Tandberg 74 - UK Vintage Radio Repair and Restoration Discussion Forum

Michael Maurice · 9th Mar 2006, 1:25 pm

I have just bought one of these on Ebay.

Its the valve one with the two magic eyes.

It had been stored in the loft and parts are somewhat corroded. The previous owner powered it up and it came on.

Because of this I didn't feel the need to 'reform' the capacitors.

The problem though is low and distorted sound, CH1 being the worst. Apart from changing the valves, has anyone had any experiences of these machines? Is there anything else I need to check or am I going to do some grafting!

DOFFERY · 9th Mar 2006, 6:50 pm

Have you tried it as a an amplifier as against playing tape ? Main trouble with the 7 as with the later model 12 was the switching ie. amp/tape/record, however try the thru amp. first.

Doffery

ppppenguin · 9th Mar 2006, 6:58 pm

As a matter of common sense it's worth checking the coupling capacitors, if only to save the valves from running into grid current. If they are any of the usual suspects then it's replace on sight. May even cure the problem.

If corrosion is a problem then the valveholders deserve some TLC.

dazzlevision · 9th Mar 2006, 9:38 pm

Hello,

As Ppenguin says, the anode to control grid coupling capacitors are very suspect. Even though Tandberg didn't use TCC or Hunts! The lozenge shaped "Wima" capacitors were very unreliable in German sets of the late 50s and 1960s. Tandberg may have used these - or similar.

I did find a very leaky capcitor across the audio output transformer primary winding in a valve Tandberg machine (many years ago).

Regards,

Dazzlevision

ricard · 31st Mar 2006, 8:55 am

There are a couple of coupling capacitors near the output valves that tend to get very hot from the valves and consequently start leaking, driving the output valves into an unfavourable operating point, with low volume and distorsion as a result. I think in later versions of the 74 they moved the capactors further away, appearantly to avoid the heat from the valves. That's not to say they can't go bad after 40+ years anyway.

There are two versions of the 74, 74 and 74B. The 'B' version basically adds a low-level playback preamp output and a different speaker selector, but the valve lineup is also changed; the 74 uses ECL82's and the 74B EL84 output valves. It's the 'B' version I have some experience with, but the same problem is bound to occur in both versions.

/Ricard

Michael Maurice · 26th May 2006, 11:11 pm

well its taken long enough but I finaly replaced those capacitors and the 680K resistors in the grid bias circuits, they had risen to over 800K.

There is now no distortion and the machine plays well.

One thing, whilst checking, I accidently shorted the HT line and damaged the selenium rectifier. I decided the best way to repair it would be to open the selenium rectifier and remove its inards and fit 4 1N5061 diodes in their place. This orkied and is hardly visible from the outside, but I needed a resistor to emulate the higher forward drop of the rectifier. I found that putting a 68R resistor in series with the AC supply cured that. I didn't want to put it on the positive side as that would affect the impedance.

Sam · 27th May 2006, 12:13 am

Quote:

Originally Posted by dazzlevision

Even though Tandberg didn't use TCC or Hunts!

They did - my 64 is full of them!

Sam

ppppenguin · 27th May 2006, 6:31 am

Quote:

Originally Posted by Michael Maurice

I found that putting a 68R resistor in series with the AC supply cured that. I didn't want to put it on the positive side as that would affect the impedance.

Surely 68R in series is 68R in series whether it's on the AC side of the rectifier or the DC side. Or am I overlooking something?

Michael Maurice · 28th May 2006, 12:08 am

Quote:

Originally Posted by ppppenguin

Surely 68R in series is 68R in series whether it's on the AC side of the rectifier or the DC side. Or am I overlooking something?

I think its to do with the AC impedance between the bridge rectifier and main smoothing capacitor.

it was also easier to fit in this way.

9th Mar 2006, 1:25 pm	#1
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,222	Tandberg 74 I have just bought one of these on Ebay. Its the valve one with the two magic eyes. It had been stored in the loft and parts are somewhat corroded. The previous owner powered it up and it came on. Because of this I didn't feel the need to 'reform' the capacitors. The problem though is low and distorted sound, CH1 being the worst. Apart from changing the valves, has anyone had any experiences of these machines? Is there anything else I need to check or am I going to do some grafting!

9th Mar 2006, 6:50 pm	#2
DOFFERY Retired Dormant Member Join Date: Aug 2004 Location: Blackpool, Lancashire, UK. Posts: 1,488	Re: Tandberg 74 Have you tried it as a an amplifier as against playing tape ? Main trouble with the 7 as with the later model 12 was the switching ie. amp/tape/record, however try the thru amp. first. Doffery

9th Mar 2006, 6:58 pm	#3
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Tandberg 74 As a matter of common sense it's worth checking the coupling capacitors, if only to save the valves from running into grid current. If they are any of the usual suspects then it's replace on sight. May even cure the problem. If corrosion is a problem then the valveholders deserve some TLC.

9th Mar 2006, 9:38 pm	#4
dazzlevision Dekatron Join Date: Sep 2003 Location: Near Swindon, North Wiltshire, UK. Posts: 3,608	Re: Tandberg 74 Hello, As Ppenguin says, the anode to control grid coupling capacitors are very suspect. Even though Tandberg didn't use TCC or Hunts! The lozenge shaped "Wima" capacitors were very unreliable in German sets of the late 50s and 1960s. Tandberg may have used these - or similar. I did find a very leaky capcitor across the audio output transformer primary winding in a valve Tandberg machine (many years ago). Regards, Dazzlevision

31st Mar 2006, 8:55 am	#5
ricard Octode Join Date: Mar 2006 Location: Lund, Sweden Posts: 1,632	Re: Tandberg 74 There are a couple of coupling capacitors near the output valves that tend to get very hot from the valves and consequently start leaking, driving the output valves into an unfavourable operating point, with low volume and distorsion as a result. I think in later versions of the 74 they moved the capactors further away, appearantly to avoid the heat from the valves. That's not to say they can't go bad after 40+ years anyway. There are two versions of the 74, 74 and 74B. The 'B' version basically adds a low-level playback preamp output and a different speaker selector, but the valve lineup is also changed; the 74 uses ECL82's and the 74B EL84 output valves. It's the 'B' version I have some experience with, but the same problem is bound to occur in both versions. /Ricard

26th May 2006, 11:11 pm	#6
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,222	Re: Tandberg 74 well its taken long enough but I finaly replaced those capacitors and the 680K resistors in the grid bias circuits, they had risen to over 800K. There is now no distortion and the machine plays well. One thing, whilst checking, I accidently shorted the HT line and damaged the selenium rectifier. I decided the best way to repair it would be to open the selenium rectifier and remove its inards and fit 4 1N5061 diodes in their place. This orkied and is hardly visible from the outside, but I needed a resistor to emulate the higher forward drop of the rectifier. I found that putting a 68R resistor in series with the AC supply cured that. I didn't want to put it on the positive side as that would affect the impedance.