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14th Oct 2025, 6:30 pm
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#101 |
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Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 8,354
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Re: Fidelity HF35
If you have 27V at the centre connection of those two resistors then there is a problem....it should be about half supply so you should be measuring about 13.5V. What does the collector of the bottom transistor (AC128) measure? I'm assuming you have your negative meter lead connected to the negative rail? There should be nothing on the collector (since it's connected to the negative rail), about 13.5 on the emitter and probably about 14V on the base (since you only need around 0.2V between emitter and base to turn on a germanium transistor).
Come back with your results.
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman..... |
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14th Oct 2025, 7:34 pm
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#102 |
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Tetrode
Join Date: Sep 2025
Location: Coventry, West Midlands, UK.
Posts: 74
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Re: Fidelity HF35
I'll check Q3 again tomorrow, but the other day was
Q3 - 22, 21, 0 I'll also recheck the resistors in case I am measuring in the wrong place. |
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14th Oct 2025, 8:36 pm
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#103 |
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Tetrode
Join Date: Sep 2025
Location: Coventry, West Midlands, UK.
Posts: 74
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Re: Fidelity HF35
Just to be sure, I was measuring at the top left where the 2 pads are joined?
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14th Oct 2025, 9:37 pm
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#104 |
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Nonode
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 2,310
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Re: Fidelity HF35
Simple as this amplifier is, I wonder if we've just hit the good old "DC-coupled amplifier" syndrome - going round in circles replacing components, only to find them failing again because by now so many are possibly implicated and none of the voltages make much sense. 104 posts so far, and not much further on. Perhaps it will be quicker to remove and test all the transistors again, and test all the Rs and Cs.
Quicker still just to rebuild the whole amplifier board with new components - there are few enough and they're all cheap enough. Mike Last edited by Boulevardier; 14th Oct 2025 at 9:47 pm. |
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15th Oct 2025, 7:15 am
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#105 | |
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Tetrode
Join Date: Sep 2025
Location: Coventry, West Midlands, UK.
Posts: 74
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Re: Fidelity HF35
Quote:
Anyway, I'll lok at it again later. |
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15th Oct 2025, 12:31 pm
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#106 |
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Tetrode
Join Date: Sep 2025
Location: Coventry, West Midlands, UK.
Posts: 74
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Re: Fidelity HF35
Checked and double checked (and realised that I am an idiot)!!
Junction of 2.2 ohm resistors = 13.35v AC128 e - 13.28v b - 13.22v c - 0v |
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15th Oct 2025, 1:05 pm
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#107 |
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Pentode
Join Date: Apr 2008
Location: Rhondda Cynon Taff, Wales, UK.
Posts: 173
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Re: Fidelity HF35
If that's the case then looks like the output transistors are fine, as this is correct and if either were shorted would get 27 or 0
Is it possible C4 is open circuit or a problem with the volume control? Maybe the amplifier is fine now. maybe a signal of some kind could be inserted at the volume control? |
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15th Oct 2025, 1:18 pm
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#108 | |
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Nonode
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 2,310
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Re: Fidelity HF35
Quote:
Have you tried the "Buzz test" again - slide off the wire connectors to the cartridge (just in case the cartidge is damaged and low resistance), and touch the now-free connecting leads with a screwdriver blade. Mike |
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15th Oct 2025, 1:56 pm
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#109 | |
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Nonode
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 2,310
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Re: Fidelity HF35
Quote:
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15th Oct 2025, 5:36 pm
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#110 |
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Tetrode
Join Date: Sep 2025
Location: Coventry, West Midlands, UK.
Posts: 74
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Re: Fidelity HF35
Re buzz test.
From the cartridge leads - nothing. From the volume control - nothing If I apply a small voltage directly to the board where the cartridge wires attach I get crackles. Measuring resistance across the volume control ranges from 370 to 368 ohns. |
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15th Oct 2025, 5:38 pm
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#111 |
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Tetrode
Join Date: Sep 2025
Location: Coventry, West Midlands, UK.
Posts: 74
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Re: Fidelity HF35
PS. Is C4 the big smoothing? cap?
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15th Oct 2025, 5:52 pm
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#112 | |
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Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 22,853
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Re: Fidelity HF35
Quote:
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
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15th Oct 2025, 5:55 pm
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#113 | |
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Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 22,853
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Re: Fidelity HF35
Quote:
If you're using the diagram attached to post #22, C4 is the capacitor connected to the wiper of the volume control. The service sheet for the HF35, where it's designated C3 gives a value of 0.1uF.
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
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15th Oct 2025, 6:00 pm
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#114 |
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Dekatron
Join Date: Sep 2015
Location: Southwold, Suffolk, UK.
Posts: 9,180
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Re: Fidelity HF35T (Transistor version).
Now at Post 113 and counting.......what circuit or version are we talking about here?
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Edward. |
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15th Oct 2025, 8:19 pm
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#115 | |
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Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 8,354
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Re: Fidelity HF35
Quote:
Don't know the value of the volume control but 370 ohms is very wrong!! I think someone said they would expect 1 megohm? That sounds right for a crystal cartridge. So...there is another thing to check. Make sure the control is connected correctly. If necessary, disconnect any leads connected to it and measure directly across the track (the two outer connections). Then check from the wiper (centre) to each outer connection while turning the control. Volume controls don't normally go short circuit but if it's been messed about with in the past, then nothing is impossible. Check the value of the control (usually marked on the side). If it measures OK when disconnected then you need to check the external connections.
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman..... |
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15th Oct 2025, 8:38 pm
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#116 |
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Tetrode
Join Date: Sep 2025
Location: Coventry, West Midlands, UK.
Posts: 74
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Re: Fidelity HF35T (Transistor version).
There are 4 tabs on the combined on/off volume control.
2 are connected to the motor winding and 2 are 240v mains. As can be seen in image. |
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15th Oct 2025, 9:00 pm
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#117 |
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Heptode
Join Date: Sep 2007
Location: Brentwood, Essex, UK.
Posts: 812
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Re: Fidelity HF35T (Transistor version).
There will be 3 more tags on the lower silver part that is the actual volume part, I guess at the back as they can’t be seen.
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15th Oct 2025, 9:01 pm
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#118 |
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Nonode
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 2,310
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Re: Fidelity HF35T (Transistor version).
Those 4 tags with the blue and brown wires are NOT for the volume control - they are for the mains switch. You'll get rather more than a tingle or a buzz if you touch them! The volume control (3 tags) will be underneath the potentiometer as shown in your photo, and will probaby be wired directly from the tags into the PCB - so awkward/impossible to access except from the tracks on the other side of the PCB.
Mike [EDIT - crossed with music-centre.] Last edited by Boulevardier; 15th Oct 2025 at 9:12 pm. |
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15th Oct 2025, 9:05 pm
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#119 |
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Tetrode
Join Date: Sep 2025
Location: Coventry, West Midlands, UK.
Posts: 74
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Re: Fidelity HF35T (Transistor version).
OK, understood,, thanks both.
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16th Oct 2025, 9:07 am
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#120 |
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Pentode
Join Date: Apr 2008
Location: Rhondda Cynon Taff, Wales, UK.
Posts: 173
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Re: Fidelity HF35T (Transistor version).
The volume control on these can be a problem, if the wiper is open circuit it could explain your no sound problem. Sometimes you can dismantle them to repair. If you just check the resistance between the middle connection on the control and either of the 2 other terminals it should change smoothly as you turn the control. If it is very high at all positions of the control (eg 1M+) this will be why you get no sound
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