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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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27th Feb 2016, 5:05 pm | #1 |
Pentode
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RCA AR88D HT problem.
Hello all
I need help, please, with low HT problem on an AR88D. My present measurements are as follows: With mode switch at 'transmit' [not for long] and all valves in place HT at V11 anode is 475. With mode switch at 'rec mod' HT at V11 anode is 70. With R30 disconnected V11 anode reads 285. Readings now with all valves removed except V14 [ Rectifier], V13 [voltage reg.] and V11 [AF o/p] with R30 connected. On 'transmit' position HT on rectifier cathode is zero. HT on V11 anode is 471. On 'rec mod' position HT on rectifier cathode is 171. HT on V11 anode is 300ish. Readings with conditions as above but with V13 removed; On 'transmit' pos. HT on rectifier cathode; zero HT on V11 anode is; zero On 'rec. mod' position HT on rectifier cathode; 332. HT on V11 anode is 275. With all valves back in place parameters are as follows; On 'transmit' pos. HT at V14 cathode is; 79. HT at V11 anode is; 75. On 'rec mod.' pos. HT at V14 cathode is; 80 HT at V11 anode is; 75. I am aware that 'transmit' position open anode circuits except V11. There are no signs of any smoke or indeed any overheating at all. And the set has been working beautifully until now. If any 88 buff has a few moments to dig out a schematic and consider these figures and a bit of advice, I would be most grateful, since having gone round in circles I am getting the horrible feeling that I might have missed something elementary! Regards to all. Ian Rose |
27th Feb 2016, 5:07 pm | #2 |
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Re: Ar88 ht problem
Have the oil filled caps been rebuilt I wonder?
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27th Feb 2016, 5:24 pm | #3 |
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Re: RCA AR88D HT problem.
Here's the circuit.
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27th Feb 2016, 5:31 pm | #4 |
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Re: RCA AR88D HT problem.
I would first measure the HT current to determine if you have too much HT current or something simple like a low emission rectifier.
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27th Feb 2016, 7:40 pm | #5 |
Pentode
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Re: Ar88 ht problem
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27th Feb 2016, 7:41 pm | #6 |
Pentode
Join Date: Aug 2010
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Re: RCA AR88D HT problem.
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27th Feb 2016, 7:51 pm | #7 |
Pentode
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Re: RCA AR88D HT problem.
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27th Feb 2016, 7:55 pm | #8 |
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Re: RCA AR88D HT problem.
High resistance connection?
Bias resistors? Lawrence. |
27th Feb 2016, 8:02 pm | #9 |
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Re: RCA AR88D HT problem.
Yes a possibilty as Colin points out low emission rectifier.Or main smoothing caps low capacity? C96/97/98
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27th Feb 2016, 8:43 pm | #10 |
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Re: RCA AR88D HT problem.
R30 powers the VR150/30 and the oscillators.
If something downstream of R30 is taking far too much current, R30 will be smoking and the regulator won't be lit. Possibilities: Low emission rectifier Bad mains transformer (on both phases, or the primary? if heaters are also low) Those resistors in the negative-to-chassis path. They do sometimes burn out, giving low HT and a remarkable amount of -ve volts on the AGC line, making the remaining stages pretty low current. It also makes a lot of neg bias for G1 of the 6V6, so R30 is the main current load on the HT +ve Cast a very jaundiced eye at R43 R44 R45 David
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28th Feb 2016, 1:30 pm | #11 |
Pentode
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Re: RCA AR88D HT problem.
Thank you Lawrence, Hamish and David.
Will look closely at those areas and report back. regards ian |
1st Mar 2016, 11:42 am | #12 |
Pentode
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Re: RCA AR88D HT problem.
Thank you all for your interest in the 'missing HT'.
And to Lawrence and David for pointing me in the direction of the bias R's. Turned out that R45 was open circuit [in fact, split]. So, good result and the 88 is now singin' its heart out!! Best regards and cheers to you all. |
1st Mar 2016, 11:55 am | #13 |
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Re: RCA AR88D HT problem.
The bias resistors are a common fault with that receiver, good news anyways.
Lawrence. |