schematic in 2013.

Looking at the schematic you will see that C37 is 32uF, this is the filter capacitor after the reservoir capacitor C38, also 32uF, so quite possibly a dual can capacitor. The first smoothing capacitor after C37 is C33 also 32uF, so it is possible that it is a triple 32uF can capacitor.

The Service manual only has an abbreviated spares listing and these capacitors are not listed, so you will need to check out exactly what capacitor (s) are fitted.

These type of capacitors can hold a significant charge after being powered off, so always allow at least 15 minutes (and disconnect the mains cable from the wall outlet) after power down to allow them to discharge before doing any replacement work.

David
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Old 9th Dec 2020, 2:46 pm   #7
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Just noticed that there is another 32uF (C17) at the start, so probably the actual reservoir cap, very rare but now there is the possibility of the can capacitor being a quad 32uF.
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Old 9th Dec 2020, 2:51 pm   #8
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Funnily enough David, I downloaded the manual and found the same as you with the parts list, so I did a list of all the caps shown on the schematic and then did a visual inspection to identify which was which and to get the working voltages. I couldn't locate any of the 4 32uf caps listed on the schematic. However, there are 2 Large upright cans which are presumably what you are referring to. I'll check if there are any markings on them and get back to you.
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Old 9th Dec 2020, 3:10 pm   #9
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

C17 is directly off the +ve output of the bridge rectifier EC25, then following the connection up to R49 (330 ohm) the other side of R49 connects to C38, then onto R48 (330 Ohm) the output of which connects to C37, which connects to R44 (2.2k) the output of which connects to C33.

Possibly from your description there may be two 32uF dual cans.

David
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Old 9th Dec 2020, 4:07 pm   #10
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Correct! I've just had a look and there are two cans 40+40uF rated at 350v. They are roughly 3 inches high by 1 inch diameter. I presume these are going to be hard to come by which means replacing them with made up DIY substitutes. From tracing the wiring C37 and C38 are in one and C17 and C33 in the other.

See pics here. https://www.vintage-radio.net/forum/...pictureid=4506
https://www.vintage-radio.net/forum/...pictureid=4507

Last edited by TerryB44; 9th Dec 2020 at 4:21 pm.
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Old 9th Dec 2020, 4:46 pm   #11
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

You can still get new parts from a few places, such as -

https://www.hotroxuk.com/jj-capacitors-caps.html

The fact that they are 40uF I wonder if that could mean that they have been replaced before (if so could well be good), do they look newish/modern, has the soldering been disturbed ?

Cannot view the pics in your links.

Update - after more attempts now able to view your pictures, difficult to be sure but they do not look new/modern.

Last edited by DMcMahon; 9th Dec 2020 at 4:49 pm. Reason: Update
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Old 9th Dec 2020, 4:51 pm   #12
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

No, they're all original. It's never been faulty before and I've had it from new!
Could this be a suitable replacement?
https://www.tubeampdoctor.com/en/32-...radial-35x50mm

Sorry, you beat me too it with HotRox! I've had valves from them in the past for my Vox amp.
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Old 9th Dec 2020, 4:55 pm   #13
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Those 2 large rectangular waxy looking capacitors on the RHS of the first picture could be candidates for replacement, are you able to identify them ?
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Old 9th Dec 2020, 5:01 pm   #14
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Quote:
Originally Posted by TerryB44 View Post
No, they're all original. It's never been faulty before and I've had it from new!
Could this be a suitable replacement?
https://www.tubeampdoctor.com/en/32-...radial-35x50mm

Sorry, you beat me too it with HotRox! I've had valves from them in the past for my Vox amp.
Good that you have had it from new, as you know all its previous history.

Yes your linked one look fine as well. What ever you get, make sure they will fit in your existing clamps and not too long so as not to foul anything.
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Old 9th Dec 2020, 5:16 pm   #15
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

The larger of the 2 waxy caps is C28 and the other C21 associated with V3 the ECC82. The new ones are 35mm diameter whereas the existing ones are 25mm, height is ok.
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Old 9th Dec 2020, 5:25 pm   #16
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

OK so low (relatively) value ( I assume 1,000 on the schematic = 1,000pF ?) used on the Bias Oscillator, in that case I would not worry about them at this stage.
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Old 9th Dec 2020, 5:35 pm   #17
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Yes they are 1000pF. I notice TubeAmp have clamp to go with the capacitor. Presumably I could use this even it meant drilling new holes for the screws and as long as the terminals are clear of the chassis.

Last edited by TerryB44; 9th Dec 2020 at 5:59 pm.
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Old 9th Dec 2020, 6:49 pm   #18
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Yes if there is space for the larger clamps and mounting holes.
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Old 9th Dec 2020, 7:01 pm   #19
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

I'll order 2 together with clamps and start with them and see what difference they make. Hopefully it will sort the problem. I really appreciate you help with this as I may be able to wield a hot soldering rod but I'm certainly no hot electronics whizz! But I am learning with the help of the likes of yourself. Many thanks!
Pic of the Truvox in its case. https://www.vintage-radio.net/forum/...pictureid=4508

Last edited by TerryB44; 9th Dec 2020 at 7:09 pm.
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Old 9th Dec 2020, 10:23 pm   #20
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Default Re: Grundig TK20 and Truvox R82. Help Required Please.

Looks a nice machine.
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