I've got one of these atrocious record players on the bench at the moment for a friend... It sounds dreadful, the speakers do not have a back as they clip together to form the "lid".
Seemingly the deck works OK, but i'll strip it down and clean it anyway..
Only fault I can find, volume seems very low. I've checked the 3300uF capacitor which is OK, but found two smaller electrolytic's black, similar to Callins which are quite out of tolerance - will replace these once I find stock.
Noting the output transistors which fail a lot with their tin whiskers, I thought i'd try out my new tester but can't make much sense of the readings. Considering the can fell of one of the A128's i'd guess it's duff! But this is what reading I get on both A128's.
Meanwhile, the AC176 (ignore the text on photo!) reads this.. Assuming it's identified the type correctly (rather than thinking it's two diodes), i'd assume this one is OK. Can anyone give any pointers on how to read these values? They don't seem to correspond at all to the transistor technical data.
I thought i'd try out my new tester but can't make much sense of the readings. Considering the can fell of one of the A128's i'd guess it's duff! But this is what reading I get on both A128's.
I'd say your tester is reading the AC128 just right, it sees it as two diodes strapped in a PNP config.
My Peak, reads and outputs in a more user-friendly manner. Here are just the first two screens reading an AC128, If I keep scrolling it gives all the hFE,Vbe,Ib,Ic on other pages.
The AC176 should read as an NPN, with the usual b,e,c. A JFET has a Gate, Source and Drain. So I'd say your tester is having trouble with reading the germanium AC176, or it's faulty. Below my Peak reading an AC176.
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The AC176 should read as an NPN, with the usual b,e,c. A JFET has a Gate, Source and Drain. So I'd say ts having trouble with the germanium AC176
I see... So maybe that one is wrong then... I'll pull the other identical transistor and test it.
The original cartridge is a SX5M, I have forgotten the cartridge numbering system but doesn’t the X stand for crystal, if it’s original then that could be the cause of low output. Have you checked by feeding a known signal into the amp.
The black Callins capacitors are worth changing on sight, they failed frequently in the 1970’s when quite young.
Failures in the output stage with the AC128 type transistors very often caused a burn up of the emitter resistors among other items. It’s a DC coupled amp so faults earlier can cause problems.
A standard approach with this type of circuit was to remove the 4 transistors (VT3,5,7 and 9) using the right hand channel as an example, test all components in the DC part of the amp then fit new transistors. Apart from the burnt out resistors and Callins capacitors the AC transistors were usually the fault.
This was usually easier and cheaper than replacing only obviously faulty transistors and reduced call backs.
Appreciate this is was a commercial decision.
Apologies, a bit of a ramble and this was 50 years ago so time can cause other problems to occur.
Frank
The original cartridge is a SX5M, I have forgotten the cartridge numbering system but doesn’t the X stand for crystal
Indeed it does, Frank.
S = Stereo
X = Crystal (or C = ceramic)
5 =series number
M = medium output.
A very typical arrangement for "entry level" stereo record players, using the Philips/Mullard (or variation thereof) four-transistor circuit.
The original cartridge is a SX5M, I have forgotten the cartridge numbering system but doesn’t the X stand for crystal, if it’s original then that could be the cause of low output. Have you checked by feeding a known signal into the amp.
The black Callins capacitors are worth changing on sight, they failed frequently in the 1970’s when quite young.
Failures in the output stage with the AC128 type transistors very often caused a burn up of the emitter resistors among other items. It’s a DC coupled amp so faults earlier can cause problems.
A standard approach with this type of circuit was to remove the 4 transistors (VT3,5,7 and 9) using the right hand channel as an example, test all components in the DC part of the amp then fit new transistors. Apart from the burnt out resistors and Callins capacitors the AC transistors were usually the fault.
This was usually easier and cheaper than replacing only obviously faulty transistors and reduced call backs.
Appreciate this is was a commercial decision.
Apologies, a bit of a ramble and this was 50 years ago so time can cause other problems to occur.
Cheers for your help, i'll get to work. Very good point about the original cart being duff. I'll have to input a signal from my MP3 player. Can be so easy to jump the gun sometimes when you have "technology". As proven here though it's not always useful...
