Hacker VHF Herald RP37A Repair - UK Vintage Radio Repair and Restoration Discussion Forum

Herald1360 · 19th May 2010, 11:09 pm

This was eventually a success, though in no way a restoration.....

After languishing in the garage for a while, my trusty old Herald stopped working- zilch output, just a thump from the speaker at switch on. Hauled off to the shed where a quick poke with the 'scope (which lasted just long enough before it, too died) revealed plenty of audio at the amplifier input. So.... dead amplifier.

This shouldn't be too difficult, I thought...... hmmmm!

Initial measurements showed up a hefty standing current accompanied by a dead short AC128 output device. No spares and no inclination to spend anything prompted the recollection of a few BD139/BD140 devices in the stash, so after tracking down a copy of the service manual, I sat down at the computer and did a little modern cheating and fired up a simulation of the Hacker circuit with the modern devices in place of the originals. A bit of iteration and I had a rebiassed design ready to go- see part circuit below.

A couple of months later (you know how it is) I got round to building up the new amplifier. Hooked it up, turned on and

. Lots of standing current and no output

At least the new devices were effectively bomb proof (I had had one bit of foresight when I set a 75mA current limit on the bench PSU I was running it from. In to work with it, then, for a bit of lunchtime consultation. Eventually, after replacing any electrolytic which looked like a possible current path,we removed the output stage test link which reduced the current a little. A check showed current was still flowing through the bottom half of the output stage. Tried removing the driver transistor- no change. Checked current in R6- high, but not flowing through T2- simply not possible given the resistors around it. This left only one possible culprit, C7 (300pF polystyrene)- about the last thing I would have suspected. It was dead short.

Replaced C7 and all the rest of the removed bits- joy- standing current now about 6mA.

Back home, reconnected in radio, now audio output but very distorted. Remembered output stage test link was still open

and hooked up meter which improved quality no end! Adjusted crossover current (RV4) and output centre point voltage (RV3) and all was finally well.

mark pirate · 20th May 2010, 1:35 am

Well done! i took the the easy route with mine, replaced the amp board with a "car boot special" removed from a very battered "painter & decorators" set obtained for 50p

When working well, these sets wipe the floor with roberts sets of the time.
While you have it in bits, worth making sure of the connection of the 5 pin plug connecting the set to the amp.

Mark

Hybrid tellies · 20th May 2010, 9:23 pm

Interesting fault glad it is all sorted. The sound from these and all these big Hackers are just amazing.

mhennessy · 21st May 2010, 7:05 pm

Excellent - no real reason why silicon can't be used in a simple circuit like this... How are the new transistors mounted - presumably you had to extend the lead-outs? A picture would be interesting (especially if you include the speaker in the shot

)

Herald1360 · 4th Aug 2010, 12:09 am

Returned to this thread yesterday and noticed request for pics......

19th May 2010, 11:09 pm	#1
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Hacker VHF Herald RP37A Repair This was eventually a success, though in no way a restoration..... After languishing in the garage for a while, my trusty old Herald stopped working- zilch output, just a thump from the speaker at switch on. Hauled off to the shed where a quick poke with the 'scope (which lasted just long enough before it, too died) revealed plenty of audio at the amplifier input. So.... dead amplifier. This shouldn't be too difficult, I thought...... hmmmm! Initial measurements showed up a hefty standing current accompanied by a dead short AC128 output device. No spares and no inclination to spend anything prompted the recollection of a few BD139/BD140 devices in the stash, so after tracking down a copy of the service manual, I sat down at the computer and did a little modern cheating and fired up a simulation of the Hacker circuit with the modern devices in place of the originals. A bit of iteration and I had a rebiassed design ready to go- see part circuit below. A couple of months later (you know how it is) I got round to building up the new amplifier. Hooked it up, turned on and . Lots of standing current and no output At least the new devices were effectively bomb proof (I had had one bit of foresight when I set a 75mA current limit on the bench PSU I was running it from. In to work with it, then, for a bit of lunchtime consultation. Eventually, after replacing any electrolytic which looked like a possible current path,we removed the output stage test link which reduced the current a little. A check showed current was still flowing through the bottom half of the output stage. Tried removing the driver transistor- no change. Checked current in R6- high, but not flowing through T2- simply not possible given the resistors around it. This left only one possible culprit, C7 (300pF polystyrene)- about the last thing I would have suspected. It was dead short. Replaced C7 and all the rest of the removed bits- joy- standing current now about 6mA. Back home, reconnected in radio, now audio output but very distorted. Remembered output stage test link was still open and hooked up meter which improved quality no end! Adjusted crossover current (RV4) and output centre point voltage (RV3) and all was finally well. Attached Thumbnails __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

20th May 2010, 1:35 am	#2
mark pirate Dekatron Join Date: Mar 2008 Location: Worthing, West Sussex, UK Posts: 5,185	Re: Hacker VHF Herald RP37A Repair Well done! i took the the easy route with mine, replaced the amp board with a "car boot special" removed from a very battered "painter & decorators" set obtained for 50p When working well, these sets wipe the floor with roberts sets of the time. While you have it in bits, worth making sure of the connection of the 5 pin plug connecting the set to the amp. Mark

20th May 2010, 9:23 pm	#3
Hybrid tellies Nonode Join Date: Jan 2004 Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area. Posts: 2,967	Re: Hacker VHF Herald RP37A Repair Interesting fault glad it is all sorted. The sound from these and all these big Hackers are just amazing. __________________ Simon BVWS member

21st May 2010, 7:05 pm	#4
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: Hacker VHF Herald RP37A Repair Excellent - no real reason why silicon can't be used in a simple circuit like this... How are the new transistors mounted - presumably you had to extend the lead-outs? A picture would be interesting (especially if you include the speaker in the shot )

4th Aug 2010, 12:09 am	#5
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Hacker VHF Herald RP37A Repair Returned to this thread yesterday and noticed request for pics...... Attached Thumbnails __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O