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Old 27th May 2019, 8:39 am   #21
cathoderay57
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It misbehaved again this morning so the OC71 was innocent. Next I will change the 400uF speaker coupling cap then start working my way back from the O/P transistors.
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Old 27th May 2019, 10:24 am   #22
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The output transistors have been known to have the whiskers problem, although not usually intermittent.
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Old 27th May 2019, 3:28 pm   #23
cathoderay57
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I think I have found the culprit - see pic. Tapping RV3 with a screwdriver reproduced the symptoms. On removing the preset from the board it measured open circuit between its ends and intermittent wiper connection. Replaced with a new one and all seems well (for now). Looking at the circuit RV3 affects the biasing of the whole amplifier chain since the transistors are all direct-coupled. When RV3 has an open circuit it throws out all the biasing hence distortion and loss of audio. Cheers and thanks for the advice. I hope I won't be back (with this radio, at least!) Jerry.
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Old 27th May 2019, 5:44 pm   #24
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Excellent, enjoy using the radio.
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Old 27th May 2019, 8:17 pm   #25
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Generally speaking, I tend to replace skeleton pre-sets with equivalent enclosd (Piher or similar) because they can fail in the way described.
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Old 30th May 2019, 9:59 am   #26
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Morning all, the fault has returned. To recap (if you'll forgive the pun) when the set is switched on it works fine for about 5-10 seconds then the sound rapidly distorts and cuts out altogether. Switching the set on and off repeats the symptoms. If I leave the set on, after a couple of minutes, the sound comes back distorted at first then slowly clears. After that it works normally until after the set has been switched off for 24 hours or so. I have copied the circuit of the AF amplifier and attached it for clarity. The only difference from the Hacker schematic as far as I can see is that on mine R10 is 82R and R11 is 180R whereas on the Hacker schematic both resistors are 150R. To date I have put replacement components in for the following, and refitted the originals because the same fault returned: C10, T6, T3. I have fitted a new RV3 which was faulty. All of the voltages on the transistors were measured close to the spec in the service sheet although I could not measure all of them while the fault was present as the set would start working before I finished measuring! This morning I measured the mid-point voltage (junction of R14 and R15) and it was 8.3v. The service sheet specifies a setting of 8.55v for an 18v supply. My supply is running at 17.7v. However, tweaking the new RV3 (which sets the mid-point voltage) increased the midpoint voltage and sound appeared immediately. Backing off RV3 caused the sound to distort and disappear. The acceptable setting range of RV3 (by which I mean the range over which I could get clear sound) seemed to increase after the set had "warmed up". To my mind this is pointing towards a leaky capacitor reforming? Maybe C8, although I have disconnected one end and reformed it OK on the Avo after which the set worked OK for a few days. Please can I have some advice, as I have just about exhausted my powers of deduction on this?. Cheers, Jerry
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Old 30th May 2019, 10:43 am   #27
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I had a similar experience in my Ekco A455 a while back with it going quiet and distorted, I thought I had a tuning cap short. Turned out it was a whiskery ac128 in the output stage, removal and a continuity test confirmed it. A replacement ac154 cured it.
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Old 30th May 2019, 10:44 am   #28
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Nicely drawn out circuit diagram. As you are seeing near enough correct voltages during the I would check out the connecting lead between the AF output board and the main chassis. Tighten up and carefully clean the plug and socket. On my RP38A this lead me a right song and dance and was giving me about three different faults. A also found a small hairline crack on the AF output board just underneath the plug and socket. Next I would change out the electrolytics on the AF output board. If the fault still persists I would use a tin of freezer spray. When the fault develops carefully apply a small squirt to the transistors T1, T2 and T4.
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Old 30th May 2019, 7:22 pm   #29
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R14 R15 are 2.2 Ohms not 2.2 KOhms. The overall current should be checked when the fault occurs, if the current increases it could be due to leakage in a capacitor or a transistor breaking down. This is more typical with a transistor drawing more current while the sound fades, though this can also be caused by the base bias resistors such as RV4 being faulty and increasing the base bias of the output transistors making the output transistors draw excessive current. It would be better to replace the electrolytic capacitors, C6 in the bass control should not be overlooked. The pF range capacitors are usually reliable but can cause unusual faults if they go leaky, C7 and C11 should be checked.
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Old 31st May 2019, 8:50 am   #30
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Thanks to all for advice. Agree with Geoff, I incorrectly marked R14 & R15; both are 2.2R. Yesterday I found 2 resistors had gone high: R8 (47k) measured 78.8k & R15 (150k) measured 211k. Both replaced. I'll continue to test other components as suggested. Cheers, Jerry
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Old 31st May 2019, 4:57 pm   #31
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Here's the latest update. As Geoff suggested, I tested the overall current drain from the power supply. Of course, when I switched on the fault had disappeared! Nevertheless I measured it and it was 18.2mA with volume at zero, peaking higher when the volume was increased, and showing a higher zero-volume current on FM than on LW or MW. I checked C6, C7 and C11 with the Avo and all showed zero leakage. I had previously checked all of the electrolytics on the amplifier board with the Avo and all charged up normally with an ultimate leakage resistance >1MOhm on the x100 range. On discharge all were holding a charge of around 12v. I then started checking the resistors on the amplifier board using a digital ohmmeter. All were within 20% except for the following: R2 (4.7k) measured 6.2k; R3 (5.6k) measured 7.2k; R12 (680R) measured 909R; R13 (330R) measured 410R. All of these were replaced with new resistors. I know what you're thinking, all resistors measuring high - was my meter reading high? I checked all of the replacement resistors using the same meter and all indicated correctly the marked resistance value. I then repeated the set-up of the quiescent current through the o/p transistors (3mA) and the midpoint voltage (8.55v) using RV4 and RV3 respectively. Here I ran into difficulty as I could not achieve both values simultaneously, otherwise the sound went distorted. The closest I could get was a quiescent current of 4mA with a midpoint voltage of about 8.6v. Even then there was slight distortion at low volume level. So I replaced T4 (Lockfit BC159) with a BC557. Set-up was repeated and hit both target values exactly, with perfectly clear sound. So I now have to wait 24 hrs and see if the fault returns.... They say patience is a virtue? Cheers, Jerry
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Old 2nd Jun 2019, 8:56 am   #32
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It's working normally this morning so unless I come back again I reckon it is fixed
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Old 3rd Jun 2019, 12:40 am   #33
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Nice work Jerry – looks like you’ve cracked it!

