LEAK TL12 PLUS Problem - UK Vintage Radio Repair and Restoration Discussion Forum

valveaudio · 13th Apr 2009, 1:12 pm

Help!

I have a pair of TL12+ amplifiers. These have had all the resistors/caps replaced by someone.
One of these amplifiers works perfectly, but the other one "motorboats" at about 20Hz, apart from this it works OK.
The motorboating starts after a fairly high level signal is input and remains when the signal is removed.
All voltages are correct and the valves have been swopped between the amplifiers and all resistors & caps. check OK. The only way to stop the problem is to change the load and jumper to 4 ohms or to reduce the feedback by doubling the value of the feedback resistor, which in turn gives a natural increase in gain and distortion.

This is driving me nuts! am I missing something?
Any ideas are welcome

Trevor

PJL · 13th Apr 2009, 1:29 pm

Is the output transformer original and the right version? Did you replace the reservoir caps?

barretter · 13th Apr 2009, 2:24 pm

What are the values of the bypass capacitors on the cathode resistors of the EL84s? If they are too high this can cause motor-boating.

AVO_VCM163 · 13th Apr 2009, 3:14 pm

When in the "boating mode" try the effect of a parallel C of 10 microfarads or more across the 8 and/or 16 microfarad HT filter caps.

If this does not cure it, check the earthing at the filter capacitors. Remove and clean the solder tag and chassis connection.

R

valveaudio · 13th Apr 2009, 3:42 pm

Ok, thanks for the replies, it's fixed, a leaky coupling cap on the EF86.

This was a brand new one fitted by me, this must have been the one in a million that failed in the MTBF tests, from now on I will check the new ones too if I have this kind of problem!

Trevor

13th Apr 2009, 1:12 pm	#1
valveaudio Hexode Join Date: Feb 2006 Location: Diss, Norfolk, UK. Posts: 386	LEAK TL12 PLUS Problem Help! I have a pair of TL12+ amplifiers. These have had all the resistors/caps replaced by someone. One of these amplifiers works perfectly, but the other one "motorboats" at about 20Hz, apart from this it works OK. The motorboating starts after a fairly high level signal is input and remains when the signal is removed. All voltages are correct and the valves have been swopped between the amplifiers and all resistors & caps. check OK. The only way to stop the problem is to change the load and jumper to 4 ohms or to reduce the feedback by doubling the value of the feedback resistor, which in turn gives a natural increase in gain and distortion. This is driving me nuts! am I missing something? Any ideas are welcome Trevor

13th Apr 2009, 1:29 pm	#2
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: LEAK TL12 PLUS Problem Is the output transformer original and the right version? Did you replace the reservoir caps?

13th Apr 2009, 2:24 pm	#3
barretter Heptode Join Date: Jan 2006 Location: Todmorden, West Yorkshire, UK. Posts: 870	Re: LEAK TL12 PLUS Problem What are the values of the bypass capacitors on the cathode resistors of the EL84s? If they are too high this can cause motor-boating.

13th Apr 2009, 3:14 pm	#4
AVO_VCM163 Retired Dormant Member Join Date: Mar 2008 Location: Hampshire, UK. Posts: 297	Re: LEAK TL12 PLUS Problem When in the "boating mode" try the effect of a parallel C of 10 microfarads or more across the 8 and/or 16 microfarad HT filter caps. If this does not cure it, check the earthing at the filter capacitors. Remove and clean the solder tag and chassis connection. R

13th Apr 2009, 3:42 pm	#5
valveaudio Hexode Join Date: Feb 2006 Location: Diss, Norfolk, UK. Posts: 386	Re: LEAK TL12 PLUS Problem Ok, thanks for the replies, it's fixed, a leaky coupling cap on the EF86. This was a brand new one fitted by me, this must have been the one in a million that failed in the MTBF tests, from now on I will check the new ones too if I have this kind of problem! Trevor