AR88 Restoration.
 

I have an original AR88 here. When I got it, the smoothing can had spilt its guts out, one of the bias network resistors (R44) had been nicely flambéed damaging the board underneath, and signs of leakage from several of the bathtub capacitors.

I replaced the smoothing can with a new metal box I had made up, and inside mounted some stripboard with 3 x 4.7uf 630v polyester capacitors as per a helpful layout seen here from another thread:

https://www.vintage-radio.net/forum/...7&d=1472224526

I replaced the 2 output valve coupling capacitors with a single one and the cap from the anode of the 6v6 to ground, tacked in a replacement for R44 and applied power.

The receiver has only a faint hum from the speaker, and the VR150 is not glowing at all. I have a spare Vr150 and get the same result. R30 reads 1.7k which is way off, but I am only seeing 50vDC wrt chassis where it meets VR150.

In TRANS mode, I see around 505 vDC at the junction of C98 and L49. Would I be correct in assuming something on the 150v rail is pulling the high voltage down? Ive replaced a few of the suspect looking micamold capacitors (C121, C52, C119) with no improvement so far.

Perhaps I need to bite the bullet and get on with the bathtub rebuilds. Is this the best way to go?

Re: AR88 Low HT
 

With respect I think you are missing something here. Not got the circuit in front of me at present.

Re: AR88 Low HT
 

550v says there's an open circuit and the power supply is off load. The VR150/30 unlit says HT to chassis is low.

Look at the series resistors from the PSU -ve end to chassis. They are known problems.

David

Re: AR88 Low HT
 

The 505v measurement was seen when the receiver was in "trans" mode, , as I understand it in trans mode SW23 in the attached is open so the 505v should be expected. I wanted to check the voltage at this point to see what the power supply was doing unloaded

I may well be missing something. I'm nowhere near as experienced as many on this forum, so thought I'd ask a little advice on how I'm approaching this.

I'll take a look at the other resistors, R44 has been changed as it had been well cooked.

Re: AR88 Low HT
 

Trans mode supplies HT to the audio output valve not to the VR150 dropper resistor so far as I remember.

Lawrence.

Re: AR88 Low HT
 

That looks correct Lawrence, so the problem is further away from the main caps. I don't see R44 and wonder what it feeds.

Re: AR88 Low HT
 

R44 is part of the bias supply series chain, it supplies bias to the 1st AF amp.

Lawrence.

Re: AR88 Low HT
 

Worth checking the rectifier: on my AR88 one half of the 'tape' filament in the directly-heated rect went O/C so the radio was essentially working with half-wave rectification, meaning hum, distortion and poor sensitivity.

Re: AR88 Low HT
 

A few years ago, my AR88D had low HT. I eventually discovered a very leaky ( almost short ) capacitor. This capacitor was a decoupling capacitor, mounted inside the RF coils box. It was dragging the HT down. I can't remember which one it was, but it was about 1 and a half inches long by an inch wide, by quarter of an inch thick, so quite chunky and dark brown. It was mounted edgeways on and close to one of the partitions inside the box. Replacing it brought the HT voltage back up and allowed the VR150 to emit its usual purple glow.

Hope you get it sorted.

Aub

Re: AR88 Low HT
 

I think I have my work cut out for me with this set ;)

I don't know the history of it, but I know the seller I bought it from had plugged it straight in to "test" it, with no speaker attached and the switch in TRANS mode, whether R44 was cooked prior to this I'm not sure.

@Aub I think I know the capacitor you mean, It stood out visually to me, I believe its C54.

Before I do that however I will replace the other bias network resistors (R43 and R45), and have a closer look at the rectifier. Unfortunately my other set has been rewired to take a GZ32 so I don't have one to swap just now, perhaps I'll order one as a spare for future. Hope to report back further in a few days.

Best regards
Adam

Re: AR88 Low HT
 

Have you tried measuring the resistance across the HT line? It won't be open circuit, as the screen grids are fed from a potential divider.

Re: AR88 Low HT
 

I can't see a potential divider across the HT.

Lawrence.

Re: AR88 Low HT
 

All to the good then, there'll be infinite resistance across the HT line.

These sets have a separate screen grid supply for some valves though, rather than using individual dropper resistors for each valve.

Re: AR88 Low HT
 

Yes, the mixer and IF screens HT is stabilized, RF screens aren't.

Ist and 2nd IF screens have no separate dropper, the rest of the RF/IF screens do so far as I can make out.

Lawrence.

Re: AR88 Low HT
 

I'm another one who doesn't like upside down American diagrams.

Years ago I had a print shop print off some AR88 diagrams on A1 sized paper and I coloured in the Anode, Screen and AGC lines in different colours.

Needless to say I can't find them now.

Re: AR88 Low HT
 

I'm not a particular fan of that schematic, same goes for the BC348.

Lawrence.

Re: AR88 Low HT
 

On the VMARS site somewhere there's a schematic [or circuit-diagram if you must be pedantically British] of the AR88 redrawn to 1950s UK-Military style rather than the sometimes-confusing-at-first-sight US style.

Re: AR88 Low HT
 

Please be reminded that the easiest method of finding if a cap is pulling down the HT volts on these sets is to simply disconnect the hot end of the cap.
If the volts jump up then replace the cap, not worth restuffing the bathtubs, just disconnect and put in new small caps

MM0HDW

Re: AR88 Low HT
 

Only just seen these replies. I did spend a very small amount of time last night looking at the receiver, quite a logistical operation getting into my workshop, I'd leave it in there but it's not the driest...

I tried disconnecting some more questionable capacitors in the RF box, no difference still.

I did happen to measure between where R30 goes to the VR150 and chassis and saw 70k rising slowly. Looking at the bias resistors it appears one may be missing but there have been some modifications to this area. I'm going to need to trace this out with the circuit digram properly. I have an LF version I can visually compare to which may also help.

I'll post back when I can provide some better information

Re: AR88 Low HT
 

The Micamold brand 4.7nF decouplers are certainly high on the suspect list here ( not actually mica dielectric, but rather average quality paper types with a moulded Bakelite case), electrically they'll vary from quite leaky to horribly so by now. The degree of leakage also tends to be very voltage-dependent, too- a capacitor that registers into megohms with a meter using a 9V battery might be uselessly leaky with HT applied. Whilst such an ohm-meter can give a rough guide with an array of capacitors, I don't even consider keeping anything that registers even slight lingering constant reading in place for this reason. With an original AR88 (or whatever), even if each leaky capacitor only takes a small DC current, the aggregate can be substantial and make mains transformer, smoothing chokes and bias resistor chain all run much hotter than they should, not to mention extra stress on the rectifier. The Micamolds start life as flat, dry and mid-brown in colour, any that look swollen/sweaty/dark coloured need evicting pronto. The oil-filled "bath-tubs" also go electrically leaky but not often to the widespread and catastrophic degree as the Micamold types.

Some folk out there get a little over-wrought in their description of the AR88- "money no object", "second-to-none" etc.etc. Yawn. It's of commercial-market origin, a good contemporary design built to "good commercial" rather than "military" standards and some of the passive componentry is a bit average in nature, so it's not surprising that a thorough and possibly long-winded overhaul is near-inevitable after all this time.