I’m interested in comment earlier in this thread re comparison between Hunter and Sovereign. Mark Hennessy said – “They sound and measure better than the Sovereigns…”. The Hunter and Sovereign use very similar (A209 and A205) AF amplifiers, though there are some differences. Should we conclude that the A209 (Hunter) is a better amplifier than the A205 (Sovereign), or is something else going on?

I ask because I have a Sovereign, and the output quality seems a bit iffy in terms of frequency response. So, I’m wondering whether to just go ahead and test and troubleshoot the A205 as is, or to put in the fairly simple mods that would convert it to A209 (having tested the existing A205 to eliminate any bad components) – the two amplifiers look as though they would be “drop-in” substitutes.

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Old 3rd Jun 2019, 10:57 am   #34
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It's not so much the amplifier; it has more to do with the loudspeaker.

I posted a series of measurements of various radios a couple of years back, including these sets: https://www.vintage-radio.net/forum/...d.php?t=135896
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Old 3rd Jun 2019, 10:15 pm   #35
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Thanks for that Mark. My RP25A has the Celestion speaker, and I've always felt it lacked treble. I think your work in the thread you linked to confirms this. Many thanks for that link - I hadn't seen it before. Loads of useful info and lots to think about.

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Old 14th Jun 2019, 8:43 am   #36
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Hello again. The radio hadn't been used for about 3 days and guess what - the fault has returned. I measured the total current drain from the dc supply while switched to FM with vol at minimum. At switch on when there was brief sound the current was 21mA and when the radio went silent the current only dropped to 20mA. However, under the same conditions the mid-point voltage changed from 9v to 12.6v. Readjusting RV3 brought the voltage back to 8.5v and sound returned but the quiescent current through the output pair had increased to 8mA (should be 3mA). Adjusting RV4 to short circuit would only bring the current down to 4mA. As happened before I couldn't set the midpoint voltage and the quiescent current to 8.55v and 3mA (per service sheet) simultaneously. I could get the set to operate without distortion by setting 6mA and 8.5v. I'm thinking something odd is happening to the output transistors. I have previously tried another AC128 but the fault was still present. I have ordered a replacement AC176 and a full set of new capacitors for the amplifier board. Cheers, jerry
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Old 14th Jun 2019, 9:26 am   #37
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Going back to my repair days we always changed both output transistors even if one measured ok. We did this in all transistorised AF output stages, silicon and ge, from portable audio equipment, Hi Fi equipment and TV sets. plus a full set up and never had any comebacks.
As you are localish to me and I am a proud owner of a Hacker Hunter RP38A I will drop you a PM.
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Old 14th Jun 2019, 11:01 am   #38
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I agree with Simon about replacements. In my full-time working days I repaired Car radios & stereos. For much of that time the DC Coupled complementary symmetrical o/p stage was the norm, and I always replaced both transistors of the o/p pair (AD157/158 or 161/162) in most sets, checked earlier transistors and other components in the circuit.
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Old 14th Jun 2019, 3:21 pm   #39
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Thanks all. Simon, you have a PM. Cheers, Jerry
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