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Goodmans Module 80 Tuner Amplifier

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Marconi_MPT4
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After a long hiatus in doing repair work, a good friend wanted his Goodmans Module 80 repaired so went round to take a look. After a few tests it was ascertained that the amplifier left hand channel was dead. On transporting back to my workbench and doing a visual inspection, the amplifier output coupling capacitors C98 L and R had bulged and vented (Sprague 3,300uF 40V). Even though they looked grim, both only measured 10% lower than marked value. Not bad innings for 1974 manufacture!

Module 80   Venting Sprague

Found replacements in stock manufactured by BC Components purchased as discounted clearance about ten years ago for another project. Reformed for a short while and slowly discharged into a 47 ohm resistor before fitting. Although replacement capacitors were correct in respect to diameter they did not have enough body length with much shorter tags. Wiring was a little tight but managed to gain enough lead length to make connections. Note, make sure the case does not touch adjacent metalwork as this amplifier has a positive ground!

Module 80   BC capacitors
Module 80 Output Coupling

Powered up but as expected still no left hand channel. A quick signal check indicated that both power amplifier boards were receiving the same audio signal, fuses intact and DC operating conditions were OK.

More to follow.

Rich

 

 
Posted : 03/03/2020 3:00 pm
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Cathovisor
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As ever: have you got about half the supply rail voltage on the output coupling electrolytic? It's my first test on any audio amp with a single rail power supply. 

 
Posted : 03/03/2020 5:16 pm
Marconi_MPT4
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@cathovisor Checked DC voltage on the negative tag of C98 L (lower capacitor, violet wire). Power supply measures approximately 55V and negative tag (also TP118) sits at -27.74V. It is roughly half supply voltage.

Measured DC potentials at various locations around the left hand power amp board and nothing appears out of spec. After being powered up for a while, a weak output was noticed at maximum volume. Might be a signal coupling problem and looking closely  there are three original electrolytic capacitors on the board to check.

Next step will be to remove the power board for closer inspection.

Rich

 

 
Posted : 04/03/2020 2:50 pm
crustytv
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For those that have access and wish to play along with Rich's diagnosis, Trader sheet 3054 for the Goodmans Module 80, now resides in the data library. Look under "Amplifiers Tuners Hi-Fi/ British Hi-fi Amps/ Goodmans".

p.s.
I suspect the manufacturers component designations vary from the Trader sheets but you should be able to figure out what's what.

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Posted : 04/03/2020 3:07 pm
Cathovisor
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Sounds good, if you'll forgive the pun! No nasty DC feedback loops to worry about.

Should be plain sailing now, you'll have it working by teatime! 

 
Posted : 04/03/2020 3:12 pm
Marconi_MPT4
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@cathovisor ?

@crustytv  Trader sheet component designations are the same except for TR being used in place of VT.

 

The good, the bad, and the ugly.

After the previous post I removed left hand power board, a process that turned out to be somewhat of a fiddle. Loosening screws of both board guides allowed removal without having to de-solder connections. Removed capacitors C90, C92 both 50μF and C93 300μF. All three were of the infamous black Callins (CCL) type used widely in early 1970's Thorn/BRC products. On measuring individual values we have the 'good' (C90) down to 41.5μF, the bad (C92) down to 11.3μF, and the ugly (C93) virtually open circuit! Actual Value was difficult to ascertain but flitted around 900pF.

Module 80 Callins Capacitors
Module 80 Callins Location

C93 bypasses VT17 emitter circuit R107 (8.2kΩ) via R108 (39Ω) but is also part of the negative feedback loop. The ratio of R107 un-bypassed to R105 collector load reduces stage gain by roughly 60%.

Replaced all three capacitors (circled red) with Panasonic FC series using two 47μF and one 330μF nearest values. Before reassembling I quickly powered up to check if working correctly, a big improvement but now right hand channel was slightly lower in volume!

Out of curiosity I applied an input signal of 1kHz and measured the output voltage on TP118 (no speakers), powered down then temporarily de-soldered one of the pins on C93. Powered up and repeated test. Calculated that C93 open circuit will cause the output signal to drop by approximately 37dB.

Reassembled left channel board into board guides, reattached transistors onto heat-sink and then repeated repair on the right channel power board to replace all three Callins capacitors.

Now fully reassembled R105 (circled green) on each channel needed to be checked. Disconnected speakers. Using a 1kHz signal input and oscilloscope connected to respective speaker output, adjusted signal level until clipping was seen. This should be symmetrical and if not adjust R105 until it is. In this case neither board required adjustment.

Left Channel Power Amplifier
Module 80 Chassis
Module 80 front

Now to put a record on, feet up and have a listen to some 70's prog.

Rich

 
Posted : 05/03/2020 8:07 pm
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Excellent stuff! Pair that with a nice Garrard AP76 and put some Floyd on... 

 
Posted : 05/03/2020 8:29 pm
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FIXITNOW2003
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Think I have one off these in a white finished case somewhere must dig it out and have a look nice, write up 

 
Posted : 07/03/2020 2:36 pm
Marconi_MPT4
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@fixitnow2003 Thanks ? 

In concluding, a few more out of spec Callins capacitors were replaced and then the amplifier was tested over a couple of days on both record and radio sources. For records, just hooked up the TD125 (*). Replaced failed dial bulb and after a quick re-check wrapped up for transport and installation.

