Replacing faulty STK series hybrids - UK Vintage Radio Repair and Restoration Discussion Forum

John_BS · 22nd Jul 2025, 10:12 am

I've just repaired a rather handsome Technics SA-200 tuner-amp - both STK 0029 modules were faulty (o/p short to +ve HT) .
I therefore built a couple of "discrete" versions mounted on an aluminium bar, and the amplifier now seems to work to spec (power / distortion).
I've posted the schematic in case anyone else has a similar problem - the 0029 is one of a series of hybrid circuits with pretty much identical topologies.

NB the bias current was set to give 30mV across the two 0R27 resistors, i.e about 50mA. T1 is mounted between the output darlingtons to provide good thermal tracking.
John

Nickthedentist · 22nd Jul 2025, 10:51 am

Clever stuff, John. There are a lot of nice bits of kit from the late 1970s around which use these modules. When they were plentiful, repair was quick and easy and not expensive. Now it's a gamble unless you resort to what you've done.

Any more photos?

John_BS · 22nd Jul 2025, 3:43 pm

Now my shoddy work is on view!

Nickthedentist · 22nd Jul 2025, 3:57 pm

No, that looks spot-on from here.

I wonder whether there's a commercially-available PCB for this kind of job?

McMurdo · 22nd Jul 2025, 4:00 pm

looks good enough to me!

FRANK.C · 22nd Jul 2025, 4:17 pm

Well done!
Looks great to me.

Radio Wrangler · 23rd Jul 2025, 2:43 am

Does the job. What's not to like? - And much better than a pretty little amp chucked in the skip. This could easily be more reliable than the original.

David

Oldmadham · 23rd Jul 2025, 3:31 am

Quote:

Originally Posted by Radio Wrangler

Does the job. What's not to like? - And much better than a pretty little amp chucked in the skip. This could easily be more reliable than the original.

David

My rule is:- Form , Fit & Function, with the foremost of these being Function!

turretslug · 23rd Jul 2025, 10:01 am

Great work there John, well done for saving an attractive and useful piece of kit that likely otherwise would have been declared BER and parts unavailable. The recent hifi receivers thread highlighted the esteem that this era of audio is held in by many here. Perfectly functional and effective repair that will last and last, probably with better heat transfer than the original,

Colin

John_BS · 23rd Jul 2025, 5:08 pm

These are the deceased amps - not a matching pair, and the one on the right failed the Isoprop test! The soldering on the pcb showed no indication of recent activity.
John

Maarten · 23rd Jul 2025, 11:43 pm

It is likely genuine, there are various kinds of printing. The numbers on the back should be plausible as well, though I don't know every detail of those.

Aub · 25th Jul 2025, 2:59 pm

Well done. Looks great and does the job.

Aub

locknut · 25th Jul 2025, 8:06 pm

Lovely, what a great solution

Daniel_CT · 22nd Nov 2025, 11:23 am

John, I just want to say thanks for the info on the STK0029 replacement.

After a few attempts (I'm not an electronics whizz by a very long way so there was some, um "hard learning" along the way) I managed to restore my mother's Technics SU 8011 that had a faulty left channel. The STK0029 was the culprit.

I couldn't get hold of the transistors in your design so I used a BDW93C paired with a BDW94C in place of the TIP102 / TIP107 pair. T1 was a BD139 mounted to the heatsink between the pair as you suggested.

For some reason I found that the value of the 390R had to be increased to 500 to get it to bias ok. I found it biased correctly at a setting of 660-680 ohms (500R + the 200R trim pot) when measuring across the bases of T1 and T3.

I replaced the other STK with a duplicate discrete circuit as well as the filter caps (these seemed to be original!) and now the unit is back in working order.

Anyway, just want to say thanks again.

John_BS · 25th Nov 2025, 4:56 pm

Daniel: glad you were able to fix it!
BTW,

Quote:

when measuring across the bases of T1 and T3.

Did you mean emitters?
Bias voltage will be slighly different due to change of darlingtons.
John

Daniel_CT · 5th Dec 2025, 4:50 pm

Quote:

Originally Posted by John_BS

Daniel: glad you were able to fix it!
BTW,

Quote:

when measuring across the bases of T1 and T3.

Did you mean emitters?
Bias voltage will be slighly different due to change of darlingtons.
John

Hi John, apologies for the delayed response. Basically what I was measuring was the cumulative resistance of R2 (the 200k trim pot) and R3 (500k) in circuit.