Well, what a pain this is proving to be. So I thought some of the transistors were duff, turns out they weren’t. The fault still persists, i.e. very low output.
I tried an MP3 player and this is still the same.
There are two 400uF electrolytic capacitors in the output stage, changed these, still the same. I then tried changing the two 10uF capacitors, nope.
I checked the large 3300uF cap with a known good one in case this was duff, nope!
The fault has changed slightly…. Now I have full volume at switch off/switch in initially for maybe a second before it “warms up” so to speak? Really odd one.
There are three 250uF LORINS caps too, which I am dubious whether it’s worth even looking at. All others are Philips tropical fish type
Okay Jamie, back to basics - voltages.
What's the voltage across the 3300uF cap (I'm assuming this is the smoother across the rectifier), and what is the voltage at the negative end of the 400uF caps? It should be about half of what the voltage across that 3300uF is.
Okay Jamie, back to basics - voltages.
What's the voltage across the 3300uF cap (I'm assuming this is the smoother across the rectifier), and what is the voltage at the negative end of the 400uF caps? It should be about half of what the voltage across that 3300uF is.
At the 3300uf it's 23V DC. And both 400uF's it's 11V DC. So that sounds about right. The circuit diagram for this is a mess, the R&TS refer to another model as being the same, but my understanding is there were two generations of this particular set. Mine being "mk2" so to speak, with two separate PCB's stuck together, instead of one.
In answer to your PM query Jamie, yes, I do have the HMV 2025 manufacturer's service manual. I'll get it scanned and will let you know when it's in the library.
![20220615 194846[1]](https://www.radios-tv.co.uk/wp-content/uploads/wpforo/attachments/884/11845-202206151948461.jpg "20220615 194846[1]")
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@crustytv Thanks for that. Im keen to try and figure the circuit out myself and find out where the issue lies, Im sure Cathovisor will lend a hand if required though. Really need to become less lazy and do some proper fault finding.
Quick and dirty scan uploaded to data library as my main scanner is currently disconnected awaiting installation in the store room. If it's not to your liking, I will connect the main scanner up and redo.
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Quick and dirty scan uploaded to data library as my main scanner is currently disconnected awaiting installation in the store room. If it's not to your liking, I will connect the main scanner up and redo.
That'll do nicely ta.
Hmmm what a strange one. Mine is different. My fault I did say it was the two part PCB this is the one part but as you can see most component positions and values are completely different…it’s dated 25th October 1971
Just to clarify, are both channels low in volume by the same amount Jamie?
Have you looked at the actual cartridge fitted to see what type it is, because if someone has replaced it in the past with a lower output ceramic, it could be unsuitable if the amp is made for a high output crystal type. A high output crystal means something like 2.2volts. A ceramic could be 200mV plus.
Clutching at straws a bit, but need to check every possibility.
To understand the black art of electronics is to understand witchcraft. Andrew.
@pye625 yep. The cart is fine, both channels are low. Confirms by external input too…
If I switch the set on I get a burst of maybe 1-2seconds of full volume it then goes right down. Same if I switch it off, so maybe a resistor is drawing way too much current in the power supply?
Obviously even with Chris’ help I’m struggling to find the correct data so testing resistors will be interesting!
So forgive me for stating perhaps the obvious, we are talking of something common to both channels at exactly the same time. Certainly in so far as the power cycling causes momentary higher volume.
You need to be monitoring the rails at this point with a scope to see if there are any sudden rises/falls that would coincide with the volume change. Perhaps a common supply resistor to the input stages may be dodgy for instance. The type of resistor fitted is prone to "split ends".
To understand the black art of electronics is to understand witchcraft. Andrew.
@cathovisor indeed.
I’ve managed to figure out the mangled circuit.
in the 1971-72 R&TS there is a revised component layout only and ONE PCB. The circuit remains the same, from the 70-71 R&TS seemingly with some minor component value changes which makes it more fun…. So am I looking in the output stage?
D'you something? From what you're describing... I reckon it's going unstable after a few seconds. Probably a poor earth somewhere allowing feedback - for both channels to do it, it must be something common like that.