Connected up the system complete with Garrard Zero-100S and Goodmans (Mezzo?) speakers. Powered on and was greeted with a dead left channel!! Heart was sinking fast and a few checks revealed the culprit as a dead short in LH speaker wiring when measured across both pins. Obvious when opening up the DIN plug with copper strands splayed everywhere.

Remaking connections cleared that problem and now have a fully working system.

Rich

* @cathovisor 1971 Floyd Echoes (Meddle)

 

 
Posted : 10/03/2020 1:24 pm
Nuvistor
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Callins capacitors, kept in us in work even when equipment was only a few years old. No idea why they were so poor compared to other makes especially the excellent blue Philips/Mullard range.

Frank

 
Posted : 10/03/2020 1:37 pm
Cathovisor
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Posted by: @marconi_mpt4

For records, just hooked up the TD125 (*)

An excellent choice of turntable, Sir! The Zero-100S is also a good choice; I have a Zero-100C as well as the later Zero-100SB.

 
Posted : 10/03/2020 3:51 pm
sideband
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Posted by: @cathovisor

Excellent stuff! Pair that with a nice Garrard AP76 and put some Floyd on... 

Funny you should mention Floyd.....I've just listened to The Wall on Vinyl. My favourite track on that is Comfortably Numb....Dave Glimour's guitar solo is played very loud.....

 
Posted : 12/03/2020 10:34 pm
Cathovisor
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One thing I didn't realise until I got the CDs of The Wall is that the album is circular - it loops back on itself.

Bit like how I didn't realise until I bought the album Sheet Music that 10cc's I'm Mandy, Fly Me (on the subsequent album How Dare You!) carries on the story of Clockwork Creep on said album, and even contains a snippet of it...

 
Posted : 12/03/2020 11:20 pm
Marconi_MPT4
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An addendum to this old thread. Recently an interesting 50 year old manufacturing fault came to light which only showed up when a CD player had been added to the system and only under certain conditions!

When listening via headphones a type of variable compression was taking place which shifted stereo balance left according to volume. Connecting a 100mV 1kHz to both auxiliary input channels nothing was immediately obvious until the test signal was increased to 1V. RH channel distortion was visible on the the positive half cycle which was curiously rounded. Increasing or decreasing input signal level did not change shape back to a sine wave until reduced to roughly 100mV or less. Oscilloscope was connected to junction of C71 and volume control.

Between the input selector switch and the volume control is a two transistor amplifier TR13 and TR14, but in order to gain access to both sides of the PCB a fair bit of dismantling had to take place. So, first remove three screws securing a metal screen above the pre amplifier board. Next remove control and tuning knobs then remove front fascia. Carefully remove the clip-on knobs from function selector switch after first ensuring all are released to the out position. Use a flat bladed screwdriver to prise button away. Undo the two 8BA bolts to release switch frame from chassis. Then unclip PCB and move board into a vertical position to allow access to both sides.

Visual inspection revealed transistor TR14 had been soldered at some time in its history as joints looked different to the surrounding area. Power supply and transistor voltages apart from one were correct. TR14 collector was -1.8V instead of between -10V to -11V. Removed TR14 for inspection and test to find a BC183L had been used whith higher gain than the specified BC172B. This checked out ok. Measuring resistors around TR14 found R77 gave a reading of just over 900Ω. Looking carefully the resistor colour code suggested this was an 820Ω ±10% !

Replaced R77 with correct 12kΩ and refitted TR14. Rechecked collector voltage which was now -10.5V. Distortion had gone!. While the board was out replaced remaining Callins electrolytic capacitors as they all had very high ESR and unable to measure a sensible value.

Goodmans Module 80   R77

 

After reassembly and rechecking of the audio performance, switching to radio and re-tuning a loud sideband screech was present when going in and out of correct tuning position (and it wasn't because it was on Radio 1). At the same time I noticed one of the tuning indicator bulbs had failed. Screech came and went When I moved my hand near the bulb wiring! Bulb replacement greatly reduced effect but stereo light flickered in and out which curiously it did before. Eventually a cure was effected by replacing the 8μF FM discriminator capacitor C51 again very high ESR. Not a Callins but manufactured by SWT together with electrolyte leakage. Used a 10uF nearest value, not only had the screech gone but now a much more stable tuning performance coupled with the stereo indicator staying lit over a wider tuning range. Also less noise on other stations.

All Callins electrolytic capacitors have now been replaced.

Now on to the Garrard Zero 100S...

Rich

 
Posted : 13/04/2023 3:10 pm
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Cathovisor
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Bit of trivia: the Radford audio oscillator and distortion analyser I own belonged to the former technical director of Goodmans, Peter Collings-Wells. I worked with his son for a while.

 
Posted : 16/04/2023 11:44 pm
Forum 142
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@cathovisor - That would be me then! - How are you? BBC TV?

 
Posted : 17/07/2023 4:43 pm
Cathovisor
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Hello Peter! Yes, I got it via Al H. and it has been doing sterling service on and off since. 

I'm freelance now, got made redundant when Sport went up to Salford.

Otherwise plodding along and looking forward to taking my pension in fifteen months. How's yourself? 

 
Posted : 17/07/2023 5:50 pm
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