I don't have any sophisticated equipment so I initially biased it using a dim bulb tester adjusting the trim pot until the bulb went completely off (this after I lost a few sets of transistors that went straight into thermal runaway until I tried this trick).

Then I ran it directly off the mains and did some fine adjusting until the voltage across R4 and R5 was in the 15-30mV range.

22nd Jul 2025, 10:51 am	#2
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 19,107	Re: Replacing faulty STK series hybrids Clever stuff, John. There are a lot of nice bits of kit from the late 1970s around which use these modules. When they were plentiful, repair was quick and easy and not expensive. Now it's a gamble unless you resort to what you've done. Any more photos?

22nd Jul 2025, 3:43 pm	#3
John_BS Octode Join Date: Dec 2008 Location: Wincanton, Somerset, UK. Posts: 1,919	Re: Replacing faulty STK series hybrids Now my shoddy work is on view! Attached Thumbnails __________________ When you don't know what you are doing, do it neatly.

22nd Jul 2025, 3:57 pm	#4
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 19,107	Re: Replacing faulty STK series hybrids No, that looks spot-on from here. I wonder whether there's a commercially-available PCB for this kind of job?

22nd Jul 2025, 4:00 pm	#5
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,787	Re: Replacing faulty STK series hybrids looks good enough to me! __________________ Kevin

22nd Jul 2025, 4:17 pm	#6
FRANK.C Heptode Join Date: Feb 2006 Location: Roscommon, Ireland Posts: 760	Re: Replacing faulty STK series hybrids Well done! Looks great to me. __________________ www.electronics.frankcuffe.ovh

23rd Jul 2025, 2:43 am	#7
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 24,743	Re: Replacing faulty STK series hybrids Does the job. What's not to like? - And much better than a pretty little amp chucked in the skip. This could easily be more reliable than the original. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

23rd Jul 2025, 10:01 am	#9
turretslug Dekatron Join Date: Nov 2011 Location: Cambridgeshire, UK. Posts: 4,758	Re: Replacing faulty STK series hybrids Great work there John, well done for saving an attractive and useful piece of kit that likely otherwise would have been declared BER and parts unavailable. The recent hifi receivers thread highlighted the esteem that this era of audio is held in by many here. Perfectly functional and effective repair that will last and last, probably with better heat transfer than the original, Colin

23rd Jul 2025, 5:08 pm	#10
John_BS Octode Join Date: Dec 2008 Location: Wincanton, Somerset, UK. Posts: 1,919	Re: Replacing faulty STK series hybrids These are the deceased amps - not a matching pair, and the one on the right failed the Isoprop test! The soldering on the pcb showed no indication of recent activity. John Attached Thumbnails __________________ When you don't know what you are doing, do it neatly.

23rd Jul 2025, 11:43 pm	#11
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,722	Re: Replacing faulty STK series hybrids It is likely genuine, there are various kinds of printing. The numbers on the back should be plausible as well, though I don't know every detail of those.

25th Jul 2025, 2:59 pm	#12
Aub Nonode Join Date: Mar 2005 Location: Nuneaton, Warwickshire, UK. Posts: 2,223	Re: Replacing faulty STK series hybrids Well done. Looks great and does the job. Aub __________________ Life's a long song, but the tune ends too soon for us all - Ian Anderson, 1971.

25th Jul 2025, 8:06 pm	#13
locknut Hexode Join Date: Mar 2022 Location: Retford, Nottinghamshire, UK. Posts: 356	Re: Replacing faulty STK series hybrids Lovely, what a great solution __________________ Paul

22nd Nov 2025, 11:23 am	#14
Daniel_CT Diode Join Date: Oct 2025 Location: Cape Town, Western Cape, South Africa. Posts: 2	Re: Replacing faulty STK series hybrids John, I just want to say thanks for the info on the STK0029 replacement. After a few attempts (I'm not an electronics whizz by a very long way so there was some, um "hard learning" along the way) I managed to restore my mother's Technics SU 8011 that had a faulty left channel. The STK0029 was the culprit. I couldn't get hold of the transistors in your design so I used a BDW93C paired with a BDW94C in place of the TIP102 / TIP107 pair. T1 was a BD139 mounted to the heatsink between the pair as you suggested. For some reason I found that the value of the 390R had to be increased to 500 to get it to bias ok. I found it biased correctly at a setting of 660-680 ohms (500R + the 200R trim pot) when measuring across the bases of T1 and T3. I replaced the other STK with a duplicate discrete circuit as well as the filter caps (these seemed to be original!) and now the unit is back in working order. Anyway, just want to say thanks again.